sedimentation of organic rich limestones in the late precambrian of southern norway abstract organic rich laminated limestones are a major facies of the late precambrian biri formation deposited around the margins of the sparagmite basin of southern norway the laminated limestones consist of an alternation of calcitic laminae and clay laminae trace element and stable isotope analyses confirm a marine origin for the carbonate and suggest that the original precipitate was aragonite the organic matter reaches 3 weight loss on ignition and apparently is finely dispersed within the sediment both cm thick limestone beds of probable storm origin and thin disrupted and folded horizons of slump origin occur within the laminated facies the organic rich laminated limestones occur at several horizons and are interbedded with carbonate grainstones of shallow water origin basinwards they pass into light to dark grey to green organic deficient mudrocks with resedimented and slumped units the facies context of the organic rich laminated limestones suggests deposition in an outer shelf setting at times of higher sea level stands the transgressive events possibly induced by changes in glacial ice cover of near or distant landmasses led to increased organic productivity through seasonal blooms of phytoplankton preservation of the organic matter in the sediments resulted from the development of anoxic or at least oxygen depleted bottom waters some physical constriction of water circulation in the sparagmite basin may have contributed towards the anoxic events the development of precambrian organic rich black shales in the sparagmite basin of southern norway does show that potential hydrocarbon source rocks were formed in the pre phanerozoic in situations comparable to some mesozoic and tertiary oil source rocks

extreme 13ccarb enrichment in ca 2 0 ga magnesite stromatolite dolomite red beds association in a global context a case for the world wide signal enhanced by a local environment abstract the palaeoproterozoic positive excursion of 13 c carb is now considered as three positive shifts of 13 c carb separated by returns to 0 which all occurred between 2 40 and 2 06 ma this isotopic event is unique in terms of both duration 300 ma and 13 c enrichment up to 18 the mechanism responsible for one of the most significant carbon isotopic shifts in earth history remains highly debatable to date 13 c of 10 to 15 cannot be balanced by organic carbon burial forg as there is no geological evidence for an enhanced corg accumulation prior to or synchronous with the excursion instead termination of these excursions is followed by formation of a vast reservoir of 13 c depleted organic material 45 at shunga and by one of the earliest known oil generation episodes at 2 0 ga none of the three positive excursions of 13 c carb is followed by a negative isotopic shift significantly below 0 as has always been observed in younger isotopic events reflecting an overturn of a major marine carbon reservoirs this may indicate that forg was constant implying that the mechanism involved in the production of corg was different onset of intensive methane cycling resulting in c change is another possibility the majority of sampled 13 c carb rich localities represents shallow water stromatolitic dolostones red beds and evaporites formed in restricted intracratonic basins and may not reflect global 13 c carb values closely spaced drill core samples n 73 of stromatolitic dolostones from the 1980 27 ma tulomozerskaya formation in the onega palaeobasin russian karelia have been analysed for 13 c carb and 18 o carb in order to demonstrate that different processes were involved in the formation of 13 c carb rich carbonates the 800 m thick magnesite stromatolite dolomite red beds succession formed in a complex combination of environments on the karelian craton peritidal shallow marine low energy protected bights barred basins evaporative ephemeral ponds coastal sabkhas and playa lakes the carbonate rocks exhibit extreme 13 c enrichment with 13 c values ranging from 5 7 to 17 2 vs v pdb mean 9 9 2 3 and 18 o from 18 6 to 26 0 vs v smow mean 22 0 1 6 the tulomozerskaya isotopic excursion is characteristic of the global 2 4 2 06 ga positive shifts of carbonate 13 c 12 c although it reveals the greatest enrichment in 13 c known from this interval an external basin s is considered to have provided an enhanced corg burial and global seawater enrichment in 13 c the global background value for the isotopic shift at tulomozero time ca 2 0 ga is roughly estimated at around 5 an explosion of stromatolite forming microbial communities in shallow water basins evaporative and partly restricted environments high bioproductivity enhanced uptake of 12 c and pene contemporaneous recycling of organic material in cyanobacterial mats with the production and consequent loss of co2 and ch4 are believed to be additional local factors which may have enhanced 13 c from 5 up to 17 such factors should be taken into account when interpreting carbon isotopic data and attempting to discriminate between the local enrichment in 13 c and globally enhanced 13 c values we propose that many previously reported 13 c values from other localities where environmental interpretations are not available or have not been taken into account may not represent the global 13 c values

geoid effects of lateral viscosity variation near the top of the mantle a 2d model abstract whereas the oceanic lithosphere is underlain by a low viscosity layer the asthenosphere it appears unlikely that this layer is well developed beneath continental cratons this difference could represent a lateral viscosity variation of two to three orders of magnitude near the top of the mantle previous mantle flow geoid models have typically neglected this variation to investigate the impact of such a lateral strength variation on mantle flow and the resulting geoid we solve for density driven flow in a 2d box of uniform viscosity fluid a dichotomous lateral viscosity variation near the top of the mantle is modeled by having a no slip boundary condition over a section of the top of the box while allowing the remainder of the top to be free slip for varying fractions of no slip on the upper boundary and various values of the wavenumber depth and phase of the driving density anomaly model geoids are found in contrast to viscous flow with only a depth dependent viscosity mode coupling in the geoid is observed this occurs principally at a wavelength equal to the lengthscale of the lateral viscosity structure with amplitudes as large as 20 40 in some cases thus coupling can produce a significant signal in the geoid at a wavelength longer than that of the driving load this result suggests that the accuracy of models of the geoid that assume a solely radial viscosity structure may be limited to the 60 80 level found in recent studies 1 2 we speculate that some of the earth s low order geoid is due to flow coupling resulting from the degree two to five components of lateral ocean continent structure

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

lateral variations in mantle rheology implications for convection related surface observables and inferred viscosity models summary over the past decade numerous analyses of convection related observables such as horizontal surface divergence geoid or gravity anomalies and dynamic surface topography have been carried out in the context of tomography based mantle flow models one of the major objectives of this modelling has been the inference of the rheological structure of the mantle with few exceptions these studies have been conducted in the framework of a viscous flow theory which assumes that the mantle rheology may be represented in terms of an effective viscosity which varies with depth only here we present a detailed assessment of the impact of lateral variations in viscosity on global convection related observables using forward modelling of buoyancy induced flow in a 3 d spherical shell we find that the resulting dynamic topography at the surface and the core mantle boundary as well as the gravitational response of the earth are affected relatively little by the inclusion of lateral viscosity variations lvv when compared with results for a purely 1 d radial viscosity model in particular we found that the effect of lvv on the global observables is significantly smaller than the variability due to uncertainties in the current seismic tomography models we also quantify the effect of lvv in the context of the viscosity inverse problem using two synthetic data sets generated with a 1 d viscosity profile and with a fully 3 d viscosity model in which the lvv span across three orders of magnitude we compared the 1 d viscosity profiles recovered from the inversions and found that lvv have virtually no effect on our inversion results the synthetic viscosity inversion further revealed that the effect of lvv is small in comparison to the uncertainties arising from the seismic tomography models the inversions also suggest that the 1 d viscosity profiles derived from actual surface data represent the depth variation of the horizontally averaged logarithm of the 3 d viscosity distribution in the mantle intr o duction constraining the rheological properties of the mantle using surface geophysical data has traditionally involved separate treatments of short seismic intermediate solid earth tides long postglacial rebound and very long mantle convection timescale processes in each case the initial focus historically speaking has been on the inversion of the data to determine a one dimensional 1 d purely radial variation of the relevant rheological parameter e g elastic moduli intrinsic attenuation and viscosity which provides the best fit to the data it has long been recognized particularly in the context of the seismic inverse problem that the search for a 1 d earth model is subject to possible bias due to the presence of lateral variations in structure which may be as significant as the radial variations especially at shallow depths this paper is concerned with the mantle viscosity field the earliest inferences of the radial i e horizontally averaged profile of viscosity based on glacial isostatic adjustment gia data e g mcconnell 1968 parsons 1972 were characterized by a pronounced increase of viscosity with depth in the top half of the mantle however these were followed by two decades of gia research that argued for an essentially isoviscous

radial models of mantle viscosity results from a genetic algorithm summary the long wavelength component of the geoid has been used to constrain radial variations in mantle viscosity using 3 d velocity anomaly models determined from seismic tomography as the buoyancy forces driving viscous flow using a genetic algorithm ga the robustness of mantle viscosity models obtained by calculusbased optimization methods is investigated gas are a relatively new class of optimization techniques which are generating a growing interest in the geophysics community these methods can be particularly useful for solving highly non linear optimization problems unlike traditional techniques gas do not require derivative information to form a jacobian matrix or sensitivity kernels models are constructed from pieces of successful models following a simple set of rules these simple rules are driven by stochastic rather than deterministic means because of the efficiency of the ga for the mantle viscosity problem it is possible to explore a greater range of potential solutions than with a traditional optimization method it is found that while both calculus based and ga optimization methods find viscosity models with a low viscosity transition zone the ga finds another class of models with a high viscosity in the transition zone that also satisfies the geoid data the geoid alone is unable to resolve low viscosity transition zone models from high viscosity transitionzone models the effect of the scaling between seismic velocity and density on the preferred viscosity model is also explored while a velocity to density scaling with a pronounced increase in the transition zone provides the best fit to the geoid the preferred vertical viscosity profile is insensitive to the velocity to density scaling

influence of the tectosphere upon plate motion this paper tests two hypotheses for the influence of thick lithosphere the tectosphere upon plate motion the first is that the tectosphere reaches down into high viscosity regions of the asthenosphere effectively slowing the motion of continental plates the second is that the tectosphere reaches down below the slow moving convective boundary layer into regions of rapid mantle convection and effectively speeds or slows the motion of continental plates our data imply that the tectosphere speeds plate motion rather than impedes plate motion and are thus most consistent with the second hypothesis

a continuous kinematic model of plate tectonic motions summary one of the many problems in the study of plate mantle coupling is the disparity between the theory of mantle dynamics which involves continuum physics and the theory of plate tectonics which employs discontinuous plates this discordance causes a variety of difficulties in geodynamic models e g infinite tractions between the plates and mantle as motions of the earth s surface do not involve discontinuous plates i e intraplate deformation is significant and plate margins have finite width it is necessary to adjust the plate tectonic model to allow for continuous surface motions here we present a model of plate motions on a sphere using analytically continuous i e infinitely differentiable functions to describe both plate geometry and plate margin width we first apply this model to the idealized system of a single rectangular plate to examine the influence of plate geometry and size on kinetic energy partitioning of plate motions on a sphere the ratio of toroidal strike slip and spin kinetic energy to poloidal convergent divergent kinetic energy is affected not only by the relative lengths of strike slip and convergent divergent margins but also on plate size which controls the magnitude of plate spin for large plates spin toroidal motion contributes a major portion of the net toroidal energy basic concepts from this simple illustrative model are then expanded to derive an analytically continuous model of present day plate tectonic motions the plate boundary for any given plate is smoothed and expressed as a single valued differentiable function this function is employed to generate the analytically continuous shape function of the plate the shape function is then used to model the plate s motion about its contemporaneous euler pole this technique is carried out for all the plates and their motions are superposed to yield a complete yet simplified model of present day plate motions we use this model to examine the influence of plate margin width on energy partitioning for the earth s plates this approximately indicates the extent to which energy partitioning is influenced by intraplate deformation the calculations indicate that the introduction of finite margin width allows spin vorticity to make a larger contribution to the toroidal energy depending on margin width the partitioning of energy is possibly reversed with toroidal energy assuming a larger proportion of the net kinetic energy the model of continuous plate motions proposed here may not only ease the disparity between the theories of plate tectonics and mantle convection but is a first simple step toward incorporating intraplate deformation into plate tectonics

violent volcanism stagnant oceans and some inferences regarding petroleum strata bound ores and mass extinctions past changes from a well mixed aerated ocean to a stratified stagnant ocean are indicated by decreasing 13c of limestone opposite to prior conclusions as well as by increasing 34s of marine sulfates the proposed stagnant ocean model includes a warm co2 enriched atmosphere and a bathyal to abyssal marine system dominated by bacterial components in the food web and by consequent light carbon in organic material and biogenic carbonates culminations of prolonged stagnant episodes correspond with marine faunal extinctions of late permian and late cretaceous time and the protracted changes and correlations provide evidence against any hypothesis of mass extinction by sudden worldwide catastrophe including the asteroid impact hypothesis the cretaceous stagnant ocean taken as the prime example is attributed to climatic warming triggered by volcanic co2 greenhouse effect and several feedbacks including decreased earth albedo and increased sinking of warm evaporite brines instead of aerated polar waters marine extinctions are attributed to upward expansion of the oxygen minimum zone and to catastrophic mixing of surface waters with poisonous sulfidic waters of the deeps the stagnant ocean provided a counterbalance between deep reduction and shallow oxidation conditions that favored maximal formation of black sulfidic shales protopetroleum and sedimentary sulfide ores and shallow to mid depth barite phosphorite iron ore cherty iron formation and sulfate bearing red beds trace elements concentrated in stagnant ocean sediments include chalcophile and pt group metals negating claims that ir provides a unique fingerprint of meteorite impact and cosmic accretion

carbon isotopic profile across the bilara group rocks of trans aravalli marwar supergroup in western india implications for neoproterozoic cambrian transition abstract the rocks of marwar supergroup in the trans aravalli sector in western india are presumed to span the time interval between neoproterozoic and early cambrian this predominantly unfossiliferous marine sedimentary sequence is characterized by a lower arenaceous facies jodhpur group middle carbonate facies bilara group and upper argillaceous arenaceous facies nagaur group rocks the sedimentation has been essentially in a shallow basin described either as the fore land slope of the rising aravalli mountains or a sag basin which developed and evolved due to subsidence of the updomed crust during neoproterozoic malani magmatism that failed to open rifts the carbon isotopic profile for the bilara group carbonate rocks in the lower part shows marked oscillations and broadly negative 13 c character with negative anomalies as low as pdb observed near the base of dhanapa formation lower unit and pdb in the overlying gotan formation middle unit the upper part of the profile shows a gradual positive shift the carbon isotopic signatures of the bilara group rocks can be correlated with the end neoproterozoic early cambrian vendian tommotian carbon isotopic evolution curve extremely low 13 c values indicate the glaciation related cold climatic postulates of the end neoproterozoic followed by the warmer climatic conditions as indicated by the positive shift the carbon isotopic data for gotan formation carbonates at variance with the globally observed 13 c trends for early tertiary do not support the recently proposed tertiary age for the bilara group

paleolithospheric structure revealed by continental geoid anomalies abstract lithospheric geoid anomalies record changes in elevation and potential energy experienced by continental lithosphere estimates of local isostatic equilibration and potential energy in tandem with lithosphere related geoid anomalies can be used to estimate paleolithospheric thickness providing a clearer understanding of how and why continental topography is developed we employ several simplifying assumptions about the crustal and mantle lithosphere density and structure and readily acknowledge that our results are therefore first order approximations to predict the pre orogenic structure of the lithosphere at the outset we emphasize that while this approach does not provide an exhaustive evaluation of the deformation mechanism it does serve to quantify the relative role played by the variations in the crustal and upper mantle components of the lithosphere in this way we are able to use independent measurement of lithospheric geoid anomalies current post orogenic elevation and lithospheric structure and paleoelevation information to estimate topographic development and structural support over time application of this technique to the southwestern united states indicates that the uplift of the colorado plateau is the result of processes in both the crust and mantle lithosphere and that the lithosphere of the pre orogenic southern basin and range was thinned relative to the northern basin and range and colorado plateau although we use the southwestern u s as an example this method can help constrain uplift mechanisms for any region for which the structure and geoid anomaly of the modern lithosphere is well understood

contributions of a strengthened early holocene monsoon and sediment loading to present day subsidence of the ganges brahmaputra delta the contribution of subsidence to relative sea level rise in the ganges brahmaputra delta gbd is largely unknown and may considerably enhance exposure of the bengal basin populations to sea level rise and storm surges this paper focuses on estimating the present day subsidence induced by holocene sediment in the bengal basin and by oceanic loading due to eustatic sea level rise over the past 18 kyr using a viscoelastic earth model and sediment deposition history based on in situ measurements results suggest that massive sediment influx initiated in the early holocene under a strengthened south asian monsoon may have contributed significantly to the present day subsidence of the gbd we estimate that the holocene loading generates up to 1 6 mm yr of the present day subsidence along the gbd coast depending on the rheological model of the earth this rate is close to the twentieth century global mean sea level rise 1 1 1 7 mm yr thus past climate change by way of enhanced sedimentation is impacting vulnerability of the gbd populations

a widespread positive 13ccarbanomaly at around 2 33 2 06 ga on the fennoscandian shield a paradox carbon isotope measurements carried out on 201 carbonate samples from the early proterozoic of the kola peninsula n karelia and norway yield 13c pdb spanning 20 5 to 11 a general 13c secular trend shows that prior to 2 33 ga values are typically normal marine averaging around 3 0 between 2 33 and 2 06 ga in jatulian time there follows a rapid excursion to positive 13c of around 6 post jatulian time is characterized by 13c of sedimentary carbonates fluctuating between 5 and 3 also it is remarkable for the first pronounced development of diagenetic carbonates which have 13c between 14 and 6 the c 6 positive 13c shift with a duration of about 270 myr coincides with a maximum in the diversity and abundance of stromatolites and with widespread development of red beds but does not coincide with the maximum of buried corg mass the fennoscandian shield represents the largest isotoically anomalous carbonate province yet reported and the positive 13c excursion together with a series of major global palaeoenviromental changes seems to be more intense than the precambrian cambrian transition events however it is still not clear what kind of mechanism this phenomenon could be attributed to an increase of the ronov ratio and or broecker ratio and other possible models are discussed as the target for future investigations

effect of lateral viscosity variations in the top 300 km on the geoid and dynamic topography summary we have investigated the impact of lateral viscosity variations lvv in the top 300 km of the mantle on the long wavelength gravitational response of the earth in contrast to the previous studies we demonstrate that the lvv may play a crucial role if a model with imposed plate velocities and partial layering is considered assuming lvv in the top mantle associated with the continental roots and the oceanic asthenosphere we are able to explain a significant portion of the geoid 85 per cent and of the free air gravity data 55 per cent without invoking a complex radial viscosity profile or assuming a lithosphere with an unrealistically low uniform viscosity the best fit to the data is found for models in which mass anomalies located at the boundary between upper and lower mantles reduce the mass flux across the 660 km discontinuity by a factor of 3 these models are characterized by a low average value of viscosity in the asthenosphere below the oceans 10 19 pa s and a large viscosity contrast 100 between the young suboceanic mantle and the deep continental roots the viscosity in the deep upper mantle is found to be around 3 10 20 pa s while a value higher by at least two orders of magnitude is typical for the lower mantle the predicted dynamic surface topography is influenced by the surface plate velocities and its amplitude amounts to a few hundred metres besides investigating the viscosity structures that reflect the different thicknesses of the lithosphere in continental and oceanic regions we have searched for a large scale pattern of lateral viscosity variation in the top 300 km that yields the best fit to the geoid the pattern obtained by this inversion shows a good correlation with the ocean continent distribution and 90 per cent of the geoid and 67 per cent of the free air gravity can be explained

global plate motion and the geoid a physical model summary the effect of lateral variations in the sub lithospheric viscosity i e in the lithosphere asthenosphere coupling is incorporated in dynamic earth models driven by plate velocities if the coupling is stronger under fast moving continents it is shown that a geoid low will develop in their wake and a high in front of them thus the well known low in the indian ocean could be explained at least in part in terms of induced upper mantle dynamics similarly if the thickness of oceanic plates increases with age the models show that trenches should be associated with marked geoid highs whereas ridges could correspond to much weaker geoid lows this also seems to agree with some features of the observed geoid at very long wavelengths the mathematical framework of such dynamic earth models is developed extensively here these models are characterized by lateral viscosity variations inside an outer shell and a purely radial viscosity structure at greater depth their internal flow patterns are only driven by imposed surface velocities not by internal loads

current plate velocities relative to the hotspots incorporating the nuvel 1 global plate motion model nuvel 1 is a new global model of current relative plate velocities demets et al 1990 which differ significantly from those of prior models here we incorporate nuvel 1 into hs2 nuvel1 a new global model of plate velocities relative to the hotspots hs2 nuvel1 was determined from the hotspot data and errors used by minster and jordan 1978 to determine am1 2 which is their model of plate velocities relative to the hotspots am1 2 is consistent with minster and jordan s relative plate velocity model rm2 here we compare hs2 nuvel1 with am1 2 and examine how their differences relate to differences between nuvel 1 and rm2 hs2 nuvel1 plate velocities relative to the hotspots are mainly similar to those of am1 2 minor differences between the two models include the following 1 in hs2 nuvel1 the speed of the partly continental apparently non subducting indian plate is greater than that of the purely oceanic subducting nazca plate 2 in places the direction of motion of the african antarctic arabian australian caribbean cocos eurasian north american and south american plates differs between models by more than 10 3 in places the speed of the australian caribbean cocos indian and nazca plates differs between models by more than 8 mm yr although 27 of the 30 rm2 euler vectors differ with 95 confidence from those of nuvel 1 only the am1 2 arabia hotspot and india hotspot euler vectors differ with 95 confidence from those of hs2 nuvel1 thus substituting nuvel 1 for rm2 in the inversion for plate velocities relative to the hotspots changes few euler vectors significantly presumably because the uncertainty in the velocity of a plate relative to the hotspots is much greater than the uncertainty in its velocity relative to other plates

long wavelength geoid the effect of continental roots and lithosphere thickness variations summary the existence of continental roots and the gradual thickening of the cooling oceanic lithosphere give rise to large scale rheological heterogeneities in the uppermost mantle the effect of these heterogeneities on the long wavelength geoid is investigated using a 3 d mantle flow model involving a low viscosity asthenosphere beneath the oceanic lithosphere and the tectonically active continental regions and a thick highly viscous lithosphere beneath ancient continents below 400 km the mantle viscosity is laterally homogeneous with a lower mantle more viscous than the overlying layer the mantle circulation is driven by imposed surface velocities nuvel 1 hs2 gripp gordon 1990 and by the density anomalies inferred from the tomographic models p16b30 and s16b30 masters et al 1996 the geoid heights both due to plate motion and due to internal loading differ by as much as 30 per cent between the models with and without lateral viscosity variations in contrast to what was suggested in previous studies spherical harmonics 2 and 3 are strongly affected by the lateral viscosity variations these differences in the forward problem suggest that the response to the inverse problem that consists of finding the profile of viscosity as a function of depth providing the best fit to the geoid should be considerably affected by the lateral viscosity variations the shear stresses at the base of the plates induced by the imposed surface velocities and those induced by the internal loading are sensitive to the lateral viscosity variations this suggests that the lateral viscosity variations are very important for understanding the stress field or the forces acting on the plates the direction of these shear stresses which should be linked to the anisotropy direction is very different from the plate motion direction

secondary convection and the growth of the oceanic lithosphere abstract this paper presents a study of the evolution of the oceanic lithosphere from a thermomechanical approach we have investigated the finite amplitude development of secondary convection cells beneath the oceanic plates by means of the single mode mean field equations and the fully 2 dimensional convection equations using finite element techniques both newtonian and non newtonian rheologies with highly temperature and pressure dependences and an activation energy of 100 kcal mol 1 have been employed the temperature at the base of the convecting medium governs the final thickness of the lithosphere the mean interior temperature varies only slightly during the temporal evolution non newtonian rheology has the tendency to induce oscillatory time dependent behavior of the flow heat flow topography and gravity are influenced by secondary convection in two ways small scale perturbations with wavelengths of around 600 km arise from the lateral thermal differences between the uprising and descending convective limbs large scale features are also produced as a consequence of lithospheric growth the calculated quantities of the heat flow topography and gravity associated with small scale convection are typically in the range of o hfu o 102m and o 10 mgal the horizontal mean temperature profiles from the convection model are used to calculate long wavelength geophysical observables as a function of age convective processes are found to reduce the rate of lithospheric thickening predictions from our model can fit well the observed data of heat flow ocean floor topography and geoid offsets along fracture zones the last data base exhibiting the most sensitivity to thermal perturbations below the lithosphere our calculations show that the oceanic lithosphere is able to grow continuously up to o 109 y long past the flattening of the seafloor we report here that the thickness of a thermally equilibrated lithosphere could reach 250 km which lies within the appropriate range of values for the continental lithosphere as inferred from studies in seismology flexure observations and secular polar motions

geoid anomalies and the structure of continental and oceanic lithospheres geoid anomalies in the wavelength range corresponding to spherical harmonies 6 to 30 are used to bring new constraints on the deep structure of the continental lithosphere on the nature of upper mantle seismic anomalies and on the thermal evolution of the oceanic lithosphere for the shallow lithospheric density anomalies considered here the geoid anomaly is shown to follow approximately the moment law derived from isostasy by a procedure of least squares inversion the geoid over old cratons is found to be equal to that over relatively young 50 myr oceanic lithosphere the oceanic geoid as a function of age follows the expected linear trend with flattening at old ages these results are independent of the degree at which the spherical harmonics are truncated if the tectosphere consisted only of a thick thermal boundary layer marked geoid lows with an amplitude larger than 20 m should be present over cratons in disagreement with the present observation the predicted geoid agrees with the observations if the negative density anomalies associated with a depleted peridotitic layer are present to the bottom of the lithosphere this analysis of the geoid confirms the petrological inferences that the deep roots observed seismically in the top upper mantle form a chemically differentiate reservoir the density of these cold roots is similar to that of mantle material elsewhere because the increase in density resulting from their lower temperature is largely balanced by the decrease in density caused by their depletion analysis in wavelength range corresponding to harmonics 6 to 30 in the oceans confirms that the geoid flattens at old ages and thus supports the view that an important amount of heat is transferred at the base of the lithosphere by small scale convection

geoid anomalies in a dynamic earth in order to obtain a dynamically consistent relationship between the geoid and the earth s response to internal buoyancy forces we have calculated potential and surface deformation love numbers for internal loading these quantities depend on the depth and harmonic degree of loading they can be integrated as green functions to obtain the dynamic response due to an arbitrary distribution of internal density contrasts spherically symmetric self gravitating flow models are constructed for a variety of radial newtonian viscosity variations and flow configurations including both whole mantle and layered convection we demonstrate that boundary deformation due to internal loading reaches its equilibrium value on the same time scale as postglacial rebound much less than the time scale for significant change in the convective flow pattern by calculating relaxation times for a series of spherically symmetric viscous earth models for uniform mantle viscosity the geoid signature due to boundary deformations is larger than that due to internal loads resulting in net negative geoid anomalies for positive density contrasts geoid anomalies from intermediate wavelength density contrasts are amplified by up to an order of magnitude geoid anomalies are primarily the result of density contrasts in the interior of convecting layers density contrasts near layer boundaries are almost completely compensated layered mantle convection results in smaller geoid anomalies than mantle wide flow for a given density contrast viscosity stratification leads to more complicated spectral signatures because of the sensitivity of the dynamic response functions to model parameters forward models for the geoid can be used to combine several sources of geophysical data e g subducted slab locations seismic velocity anomalies surface topography to constrain better the structure and viscosity of the mantle

correlation of late precambrian and early palaeozoic sequences by eustatic sea level changes and the selection of the precambrian cambrian boundary abstract the author argues for a pragmatic approach to the problem of defining the precambrian cambrian boundary a boundary at first occurrence of shelly fauna as recommended by the precambrian cambrian working group will create severe difficulties in worldwide correlation by placing the boundary stratigraphically somewhat higher i e the fallotaspis zone with a more abundant fauna there is a much better basis for biostratigraphic correlation eustatic sea level changes may be another important tool for late precambrian early palaeozoic correlation and more detailed facies interpretation of such sequences should be carried out in order to recognise these eustatic sea level changes

effects of lateral viscosity variations on the geoid 1 we investigate the effects of lateral viscosity variations lvvs on mantle circulation model predictions of the global geoid the present study is motivated in part by earlier findings that lvvs due to stiff slabs in the lower mantle have a strong influence on the geoid and that slabs in the lower mantle are perhaps no stronger than the ambient mantle however more recently it has been argued based on global seismic tomography models that lvvs are a minor effect on the geoid in the light of these seemingly contradictory results we re visit the problem of slab strength in the lower mantle we substantiate that the geoid calculated from tomography is hardly affected by the presence of lvvs whereas the geoid computed from global slab models yields a poor fit to the observed geoid when lvvs are considered however this degradation of fit only occurs for very long wavelengths of flow indicating inherent differences between the slab and tomography models we also investigate the effects on the geoid due to weak plate boundaries strong cratonic keels and a low viscosity region in the d layer due to post perovskite in addition to the geoid we attempt to fit plate motions with a circulation model that has prescribed weak zones at plate boundaries motions are matched well and taking into account lvvs the geoid with appropriate surface velocity boundary conditions agrees with the observed geoid as well as for free slip cases

integrated ediacaran chronostratigraphy wernecke mountains northwestern canada abstract terminal neoproterozoic ediacaran ca 600 543 ma strata of the upper part of the windermere supergroup are well exposed in the mackenzie mountains and the wernecke mountains of northwestern canada windermere strata in the mackenzie mountains contain an exceptional ediacaran biostratigraphic and isotopic c and sr record while the sequence stratigraphic record is subtle throughout this predominantly deep water succession coeval strata in the wernecke mountains can be correlated with the succession in the mackenzie mountains on a formational level in contrast to the deep water setting of the mackenzies succession the werneckes succession preserves a predominantly shallow water succession amenable to detailed sequence stratigraphy integrated lithostratigraphy chemostratigraphy biostratigraphy and sedimentology partitions the wernecke mountains succession into five depositional sequences and constrains correlation with strata in the mackenzie mountains the siliciclastic and carbonate sediments deposited along the margin of the proto pacific ocean record accumulation in continental slope neritic and terrestrial paleoenvironments distinctive temporal chemostratigraphic and biostratigraphic attributes allow these strata to be correlated with other ediacaran successions worldwide integrating sequence stratigraphy with these attributes is a robust yet under utilized correlation tool and is a step toward erecting a more detailed working neoproterozoic chronostratigraphy

distinctive hydrocarbon biomarkers from fossiliferous sediment of the late proterozoic walcott member chuar group grand canyon arizona abundant extractable hydrocarbons in association with well preserved kerogen have been recognised in a dolostone from the chuar group approx 850 ma the biomarkers present include c15 c30 acyclic isoprenoids c26 c28 steranes c27 c35 hopanes and extended tricyclic terpanes among the striking characteristics of the distributions are the dominance of steranes and methyl steranes lacking side chain alkylation and the presence of putative c29 c35 neohopanes and gammacerane the high abundance of steranes is consistent with the presumed eukaryotic affinities of several types of microfossils including chuaria present in associated mudstones this stratum is the oldest in which gammacerane has been found and suggests contributions from protozoa to the depositional environment which appears to have been hypersaline there is a strong correlation between the carbon isotopic composition of the kerogen and that of the various polarity fractions of the bitumens chemical degradation and pyrolysis of the kerogens yielded n alkanes with similar characteristics to the bitumens but there are differences in the patterns of distribution of the acyclic isoprenoid sterane and triterpane biomarkers

the study on the source of the oil seeps and bitumens in the tianjingshan structure of the northern longmen mountain structure of sichuan basin china abstract the tianjingshan structure of the northern longmen mountain is located in the north of the sichuan province china two primary source rocks are present in this region the overmature infracambrian lower cambrian source rocks and the mature upper permian source rocks oil seeps and solid bitumens occur extensively in the mesozoic and paleozoic marine strata indicating good exploration prospects in this area however oil to source correlation in the tianjingshan structure is difficult due to both high maturity of the lower cambrian source rocks and severe biodegradation of oil seeps and bitumens in this work one oil seep from the permian qixia formation and two severely degraded bitumens from the cambrian strata in the tianjingshan structure were correlated with their potential source rocks hydropyrolysis was used to release biomarkers bound to kerogen of the overmature lower cambrian source rock the released biomarkers were used for the oil to source correlation together with those indexes resistant to thermal degradation and biodegradation based on the geochemical data and the petroleum geological settings of the area it can be inferred that the cambrian ordovician and silurian bitumens with light bulk isotopic values commonly 13 c

sea level changes facies changes and the late precambrian early cambrian evolutionary explosion abstract an explosive increase in the fossil record coincided with a major rise in sea level during latest precambrian early cambrian times the artefactual and real effects of an onshore offshore environmental gradient upon taxonomic diversity are considered with particular reference to 14 fossiliferous sections from around the world within taxon and within habitat evolution through the early cambrian are also discussed the evidence suggests that evolution was a diversity dependent i e kinetic growth process made possible by the colonisation and gradual filling of habitats that were growing both in variety and area archaeocyatha diversified rapidly at first but may have suffered later decline and extinction because facies changes in late early cambrian times reduced their area of habitat

carbon isotope stratigraphy and the problem of a pre tommotian stage in siberia carbon isotopic oscillations are useful to elucidate the stratigraphy and biogeochemical events around the precambrian cambrian transition new isotopic data from the manykaj and emyaksin formations of the eastern anabar uplift siberia help to correlate the lower cambrian and neoproterozoic cambrian transitional beds across the siberian platform the similarity of trends and amplitudes of the carbon isotopic curves together with biostratigraphic and sequence stratigraphic markers from the anabar uplift provide a precise correlation with the southern part of the siberian platform diagenesis of argillaceous limestones of the emyaksin formation has appar ently not affected the primary isotopic variations the resulting curve is nearly identical in sections about 100 km apart in the tommotian atdabanian portion of the formation relatively frequent and pronounced isotopic oscillations in the lower beds of the emyaksin formation fit between features i and ii of the southern siberian isotopic reference scale but are undetected therein owing to the deposi tional hiatus at the base of the tommotian stage in its type section this confirms the transgressive onlap from the north suggested by previous studies and makes the appearance of the cambrian skele tal fossils on the siberian platform less abrupt the hiatus in the south appears to embrace at least two biostratigraphic zones as recognized in the north the case is strengthened for a pre tommotian cambrian stage in siberia the biostratigraphic framework for which has been elaborated earlier

grim gravity model improvement using lageos grim3 l1 improved modeling of the earth s geopotential is one of the major tasks of satellite geodesy the availability of a new set of precise lageos satellite laser ranging slr data as well as seasat acquired observations enabled us to compute an improved solution in the grim model series the new geopotential grim3 l1 recombined the satellite perturbation normals of the former grim3 solution with new normal equation subsystems derived from an additional 16 months of lageos slr data oceanic 1 1 mean free air anomalies recovered from seasat altimetry an updated set of terrestrial 1 1 mean free air anomalies and several condition equations for zonal and resonant harmonics the combined normal equations were solved for a complete set of spherical harmonics up to degree and order 36 and the coordinates of 109 tracking stations comparisons with altimetry derived quantities terrestrial gravity anomalies from satellite to satellite tracking longitude accelerations of 24 hour satellites and satellite orbit fits show a substantial improvement over previous grim solutions and indicate that the lageos data have contributed considerably to the reliable estimation of the long wavelength portion of the gravity field

heat flow and structure of the lithosphere in the eastern canadian shield twenty two new determinations of heat flow and radiogenic heat production in the superior and grenville provinces of the canadian shield are presented the new data and previously published measurements strongly constrain the thermal structure of the eastern canadian shield in the abitibi greenstone belt heat flow gradually increases from 29 mw m 2 near the grenville front to 44 mw m 2 east of the kapuskasing uplift this heat flow variation is interpreted in terms of crustal thickening and increased thickness of a tonalitic layer with average heat production of about 1 1 w m 3 this interpretation based on estimated heat production of major rock types in the region is consistent with crustal models derived from recent seismic reflection and refraction studies it also leads to an estimate of about 12 mw m 2 for the mantle heat flow beneath the area the average heat flow in the grenville province 41 10 mw m 2 is the same as that of the superior province this similarity and the lack of significant variation of heat flow across the grenville front indicate that the crust on both sides of the front has similar heat production and thus composition in the western part of the grenville province heat flow reaches high values in the vicinity of the boundary between the allochtonous polycyclic and monocyclic belts where enriched granitic plutons are found in the crystalline terranes in the central part of the grenville province heat flow and heat production are related to each other the parameters of the linear heat flow heat production relationship qr 30 2 in mw m 2 and d 7 1 1 7 km are close to those of the much younger appalachian province implying that the higher appalachian heat flow is due solely to higher heat production in the upper crust the data provide no evidence for variation of mantle heat flow between the superior grenville and appalachian provinces whose tectonic ages range between 2700 and 400 ma the small value of the mantle heat flow about 12 mw m 2 implies that the depth to the 450 c isotherm which controls the effective elastic thickness of the lithosphere is very sensitive to crustal heat production

simple considerations on forces driving plate motion and on the plate tectonic contribution to the long wavelength geoid summary in order to gain some insights into the role of deep mass anomalies on surface velocity and geoid anomalies cartesian models involving strong lateral viscosity contrasts and where plate boundaries are mimicked by zones of very low viscosity are performed the response of the system to mass anomalies at various depths and various distances from the plate boundary is computed it is compared with the responses obtained using the three other methods used in global geoid models for mimicking the mobility of the lithosphere models with layered viscosity and a uniform low viscosity lithosphere models computing plate velocities from horizontal force balance and models where the surface velocities are set equal to the observed ones with plates moved by the hand of god contrary to what occurs in models with layered viscosity the geoid response for the weak zone model proposed here changes drastically from positive to negative depending upon the position of the mass anomaly with respect to the plate boundary this arises from the fact that only mass anomalies situated close to the plate boundary are able to move the plates efficiently changing the location of an upper mantle mass anomaly by a few hundred kilometres considerably modifies the amplitude of the predicted plate velocities and geoid anomalies consequently upper mantle density anomalies deduced from global tomography analysis or density anomalies assumed to be known a priori subducted slabs but truncated by a spherical harmonic decomposition should not be used in models aimed at computing plate motions and the associated geoid when a mass anomaly is situated just below a plate boundary the sign of the predicted geoid anomalies can be the same as for models with a layered viscosity and a low viscosity lithosphere however the shape of the predicted geoid anomaly and its amplitude as a function of the depth of the mass anomaly cannot be mimicked by models with a uniform viscosity lithosphere whatever the lithospheric viscosity for mass anomalies situated at a large distance from the plate boundaries the induced plate velocities are small and the geoid is equal to that computed for a highly viscous continuous lithosphere the force balance method is strongly affected by the cut off in the harmonic decomposition only models with a lithospheric layer below the surface reproduce the geoid successfully for mass anomalies situated away from a plate boundary the plate velocities for the force balance method and for the weak zone model proposed here are not proportional due to differences in the structure of the plate boundary in short only mass anomalies very close to a plate boundary are able to influence the plate velocities but these velocities depend strongly on the mechanical details of the plate boundary as a consequence we believe that it is impossible to compute plate velocities from what we know from mass anomalies in the earth precisely enough to include them in a geoid model on the other hand actual plate velocities are known we show that an accurate geoid can be computed by imposing the observed plate velocities onto the earth s surface even if the details of the mechanical structure of the lithosphere are not respected it is this method that is suggested for introducing the effect of plate tectonics in global geoid models

present day subsidence in the ganges brahmaputra meghna delta eastern amplification of the holocene sediment loading contribution the subsidence of the ganges brahmaputra meghna delta gbmd drastically increases the adverse impacts of coastal flooding and exacerbates the vulnerability of populations from ongoing rapid sea level rise we focus here on estimating the present day subsidence rates induced by the loading of sediments continuously deposited within the gbmd over the past 11 000 years by constructing a realistic gbmd 3 d numerical model with laterally variable mantle and lithospheric structure we demonstrate for the first time that the presence of the strong indian craton and the weakened indo burma margin results in significant amplification of subsidence driven by sediment loading in the eastern part of the delta where the population density is the highest 1 000 habitants per km 2 although uncertainties remain regarding the amplitude of subsidence the rate estimates 2 3 mm year are found to be comparable to the present day global mean sea level rise

a global geoid model with imposed plate velocities and partial layering most inversions of the long wavelength geoid in conjunction with the seismic tomographic information have so far been carried out under the assumption of either purely whole mantle or perfectly layered circulation moreover modeling the lithosphere as a spherical shell with a uniform low viscosity was found to yield the best fit to the observed geoid we have tested whether a good prediction of the geoid can also be achieved by including two constraints a semipermeable behavior of the 660 km discontinuity and surface plate velocities equal to the observed ones the mass transfer between upper and lower mantle has been changed by imposing a surface density anomaly at a depth of 660 km which is proportional to the mass anomaly needed to achieve perfectly layered circulation on the top of the mantle we assume a stiff lithosphere that moves with a velocity corresponding to the observed plate motion the viscosity only varies with depth considering a simple three layer viscosity structure and changing the permeability of the 660 km interface we have obtained a satisfactory variance reduction of the geoid data 75 for degrees 2 12 the best fit to the geoid is obtained if the mass transfer across the 660 km boundary is reduced to one third in comparison with the purely whole mantle model the best fitting viscosity profile is characterized by a clearly defined asthenosphere and a viscosity increase by at least 2 orders of magnitude in the lower mantle the amplitudes of dynamic topography predicted by our model are remarkably small 100 m thus fully compatible with the observation

new carbon isotope stratigraphy of the ediacaran cambrian boundary interval from sw china implications for global correlation the yangtze platform preserves relatively thick carbonate successions and excellent fossil records across the ediacaran cambrian boundary interval the intensely studied meishucun section in east yunnan was one of the global stratotype section candidates for the precambrian cambrian boundary however depositional breaks were suspected in the section and the first appearance of small shelly fossils could not be verified the laolin section located in ne yunnan is more continuous and shows great potential for global correlation of carbon isotope features across the precambrian cambrian boundary however the stratigraphic framework and correlations were controversial we studied and systematically sampled the laolin section and present here new carbon isotope data for this section the laolin section consists of in ascending order the baiyanshao dolostone of the dengying formation the daibu siliceous dolostone zhongyicun dolomitic phosphorite lower dahai dolostone and upper dahai limestone of the zhujiaqing formation and the black siltstone of the shiyantou formation our data reveal a large negative 13 c excursion 7 2 l1 in the daibu member which matches the previously published data for the laolin section and a large positive excursion 3 5 l4 in the dahai member which was not shown in the published data the excursion l1 correlates well with the similarly large negative excursion near the first appearance of small shelly fossils in siberia and mongolia similar magnitude excursions are also known from morocco and oman for which there are no robust fossil constraints but from where volcanic ash beds have been dated precisely at 542 ma thus confirming a global biogeochemical event near the ediacaran cambrian boundary our data also indicate that deposition was more continuous at the laolin section compared with the meishucun section where there are no records of a comparable negative excursion near the ediacaran cambrian boundary nor any comparable positive excursion in the dahai member therefore the laolin section has proven potential to be a supplementary global stratotype section for the ediacaran cambrian boundary on the yangtze platform

plate tectonics and aspherical earth structure the importance of poloidal toroidal coupling we derive poloidal flow internal loading green functions for incompressible fluid models of the mantle consisting either of a single constant viscosity spherical shell or of two adjacent spherical shells having different viscosities from the green function we obtain kernels connecting the surface divergence the geoid and the surface topography fields to the lateral density heterogeneity inferred on the basis of the application of seismic tomographic imaging techniques and with these kernels we argue that both the surface divergence and geoid fields consisting of harmonic degrees 2 5 may be reasonably fit with only a factor of 8 viscosity increase at a depth of 1200 km we point out however that the coupling from poloidal to toroidal flow which is required to understand surface velocity spectra may allow these observations to be understood in terms of a viscosity increase at depth which is smaller than required by nonhydrostatic geoid data in the context of pure poloidal models using the observed kinetic energy in the surface plate motions as a constraint allows us to infer a value for the steady state upper mantle viscosity of 2 0 0 5 1021 pa s we comment on the implication of the difference between this number and the somewhat lower value 1 1021 pa s which has been derived on the basis of analyses of signatures of the glacial isostatic adjustment process

a molecular and isotopic geochemical study of meso to neoproterozoic 1 73 0 85 ga sediments from the jixian section yanshan basin north china abstract this paper systematically describes the molecular and isotopic characteristics of meso to neoproterozoic 1 73 0 85 ga sediments from the jixian section yanshan basin north china a correlation is made between molecular fingerprints and paleoenvironmental and paleontologic data acquired from these strata the agreement between these parameters suggest that the biomarkers preserved in these proterozoic sediments are good indicators of the evolution of the paleoenvironment and early life in the yanshan basin the fact that extended regular isoprenoids c20 were detected in most of the studied samples suggest a sustained lipid input to sedimentary organic matter from halophiles and other possible archaebacteria through meso to neoproterozoic two dehydroabietin isomers i e 18 and 19 norabieta 8 11 13 trienes were unexpectedly present in these samples which provides evidence suggesting that the compounds with abieta structure could be derived from primitive organisms such as algae the stable carbon isotope characteristics of kerogen and coexisting carbonate show a rapid decline in atmospheric co2 concentration from 1 73 to1 58 ga and a stable concentration afterwards the difference c in isotopic composition between carbonate and associated kerogen lies in zonal variations along the stratigraphic sequence which can be attributed to the alternations of marine transgressions and regressions that occurred in the yanshan basin during the proterozoic the carbon isotope relationships between kerogen and alkanes from these strata are consistent with the hypothesis that before 1 4 ga both n alkanes and isoprenoids had been derived mainly from heterotrophic halophiles and other archaebacteria but after that only isoprenoids originated from archaebacteria whereas n alkanes were possibly derived from photosynthetic organisms changes in the source of n alkanes around 1 4 ga may suggest a decline of heterotrophic reworking to primary lipids which can be related to the formation of a quiet shallow water environment of an epicontinental sea at this stage in the yanshan basin

oxygen carbon and strontium isotope records of ediacaran carbonates in central iberia spain carbon oxygen and strontium isotope ratios were determined in the uppermost neoproterozoic carbonate rocks at pastores sw salamanca central iberia the low 18o v smow values 15 19 indicate an alteration of the whole stratigraphic series however the alteration process does not seem to have affected most of the 13c and 87sr 86sr data these data reveal that only one particular level of the carbonate section shows simultaneous relative increases in mn sr and 87sr 86sr ratios decreases in the 18o and 13c values and covariations between 87sr 86sr and the other parameters despite this most of the carbonate section lacks these covariations and its 13c 4 6 to 0 5 and 87sr 86sr 0 70845 0 70853 ratios probably reflect the seawater isotopic features at the time of sedimentation the data for c and sr isotopes at the pastores carbonate section and the presence of cloudina in platform carbonates together with an overlying major sub cambrian unconformity following a relative sea level fall are features comparable to those frequently shown by proterozoic cambrian boundary series across the world thus our results consistently support the notion that the carbonates at pastores are equivalent to other carbonates deposited close to the ediacaran cambrian boundary

molecular markers carbon isotopes and rare earth elements of highly mature reservoir pyrobitumens from sichuan basin southwestern china implications for precambrian lower cambrian petroleum systems abstract the moxi gaoshiti bulge of the sichuan basin in southwestern china is unique because it provides insights into the geochemistry of lower cambrian reservoir bitumens and their geological significance for paleo oil reservoirs analyses of biomarkers carbon isotopes trace elements te and rare earth elements ree were conducted on reservoir bitumens and the associated source rocks to constrain sedimentary environments and bitumen paleo oil sources based on biomarkers carbon isotopes te ratios and ree distribution patterns both of the cambrian qiongzhisi 1q and sinian dengying z2dn source rocks can be hydrocarbon sources for the cambrian longwangmiao 1l bitumens in the mg bulge lower cambrian doushantuo formation not discussed here abundant n alkanes relative to isoprenoids significant c21 and c22 steranes predominance of dibenzothiophene dbt and low ratios of biomarkers such as pr ph dbt p c22tt c21tt and c35h 22s c34 h 22s indicated that the source rock was characterized as shale from an overall highly reducing marine depositional environment the average 13c values from the compositions of the solid bitumens show an abnormal order saturate hydrocarbons

carbon isotope excursions in precambrian cambrian boundary beds morocco the precambrian cambrian boundary is one of the most significant points on the geological timescale because it marks the change in the fossil record from soft bodied organisms to skeletonized faunas the cambrian explosion or radiation event1 saw the evolution of many new life forms especially various molluscs and archaeocyathids from the base of the cambrian tommotian stage others including the trilobites and bivalves evolved a little later in the succeeding atdabanian stage2 3 a massive increase in the biomass of the photic zone through phytoplankton blooms has also been postulated for this time4 particularly in the late riphean early vendian and in the early cambrian5 there has been much discussion concerning where precisely to place the boundary6 7 and here are several contenders in the world for the type section8 i present here a carbon isotope profile from the precambrian cambrian boundary beds in morocco where two distinctive positive 13c excursions are revealed these are interpreted as records of increased organic productivity as part of the precambrian cambrian explosion of life 13c excursions could be useful points for correlation between boundary sections

phosphogenesis around the proterozoic phanerozoic transition there was a very significant and widespread phosphogenic event around the proterozoic phanerozoic transition this is reflected in the occurrence of many major and minor latest proterozoic and cambrian phosphate deposits phosphorites in asia australia africa and south america as well as minor occurrences in many other parts of the world the major peak in phosphogenesis was in the tommotian with lesser events in the ediacaran and the lenian bracketed by other minor though significant events recent evidence on the timing of proterozoic phosphogenesis does not now support the suggestion of a phosphogenesis glaciation link these peaks in phosphogenesis are taken as evidence that there was a marked increase in the phosphate content of the shallow seas at about this time possibly as a result of the circulation of deep phosphorus rich ocean waters into the photic zone not only was this responsible for the formation of phosphorites but it also had a profound effect on the biota and may have been a driving mechanism for the cambrian faunal explosion and the initiation of biomineralization

neoproterozoic early cambrian isotopic variation and chemostratigraphy of the lesser himalaya india eastern gondwana the rodinia supercontinent had fragmented by 750 ma and east gondwana india australia and antarctica separated from west laurentia baltica africa and south america occupied other side of the rodinia neoproterozoic rifting breakup of rodinia low latitude glaciation and global warming events have been recorded from the lesser himalaya of india chemostratigraphy of the blaini krol tal succession indicates precambrian cambrian transition lies in the lower tal formation 13 c 4 pdb the krol belt in the lesser himalaya is characterized by positive 13 c values 1 to 6 pdb the appearance of multicelluler ediacaran life in the upper krol is consistent with hypothesis that increase in atmospheric oxygen played a major role in metazoan evolution and cambrian diversification of life on earth the base of the terminal proterozoic system in the lesser himalaya is established in the blaini formation the pink cap carbonate of the blaini formation shows negative 13 c value 3 pdb and this blainian glaciation is correlated with marinoan glacial event other regions of the eastern gondwana and south china parts of siberia and north africa neoproterozic early cambrian chemostratigraphy in the lesser himalaya is consistent in the isotopic variation where there is carbon isotopic similarity between neoproterozoic bambui group in central brazil south america and the krol formation of the lesser himalaya india in the northeastern lesser himalaya india the neoproterozoic sedimentary succession shows well developed carbonate the buxa dolomite shows significantly positive c isotope ratios 13 c 3 7 to 5 4 pdb the o isotopic data also shows remarkable consistency with the 18 o values fluctuating within a narrow range between 8 9 and 7 2 vpdb these isotopic results from the eastern lesser himalaya correspond to the terminal proterozoic c isotopic evolution followed by oscillations during the precambrian cambrian transition in the lesser himalaya in eastern gondwana

partial advection of equidensity surfaces a solution for the dynamic topography problem although one layer dynamic models of the earth s mantle have successfully explained the geoid they generate a surface dynamic topography that seems too large relative to geological observations in this study we hypothesize the possibility of partial advection of mantle equidensity surfaces by vertical motion induced by driving loads these large scale flow dependent loads would greatly reduce the dynamic topography amplitude while preserving a good fit to the observed geoid various physical processes related to nonequilibrium phase changes or to the existence of chemical heterogeneity in the mantle could justify a partial advection of the mean density in this paper we simply consider the flow dependent loads as proportional to the vertical flow velocity two density mantle models are considered one from subduction reconstruction ricard et al 1993 and one from seismic tomography li and romanowicz 1995 we show that a very moderate entrainment a few kilometers of the equidensity surfaces in the transition zone is sufficient to reduce dynamic topography amplitude by a factor of 2 or 3 the seismic velocity signal associated with this entrainment would be hidden by the signal of thermal origin using this new hypothesis we compute sea level changes associated with epeirogeny for the cretaceous paleocene and oligocene periods the amplitude and phase of these changes are in fairly good agreement with geological hypsometric curves our results suggest that not only the thermodynamics but also the kinetics of mineralogical phase changes in the transition zone are of crucial importance

subducted slabs and the geoid constraints on mantle rheology and flow geoid anomalies are primarily the result of the density contrasts driving mantle convection and plate motion the total geoid anomaly resulting from a given density contrast in a convecting earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries as well as by the density contrast itself these boundary deformations and hence the total gravity field depend on the radial distribution of effective viscosity if the internal density contrasts can be estimated as is the case for subducted slabs useful constraints can be placed on the depth and on the variation of viscosity with depth of the convecting system the degree 4 9 components of the observed long wavelength geoid are highly correlated with those predicted by a density model for seismically active subducted slabs the positive sign of the correlation requires that the effective viscosity increases with depth by a factor of 30 or more the amplitude of the correlation cannot be explained by the density contrasts associated with just the seismically active parts of subducted slabs however the amplitude can be explained if the density contrasts associated with subduction extend into the lower mantle or if old lithosphere is piled up at the base of the upper mantle beneath subduction zones to a thickness in excess of 350 km over horizontal distances of thousands of kilometers mantlewide convection in a mantle that has a viscosity increasing with depth provides a simple explanation of the long wavelength geoid anomalies over subduction zones

mass extinctions the view of a sceptic the concept of periodic mass extinctions by extraterrestrial impacts has contributed very strongly to the current revival of interest in the problem of mass extinctions in this context several radical claims have been made asserting that the newly emerging picture of mass extinctions as a separate class of biological phenomena caused by a separate class of processes demands a far reaching revision of the the neodarwinian paradigm of evolution and implies a vindication of the cuvierian perspective on earth9s history the validity of these claims however is questionable at all levels from the interpretation of particular events to the concept of mass extinctions as a special class of phenomena to their proposed causation by impacts and to their alleged historical pedigree and implications for current evolutionary theory mass extinctions may actually be clusters of more or less accidentally aggregated in time but causally distinct extinction episodes

carbon isotope geochemistry and palaeontology of neoproterozoic to early cambrian siliciclastic successions in the east european platform poland a well preserved stratigraphic record of the organic carbon isotopic composition across the precambrian cambrian transition has been obtained from a sequence of entirely detrital rocks underlying the lublin slope east european platform poland the observed 13 c pattern for organic carbon is comparable to isotope data determined for coeval carbonate platform sequences elsewhere these results underline the truly global nature of these secular variations within the carbon cycle during the terminal proterozoic and early cambrian largely independent of host lithology and facies the direct combination of micropalaeontological and geochemical results from sedimentary organic matter allows a detailed interpretation of the isotope record as probably caused by secular changes in the net balance of burial and erosion rates of repositories of organic carbon furthermore these variations appear to be directly correlatable with changes in the diversity and abundance of the biota and radiation extinction events

trace elements and sr and c isotopic signatures in late neoproterozoic and earliest cambrian sedimentary organic matter from siliciclastic successions in the east european platform sedimentary organic matter deriving from tubes of sabelliditids vendotaenids sapropelic films and kerogens was extracted by acid processing from upper vendian siliciclastic successions in the east european platform elemental composition obtained by ins

phosphorus phosphorites and skeletal evolution at the precambrian cambrian boundary clustering of sedimentary phosphate deposits around the precambrian cambrian boundary is related to a global phosphogenic event this event probably resulted from a complex interplay of factors and had a profound effect on the biota with high concentrations of phosphorus in the photic zone producing an increase in the total shallow marine biomass faunal diversification the precipitation of skeletal structures and explosive evolution

tectonic stresses in the lithosphere various types of observables earthquake focal mechanisms in situ measurements and geological deformations give information about the large scale lithospheric stress field the latter has often been explained by postulating appropriate forces acting at the edges and beneath the plates this approach ignores the role of mass heterogeneities within the lithosphere here we analyze the effect of both boundary and internal forces on the stress pattern and show that both contributions are of comparable magnitude the presence of internal sources makes the problem three dimensional we show however that it can be reduced to a two dimensional plane stress formulation whereby the edge forces are expressed by the non hydrostatic stresses and the basal shear is increased by the addition of a term proportional to the gradient of the mean vertical stress for the oceanic lithosphere we derive a compression that increases with age the comparison with geophysical observables yields an upper bound of a few bars on the magniude of the basal drag for the continents we infer the existence of an underlying upper mantle somewhat denser than under oceans

palaeoenvironment and carbon isotope stratigraphy of upper proterozoic carbonates of the yangtze platform the 13c values for phanerozoic marine carbonates display secular trends mainly in the range 1 to 2 1 these have been interpreted as primary features reflecting changes in the carbon isotope composition of dissolved bicarbonate in seawater brought about largely by variations in the proportions of carbon being sequestered in the oxidized and reduced carbon reservoirs1 2 carbon isotope trends are not as well established for the precambrian but there are indications that upper proterozoic sequences contain carbonates with unusually high 13c values3 9 here we present the results of the first carbon isotopic study of the upper proterozoic carbonate strata from the type locality of the sinian system in the yangtze gorges sections near yichang south china and discuss their implications for accumulation of organic matter we also compare the results for these chinese strata with 13c trends in other upper proterozoic cambrian sequences and assess the extent to which such isotopic data can be used to make stratigraphic correlations between widely spaced sequences

carbon isotopes and precambrian cambrian boundary geology south australia ocean basin formation seawater chemistry and organic evolution a carbon isotope stratigraphy from the lowest cambrian strata in the adelaide fold belt of south australia is interpreted in terms of the geological context of the sequence major transgression developing circulation patterns and upwelling resulting from an adjacent opening ocean and biomass increases stemming from the cambrian radiation event there is also evidence from the textures of ooids and cements in late precambrian cambrian strata for subtle variations in seawater chemistry across the boundary and these may well have been instrumental in the development of calcareous skeletons in the early metazoans

stable isotope geochemistry and palynology of the late precambrian to early cambrian sequence in newfoundland a sequence of clastic sediments in southeastern newfoundland straddling the precambrian cambrian boundary has been investigated for its stable isotope geochemistry of carbon and sulfur and acid resistant organic walled microfossils a detailed study of the chapel island formation which includes the boundary interval has revealed fluctuations in the isotopic composition of organic carbon these are largely interpreted as caused by differences in the depositional environments highly variable sulfur isotopic compositions indicate bacterial sulfate reduction as a pyrite forming process sometimes under sulfate limited conditions palynological results are quite limited with respect to diagnostic microfossils

enhanced deepwater circulation and shift of sedimentary organic matter oxidation pathway in the okinawa trough since the holocene 1 high resolution biogeochemical and magnetic property analyses of a 20 kyr sediment core in the okinawa trough revealed a significant upward decrease in total sulfur content ts at around 10 cal ka bp which coincides with a transformation from foliation to anomalous sedimentary magnetic fabric which indicates an enhancement of deepwater circulation synchronous change in biogeochemical and magnetic properties in sediments was likely triggered by the intensified kuroshio current which strengthened surface water productivity and the relative input of marine organics to the sediments however this event did not promote sedimentary sulfur burial instead the synchronously enhanced deepwater circulation switched the organic carbon oxidation pathway from sulfate reduction to aerobic respiration which resulted in ts depleted sediments

tectonic controls of phanerozoic sedimentary rock cycling the cyclic nature of the phanerozoic sedimentary rock distribution carbon sulphur coupling and material transfer among sedimentary reservoirs appears to be controlled by tectonic factors the distribution of preserved sedimentary mass in terms of rock mass remaining u geologic age shows a minimum c 300 350 ma ago which separates two subcycles of erosion and deposition of sedimentary rocks the older subcycle was interrupted because of the major continental collisions of the devonian and late carboniferous these collisions resulted in a reduction of outcrop areas of rocks of the older cycle relative to their masses leading to a decline in the probability of destruction and an increase in half life of these older sediments a strong correlation exists between the long term cyclicity in the phanerozoic global sea level curve and the distribution of carbon and sulphur among their major exogenic reservoirs this correlation is related to two principal tectonic modes of the phanerozoic oscillatory and submergent it is postulated that the submergent mode of active plate convergence obduction and subduction of sediments large ridge volume and high sea level gave rise to low erosion and sedimentation rates less restricted environments of carbonate deposition and relatively high atmospheric co 2 levels high temperatures resulting from an increased rate of production of co 2 from diagenetic and metamorphic reactions at subduction zones as sea level rose carbon was transferred from the sedimentary reservoir of reduced organic carbon to that of oxidized inorganic carbon in limestones whereas sulphur moved from the oxidized sulphate reservoir to the reduced sulphide reservoir as sea level fell reservoir transfers were opposite to those above culminating in the oscillatory mode of generally elevated continental interiors these reservoir transfers are consistent with secular changes observed in the distribution of 13 c and 34 s in sedimentary materials during the phanerozoic petrographic examination of phanerozoic oolite formations shows that ooids with preserved calcitic relict textures are characteristic of pre carboniferous carbonate rocks whereas ooids with relict textures indicative of initial aragonite mineralogy are dominant in rocks of younger age these changes in ooid mineralogy may be interpreted as reflecting changes in co 2 levels of the ocean atmosphere system consistent with the above tectonic considerations atmospheric co 2 levels were higher prior to carboniferous time favouring formation of calcitic ooids and skeletal parts after the carboniferous co 2 levels fell and aragonite and mg calcites of greater than 8 mol mg increased in abundance as precipitates

a global analysis of heat flow from precambrian terrains implications for the thermal structure of archean and proterozoic lithosphere previous studies of heat flow from precambrian terrains have yielded two different relationships a global temporal relationship between heat flow and tectonic age and a regional spatial relationship between heat flow and the proximity of archean cratons we analyze heat flow from tectonically stable precambrian terrains worldwide to address questions associated with these two heat flow patterns 1 is the spatial relationship a global pattern 2 do the two heat flow relationships have a common underpinning in answer to the first question our data analysis reveals a widespread spatial pattern between heat flow and the proximity of archean cratons which is characterized by low heat flow in archean cratons and proterozoic terrains adjacent to cratonic margins and higher heat flow in proterozoic terrains that are more than a few hundred kilometers from a craton to address the second question we examine three previous interpretative models of precambrian heat flow 1 simple cooling of a thermal boundary layer 2 thicker lithosphere in archean terrains than in proterozoic and 3 greater heat production in proterozoic crust than in archean model 1 predicts essentially no change in heat flow in terrains older than 1 5 ga and therefore does not likely provide a common underpinning for the precambrian heat flow patterns models 2 and 3 when combined with the special structural configuration of sutures can independently yield both the spatial and temporal heat flow distributions and thus alone or together may be considered candidates to explain the precambrian heat flow patterns however thermal models of the lithosphere in which the heat flow patterns are explained entirely by variations in crustal heat production cannot satisfy constraints on upper mantle temperatures beneath archean cratons derived from xenolith thermobarometry the thermobarometry constraints can be satisfied by models in which differences in lithospheric thickness are the principal factor controlling the surface heat flow distributions if the heat flow patterns result primarily from variations in lithospheric thickness then a different temporal heat flow relationship for precambrian terrains can be proposed one in which heat flow varies inversely with the age of stabilization of the lithosphere

northwest africa s ediacaran to early cambrian fossil record its oldest metazoans and age constraints for the basal taroudant group morocco abstract the anti atlas area in southern morocco preserves one of the most complete latest ediacaran to cambrian shallow marine carbonate records in the world and serves as a standard for global chemostratigraphic 13ccarb curves contrary to the excellent chemostratigraphic coverage neither metazoan fossils only stromatolites and calcified algae nor radiometric age constraints were hitherto available for the basal part of this carbonate record the tifnout member of the adoudou formation of the taroudant group we report an ediacara type fauna from shallow marine sandstones from the basal tabia member and microfossils from dolostones from both the tabia and the tifnout members of the adoudou formation the former are discoidal fossils and share many characteristics with the form genus aspidella and exhibit a peculiar style of preservation pointing towards an early diagenetic two stage carbonate followed by silica cementation history leading to good preservation of these supposedly once soft bodied organisms the dolostone hosted microfossils are preserved as sparite filled molds enclosed by a thin opaque envelope despite their small size they resemble late ediacaran calcifying metazoans such as namacalathus and tubular small shelly fossils together these fossils constitute the oldest known direct evidence for presumably metazoan life from northwest africa we also report new time constraints for the adoudou formation and fossils enclosed therein based on combined la icp ms and high precision ca id tims u pb zircon geochronology of detrital and volcanic zircons together with new chemostratigraphic 13ccarb and literature data these new age constraints suggest a late ediacaran age time span between ca 561 and 542 ma for the discoidal ediacara type fossils from the basal tabia member and a latest ediacaran to earliest cambrian time span between ca 544 and 540 ma and an early cambrian age time span between ca 528 and 523 ma for the two microfossil faunas from the tifnout member

facies distribution of the lower cambrian cryptic microbial and epibenthic archaeocyathan microbial communities western anti atlas morocco marine microbial communities recorded in the moroccan anti atlas were unaffected across the neoproterozoic cambrian transition a stromatolite dominated consortium was replaced at the beginning of the atdabanian ca 20 myr after the neoproterozoic cambrian boundary by shelly metazoan and thromboid consortia which contain the oldest biostratigraphically significant fossils of the moroccan cambrian the associated collapse of microbial mat stromatolitic growth appears to coincide with a change from pre atdabanian shallow water restricted conditions into atdabanian deeper open sea conditions it is postulated that this environmental change led to an episode of improved water circulation over carbonate platform interiors promoting shelly metazoan immigration into the region the tiout amouslek lithostratigraphic contact in the early atdabanian marks the end of an episodically unstable seafloor as suggested by the abundance of slumping and sliding structures and synsedimentary microfaults and cracks recorded in the underlying tiout member concurrent with the transition is the occurrence of a network of cryptic fissures and cavities that provided habitats for a coelobiontic chemosynthetic heterotrophic microbial community composed of stromatolitic crusts renalcis epiphyton girvanella intergrowths and kundatia thalli in the overlying amouslek formation archaeocyathan thromboid reefs were constrained by substrate stability water depth and subsidence rate four reef geometries are distinguished i patch reefs surrounded by shales ii bioherms in which flank beds intercalate laterally with carbonate and shale inter reef sediments iii biostromes or low relief structures formed as a result of lateral accretion of patch reefs and iv kalyptrate complexes that nucleated because of a marked tendency for aggregation and in which patch reefs and bioherms occur stacked together bounded by clay marl silt seams

intermittent layered convection in a model mantle with an endothermic phase change at 670 km it is not yet known whether convection in the earth s mantle occurs in cells that extend throughout the entire depth of the mantle or if the upper and lower mantle convect in independent layers both possibilities seem to be consistent with geochemical and seismologi cal observations1 4 it is generally accepted however that the seismologically observed boundary between the upper and lower mantle at 670 km depth arises from the endothermic phase change spinel perovskite mgo here we investigate the results of including this phase change in a model of mantle convection for realism and to eliminate scaling problems our calculations use commonly accepted values of geophysical parameters for a clapeyron slope y 2 106 a value close to experimental results and theoretical expectations we observe local intermittent mixing between the upper and lower mantle such behaviour may offer a way to reconcile the existence of geophysical evidence for both whole mantle and layered convection

geochemical changes across the proterozoic cambrian transition in the durmala phosphorite mine section mussoorie hills garhwal himalaya india abstract the transition from the neoproterozoic nemakit daldynian 1 krol e dolomite to the tommotian lower tal phosphorite chert black shale sequence denotes a period of drastic change in the depositional environment and the chemical milieu of the sedimentation basin these changes are reflected by a marked negative excursion of carbon isotopes phosphorus enrichment trace metal content high organic carbon burial and notably the first appearance of prolific small shelly fauna and burrowing organisms recent studies have documented these changes along several stratigraphic profiles in the mussoorie hills of the garhwal himalaya for which a single profile from the durmala phosphorite mine provides a key section discussed here

carbon isotope stratigraphy of the precambrian cambrian sukharikha river section northwestern siberian platform a high resolution carbon isotope profile through the uppermost neoproterozoic lower cambrian part of the sukharikha section at the northwestern margin of the siberian platform shows prominent secul

precambrian cambrian transition death valley united states the death valley region contains one of the best exposed and often visited precambrian cambrian successions in the world but the chronostratigraphic framework necessary for understanding the critical biologic and geologic events recorded in these sections has been inadequate the recent discovery of treptichnus phycodes pedum within the uppermost parasequence of the lower member of the wood canyon formation allows correlation of the precambrian cambrian boundary to this region and provides a necessary global tie point for the death valley section new carbon isotope chemostratigraphic profiles bracket this biostratigraphic datum and record the classic negative carbon isotope excursion at the boundary for the first time biostratigraphic chemostratigraphic and lithostratigraphic information from pretrilobite strata in this region can be directly compared with similar data from other key sections that record the precursors of the cambrian explosion few precambrian cambrian boundary sections contain both the facies restricted boundary fossil t pedum and carbon isotope data as found in death valley thus the death valley succession provides a critical link toward our understanding of the correlation between siliciclastic dominated and carbonate dominated precambrian cambrian transition sections

revising the spectral method as applied to modeling mantle dynamics 1 the observed geoid dynamic topography and surface plate velocities are controlled by various factors such as density and viscosity variations in the earth s mantle and strength of the lithosphere previous studies have shown that the geoid signal cannot be resolved in details within the framework of a simplified model of the mantle flow considering only radial viscosity variations thus a modeling technique handling both radial and lateral variations of viscosity and other parameters should be used the spectral method provides a high accuracy semianalytical solution of the navier stokes and poisson equations when viscosity is only depth radially dependent in this study we present the numerical approach built up on the substantially revised method originally proposed by zhang and christensen 1993 for solving the navier stokes equation in the spectral domain with lateral variations of viscosity lvv this approach incorporates a number of numerical algorithms providing efficient calculations of the instantaneous stokes flow in the sphere and taking into account the effects of lvv self gravitation and compressibility in contrast to the traditionally used propagator method our approach suggests a continuous integration over depth without introducing internal interfaces various numerical tests have been employed to test accuracy and efficiency of the proposed technique benchmarking of the code shows its ability to solve the mantle convection problems implying strong lvv with high resolution

the rise and decline of the ediacaran biota palaeobiological and stable isotopic evidence from the nw and ne lesser himalaya india