selective alteration of sargassum lipids in anoxic sediments of the orca basin abstract the orca basin represents a unique depositional environment where sediments are overlain by several hundred meters of anoxic brine layers of the seaweed sargassum are found as discrete layers within the sediments and appear intact by visual examination to examine lipid diagenesis in this environment major lipid classes sterols fatty acids and alcohols in plants collected in the water column were compared to those of plants found in sediments and dated at 380 years old by 14 c measurements plant fronds stipes and air bladders were analyzed separately to compare the preservation of lipids among major sargassum tissues all lipids showed significant changes in distribution and abundance in preserved plants with a general loss of unsaturated components the concentration of several saturated fatty acids however remained unchanged or increased in older plant tissues suggesting that thet were more resistant to decomposition and that reduction of unsaturated acids to saturated forms may have occurred over time sterols showed contrasting results with nuclear saturated sterols stanols decreasing in abundance by an average of 62 in preserved tissues while nuclear unsaturated sterols stenols showed only a mean decrease of 41 two major sterols cholest 5 en 3 ol and cholesta 5 24 dien 3 ol were reduced on average by 92 2 and 93 5 respectively while the major sterol in all sargassum tissues 24 ethylcholesta 5 24 28 e dien 3 ol showed no significant change in concentration over time results of lipid analysis for preserved and fresh sargassum suggests that selective degradation may occur in anoxic marine environments counter to pathways typically seen in other systems

factors that control the angle of shear bands in geodynamic numerical models of brittle deformation abstract numerical models of brittle deformation on geological timescales typically use a pressure dependent mohr coulomb or drucker prager plastic flow law to simulate plastic failure despite its widespread usage in geodynamic models of lithospheric deformation however certain aspects of such plasticity models remain poorly understood one of the most prominent questions in this respect is what are the factors that control the angle of the resulting shear bands recent theoretical work suggest that both roscoe 45 coulomb angles 45 2 where is the angle of internal friction and arthur angles 45 4 where is the dilation angle as well as all intermediate angles are possible published numerical models however show a large range of shear band angles with some codes favoring arthur angles whereas others yield coulomb angles in order to understand what causes the differences between the various numerical models here i perform systematic numerical simulations of shear localization around an inclusion of given length scale both numerical element type geometrical and rheological viscoplastic versus viscoelastoplastic effects are studied results indicate that the main factor controlling shear band angle is the non dimensional ratio between the length scale of the heterogeneity d and the size of the numerical mesh x coulomb angles are observed only in cases where the inclusion is resolved well d x 5 10 and in which it is located sufficiently far from the boundary of the box in most other cases either arthur or roscoe orientations are observed if heterogeneities are one element in size coulomb angles are thus unlikely to develop irrespective of the employed numerical resolution whereas differences in element types and rheology do have consequences for the maximum obtainable strain rates inside the shear bands they only have a minor effect on shear band angles shear bands initiated from random noise or from interactions of shear bands with model boundaries or other shear bands result in stress heterogeneities with dimensionless length scales d x 1 2 such shear bands are thus expected to form roscoe or arthur orientations consistent with the findings in previous numerical models

geochemistry of the alginite and amorphous organic matter from type ii s kerogens abstract maceral fractions of the type ii s kerogens from the monterey formation miocene california u s a and duwi formation campanian maastrichtian egypt were separated by density gradient centrifugation the monterey fm kerogen sample was comprised chiefly of light red fluorescing amorphous organic matter aom the flash pyrolyzate of which was characterized by a predominance of alkylbenzenes alkylthiophenes and alkylpyrroles in contrast the pyrolyzates of its alginite concentrate showed a highly aliphatic character typical of this maceral with the series of n alkenes and n alkanes c6 c26 predominating the pyrolyzate of the dominant light brown fluorescing aom of the duwi fm kerogen had a relatively high concentration of alkylbenzenes and alkylthiophenes while its alginite concentrate showed a more aliphatic character upon pyrolysis there was a marked enrichment of thiophenic sulfur in the light colored aom of both samples and also pyrrolic nitrogen in the case of the monterey relative to the alginite the results support a bacterially mediated degradative origin for type ii s amorphous organic matter with algal remains as the primary source of the kerogen

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

inferring the viscosity and the 3 d density structure of the mantle from geoid topography and plate velocities summary in a dynamic earth mantle mass heterogeneities induce gravity anomalies surface velocities and surface topography these lateral density heterogeneities can be estimated on the basis of seismic tomographic models recent papers have described a realistic circulation model that takes into account the observed plate geometry and is able to predict the rotation vectors of the present plates the relationship between the surface observables and the heterogeneities is sensitive to the viscosity stratification of the mantle here we use this model combined with a generalized least squares method in order to infer the viscosity profile of the earth from the surface observations and to get some new insight into the 3 d density structure of the mantle the computed radial viscosity profile presents a continuous increase of more than two orders of magnitude the asthenosphere has a viscosity close to 2 x lo2 pa s no sharp discontinuity is requested at the upper lower mantle interface the largest viscosity 7 x 10 pas is reached in the middle of the lower mantle at greater depth approaching the core mantle boundary the viscosity decreases by one order of magnitude the model suggests that the well known degree 2 and order 2 anomaly in the transition zone of the upper mantle is merely the signature of the slabs it also slightly increases the degree 2 and order 0 in the lower mantle and decreases it in the upper mantle in other words the inversion requests a hotter lower mantle beneath the equator and a colder upper mantle at the same latitudes

sulphur and oxygen sequestration of n c37 and n c38 unsaturated ketones in an immature kerogen and the release of their carbon skeletons during early stages of thermal maturation sedimentary rock from the gessoso solfifera formation messinian in the vena del gesso basin northern italy containing immature ro 0 25 s rich organic matter was artificially matured by hydrous pyrolysis at temperatures from 160 to 330 c for 72 h to study the diagenetic fate of n c37 and n c38 di and tri unsaturated methyl and ethyl ketones alkenones biosynthesised by several prymnesiophyte algae during early diagenesis the alkenones are incorporated into the kerogen by both sulphur and oxygen cross linking as indicated by chemical degradation experiments with the kerogen of the unheated sample heating at temperatures between 160 and 260 c which still represents early stages of thermal maturation produces large amounts up to 1 mg g toc of s bound o bound and both s and o bound n c37 and n c38 skeletons saturated n c37 and n c38 methyl ethyl and mid chain ketones c37 and c38 mid chain 2 5 di n alkylthiophenes c37 and c38 1 2 di n alkylbenzenes and c37 and c38 n alkanes with increasing thermal maturation three forms of the n c37 and n c38 skeletons are relatively stable saturated hydrocarbons 1 2 di n alkylbenzenes and saturated ketones whereas the s and o bound skeletons are relatively labile these results suggest that in natural situations saturated ketones with an n c37 and n c38 skeleton can be expected as well as the corresponding hydrocarbons

steady state laboratory flow laws alone fail to explain postseismic observations abstract we test whether laboratory derived steady state flow laws for wet olivine can be used to explain postseismic relaxation in the upper mantle following the 1999 hector mine earthquake this is accomplished using rate changes in gps observed displacement time series at far field stations as constraints on the parameters associated with laboratory derived temperature and stress dependent flow laws for diffusion and dislocation creep of olivine we primarily concentrate on the influences of temperature grain size and background strain rate to explain observed fast early displacement rates that slow rapidly in the 7 years of postseismic observations we find that a reasonable fit to the observed time series cannot be achieved by any combination of flow law parameters and plausible assumptions for the tectonic environment of the mantle beneath the mojave desert we conclude that postseismic relaxation within the mantle following the hector mine earthquake likely includes an initial transient weakening phase suggesting that steady state flow laws by themselves are insufficient to characterize the flow of mantle rocks when stress conditions evolve

simultaneous inversion of mantle properties and initial conditions using an adjoint of mantle convection through the assimilation of present day mantle seismic structure adjoint methods can be used to constrain the structure of the mantle at earlier times i e mantle initial conditions however the application to geophysical problems is restricted through both the high computational expense from repeated iteration between forward and adjoint models and the need to know mantle properties such as viscosity and the absolute magnitude of temperature or density a priori we propose that an optimal first guess to the initial condition can be obtained through a simple backward integration sbi of the governing equations thus lessening the computational expense given a model with known mantle properties we show that a solution based on an sbi generated first guess has smaller residuals than arbitrary guesses mantle viscosity and the effective rayleigh number are crucial for mantle convection models neither of which is exactly known we place additional constraints on these basic mantle properties when the convection induced dynamic topography on earth s surface is considered within an adjoint inverse method besides assimilating present day seismic structure as a constraint we use dynamic topography and its rate of change in an inverse method that allows simultaneous inversion of the absolute upper and lower mantle viscosities scaling between seismic velocity and thermal anomalies and initial condition the theory is derived from the governing equations of mantle convection and validated by synthetic experiments for both one layer viscosity and two layer viscosity regionally bounded spherical shells for the one layer model at any instant of time the magnitude of dynamic topography is controlled by the temperature scaling while the rate of change of topography is controlled by the absolute value of viscosity for the two layer case the rate of change of topography constrains upper mantle viscosity while the magnitude of dynamic topography determines the temperature scaling lower mantle viscosity when upper mantle lower mantle density anomaly dominates the flow field this two stage scheme minimizes the tradeoff between temperature and lower mantle viscosity for both cases we show that the theory can constrain mantle properties with errors arising through the adjoint recovery of the initial condition for the two layer model this error is manifest as a tradeoff between the temperature scaling and lower mantle viscosity

introduction to special issue of canadian journal of earth sciences the lake nipigon region geoscience initiative the lake nipigon region geoscience initiative lnrgi was a 3 year collaborative geoscience data acquisition and compilation project that focussed on lake nipigon in ontario and involved partnerships between mineral industry university and provincial and federal government geoscientists a major objective of the initiative was to collect new geoscience information to better understand the geological history of the nipigon region to inform land use planning identify areas of high mineral potential and attract mineral exploration the special issue on the lnrgi highlights the scientific successes of the initiative in the fields of geology petrology geochemistry geochronology stratigraphy and basin analysis mineral deposits and surficial geology and geochemistry

environmental conditions in the south atlantic angola basin during the early cretaceous abstract the south atlantic has experienced periods of intense anoxia and black shale deposition that form an important hydrocarbon source here we have investigated the depositional environment during the initial opening phase of the south atlantic during the early cretaceous the period is crucial as it is characterized by extensive source rock deposition and because it sets the stage for subsequent periods of anoxia within the northern sub basin of the south atlantic within an aptian sequence of organic rich sediments up to 40 total organic carbon toc from deep sea drilling project dsdp site 364 we found a distinct biomarker distribution including the presence of isorenieratane and an array of thiophenic s containing compounds our results indicate that during the time of deposition corresponding to the initial phase of opening of the south atlantic most of the water column in the angola basin was hypersaline 40 and euxinic with euxinia episodically reaching the photic zone the low relative abundance of marine biomarkers in the samples suggests that these extreme conditions were unfavourable for typical marine organisms stratigraphically up section the biomarker distribution changed as the toc content gradually decreased

shear heating induced lithospheric scale localization does it result in subduction abstract even though it is a well established fact that the earth is currently in a plate tectonics mode the question on how to break lithospheric plates and initiate subduction remains a matter of debate here we focus on shear heating as a potential mechanism to cause lithospheric shear localization and subsequent subduction initiation in oceanic plates it is shown that shear heating under some conditions i facilitates the formation of a lithospheric scale shear zone and ii is capable of stabilizing a lithospheric scale shear zone thus creating the necessary condition for subduction initiation to occur furthermore we demonstrate that not only the localization process is of importance but also the post localization stage where rapidly growing convective instabilities might prevent an incipient subduction zone from developing we develop scaling laws for both the localization and the post localization stages in the case of lithospheric localization the characteristic length scale equals a quarter of the dominant folding wavelength in the lithosphere thus of the order of the competent layer thickness this shows that oceanic lithosphere develops an intrinsic perturbation length scale that is largely independent of smaller scale heterogeneities we compare the subduction initiation potential of wet olivine and dry olivine rheologies and find that for earth like conditions a dry olivine rheology is more likely to result in subduction initiation a large plate age does not always increase the potential for subduction initiation as it increases the potential for convective instabilities to occur instead an optimal plate age exists for shear heating induced subduction initiation which is around 40 ma for a wet olivine rheology

long chain alkenones preserved in miocene lake sediments abstract over the last few decades the occurrence of long chain alkenones lcas in quaternary lacustrine settings has been reported the distributions being remarkably different from those of open ocean settings we report here the occurrence of lcas with modern lake type composition from middle miocene sedimentary strata in the dahonggou section qaidam basin northern tibetan plateau this is the first report of lcas with preserved lake compositions in pre quaternary lacustrine sediments the discovery implies that the producers of modern lacustrine lcas could originate from similar lake species living at least as far back as the miocene epoch it also sheds new light on the potential of alkenones as palaeotemperature indicators in lacustrine systems back to at least 10 5 ma

identification of a novel c25 highly branched isoprenoid thiophene in sediments a novel c25 highly branched isoprenoid hbi thiophene 2 2 methylpropyl 4 9 2 6 12 trimethyltetradecyl thiophene has been identified in three sediment samples by isolation followed by characterization by 1h nmr gc ms and raney ni desulphurization this component is probably formed by incorporation of inorganic sulphur species into a specific sedimentary c25 hbi alkadiene this latter compound is probably derived from diatoms

mantle circulation models with variational data assimilation inferring past mantle flow and structure from plate motion histories and seismic tomography summary mantle convection models require an initial condition some time in the past because this initial condition is unknown for earth we cannot infer the geological evolution of mantle flow from forward mantle convection calculations even for the most recent mesozoic and cenozoic geological history of our planet here we introduce a fluid dynamic inverse problem to constrain unknown mantle flow back in time from seismic tomographic observations of the mantle and reconstructions of past plate motions using variational data assimilation we derive the generalized inverse of mantle convection and explore the initial condition problem in high resolution 3 d spherical mantle circulation models for a time period of 100 myr roughly comparable to half a mantle overturn we present a synthetic modelling experiment to demonstrate that mid cretaceous mantle structure can be inferred accurately from fluid dynamic inverse modelling assuming present day mantle structure is well known even if an initial first guess assumption about the mid cretaceous mantle involved only a simple 1 d radial temperature profile we also demonstrate that convecting present day mantle structure back in time by reversing the time stepping of the energy equation is insufficient to model the mantle structure of the past the difficulty arises because such backward convection calculations ignore thermal diffusion effects and therefore cannot account for the generation of thermal buoyancy in boundary layers as we go back in time inverse mantle convection modelling should make it possible to infer a number of flow parameters from observational constraints of the mantle

artificial maturation of an immature sulfur and organic matter rich limestone from the ghareb formation jordan abstract an immature r o 0 39 s rich s org c 0 07 organic matter rich 19 6 wt toc limestone from the ghareb formation upper cretaceous in jordan was artificially matured by hydrous pyrolysis 200 220 300 c 72 h to study the effect of progressive diagenesis and early catagenesis on the amounts and distributions of hydrocarbons organic sulfur compounds and s rich geomacromolecules the use of internal standards allowed the determination of absolute amounts with increasing thermal maturation large amounts of alkanes and alkylthiophenes with predominantly linear carbon skeletons are generated from the kerogen the alkylthiophene isomer distributions do not change significantly with increasing thermal maturation indicating the applicability of alkylthiophenes as biomarkers at relatively high levels of thermal maturity for a given carbon skeleton the saturated hydrocarbon alkylthiophenes and alkylbenzo b thiophenes are stable forms at relatively high temperatures whereas the alkylsulfides are not stable the large amount of alkylthiophenes produced relative to the alkanes may be explained by the large number of monosulfide links per carbon skeleton these results are in good agreement with those obtained previously for an artificial maturation series of an immature s rich sample from the gessoso solfifera formation

dynamic constraints on the crustal scale rheology of the zagros fold belt iran thin skinned fold and thrust belts are generally considered as the result of contractional deformation of a sedimentary succession over a weak decollement layer the resulting surface expression frequently consists of anticlines and synclines spaced in a fairly regular manner it is thus tempting to use this spacing along with other geological constraints to obtain insights into the dynamics and rheology of the crust on geological time scales here we use the zagros mountains of iran as a case study as it is one of the most spectacular well studied thin skinned fold and thrust belts in the world both analytical and numerical models are employed to study what con trols fold spacing and under what conditions folding dominates over thrusting the models show that if only a single basal decollement layer is present underneath a brittle sedimentary cover deformation is dominated by thrusting which is inconsistent with the data of the zagros fold belt if we instead take into account additional decollement layers that have been documented in the fi eld a switch in deformation mode occurs and crustal scale folding is obtained with the correct spacing and time scales we show that fold spacing can be used to constrain the friction angle of the crust which is 5 the zagros fold belt this implies that on geological time scales the upper crust is signifi cantly weaker than previously thought possibly due to the effect of fl

inferences of mantle viscosity from tectonic plate velocities the present day velocities of the earth s tectonic plates provide direct constraints on the absolute value of the mantle viscosity we present the results of a formal inversion of plate motion data and demonstrate its utility in constraining the depth variation of absolute viscosity

configuration of the indian moho beneath the nw himalaya and ladakh 1 teleseismic receiver function analysis of seismograms recorded on a 700 km long profile of 17 broadband seismographs traversing the nw himalaya shows a progressive northward deepening of the indian moho from 40 km beneath delhi south of the himalayan foredeep to 75 km beneath taksha at the karakoram fault similar studies by wittlinger et al 2004 to the north of the karakoram fault show that the moho continues to deepen to 90 km beneath western tibet before shallowing substantially to 50 60 km at the altyn tagh fault the continuity of the indian moho imaged in the receiver functions reported here along with those of wittlinger et al 2004 suggest that in this part of the himalayan orogen the indian plate may penetrate as far as the bangong suture and possibly as far north as the altyn tagh

3 d multi observable probabilistic inversion for the compositional and thermal structure of the lithosphere and upper mantle ii general methodology and resolution analysis 1 here we present a 3 d multi observable probabilistic inversion method particularly designed for high resolution regional thermal and compositional mapping of the lithosphere and sub lithospheric upper mantle that circumvents the problems associated with traditional inversion methods the key aspects of the method are as follows a it exploits the increasing amount and quality of geophysical datasets b it combines multiple geophysical observables rayleigh and love dispersion curves body wave tomography magnetotelluric geothermal petrological gravity elevation and geoid with different sensitivities to deep shallow thermal compositional anomalies into a single thermodynamic geophysical framework c it uses a general probabilistic bayesian formulation to appraise the data d no initial model is needed e compositional a priori information relies on robust statistical analyses of a large database of natural mantle samples and f it provides a natural platform to estimate realistic uncertainties in addition the modular nature of the method algorithm allows for incorporating or isolating specific forward operators according to available data the strengths and limitations of the method are thoroughly explored with synthetic models it is shown that the a posteriori probability density function i e solution to the inverse problem satisfactorily captures spatial variations in bulk composition and temperature with high resolution as well as sharp discontinuities in these fields our results indicate that only temperature anomalies of t 150 c and large compositional anomalies of mg 3 or bulk al2o3 1 5 can be expected to be resolved simultaneously when combining high quality geophysical data this resolving power is sufficient to explore some long standing problems regarding the nature and evolution of the lithosphere e g vertical stratification of cratonic mantle compositional versus temperature signatures in seismic velocities etc and offers new opportunities for joint studies of the structure of the upper mantle with unprecedented resolution

three dimensional velocity field of present day crustal motion of the tibetan plateau derived from gps measurements 1 using the measurements of 750 gps stations around the tibetan plateau for over 10 years since 1999 we derived a high resolution 3 d velocity field for the present day crustal movement of the plateau the horizontal velocity field relative to stable eurasia displays in details the crustal movement and tectonic deformation features of the india eurasia continental collision zone with thrust compression lateral extrusion and clockwise rotation the vertical velocity field reveals that the tibetan plateau is continuing to rise as a whole relative to its stable north neighbor however in some subregions uplift is insignificant or even negative the main features of the vertical crustal deformation of the plateau are the following a the himalayan range is still rising at a rate of 2 mm yr the uplift rate is 6 mm yr with respect to the south foot of the himalayan range b the middle eastern plateau has a typical uplift rate between 1 and 2 mm yr and some high mountain ranges in this area like the longmen shan and gongga shan have surprising uplift rates as large as 2 3mm yr c in the middle southern plateau there is a basin and endorheic subregion with a series of ns striking normal faults showing obvious sinking with the rates between 0 and 3 mm yr d the present day rising and sinking subregions generally correspond well to the cenozoic orogenic belts and basins respectively e at the southeastern corner of the plateau there is an apparent trend that the uplift rate is gradually decreasing from between 0 8 and 2 3 mm yr in the inner plateau to between 0 5 and 1 6 mm yr outside the plateau with the decrease of terrain height

how flat is tibet high resolution digital topography three arc second grid for most of tibet provides new information to characterize the relief of the highest and largest plateau on earth the arid to semiarid central and northern part of the plateau interior has low relief average slopes of over 250 m windows and a mean elevation of 5023 m above sea level at moderate wavelengths of m relief is or less for most of tibet as opposed to the much higher relief of up to 6 km on the plateau edges where glacial and fluvial dissection is greater because of higher levels of precipitation the only faults manifesting significant topographic relief are the relatively small scale generally north trending graben systems primarily in southern tibet and several large scale fault systems near the edges of tibet the flatness of tibet implies that 1 there has been little deformation especially shortening of the uppermost crust north of the graben systems during the late cenozoic and 2 shallow crustal isostatic compensation has been acting to level the surface of the plateau

lithospheric deformation induced by loading of the hawaiian islands and its implications for mantle rheology the long term rheological properties of the lithosphere are fundamental for understanding both surface tectonics and mantle dynamics on earth in this study we have developed 3 d finite element models for computing the load induced surface deformation and stress for lithosphere and mantle with realistic nonlinear viscoelastic rheology including the frictional sliding low temperature plasticity and high temperature creep we have determined the lithospheric deformation and stress due to volcano loading in the hawaiian islands region for the last few million years by comparing model predictions with seismic observations of the depth to the top of oceanic crust and depth dependence of seismicity in the hawaiian islands region we have sought to constrain lithospheric rheology our calculations show that the load induced surface deformation is controlled by low temperature plasticity and frictional sliding but is insensitive to high temperature creep lithospheric strength predicted from laboratory derived low temperature plasticity needs to be reduced significantly and a frictional coefficient f ranging from 0 1 to 0 7 is required in order to account for the observations however f 0 1 weakens the shallow part of the lithosphere so much that it causes the minima in strain rate and stress to occur at too large depths to be consistent with the observed depth distribution of seismicity our results therefore suggest a value for f between 0 25 and 0 7 finally the maximum stress that accumulates in the deformed lithosphere beneath the hawaiian islands is about 100 200 mpa for models that match the observations and this stress may be viewed as the largest lithospheric stress on earth

radial profile of mantle viscosity results from the joint inversion of convection and postglacial rebound observables we present new inferences of the radial profile of mantle viscosity that simultaneously fit long wavelength free air gravity harmonics associated with mantle convection and a large set of decay times estimated from the postglacial uplift of sites within previously glaciated regions hudson bay arctic canada and fennoscandia the relative sea level variation at these latter sites is constrained by age height pairs obtained by geological survey rather than the subjective trends which are commonly used in glacial isostatic adjustment gia studies our viscosity inferences are generated using two approaches first we adopt a relative viscosity profile which is known to provide a good fit to the free air gravity harmonics and determine an absolute scaling which yields a best fit to the gia decay time constraints second we perform an iterative nonlinear joint inversion of the two data sets in both cases our inferred profiles are characterized by a significant increase of viscosity 2 orders of magnitude with depth to values of 1022 pa s in the bottom half of the lower mantle the new viscosity profiles are shown to satisfy constraints based on the postglacial uplift of both fennoscandia the classic haskell 1935 number and hudson bay which have commonly been invoked to argue for an isoviscous mantle furthermore the models are used to predict a set of long wavelength signatures of the gia process these include predictions of gia induced variations in 1 the length of day over the late holocene period 2 the earth s precession constant and obliquity over the last 2 6 myr and 3 the present day zonal harmonics of the geopotential jl l 7 the predictions 1 and 3 bound the late holocene and ongoing mass flux between the large polar ice sheets greenland and antarctic and the global oceans to small values 0 4 mm yr equivalent eustatic sea level rise

the boundary between the indian and asian tectonic plates below tibet the fate of the colliding indian and asian tectonic plates below the tibetan high plateau may be visualized by in addition to seismic tomography mapping the deep seismic discontinuities like the crust mantle boundary moho the lithosphere asthenosphere boundary lab or the discontinuities at 410 and 660 km depth we herein present observations of seismic discontinuities with the p and s receiver function techniques beneath central and western tibet along two new profiles and discuss the results in connection with results from earlier profiles which did observe the lab the lab of the indian and asian plates is well imaged by several profiles and suggests a changing mode of india asia collision in the east west direction from eastern himalayan syntaxis to the western edge of the tarim basin the indian lithosphere is underthrusting tibet at an increasingly shallower angle and reaching progressively further to the north a particular lithospheric region was formed in northern and eastern tibet as a crush zone between the two colliding plates the existence of which is marked by high temperature low mantle seismic wavespeed correlating with late arriving signals from the 410 discontinuity poor sn propagation east and southeast oriented global positioning system displacements and strikingly larger seismic sks anisotropy

present day crustal motion within the tibetan plateau inferred from gps measurements 1 using the measurements of 726 gps stations around the tibetan plateau we determine the rigid rotation of the entire plateau in a eurasia fixed reference frame which can be best described by an euler vector of 24 38 0 42 n 102 37 0 42 e 0 7096 0 0206 ma the rigid rotational component accommodates at least 50 of the northeastward thrust from india and dominates the eastward extrusion of the northern plateau after removing the rigid rotation to highlight the interior deformation within the plateau we find that the most remarkable interior deformation of the plateau is a glacier like flow zone which starts at somewhere between the middle and western plateau goes clockwise around the eastern himalayan syntaxis ehs and ends at the southeast corner of the plateau with a fan like front the deformation feature of the southern plateau especially the emergence of the flow zone could be attributed to an eastward escape of highly plastic upper crustal material driven by a lower crust viscous channel flow generated by lateral compression and gravitational buoyancy at the later developmental stage of the plateau the first order feature of crustal deformation of the northeastern plateau can be well explained by a three dimensional elastic half space dislocation model with rates of dislocation segments comparable to the ones from geological observations in the eastern plateau although gps data show no significant convergence between the eastern margin of the plateau and the sichuan basin a small but significant compressional strain rate component of 10 5 2 8 nstrain yr exists in a relatively narrow region around the eastern margin in addition a large part of the eastern plateau northeast of the ehs is not undergoing shortening along the northeastward convergence direction of the ehs but is stretching

mantle fluids in the karakoram fault helium isotope evidence abstract the karakoram fault kkf is the 1000 km long strike slip fault separating the western himalaya from the tibetan plateau from geologic and geodetic data the kkf is argued either to be a lithospheric scale fault with hundreds of km of offset at several cm a or to be almost inactive with cumulative offset of only a few tens of kilometers and to be just the upper crustal localization of distributed deformation at depth here we show 3he 4he ratios in geothermal springs along a 500 km segment of the kkf are 3 100 times the normal ratio in continental crust providing unequivocal evidence that a component of these hydrologic systems is derived from tectonically active mantle mantle enrichment is absent along the indus yarlung suture zone isz just 35 km southwest of the kkf suggesting that the mantle fluids flow only within the kkf within the last few ma the kkf must have accessed tectonically active tibetan mantle northeast of the mantle suture which we therefore locate vertically beneath the kkf very close to the surface trace of the isz hence in southwestern tibet indian crust may not now be underthrusting substantially north of the isz even though miocene underthrusting may have placed indian crust north of the isz in the lower half of the tibetan plateau crust this is in significant contrast to central and eastern tibet where underthrust indian material not only forms the lower half of the tibetan crust but is also currently underthrusting for 200 km north of the isz our new constraint on kkf penetration to the mantle allows an improved description of the continuously evolving karakoram fault as a tectonically significant yet perhaps geologically ephemeral lithospheric structure

a synopsis of mineral deposits in the archean and proterozoic rocks of the lake nipigon region thunder bay district ontario a variety of metallic and non metallic mineral deposit types occur within archean and proterozoic rocks in the area encompassing the lake nipigon region geoscience initiative archean deposit types include algoma type banded iron formation hosted iron e g lake nipigon iron range volcanogenic massive sulphide copper zinc e g onaman tashota belt ultramafic intrusion hosted chromium e g puddy chrome lakes mafic to ultramafic intrusion hosted copper nickel platinum group element pge e g lac des iles and pegmatite hosted deposits of rare metals li ta be uranium and molybdenum e g georgia lake field black sturgeon lake and anderson lake respectively mesothermal lode gold deposits are prominent in the beardmore geraldton camp superior type iron formation occurs in paleoproterozoic gunflint formation red bed copper occurs in mesoproterozoic midcontinent rift related osler group volcanic and interflow sedimentary rocks native copper and copper sulphides occur in mesoprote

geodynamic inversion to constrain the non linear rheology of the lithosphere a common method to determine the strength of the lithosphere is through estimating its effective elastic thickness from the coherence between gravity and topography this method assumes a priori that the lithosphere is a thin elastic plate floating on a viscous mantle whereas this seems to work well with oceanic plates it has given controversial results in continental collision zones usually continental collisions zones are well studied areas for which additional geophysical datasets such as receiver functions and seismic tomography exist that constrain the geometry of the lithosphere and often show that it is rather complex yet lithospheric geometry by itself is insufficient to understand the dynamics of the lithosphere as this also requires knowledge of the rheology of the lithosphere experimental results show significant variability between various rock types and there are large uncertainties in extrapolating laboratory values to nature which leaves room for speculation

free and sulphurized hopanoids and highly branched isoprenoids in immature lacustrine oil shales abstract a study of the solvent extracts of four samples from two immature oil shales from tertiary lacustrine basins ribesalbes and campins southern european rift system deposited under reducing conditions has allowed the identification of s containing hopanoids and novel highly branched isoprenoids hbi hbi derivatives occur only in the ribesalbes shales where two new structural types of c 25 s containing hbi isomers have been identified these compounds probably have different precursor organisms from those in recent marine systems they are present as thiophenes thiolanes and thiolane sulphoxides in one case and only as thiolanes in the other 17 21 h 17 21 h and 17 21 h thienyl and methylthienylhopanes have been found in the shales from the two basins in the campins samples these compounds occur together with thiolanyl and methylthiolanylhopanes distributions of 17 21 h and 17 21 h hopanes hop 17 21 enes and 2 methylhop 17 21 enes ribesalbes only are also encountered comparison of the hopane distributions from both solvent extractable aliphatic and thiophene fractions and the sulphur and oxygen bound macromolecular matter in ribesalbes basin shows a remarkable similarity between all fractions this resemblance points to rather uniform inputs and depositional processes for the accumulation of these compounds both as small molecules or bound to macromolecules e g early bonding of bacteriohopanepolyols to sulphur and to the macromolecular organic matter the good correlation among the macromolecular fractions in ribesalbes basin is reinforced when comparing the 13 c composition of phytane c 20 and c 25 hbi and the c 35 hopane homologues in the different fractions

constraining effective rheology through parallel joint geodynamic inversion abstract the dynamics of crust and lithosphere is to a large extent controlled by its effective viscosity unfortunately extrapolation of laboratory experiments indicates that viscosity is likely to vary over many orders of magnitude additional methods are thus required to constrain the effective viscosity of the present day lithosphere using more direct geophysical observations here we discuss a method which couples 3d geodynamic models with observations surface velocities and gravity anomalies and with a bayesian inversion scheme on massively parallel high performance computers we illustrate that the basic principle of a joint geodynamic and gravity inversion works well with a simple analytical example in a next step we test our approach using a synthetic 3d model of salt tectonics with erosion and sedimentation and check how much noise conditions model resolution and sparse data coverage affect the resolving power of the method results show that it is possible to constrain the effective viscosity and density of layers that contribute to the large scale dynamics provided that those layers are numerically well resolved the properties of thin layers that do not contribute much to the overall dynamics cannot be constrained but noise or sparse data sampling does not significantly affect the inversion results this thus illustrates that a joint geodynamic and gravity inversion is a potentially powerful method to constrain the dynamics of the crust and lithosphere having better constraints on the structure of the present day crust and lithosphere will help to narrow the parameter space for models that aim to unravel lithosphere dynamics on a geological time scale

comparison of two u37k sea surface temperature records for the last climatic cycle at odp site 658 from the sub tropical northeast atlantic abstract u37k is a proxy indicator of sea surface temperature of the euphotic zone two records of u37k are compared to assess the validity of this indicator as a proxy of paleo sst the site studied odp leg 108 site 658 is located below the upwelling cell off northwest africa the 658a b record has already been reported and the main finding was the detection of rapid and large sst oscillations in terminations magnetic susceptibility data are used to cross correlate the new hole 658c record with that of 658a and b the oxygen isotope stratigraphic record is used to produce a preliminary time scale the two sst records of the last 120 ka reveal similar long term changes for the last climatic cycle which are in agreement with other paleoclimatic indicators such as 18o both the new 658c u37k record and the magnetic susceptibility data suggest that the missing record of stage 4 reported for 658a and b has been recovered in hole 658c detailed comparison of the high resolution records of the first 5 m 25 ka demonstrates that the u37k index is a reproducible measure for sst reconstruction for this segment of the record all features of sst changes reported in the 658a and b curve are confirmed in the new 658c curve furthermore sst oscillations of roughly the same magnitude and duration as the ones reported to occur during terminations have been identified in stages 3 and 4 these new findings indicate that rapid and large sst oscillations can occur not only during the deglaciation transition but also throughout the interglacial and glacial stages other complimentary proxy studies are needed to understand the mechanisms of these sst changes

computational approaches to studying non linear dynamics of the crust and mantle we outline a mathematical formulation for mantle convection which can deal with the viscoelastic plastic rheology of the cool parts of the lithosphere this formulation is then analyzed to expose the numerical challenges inherent in the equations and a suitable solution strategy is outlined with this strategy in place we discuss a parallel implementation explaining how we maintain computational efficiency in a tiered and modular software environment we show two examples from geodynamics which demonstrate the capability of the numerical method and its implementation c 2007 elsevier b v all rights reserved

models of isostatic and dynamic topography geoid anomalies and their uncertainties knowledge of the dynamic topography at earth s surface caused by sublithospheric density contrasts would provide crucial constraints on models of mantle dynamics we calculate global models of this dynamic topography by subtracting from the observed topography estimates of the topography caused by assumed isostatically compensated density variations in the crust the oceanic lithosphere and the continental tectosphere we also calculate the isostatic geoid anomaly that would result from these compensated near surface density anomalies because there is controversy about the thermal structure of old oceanic lithosphere we investigate two models of the cooling of the oceanic lithosphere half space and plate and calculate the effect of each on the inferred dynamic topography peak amplitudes of dynamic topography from sublithospheric sources expanded to spherical harmonic degree 6 are approximately 1 km we estimate the uncertainties in the calculated dynamic topography and geoid by combining the errors in the data and systematic errors to obtain the spectral coefficients of the fields we use weighted least squares inversions the calculated dynamic topography arising from sublithospheric sources residual geoid and errors associated with these fields can be used in studies of mantle dynamics

organic geochemistry and paleoenvironment of the early eocene pesciara di bolca konservat lagerstatte italy abstract exceptional preservation of fossils in so called konservat lagerstatten requires specific depositional regimes excluding disturbance of bottom sediments by either wave actions and currents or by benthic fauna we here describe a depositional model for the eocene pesciara di bolca konservat lagerstatte based on sedimentological paleoecological and detailed organic geochemical results sediments were deposited in a lagoonal like basin with stagnant bottom waters located on an extended carbonate platform that was sheltered from open marine waters by a submarine threshold run off from nearby land areas provided nutrients to support an algal community dominated by diatoms no fossil diatom shells have been identified but evidence for their presence is given by the high abundance of highly branched isoprenoids in extractable bitumens influx of terrigenous organic matter into the lagoon occurred in particular during deposition of the basal fish bearing level l1 here not only plant macrofossils amber spores and pollen but also the lipid composition indicated notable input of land plants via the presence of n c 24 to n c 32 carboxylic acids long chain n alkanes n c 27 n c 29 n c 31 and angiosperm wax triterpenoids the redox regime in general was strongly reducing as evidenced by the high concentration of sulfur vs organic carbon excellent kerogen preservation as shown by high hydrogen indices and low pristane phytane but high phytane n c 18 ratios the water column was highly stratified with anoxic saline bottom and fresh surface waters euxinic conditions with free reduced sulfur present in the photic zone could only be detected in sediments from the l1 horizon via traces of aromatic carotenoids derived from green sulfur bacteria chlorobiaceae which utilize h 2 s in anoxygenic photosynthesis the depositional regime is thus comparable to the lithographic limestones of solnhofen but based on biomarker evidence lacks the high salinities postulated for the latter biomarker composition indicates that best preservation conditions prevailed in the basal part of the studied section 0 7 m above datum but declined upon deposition of the upper part we interpret the body of the pesciara as a parasequence of the 4th order 0 01 0 5 ma with the lower part representing a relative sea level lowstand and the upper part a relative sea level highstand

from phytoplankton to oil shale reservoirs a 19 million year record of the late cretaceous tethyan upwelling regime in the levant basin abstract the complexity of sedimentary organic matter formation its source to burial and influence on bottom water communities are addressed in this study of the 19 million years long upper cretaceous tethyan upwelling regime in the levant a multi proxy approach is amalgamated into a model through which the paleoceanographic complexity of an intense and long lasting upwelling system is better understood a shift in primary producer assemblages and reorganization of the bottom water bacterial consortium followed the campanian maastrichtian boundary enforcing conspicuous sedimentological and faunal changes the siliceous and phosphatic bearing sediments of the campanian gave way to maastrichtian uniform and regionally distributed organic rich carbonates accompanied by an increase in sedimentation rates from 1 to 6 cm kyr this has facilitated both the increase in organic matter production and its excellent preservation and through diagenetic pathways affected its elemental composition these paleoceanographic modifications have also affected foraminiferal assemblages from massive blooms of triserial buliminid benthic foraminifera during the campanian which have sequestered diatom chloroplasts to diverse trochospiral forms during the maastrichtian using nitrate instead of oxygen for their respiratory pathways we propose that shifts in the type of primary producers promote a change in the amount and type of preserved organic matter in bottom water communities on the lithological composition of the rock mass and ultimately on the hydrocarbon generation potential of the studied source rock the continued rise of southern tethyan sea level and global highstands during the upper maastrichtian culminating into arabian plate wide maximum flooding surfaces cause the migration of upwelling activity away from the study area terminating the high productivity sequence in the levant

stress strength relationship in the lithosphere during continental collision lithospheric strength profiles generated for a shortening continental lithosphere generally predict excessively high differential stresses in the sub moho continental mantle this seems inconsistent with the relative scarcity of earthquakes at this depth this inconsistency was put forward as evidence for weak mantle rheology however this argument implicitly assumes that strength envelopes are valid in actively deforming regions we test this assumption on two end member model lithospheres having identical upper crustal rheologies but with 1 a weak lower crust and strong mantle and 2 a strong lower crust and weak mantle for this purpose we compare one dimensional 1 d with 2 d visco elastoplastic numerical models of continental shortening the 2 d models show that strongly heterogeneous deformation typically follows initially homogeneous deformation lithospheric scale buckle folds and shear zones result in strain rate variations of as much as three orders of magnitude differential stresses in the upper crust are close to yield as predicted by 1 d models stresses in deeper lithospheric regions however are significantly smaller than in 1 d models especially in actively deforming regions systematic numerical simulations as a function of temperature and deformation rate reveal that 1 d models are reliable in hot and or slowly deforming lithospheres only the relative scarcity of earthquakes at mantle levels should thus be interpreted as an intrinsic consequence of strong lithospheric deformation rather than as evidence for a weak upper mantle

bacterial and algal markers in sedimentary organic matter deposited under natural sulphurization conditions lorca basin murcia spain abstract free lipids sulphur bound lipids present in macromolecular fractions and kerogen pyrolysis products have been studied in shales and early diagenetic carbonates replacing gypsum from a messinian sequence of lorca basin the high abundance of phytane 2 3 dimethyl 5 2 6 10 trimethylundecyl thiophene mid chain c 20 isoprenoid thiophenes and bithiophenes and bis o phytanyl and o phytanyl o sesterterpanyl glycerol ethers indicates that the organic matter in all these samples was deposited under hypersaline conditions the isopranyl glycerol ethers are essentially found in the sulphur bound macromolecular matter which contrasts with the low concentrations of these compounds as free lipids however the distributions of these isopranyl glycerols is paralleled by the occurrence of free phytanic acid shales and laminated carbonate and 3 7 11 15 19 pentamethyleicosanoic acid only in the laminated carbonate the bacterial inputs are represented by 2 hydroxytetracosanoic acid n octadec 11 enoic acid hopanoic acids and the distributions of iso and anteiso c 15 and c 17 homologues and minor amounts of iso c 16 and anteiso c 14 these branched fatty acids are characteristic of sulphate reducing bacteria the relative proportions of the iso and anteiso compounds in the total fatty acid distributions are correlated with the proportion of reduced sulphur in the sediments

geochemistry of the mesoproterozoic intrusive rocks of the nipigon embayment northwestern ontario evaluating the earliest phases of rift development the intrusive rocks of the nipigon embayment comprise a series of four mafic to ultramafic intrusions and a number of laterally extensive diabase sills that are among the oldest expression of the

where is the real transform boundary in california 1 the zone of highest geodetically defined strain rate in california does not everywhere coincide with the surface trace of the san andreas fault saf to determine whether this reflects the pattern of long term permanent deformation we analyzed the velocity field on swaths across the transform located so as to avoid intersections among the major fault strands slip rates and flexural parameters for each fault were determined by finding the best fit to the velocity profile using a simple arctan model representing the interseismic strain accumulation our slip rates compare well with current geologic estimates which suggests that the present day velocity field is representative of long term motions we find that the transform is a zone of high strain rate up to 80 km wide that is straighter than the saf and has an overall trend closer to the relative plate motion vector than the saf most sections of the saf take up less than half of the total slip rate and slip is transferred from one part of the system to another in a way that suggests that the saf should not be considered as a unique locator of the plate boundary up to half of the total displacement takes place on faults outside the high strain rate zone distributed over several hundred kilometers on either side our findings substantiate previous suggestions that the transform has the characteristics of a macroscopic ductile shear zone cutting the continental lithosphere around which stress and strain rate decrease on a length scale controlled by the length of the transform

subduction of the indian lithosphere beneath the tibetan plateau and burma abstract subduction of the indian lithosphere under eurasia plays an important role in the tectonic evolution of the tibetan plateau and surrounding regions to improve our knowledge of pertinent mantle structures through tomographic imaging we combine p wave arrival time data from temporary arrays in tibet and stations of the chinese seismograph network with reprocessed data from the international seismological centre the new images reveal considerable spatial variations in mantle structure along the collision zone and the horizontal distance over which presumed continental indian lithosphere slides northward beneath the plateau decreases from west where it underlies the himalayas and the entire plateau to east where no indication is found for present day underthrusting beyond the himalayan block and indus tsangpo suture p wavespeed appears low in the shallow mantle beneath much of central and eastern tibet these observations suggest that indian lithosphere underlies only the southwestern part of the plateau and that the central and northeastern part is underlain by lithosphere of asian origin the continental parts of the indian plate that currently underthrust or subduct below the plateau appear generally detached from the oceanic slab fragments that subducted longer ago and that have been detected deeper in the mantle except perhaps in the central part of the collision zone 80 90 e this suggests that ongoing india eurasia collision must increasingly be driven by other forces such as subduction of the indian plate further west and east e g beneath indochina our images reveal structures associated with eastward subduction along the burmese arc southeast of the eastern himalayan syntaxis the tengchong volcanic complex in southwest china is marked by slow wave propagation to 150 km depth suggesting a causal link to subduction along the burmese arc the low velocities beneath the red river fault region extend to greater depth and may be related to upper mantle processes further southeast

continuous deformation of the tibetan plateau from global positioning system data global positioning system velocities from 553 control points within the tibetan plateau and on its margins show that the present day tectonics in the plateau is best described as deformation of a continuous medium at least when averaged over distances of 100 km deformation occurs throughout the plateau interior by ese wnw extension and slightly slower nne ssw shortening relative to eurasia material within the plateau interior moves roughly eastward with speeds that increase toward the east and then flows southward around the eastern end of the himalaya crustal thickening on the northeast ern and eastern margins of the plateau occurs over a zone 400 km wide and cannot be the result of elastic strain on a single major thrust fault shortening there accommodates much of india s penetration into eurasia a description in terms of movements of rigid blocks with elastic strain associated with slip on faults between them cannot match the velocity field

forward and inverse modelling of gravity revealing insight into crustal structures of the eastern alps abstract the crustal structure of the eastern alps is less known than that of the western alps as participants of the european geotraverse and particular swiss projects investigated the alpine crust there recently the eastern alps were subject to seismic research which was conducted in the context of the german austrian and italian transalp project the 3d modelling of the density structure which is presented in this paper belonged to a series of piggy back projects which were closely accomplished using the seismic reflection studies we present a combined gravity seismic interpretation which is based on the results of both older deep seismic profiles and the new transalp profile special emphasis was laid on geologic and tectonic information which served as model constraints for near surface structures a 3d forward modelling of both the bouguer gravity field and the geoidal undulations provides in depth insight into the lithosphere and considers the constraints due to the uncertainties of used constraints particularly in depths of the eastern alpine moho an inversion technique complemented this study and provided insight into the shape and density contrast at the crust mantle interface the modelling indicates that the bouguer gravity field in the eastern alps is mainly caused by two sources the density contrast at the crust mantle boundary 350 kg m3 and the density inhomogeneities in the upper 10 km of the crust which contributes to the overall gravity field by amounts of approximately 30 10 5 m s2 the results of the forward and inverse modelling produced a shallower crust mantle boundary in the southern adriatic area in comparison with the northern part and differs from the seismic findings of the transalp group to some extent this observation and observable mismatch in the long wavelength model and the observed geoidal undulations point to density inhomogeneities in sub crustal and even sub lithospheric depths these deep seated inhomogeneities should have a substantial bearing on the isostatic balance of the eastern alps

thermal stability of thiophene biomarkers as studied by hydrous pyrolysis abstract an immature r o 0 25 sulphur rich calcareous shale from the gessoso solfifera formation messinian in the vena del gesso basin northern italy was artificially matured by hydrous pyrolysis at constant temperatures ranging from 160 to 330 c for 72 h to study the applicability of alkylthiophenes as molecular indicators during progressive diagenesis and early catagenesis artificial maturation at temperatures up to 330 c generated large amounts of alkylthiophenes from the thermal degradation of kerogen the importance of the kerogen in the production of alkylthiophenes is supported by the observation that the isomer distributions of alkylthiophenes at high temperatures coincide with the isomer distributions of alkylthiophenes in the curie point pyrolysate of the isolated kerogen of the original sample the generation profiles of the thiophenes with a phytane carbon skeleton and the 3 4 di n alkylthiophenes suggest that continuous degradation of alkylthiophenes is also taking place so that the overall effect is alkylthiophene concentrations with intermediate values the structural isomer distribution of linear alkylthiophenes does not change significantly when these compounds are generated at different temperatures this implies that important information regarding the original positions of functionalities in the precursor molecules is retained and thus that alkylthiophenes are useful biomarkers at relatively high levels of thermal maturity up to r o 0 95 1 1

secular and environmental constraints on the occurrence of dinosterane in sediments abstract the distribution patterns of sedimentary a ring methylated steranes have changed markedly over geological time although dinosterane and its isomer 24 ethyl 4 methylcholestane have been tentatively identified in three proterozoic rock units they are either not detectable or occur in low abundance relative to 3 methyl steranes throughout most of the palaeozoic between permian and middle triassic times 260 220 ma ago 4 methyl sterane abundances in marine sediments increased markedly the presence of dinosterane in some middle triassic marine sediments is contemporaneous with the appearance of fossil cysts of uncontested dinoflagellate affinity 4 methyl steranes including dinosterane or their precursor sterenes and sterols then show a continuous presence often in high abundance in marine sediments from the late triassic through to the present day assemblages of 4 methyl steranes and their precursors but with dinosterane absent or in low relative abundance are often the predominant steroids in lacustrine sediments in the cainozoic dinosterane appears to arise predominantly from marine dinoflagellates and as a consequence is a useful biological marker for mesozoic and cainozoic marine organic matter the isomer 24 ethyl 4 methylcholestane is likely to have multiple origins although its very high abundance in tertiary lacustrine sediments and oils compared to older materials suggests that dinoflagellates could also be the source in these cases

recognition of paleobiochemicals by a combined molecular sulfur and isotope geochemical approach study of organic matter in immature sediments from a messinian evaporitic basin shows that consideration of structures modes of occurrence and carbon isotopic compositions of free and sulfur bound carbon skeletons allow identification of biochemical precursors detailed information concerning biotic communities present during deposition of sediments can be retrieved in this way moreover unprecedented biochemicals were recognized these extend the horizon of biomarker geochemistry

the role of crustal strength variations in shaping orogenic plateaus with application to tibet 1 the tibetan plateau is the type example of a large orogenic plateau formed as a result of continent continent collision the morphology of the plateau and its margins suggests that preexisting variations in crustal strength have influenced the growth of the plateau we have developed a three dimensional numerical model of deformation in a viscous crust in order to investigate the effects of lateral heterogeneities on plateau growth the model includes a two layer crust and allows for lateral variation of viscosity in both the upper and lower layers model results indicate that crustal strength variations have a dramatic effect on the morphology and dynamics of a developing plateau a region of strong crust is characterized by a very steep plateau margin that propagates extremely slowly does not accommodate significant shortening strain and is subparallel to local upper and lower crustal velocities a weak crustal region develops a gently sloping margin uplift propagates rapidly across the weak zone and crustal material within the plateau is diverted toward the low strength region with a relatively simple distribution of strength variations corresponding to strong tarim and sichuan basin crust and a weak southeastern corner our model produces a plateau with many similarities to the tibetan plateau including the overall morphology rotation around the eastern syntaxis and e w extension analysis of model results suggests that e w extension of the central plateau may be related to the rapid eastward flow of crustal material into a weak zone in the se corner without significant change in plateau elevation

gps velocity field for the tien shan and surrounding regions 1 measurements at 400 campaign style gps points and another 14 continuously recording stations in central asia define variations in their velocities both along and across the kyrgyz and neighboring parts of tien shan they show that at the longitude of kyrgyzstan the tarim basin converges with eurasia at 20 2 mm yr nearly two thirds of the total convergence rate between india and eurasia at this longitude this high rate suggests that the tien shan has grown into a major mountain range only late in the evolution of the india eurasia collision most of the convergence between tarim and eurasia within the upper crust of the tien shan presumably occurs by slip on faults on the edges of and within the belt but 1 3 mm yr of convergence is absorbed farther north at the dzungarian alatau and at a lower rate with the kazakh platform to the west the tarim basin is thrust beneath the tien shan at 4 7 mm yr with respect to eurasia the ferghana valley rotates counterclockwise at 0 7 myr 1 about an axis at the southwest end of the valley thus gps data place a bound of 4 mm yr on the rate of crustal shortening across the chatkal and neighboring ranges on the northwest margin of the ferghana valley and they limit the present day slip rate on the right lateral talas ferghana fault to less than 2 mm yr gps measurements corroborate geologic evidence indicating that the northern margin of the pamir overthrusts the alay valley and require a rate of at least 10 and possibly 15 mm yr

teleseismic imaging of subducting lithosphere and moho offsets beneath western tibet teleseismic images suggest that the tarim plate plunges 45 s down to 300 km depth beneath nw tibet the 410 km discontinuity shallows by 10 km under the plateau implying 100 c cooler upper mantle the deepest moho on record 90 km lies under w qiangtang it rises abruptly by 20 and 10 km beneath the altyn tagh fault and bangong suture respectively vp vs ratios are normal except in the yecheng flexural basin and deep under the south karakax volcanics 1 92 w kunlun s neogene tectonics are simply accounted for by oblique subduction of lithospheric mantle beneath an upward extruding thrust wedge of the tarim crust

adjoint based estimation of plate coupling in a non linear mantle flow model theory and examples we develop and validate a systematic approach to infer plate boundary strength and rheological parameters in models of mantle flow from surface velocity observations based on a realistic rheological model that includes yielding and strain rate weakening from dislocation creep we formulate the inverse problem in a bayesian inference framework to study the distribution of parameters that are consistent with the observations we compute the maximum a posteriori map point gaussian approximations of the parameter distribution around that map point and employ markov chain monte carlo mcmc sampling methods the computation of the map point and the gaussian approximation require first and second derivatives of an objective function subject to non linear stokes equations these derivatives are computed efficiently using adjoint stokes equations we set up 2 d numerical experiments with many of the elements expected in a global geophysical inversion this setup incorporates three subduction zones with slab and weak zone interplate fault geometry consistent with average seismic characteristics with these experiments we demonstrate that when the temperature field is known we can recover the strength of plate boundaries the yield stress and strain rate exponent in the upper mantle when the number of uncertain parameters increases there are trade offs between the inferred parameters these trade offs depend on how well the observational data represents the surface velocities and on the weakness of plate boundaries as the plate boundary coupling drops below a threshold the uncertainty of the inferred parameters increases due to insensitivity of plate motion to plate coupling comparing the trade offs between inferred rheological parameters found from the gaussian approximation of the parameter distribution and from mcmc sampling we conclude that the gaussian approximation which is significantly cheaper to compute is often a good approximation in particular locally around the map point thus the method can be applied to the global problem of inferring non linear constitutive parameters and plate coupling factors for each subduction zone in a global geophysical inversion with known slab structure

evidence for sulphide links in a crude oil asphaltene and kerogens from reductive cleavage by lithium in ethylamine abstract reduction of a sulphur rich asphaltene from a crude oil rozel point with lithium in ethylamine li etnh2 affords an alkane fraction similar to that obtained from desulphurisation with raney nickel but in higher yield using li etnd2 deuterium labelled alkanes were obtained thereby confirming the presence of sulphide linkages in the asphaltene fraction reduction of two sulphur rich kerogens with similar sulphur contents serpiano shale m triassic and jurf ed darawish jed u cretaceous afforded significant quantities of polar solvent extractable material both kerogens also yielded low amounts of hydrocarbons although the amount released from jed kerogen was significantly higher the results indicate that whereas the kerogens contain hydrocarbon moieties intermolecularly linked by sulphide bridges the distributions of the precursors lipids which were incorporated were different for example gc ms analysis of the labelled products from jed kerogen revealed them to be dominated by steranes of algal origin with the position s of deuterium labelling providing evidence of sulphide linking and cross linking on the other hand the labelled products from serpiano kerogen were dominated by hopanes of bacterial origin which were linked terminally via the side chain

characterizing the main himalayan thrust in the garhwal himalaya india with receiver function ccp stacking abstract we use common conversion point ccp stacking of ps receiver functions to image the crustal structure and moho of the garhwal himalaya of india our seismic array of 21 broadband seismometers spanned the himalayan thrust wedge at 79 80 e between the main frontal thrust and the south tibet detachment in 2005 2006 our ccp image shows the main himalayan thrust mht the detachment at the base of the himalayan thrust wedge with a flat ramp flat geometry seismic impedance contrasts inferred from geologic cross sections in garhwal imply a negative impedance contrast velocity decreasing downward for the upper flat located beneath the lower himalaya and a positive impedance contrast velocity increasing downward for the ramp located beneath the surface trace of the munsiari thrust or mct i at the lower flat located beneath the higher himalaya spatially coincident measurements of very high electrical conductivities require the presence of free fluids and we infer a negative impedance contrast on the mht caused by ponding of these fluids beneath the detachment our seismic image indicates that the upper flat of the mht is 10 km below sea level and dips north at 2 connecting to a mid crustal ramp which is 10 km high and dips at 16 the lower flat is 20 25 km below sea level and dips at 4 the main central thrust mct appears as a negative impedance contrast dipping at 16 the moho is nearly horizontal at 35 45 km depth beneath the sub himalaya and lower himalaya deepening to 50 km or more beneath the higher himalaya this depth is 10 25 km shallower than in the nw indian himalaya and 5 10 km shallower than in central nepal requiring significant along strike variations in crustal thickness the observed thickness of subducted indian crust in garwhal is 20 28 km

comparisons of the kinematics and deep structures of the zagros and himalaya and of the iranian and tibetan plateaus and geodynamic implications 1 we compare the geologic histories the deep structures and the present day kinematics of deformation of the himalaya and the adjacent tibetan plateau with those of the zagros and iranian plateau to test geodynamic processes of continental collision shortly after india and arabia collided with eurasia horizontal shortening manifested itself by folding and thrust faulting of sedimentary rock detached from india s and arabia s underlying crystalline basement subsequently slip on thrust faults stacked slices of india s basement to build the himalaya on india s northern margin such faulting has not yet developed in the zagros where collision is more recent and arabia penetrates into eurasia more slowly than india does so that postcollision convergence with eurasia is less the greater elevation thicker crust and more marked heterogeneity of the upper mantle beneath the tibetan than beneath the iranian plateau also reflect a more advanced stage of development moreover while thrust or reverse faulting and crustal shortening continue on the margins of both plateaus normal faulting suggesting horizontal extension and crustal thinning occurs within tibet but not in iran hence the balance of forces that built the high tibetan plateau must have changed apparently some time since 15 ma removal of tibetan mantle lithosphere could have altered that balance if mantle lithosphere beneath the iranian plateau has been removed however the change in force balance has been too small to initiate normal faulting low seismic wave speeds in the uppermost mantle just beneath the moho of both plateaus suggest to us that lithosphere beneath both is thin consistent with late cenozoic removal of it but alternative explanations might account for these low speeds despite its apparently thin and hence presumably weak mantle lithosphere much of central iran undergoes little deformation it illustrates how a crustal block can behave rigidly not necessarily because it is strong but because deviatoric stresses can be small whereas differences between the two regions clearly depend on the amount that arabia and india have penetrated into eurasia which scales with both the dates of collision and rates of convergence we see no differences in the operative processes that depend on the present day rates of convergence

slab1 0 a three dimensional model of global subduction zone geometries 1 we describe and present a new model of global subduction zone geometries called slab1 0 an extension of previous efforts to constrain the two dimensional non planar geometry of subduction zones around the focus of large earthquakes slab1 0 describes the detailed non planar three dimensional geometry of approximately 85 of subduction zones worldwide while the model focuses on the detailed form of each slab from their trenches through the seismogenic zone where it combines data sets from active source and passive seismology it also continues to the limits of their seismic extent in the upper mid mantle providing a uniform approach to the definition of the entire seismically active slab geometry examples are shown for two well constrained global locations models for many other regions are available and can be freely downloaded in several formats from our new slab1 0 website http on doi gov d9arbs we describe improvements in our two dimensional geometry constraint inversion including the use of average active source seismic data profiles in the shallow trench regions where data are otherwise lacking derived from the interpolation between other active source seismic data along strike in the same subduction zone we include several analyses of the uncertainty and robustness of our three dimensional interpolation methods in addition we use the filtered subduction related earthquake data sets compiled to build slab1 0 in a reassessment of previous analyses of the deep limit of the thrust interface seismogenic zone for all subduction zones included in our global model thus far concluding that the width of these seismogenic zones is on average 30 larger than previous studies have suggested

the radiogenic isotope characteristics of dikes and sills associated with the mesoproterozoic midcontinent rift near thunder bay ontario canada abstract new radiogenic isotope data are presented for intrusive rocks of the mesoproterozoic midcontinent rift from the north shore of lake superior these mafic rocks are amongst the older portions of the rift sequence and offer insights into the early development and evolution of the rift two sill suites are recognized within the study area an earlier spatially restricted ultramafic unit termed the riverdale sill and the predominant logan sills as well as three dike sets the north east trending pigeon river swarm the north west trending cloud river dikes and the mt mollie dike in addition an enigmatic extrusive unit termed the devon basalts is also present the logan sills are characterized by 87sr 86sri values of 0 7048 0 7119 with 143nd 144nd values from 0 512012 to 0 512187 whereas the riverdale sill is characterized by 87sr 86sri values ranging from 0 7037 to 0 7078 and 143nd 144nd of 0 511938 0 511984 both the cloud river and pigeon river dikes show a range of isotope data with the cloud river dikes having 87sr 86sri values ranging from 0 7042 to 0 7064 and 143nd 144nd values from 0 512222 to 0 512379 whereas the pigeon river dikes have 87sr 86sri values from 0 7041 to 0 7243 and 143nd 144nd from 0 511640 to 0 512383 the mt mollie dike is characterized by 87sr 86sri of 0 7041 0 7074 and 143nd 144nd values range from 0 512014 to 0 512371 the majority of samples are characterized by negative ndt which are interpreted to be the result of contamination of a plume related mantle source by older crustal material the lack of correlation with other indices of fractionation e g sio2 for the majority of samples implies interaction with subcontinental lithospheric mantle either through assimilation or melting of the mantle rather than contamination during emplacement however a small number of samples appear to have assimilated material during emplacement resulting in strongly negative ndt values the radiogenic isotope data are consistent with a suite of rocks that have undergone complex contamination histories requiring variable crustal residence times and assimilation of material both at depth in large magma chambers and during emplacement

underplating in the himalaya tibet collision zone revealed by the hi climb experiment we studied the formation of the himalayan mountain range and the tibetan plateau by investigating their lithospheric structure using an 800 kilometer long densely spaced seismic array we have constructed an image of the crust and upper mantle beneath the himalayas and the southern tibetan plateau the image reveals in a continuous fashion the main himalayan thrust fault as it extends from a shallow depth under nepal to the mid crust under southern tibet indian crust can be traced to 31 n the crust mantle interface beneath tibet is anisotropic indicating shearing during its formation the dipping mantle fabric suggests that the indian mantle is subducting in a diffuse fashion along several evolving subparallel structures

geochemical characterization of malm zeta laminated carbonates from the franconian alb sw germany ii abstract during jurassic times especially within the malm stage local depressions formed on the eastern bavarian carbonate platform that were surrounded by wall reefs this created a unique depositional environment where in an open marine setting stagnant and anoxic bottom waters developed in an intra reef depression anoxic conditions were stabilized below wave base by enhanced salinity in the bottom waters and establishment of a density stratification biomarker analysis was applied to characterize palaeosalinity and redox conditions utilizing organic sulfur compound methylated chroman hopanoid and saturated isoprenoid distributions prevention of terrigenous influx by protecting reef walls lead to iron deficiency in intra reefal sediments causing early diagenetic sulfurization of functionalized lipids absence of clay mineral catalysts and early sulfurization favored an unusual steroid distribution lacking rearranged analogues but providing coexistence of saturated steranes with and configuration 13 17 spirosterenes unknown sterenes and steradienes as well as mono di and triaromatic steroids desulfurization of the polar fraction was carried out for two samples to verify that palaeoenvironment reconstruction based on free hydrocarbon and heterocompound distribution was not invalidated by sulfur quenching of selected compounds gammacerane was found to be the only component to occur exclusively in the sulfur bound fraction only minor amounts of hydrocarbons were released upon desulfurization implying that free bitumen analysis was applicable for organofacies characterization organic petrological investigation of organic mats revealed the presence of two different types of such structures comparison of organoclast fluorescence spectra with those of extracted porphyrin fractions indicate an origin of porphyrins exclusively from cyanobacterial mats whereas algal dominated mats yield no porphyrins this aims towards a better localization of specific biomarker origin in geological samples

tibetan and indian lithospheres in the upper mantle beneath tibet evidence from broadband surface wave dispersion broadband seismic experiments over the last two decades have produced dense data coverage across tibet yet the mechanism of the india asia lithospheric convergence beneath it remains a puzzle with even its basic features debated and with very different end member models advocated today we measured highly accurate rayleigh and love wave phase velocity curves in broad period ranges up to 5 200 s for a few tens of pairs and groups of stations across tibet combining in each case hundreds to thousands of interstation measurements made with cross correlation and waveform inversion methods robust shear velocity profiles were then determined by extensive series of nonlinear inversions of the data designed to constrain the depth dependent ranges of isotropic average shear speeds and radial anisotropy temperature anomalies in the upper mantle were estimated from shear velocities using accurate petrophysical relationships our results reveal strong heterogeneity in the upper mantle beneath tibet very large high velocity anomalies in the upper mantle are consistent with the presence of underthrust beneath southwestern tibet and subducted beneath central and eastern tibet indian lithosphere the corresponding thermal anomalies match those estimated for subducted indian lithosphere in contrast to the indian lithosphere tibetan lithosphere and asthenosphere display low to normal shear speeds tibetan lithosphere is thus warm and thin radial anisotropy in the upper mantle is weak in central and strong in northeastern tibet possibly reflecting asthenospheric flow above the subducting indian lithospheric slab

the geochemistry geochronology and paleomagnetism of dikes and sills associated with the mesoproterozoic midcontinent rift near thunder bay ontario canada abstract the midcontinent rift mcr of north america comprises a series of basaltic sheets flows and intrusive rocks emplaced in the lake superior region during the mesoproterozoic the mafic rocks preserved on the northern flank of lake superior represent the older portions of the rift sequence and offer insights into the early development of the rift new geochronological geochemical and paleomagnetic data are presented for the dikes and sills located in and south of thunder bay ontario three sill suites are recognized within the study area an earlier spatially restricted ultramafic unit termed the riverdale sill the predominant logan sills and nipigon sills in the north of the study area in addition three dike sets are recognized the north east trending pigeon river swarm the north west trending cloud river dikes and the mt mollie dike the geochemical data demonstrate that the majority of sills south of thunder bay are of logan affinity and distinct from those of broadly similar age in the nipigon embayment to the north the pigeon river dikes that intrude the sills are geochemically coherent but distinct from the logan sills and could not be feeders to the sills the new age of 1109 2 4 2 ma for the cloud river dike and its r polarity are consistent with published magnetostratigraphy the mt mollie dike age 1109 3 6 3 ma indicates that it is not coeval with the spatially associated crystal lake gabbro as previously thought the complexity of the dike and sill suites on the northern flank of suggests that the early phases of rifting occurred in distinct and changing stress fields prior to the main extensional rifting preserved in younger rocks to the south the geochemistry and geochronology of the intrusions suggest a long lived and complex magmatic history for the midcontinent rift

multimedia exploration strategies for pges insights from the surficial geochemistry case studies project lake nipigon region geoscience initiative northwestern ontario the surficial geochemistry case studies project in the lake nipigon region involved detailed quaternary mapping and multimedia geochemical sampling within five case study areas two of these areas lac des iles and tib lake contain known platinum group element pge mineralization the other three case study areas feature drainage catchments with lakes that contain anomalous levels of pges in bottom sediment surficial media sampled included till soils stream sediment lake sediment peat surface water and groundwater over archean terrain such as at lac des iles and tib lake there is excellent geochemical contrast between the pge prospective rock type mafic to ultramafic intrusive and the surrounding rock type e g granitoid rocks this geochemical contrast is mirrored in the geochemistry of most surficial media sampled during this project over the nipigon embayment the geochemical contrast between mafic and ultramafic rocks e g seagull type intrusions and the surrounding diabase sill rocks is inherently weaker due to the relatively high background levels for copper palladium and gold in the nipigon diabase sills however the results of stream water geochemistry over the seagull and disraeli ultramafic intrusions highlight their unique geochemical footprint on the surficial landscape in particular with respect to cr mg and ni compared with the surrounding nipigon diabase in addition lake sediments underlain by the seagull disraeli and hele intrusions have distinctly elevated cr concentrations and gd yb ratios compared with the surrounding areas underlain by nipigon diabase sills or sibley group rocks therefore exploration value can be maximized over the nipigon embayment by exploiting these geochemical contrasts in surficial media to discriminate between ultramafic rocks and the surrounding nipigon diabase sills the results of this study highlight the importance of chromium concentrations in surficial media as a diagnostic feature for the presence of ultramafic rocks regardless of their age or location in general a cost effective exploration strategy for pge mineralization includes targeting the associated metals cr ni within drift deposits c horizon till and drainage media stream sediment lake sediment to vector to prospective mafic ultramafic intrusive rocks prior to detailed property scale follow up involving the determination of base metals and pges within soil till stream sediment and peat samples relative to the metals copper nickel and chromium ppm levels the pges have significantly lower initial concentrations ppb levels are less mobile in the surficial environment have significantly shorter glacial dispersion trains and are less reliably determined at the laboratory

molecular stratigraphy a new tool for climatic assessment variations in sea surface temperatures over the past 500 000 years are inferred from the relative abundance behaviour of two organic compounds c37 alkenones over the upper 8 metres of a sediment core from the eastern equatorial atlantic this molecular record ascribed to contributions from prymnesiophyte algae correlates well with the variations in the 18 signal for the calcareous skeletons of certain planktonic foraminifera thus providing the first demonstration of a new stratigraphical technique which may be especially valuable where methods based on carbonate 18 fail

depositional environments of the senonian chert phosphorite and oil shale sequence in israel as deduced from their organic matter composition in the upper cretaceous sequence of the negev southern israel the organic matter in phosphorites and cherts differs from that associated with oil shales in its higher content of humic substances and lower kerogen content and in its more intensive microbial alteration the n alkane distribution pattern of the oil shales phosphorites and cherts indicates that marine biota probably algae are the main organic precursors of their organic matter in some of the oil shales however some contribution of terrestrial organic matter is also evident similar high phytane pristane ratios indicate that the organic matter in the oil shales as well as in the phosphorites and cherts accumulated under reducing conditions the main differences in the organic matter composition are attributed to early diagenetic processes rather than to different biotic precursors or to late modifications due to temperature induced maturation the depositional model suggested for the sequence involves upwelling conditions at the boundary between the deep tethys and the shallow shelf which induced high organic productivity deep into the inner shelf bottom water circulation enabled intensive microbial alteration of the organic matter followed by a winnowing process leading to phosphorite formation since humification is considered an oxygen consuming reaction these processes favoured the formation of oxygen enriched humic substances and the oxidation of humic substances already present these humic substances are relatively resistant to further alteration and their conversion into kerogen is thus retarded subsequently syndepositional tectonic activity resulted in the introduction of less saline water restriction of bottom water circulation and the establishment of a density stratification in the water body consequently aeration of the bottom layer and the sediments was inhibited microbial alteration was reduced and later winnowing processes were prevented such conditions favoured the formation of kerogen directly rather than through humic substances and also favoured the preservation of most of the organic matter in the form of oil shale deposits instead of phosphorites

mantle heterogeneities geoid and plate motion a monte carlo inversion seismic tomography in both the upper and the lower mantle as well as subducting oceanic slabs defined by seismicity has been translated into density heterogeneities to generate models of mantle circulation these models can predict both the surface velocities and the geoid which can be compared with plate tectonics and gravity data a given model is specified by 6 parameters related to the viscosities of 3 mantle layers and the absolute amplitudes of density variations in the upper and lower mantle as well as in the slabs the values of these parameters are chosen at random within an acceptable range each model is submitted to an appropriate test comparing observations with predictions the results of the most successful models selected by this monte carlo inversion are displayed they yield preferred mantle viscosity structures exhibiting large variations at depth with a physical interface between upper and lower mantle i e with the possibility for the circulation to penetrate the 650 km discontinuity two classes of viscosity profiles stand out the first one implies a regular increase of the viscosity in the sublithospheric mantle with reasonable values for the density parameters the second one is unexpected in the sense that it predicts a very stiff bottom for the upper mantle it also requires vanishingly small amplitudes for the upper mantle density heterogeneities defined by tomography which would thus have to be of lithological rather than thermal origin with a chemical interface at 650 km the outcome is very similar the same two classes of viscosity structures do yield a satisfactory geoid prediction however only the class of models with a stiff layer at midmantle depths predicts acceptable surface velocities altogether the best models out of some 60 000 which have been tested only explain one third of the geoid and two thirds of the surface divergence for spherical harmonic degrees 1 to 6 nevertheless the main features of these two observed patterns are present in the computed maps and 4 out of 6 correlation coefficients lie close to the 90 confidence level this is true for the geoid as well as for the surface divergence of the displacement velocity however as the internal viscosity structure has been assumed to have spherical symmetry the rotational component of the surface velocities cannot be predicted

isotopic composition and speciation of sulfur in the miocene monterey formation reevaluation of sulfur reactions during early diagenesis in marine environments the timing and pathways of early diagenetic sulfur transfer from dissolved species in pore waters to solid inorganic and organic compounds in sediments have been studied in the miocene monterey formation santa maria basin onshore california correlation between concentrations of total organic carbon toc and total sulfur ts in addition to concentrations of titanium aluminum total iron and reactive iron have been used to infer organic matter reactivity redox conditions and relative rates of clastic and biogenic input for each lithofacies isotopic compositions of six sulfur species acid volatile disulfide kerogen bitumen sulfate and elemental have provided information regarding relative timing of sulfur incorporation sulfate diffusivity in the upper centimeters of the sediments and the sources of sulfur for individual species isotopically the disulfide species expresses the greatest fractionation relative to estimated values of miocene seawater sulfate 22 cdt on average disulfide is depleted in 34s by 10 4 relative to kerogen and by 9 9 relative to acid volatile sulfide the 34s of bitumen shows no systematic change relative to 34skeregon suggesting the presence of migrated bitumen isotopic similarity of sulfate and elemental sulfur to sulfides and bitumen indicates that sulfate and elemental sulfur are chemical and or biological oxidation products derived from sulfides and bitumen consistent ordering of isotopic values for sulfur species disulfide acid volatile sulfide kerogen indicates that pyrite precipitated nearest to the sediment water interface under mildly reducing conditions and with little or no decrease in sulfate concentration relative to seawater enrichment of 34s in acid volatile sulfide and kerogen sulfur resulted from formation of these species at greater depths or in restricted micro environments under more reducing conditions and with low concentrations of porewater sulfate the formation of acid volatile sulfide after pyrite but during early diagenesis is significant because it implies more strongly reducing conditions than are generally recognized in deep water marine sediments

radiogenic isotope characteristics of the mesoproterozoic intrusive rocks of the nipigon embayment northwestern ontario the ca 1110 1120 ma diabase sills of the nipigon embayment represent the oldest phase of the midcontinent rift recognized to date the sills can be subdivided into five geochemically distinct suites nipigon mcintyre inspiration shillabeer and jackfish sm nd rb sr and pb pb isotope data for the sills can be used to evaluate the source regions and emplacement histories of these sills the endt values for all the sill suites are consis tently negative 0 5 to 6 6 but show coherent variations both within and between suites the negative endt values can be interpreted as the result of contamination of a plume related mantle source by older crustal material the initial strontium sri values 0 7032 0 7068 for the sills indicate that at least two distinct contaminants are required 1 a source with strongly negative endt and lower sri likely archean metasediment or granite and 2 one with elevated sri likely sedimentary rocks of the proterozoic sibley group the pb data for the nipigon and inspiration sills form distinct linear arrays consistent with variable contamination of a source comparable to that of other igneous suites of the midcontinent rift the radiogenic isotopes preserve a complex contamination history best interpreted as the result of 5 contamination by a variety of archean and proterozoic sources the ultramafic jackfish sill shows the least ev idence of contamination and is interpreted to have been emplaced along a crustal scale fault with little interaction with crustal material in contrast the other sill suites have undergone complex contamination histories requiring variable crustal residence times and assimilation of material both at depth in large magma chambers and during emplacement

further refinement to the timing of mesoproterozoic magmatism lake nipigon region ontario the midcontinent rift in the lake superior region of north america is one of the best preserved examples of an aborted precambrian intercontinental rift one that hosts a diverse suite of rock types in addition to the well studied and voluminous rift fill flood basalts although there is a growing database of high precision age information for the main volcanic packages and the largest mafic intrusions there is relatively little information available on the absolute timing of mafic ultramafic intrusions dyke swarms and alkaline complexes especially in the ontario portion of the rift we report new high precision u pb ages for 29 samples primarily collected in the lake nipigon area ontario from these new age results it is now possible to expand the known distribution of geon 15 magmatism in the region confirm an early stage of midcontinent rift mafic magmatism between 1150 and 1130 ma provide evidence that significant mafic ultramafic magmatism occurred in the lake nipigon region slightly earlier

organic carbon as a palaeoenvironmental indicator in the marine realm recent developments in biogeochemistry have made accessible all information currently known to be present in sedimentary organic compounds i e the carbon skeleton structure the positions of functional groups and stable carbon isotope ratios this combined information has strengthened the applications of sedimentary organic compounds as biomarkers and palaeoenvironmental indicators this review summarizes the most important biomarkers and their applications in reconstructing marine palaeoenvironments and processes the marine environment is characterized by the combined presence of specific compounds such as organic sulphur compounds dinosteroids 24 n propylsteroids even in trace amounts high amounts of c hbi hydrocarbons and the absence of long chain n alkanes with odd over even predominance bicadinanes and specific higher plant triterpenoids e g oleanane furthermore palaeoenvironmental conditions such as photic zone anoxia hypersalinity microbial sulphate reducing activity algal blooming palaeo upwelling and palaeo surface seawater temperature are indicated by molecular and or isotopic characteristics biomarkers from terrestrial sources are included because land derived organic matter is an important contributor to coastal and shallow marine environments a short case study within a holocene terrestrial marine setting florida bay usa is also presented to illustrate the reconstruction of transgressive regressive trends using selected organic compounds

overburden geochemical signature of the lac des iles platinum group element deposit northwestern ontario canada many previously published studies of the behaviour of pt and pd in till and soils have been done in areas of complex stratigraphy or very thin overburden cover making the interpretation of soil results difficult because of the many variables associated with these settings at the lac des iles mine site in northwestern ontario there are excellent exposures of the overburden in a series of exploration trenches glacial dispersal trains can be observed in till c horizon geochemistry e g ni cr cu and co regional geochemical dispersal trains of elements such as ni cr mg and co associated with the north lac des iles intrusion can be detected for about 4 km beyond the western margin of the mine block intrusion entire dispersal trains range from 5 to 7 km in length and about 1 to 2 km in width the dispersal of north lac des iles intrusion rock fragments tends to mask the response of the mine block intrusion dispersal trains of pt and pd are not well defined and tend to be very short 1 km in

thiophenic biomarkers for palaeoenvironmental assessment and molecular stratigraphy reconstruction of ancient depositional environments is an important objective in the earth sciences the complex assemblages of organic molecules in sediments include compounds that can be related to known biochemicals produced by specific biota from well defined habitats1 3 and these chemical fossils are applied frequently to reconstruct palaeoenvironments especially in studies of organic and petroleum geochemistry and in chemical oceanography most of the compounds used in this way are hydro carbons2 4 although oxygenated for example alcohols acids and ketones and nitrogen containing for example porphyrins compounds have also been applied successfully1 2 here we report on the use of organic sulphur compounds as geochemical tools for palaeoenvironmental and stratigraphic assessment depth profiles of specific sulphur compounds related to certain organisms show significant variations reflecting changes both in sources of organic matter and in physical conditions such as salinity of the depositional environment in a number of cases these variations are not otherwise revealed by changes in lithology geochemical bulk data and or absolute and relative amounts of hydrocarbon biological markers moreover several organic sulphur compounds reveal the presence of unprecedented biochemicals in these palaeoenvironments

recognition of alkenones in a lower aptian porcellanite from the west central pacific odp leg 198 drilling on shatsky rise recovered a lower aptian porcellanite 120 5 ma deposited during oceanic anoxic event oae 1a that contains c36 c39 alkadienones c37 2 and c39 2 alkadien 2 ones and c36 2 and c38 2 alkadien 3 ones this alkenone distribution differs from that typical of contemporary sediments and haptophyte algae but resembles that of cretaceous sediments from the blake bahama basin the discovery of alkenones in the early aptian extends their sedimentary record by 15 m y to 120 5 m y and demonstrates the potential for long term survival of these diagnostic functional lipids under favorable depositional conditions and subsequent shallow burial it also contributes to the understanding and reconstruction of evolutionary developments in alkenone distributions and biosynthesis over geologic time 2003 elsevier ltd all rights reserved

sulfur diagenesis and partitioning in holocene peru shelf and upper slope sediments abstract sediments of the peru margin were sampled in two transects across the shelf and upper slope 75 to 1000 m in october of 1992 this region is characterized by high surface water productivity and consequent high organic carbon fluxes low terrigenous inputs as well as an intense oxygen minimum zone omz impinging on the slope at depths between 75 and 550 m sediments analyzed in this study were collected from one ship deployed box core and five submersible deployed push cores organic carbon toc concentrations in surface sediments range from 1 to 18 on a dry weight basis formation of pyrite begins at the sediment water interface swi maximum concentrations of pyrite sulfur in the cores from this study range from 0 5 to 1 8 wt concentrations of pyrite are influenced by availability of reactive iron and an oxidant capable of converting avs to pyrite highly reactive iron is stripped from terrigenous detritus in the core of the omz some of this iron may contribute to glauconite formation in sediments near the base of the omz where it is concentrated into a glauconitic green sand as a result of reworking and winnowing in a shallow water site 109 m at the upper edge of the omz high concentrations of pyrite total iron and iron aluminum ratios indicate that easily reduced iron species are available for sulfidation at the swi pyrite formation at this site is nearly complete in the upper 4 cm of the sediment in contrast pyrite concentrations are slightly lower at the surface of two of the deeper water sites at 309 m in the center of the omz and to a lesser extent at 671 m at the bottom edge of the omz and appear to be influenced in part by availability of reactive iron concentrations of acid volatile sulfides avs are low in all sediments 0 04 wt but are slightly elevated in sediments from the center of the omz the ratio of organic sulfur s org to organic carbon toc at the sediment surface of the cores examined here is remarkably constant 0 012 0 002 and within the range observed in marine biota 0 01 to 0 02 suggesting that organic sulfur at the swi is dominantly biogenic although s org toc ratios increase downcore there is no clear correlation between concentrations of organic sulfur and organic carbon below the swi there is also no clear relation between concentrations of organic sulfur and total iron organic sulfur concentrations vary from site to site 0 06 to 0 33 wt but are relatively constant downcore at any given site maximum concentrations are in cores from the center of the omz

on the effect of temperature and strain rate dependent viscosity on global mantle flow net rotation and plate driving forces summary global circulation models are analysed using a temperature and strain rate dependent rheology in order to refine previous estimates of the nature of mantle flow and plate driving forces based on temperature inferred from a tectonic model and seismic tomography the suboceanic viscosity is lower than underneath continents by one order of magnitude if net rotations of the lithosphere with respect to a stable lower mantle reference frame are accounted for the patterns of flow in the upper mantle are similar between models with layered and those with laterally varying viscosity the excited net rotations scale with the viscosity contrast of continental roots to the ambient mantle this contrast is dynamically limited by the power law rheology surface net rotations match the orientation of hotspot reference frame euler poles well and amplitudes are of the right order of magnitude i compare prescribed surface velocity models with free slip computations with imposed weak zones at the plate boundaries velocity fields are generally consistent models based on laboratory creep laws for dry olivine are shown to be compatible with average radial viscosity profiles plate velocities in terms of orientation and amplitudes plateness of surface velocities toroidal poloidal partitioning and fabric anisotropy formation under dislocation creep in the upper mantle including temperature dependent variations increases the relative speeds of oceanic versus continental lithosphere makes surface velocities more plate like and improves the general fit to observed plate motions these findings imply that plate driving force studies which are based on simpler mantle rheologies may need to be revisited

constraining lithospheric flow the motion of earth s tectonic plates the lithosphere is driven by the subduction of relatively cold and dense oceanic plates into the mantle the resulting forces drive the motions of continental plates but the manner in which this happens depends on the effective viscosities of the lithosphere and mantle on page 1515 of this issue liu and hasterok 1 discuss a novel method of constraining viscosities of the lithosphere from geophysical data

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