hydrocarbon biomarkers of neoproterozoic to lower cambrian oils from eastern siberia the neoproterozoic era is of widespread geobiological interest because it marks the critical transition from a world of microbes to one where animals become an established feature of the landscape much research into this time period has focused on the ventilation of the oceans as this is widely considered a primary factor driving the diversification of complex multicellular life in this study proterozoic to cambrian aged oils from eastern siberia were analyzed for their hydrocarbon biomarker contents and compound specific carbon isotopes in order to further our understanding of the prevailing environment and its microbial and metazoan communities geochemically these oils are broadly comparable to those of the ediacaran cambrian sedimentary rocks and oils of the south oman salt basin organic matter in the source sedimentary rocks included significant contributions from green algae demosponges and bacteria including cyanobacteria and methanotrophic proteobacteria although the ages of the siberian oils and putative parent source rock intervals are poorly constrained the geochemical similarities between the ediacaran oman huqf and nepa botuoba katanga family of siberian oil samples are impressive leading to the inference that their source rocks are coeval on the other hand oils from the baykit high are distinctive likely older and possibly of cryogenian age

maceral composition and origin of organic matter input in neoproterozoic lower cambrian organic rich shales of salt range formation upper indus basin pakistan abstract the appropriate thermal maturity oil window or lower of the neoproterozoic rocks is a prerequisite for preserving robust and primary biological assemblages the neoproterozoic lower cambrian organic rich shales present in the upper part of salt range formation pakistan with abundant organic matter om total organic carbon toc up to 46 at relatively low thermal maturity protobitumen reflectance bro 0 2 0 4 represents by far the lowest thermal maturity neoproterozoic lower cambrian rocks analyzed to date the detailed petrographic description of neoproterozoic lower cambrian organic matter in the current study was lacking from previous source rock studies of coeval strata from oman eastern siberia australia and elsewhere eight organic rich shales of the salt range formation from three different outcrops have been analyzed for maceral composition and source rock characterization using organic petrography scanning electron microscopy and standard source rock assessment techniques the organic petrography reveals the organic rich shales are predominantly composed of alginites lamalginite and telalginite bituminite protobitumen and mineral bituminous groundmass overall the maceral assemblages suggest the deposition of these shales took place under a salinity stratified water column in a shallow restricted basin where both oxygenic and anoxygenic modes of primary production were operative the top oxygenated part of the water column was populated with the phototropic mat forming cyanobacterium whereas the lower low dissolved oxygen zone was dominated by algal mats and sulphate reduction bacteria the rare and scattered thucholites present within the salt range formation shales were derived from the radiation induced polymerization of adjacent om by the radioactive grains the heat from the radioactive grains however found to have a negligible effect on the overall thermal maturity of these shales

tomographic imaging by reflected and refracted arrivals at the north sea we discuss some processing steps of a marine 3 d data set from the oseberg field north sea we compare the prestack depth migrated images obtained by the velocity fields provided by different tools velocity spectra reflection tomography and joint tomographic inversion of reflected and refracted arrivals the last ones are definitely better we also produced a synthetic example by modeling the estimated earth structure and the actual recording geometry and we reached similar conclusions the correlation between reflected and refracted signals may be unclear for later arrivals because of their reciprocal interference and multiple reflections we adopt a technique based on a surgical mute in the p domain which allows coupling the signals coming from the same elastic interface

enigmatic origin of the largest known carbon isotope excursion in earth s history carbonate rocks from the middle ediacaran period in locations all over the globe record the largest excursion in carbon isotopic compositions in earth history this finding suggests a dramatic reorganization of earth s carbon cycle named the shuram excursion for its original discovery in the shuram formation oman the anomaly closely precedes impressive events in evolution including the rise of large metazoans and the origin of biomineralization in animals instead of a true record of the carbon cycle at the time of sedimentation the carbon isotope signature recorded in the shuram excursion could be caused by alteration following deposition of the carbonate sediments a scenario supported by several geochemical indicators however such secondary processes are intrinsically local which makes it difficult to explain the coincident occurrence of carbon isotope anomalies in numerous records around the globe both possibilities are intriguing if the shuram excursion preserves a genuine record of ancient seawater chemistry it reflects a perturbation to the carbon cycle that is stronger than any known perturbations of the modern earth if however it represents secondary alteration during burial of sediments then marine sediments must have been globally preconditioned in a unique way to allow ordinary and local processes to produce an extraordinary and widespread response

effects of ridge crest trench interaction on antarctic phoenix spreading forces on a young subducting plate precise measurements of spreading rates on marine magnetic profiles collected to the west of the antarctic peninsula have enabled some consideration of the forces governing plate motion since antarctic phoenix motion has been controlled by the local rather than global force balance over the last 35 m y the total effective driving force per unit length of trench is calculated to have ranged between 2 6 and 3 6 1012 n m which is much less than is commonly thought necessary to support subduction conventional calculations may overestimate slab pull for old slabs because they neglect the effect of extensional disruption in limiting the contribution to the balance of forces at the trench the low estimate of driving forces obtained here implies that resistive forces are also smaller than is generally assumed driving forces show a strong correlation with observed spreading rates which indicates that resistive forces were largely velocity dependent fluid migration up the subduction zone may elevate temperatures in and around the shear zone reducing resistive forces below the levels required by purely conductive models changes in convergence rate may affect the depths of both the brittle ductile deformation boundary and the basalt eclogite phase change causing a negative feedback which would appear as a velocity dependent resistive force the different driving forces acting on the ne and sw parts of the phoenix plate as a consequence of older oceanic lithosphere at the trench in the ne caused antarctic phoenix spreading to take place about a near pole to the sw since 21 m y ago and ultimately resulted in disruption of the phoenix plate about 9 m y ago spreading rates decreased abruptly about 6 m y ago probably because of e w compression across the long transform faults bounding the phoenix plate however spreading on the last three segments of the antarctic phoenix ridge continued at least until 4 m y ago either spreading stopped progressively from sw to ne or the final stage took place about a very near pole to the sw a magnetic quiet zone extends up to 95 km from the margin between the tula fracture zone and the north anvers fracture zone and is thought to indicate that the ridge crest became buried by terrigenous sediment prior to collision the absence of a magnetic quiet zone associated with the most recent ridge crest trench collisions suggests a change in sedimentary regime during the late miocene anomalously fast apparent spreading rates between 23 and 21 m y ago are thought to indicate an error in this part of the magnetic reversal time scale

velocity depth ambiguity of reflection traveltimes the conversion of time horizons to depth is fundamental to exploration geophysics the interval velocity used in the conversion is often estimated from the stacking velocity assuming that each layer s interval velocity is homogeneous however even for one laterally inhomogeneous layer above a flat reflector the stacking velocity can swing violently about its average and conventional methods of velocity estimation fail i show that violent swings in the stacking velocity are a symptom of a long wavelength ambiguity between the burial depth to an interface and interval velocity lateral variations in seismic velocity with a spatial wavelength of about 2 7 d where d is the depth to the reflecting horizon cannot be unambiguously resolved from traveltime measurements the spatial wavelength of this ambiguous component varies from 2 57 d for very small source receiver separations to 2 86 d for source receiver separations equal to d spectral components of the stacking velocity at wavelengths shorter than

observation and tentative interpretation of a double bsr on the nankai slope seismic data collected during the french japanese kaiko tokai cruise of r v l atalante on the upper slope of the eastern nankai margin reveal the simultaneous presence at two distinct depths below the seafloor of two bottom simulating reflector bsr type reflectors the upper bsr is traced as a continuous reflector over about 10 km as water depth decreases from 850 m to 550 m its depth below seafloor decreases from 200 m to 40 m the lower bsr is traced at 50 100 m below the upper one the two bsrs end abruptly near the summit of the daichii tenryu knoll into an area where the 3 5 khz record suggests active gas expulsion through the seabed the observed depth of the upper bsr fits the predicted one for the base of the methane gas hydrate stability zone as estimated from present temperature and pressure conditions at the seafloor and in the slope sediments thus we interpret the upper bsr as an active methane hydrate bsr we further suggest that the lower bsr is a residual hydrate related bsr this could have followed a recent migration of the base of the methane hydrate stability zone from the lower bsr to the upper one as possible causes for this migration we discuss sea bottom warming and tectonic uplift the bsr migration could have occurred as a response to a 1 2 c sea bottom warming or with an equivalent effect an event of fast uplift of the seafloor by about 90 m we do not discard other interpretations of the lower bsr such as an active hydrate related bsr formed from a mixture of gases

the natural magnetization of a 3 kilometer section of icelandic crust the magnetization and magnetic properties of samples from a 3 km vertical section of icelandic crust the iceland research drilling project section are described the section is located at the head of reydarfjordur eastern iceland at 65 01 n 14 21 w and extends vertically from about 0 5 km to 3 5 km depth beneath the original surface the section consists of 1 km of exposed flows with minor dikes and volcaniclastics underlain by a 2 km drill core component consisting of 60 flows and minor volcaniclastics and 40 dikes the section is largely normally magnetized corresponding in age and polarity to seafloor spreading anomaly 5 geomagnetic polarity epoch 9 flows which accumulated during anomaly 5 time extend vertically for 2145 m in contrast with 1000 m thick equivalent sections which are exposed updip this great increase in downdip thickening is consistent with observations elsewhere in eastern iceland and models for icelandic crustal formation involving the combination of volcanic activity subsidence and spreading a combination of paleomagnetic and structural evidence is used to provide an age calibration for the section and to predict the local presence of magnetic overprinting in the lower part of the section this overprinting is probably in part due to the production of secondary magnetite and is developed to a varying extent in epidote bearing flows at below 2 8 km crustal original depth where moderate secondary magnetite development has occurred flows frequently show both normal and reverse stable polarities with the latter being the original secondary magnetite development is high where dike density in the epidote bearing flows is also high here very strong well defined pervasive normal magnetization characterizes the flows this also being the dominant polarity of the dikes magnetic overprinting of this type imposed shortly after crustal formation could occur at 800 to 1200 m depth in typical ocean crust and may provide a contribution to the source for the linear magnetic anomaly patterns of the ocean basins the initial susceptibility of the flows shows the best defined depth trends over the section susceptibility increases with depth to about 2 km crustal depth and then decreases systematically to the bottom of the section the reason for the initial increase is not known but the decrease at depth can be explained by transfer of iron from primary oxides to secondary oxides and silicates the decrease is sufficiently linear to warrant extrapolation to a zero value at 4 3 km crustal depth or just below the estimated level of onset of greenschist facies metamorphic conditions in typical oceanic crust the equivalent level is estimated to be at 1 2 km depth the net magnetization of the section the combined remanent and induced contributions when expressed as average values for 500 m depth intervals is everywhere positive that is is in the sense of the present field variation in net magnetization is related to the proportions of the normal and reversely magnetized units in each interval

deciphering the impact of diagenesis overprint on negative δ13c excursions using rock magnetism case study of ediacaran carbonates yangjiaping section south china rock magnetism is used here to investigate the genesis of one of the puzzling negative carbon isotopic excursions of the neoproterozoic in the yangtze platform south china a detailed characterization of the magnetic mineralogy which includes low temperature and high field magnetometry and classical magnetic measurement arm irm susceptibility was therefore performed along upper doushantuo and lower dengying formations outcropping in the yangjiaping section the derived magnetic parameters show variations that can be interpreted as variations in magnetic grains size and in oxide contents they show that the magnetic content is significantly reduced in samples presenting negative 13ccalcite values we interpret this as a result of magnetite dissolution and secondary carbonate precipitation during early diagenesis bacterial sulfate reduction combined with c and o isotopic data paleomagnetic techniques thus show that the upper doushantuo lower dengying negative excursion of the yangjiaping section is largely due to diagenesis although the preservation of a genuine 13c excursion of lower magnitude from 7 down to 0 instead of down to 9 as usually considered cannot be ruled out a corrected 13ccarbonate chemostratigraphic curve is therefore proposed the unambiguous identification of a strong diagenetic component for this excursion casts doubts on the primary nature of other potentially time equivalent negative excursions of the yangtze platform and thus to its correlation to negative excursions in other cratons i e shuram excursion more generally this study illustrates the potential of magnetic mineralogy characterization a low cost time efficient and non destructive technique as screening tool for diagenetic overprints of 13c and 18o

carbon isotope records in a mesoproterozoic epicratonic sea carbon cycling in a low oxygen world abstract carbon isotope profiles of sedimentary strata are widely used as both a tool for stratigraphic correlation and as a mechanism for inferring important changes in earth s biogeochemical cycles chemostratigraphic interpretations however often rely on the assumption that the isotopic composition of dissolved inorganic carbon dic in the surface oceans is broadly homogeneous at times of globally high sea level when epicratonic epeiric environments dominate our sedimentary record carbon isotope records reveal substantial lateral variability here we investigate lateral variability in the marine carbon isotope record from pericratonic to epicratonic environments of the mesoproterozoic 1 1 ga atar el mreiti groups of the taoudeni basin west africa restricted marine epicratonic environments are consistently 2 4 lighter than open marine pericratonic environments suggesting input of an isotopically light carbon source that preferentially affected epicratonic environments in contrast to epicratonic environments in the paleozoic where input of isotopically light carbon is generally attributed to input of terrestrial organic matter we suggest that in situ remineralization of organic carbon via anaerobic microbial cycling drove observed isotopic variability within epicratonic el mreiti group strata the extent of organic matter remineralization and thus degree of 13 c depletion is correlated with water depth and associated with distinct differences in both total organic carbon toc and pyrite concentration suggesting a potential linkage to both the persistence of anoxia and the availability of sulfate within epicratonic environments in such settings the isotopic effects of organic carbon remineralization are potentially enhanced by either methane oxidation following methanogenic decomposition or the intermittent oxidation of an enhanced dissolved organic carbon doc pool

field characteristics of laterally emplaced dikes anatomy of an exhumed miocene dike swarm in reydarfjordur eastern iceland abstract many dikes of the tertiary breiddalur dike swarm in reydarfjordur have a great lateral extent but are vertically upward and or downward discontinuous field relationships indicate that they are not the feeders of the lava pile they intrude their characteristics can be best interpreted as them constituting the fossil expression of lateral magma emplacement analogous to that occurring in the actively spreading krafla volcanic center of the neovolcanic zone average dike thickness increases clearly with depth and the thickness versus depth relationship is different from that described for other tertiary dike swarms in eastern iceland it is suggested that dikes represent the extent of finite spreading which at higher levels in the crust is expressed by fissures and other extensional structures

spatial geochemistry of upper jurassic marine carbonates iberian subplate abstract chemostratigraphy applied to ancient marine carbonates is commonly based on one dimensional stratigraphic sections or core data as demonstrated from modern oceans this approach underestimates the spatial complexity of physico chemical seawater properties here a several hundred kilometer long transect consisting of seven upper jurassic sections from settings ranging from proximal neritic middle shelf to the epioceanic distal fringe across the southern and eastern palaeomargins of the iberian sub plate reveals variability in sedimentologic stratigraphic and geochemical records the comparison of isotopic and elemental data from different carbonate materials matrix micrite carbonate cements in veinlets belemnite rostra and ammonite shells reveals differential diagenetic pathways microfacies cathodoluminescence and geochemical data retrieved from biostratigraphically well constrained sections reveal that epioceanic matrix micrite geochemical data provide valuable proxies for palaeo seawater properties our data are reviewed in the context of published late jurassic records the outcome shows a higher level of complexity including the potential admixture of marine continental and diagenetic geochemical signals in the epicontinental record the stratigraphic trend in carbon isotopes of epioceanic sections agrees upon that of upper jurassic reference sections from the northern tethyan margins while oxygen isotope ratios are relatively 18o enriched palaeo seawater temperatures across the transect investigated were estimated using 18o as tentative proxy for interpreting distance from shore differences in water masses relative depth variations and potential local forcing factors palaeoenvironmental conditions evaluated through the combined record of isotope 18o elemental mn and skeletal content contrast with relative fluctuations in sea level site specific changes in palaeoceanographic parameters such as water depth seawater temperature salinity and upwelling are considered in comparison to examples from ancient and modern oceans

fast model building using demigration and single step ray migration this work provides explorationists with simple procedures to perform depth conversion more accurately than can be achieved with simple vertical layer cake depth conversion the use of image rays which are inadequate in structurally complex areas is avoided migrated time interpretations are still used and are demigrated using the kirchhoff time migration equations this backs out the effect of the time migration prior to a ray depth migration and enables the lateral shifts between the time migrated image and a depth migrated image to be quantified these shifts can be separated into a mismigration component and a refraction component the relative size of the components define whether time or depth migration is required and may be used to justify a remigration of the seismic image furthermore the tedious layer by layer approach to ray depth migration may be avoided by using the velocity depth model from the vertical layer cake depth conversion of the time migrated data for ray depth migration of the

hydrothermal alteration of the ediacaran doushantuo formation in the yangtze gorges area south china abstract the geochemical and fossil record preserved in the ediacaran age 635 551 ma doushantuo formation of south china has been extensively examined to explore the impact of changing climate and the oxidation state of the oceans on the development and distribution of early multicellular life in the yangtze gorges area this formation shows many of the geochemical trends and features thought to typify global ocean chemistry in the ediacaran period but there are indications that post sedimentary processes modified these signals this study of clay minerals and organic matter builds a more detailed picture of the type and degree of post sedimentary alteration at different stratigraphic levels of the formation and focuses on how this alteration influenced stable carbon and oxygen isotope records in the cratonward jiulongwan and huajipo sections of the doushantuo formation its lower part members 1 and 2 consists largely of dolomitic shale rich in authigenic saponite that crystallized in an alkaline sedimentary basin saponite has been altered to chlorite via corrensite across tens of meters of strata in lower member 2 with increased alteration downward toward the cap dolostone the greater chloritization is accompanied by lower 18 o and higher d values of trioctahedral clays this pattern of alteration of trioctahedral clays is likely due to hydrothermal fluid activity in the underlying relatively permeable nantuo formation and cap dolostone a concomitant increase of solid bitumen reflectance toward the base of the formation supports this idea in the uppermost part of the formation in the yangtze gorges area member 4 a typical open water marine dolomitic shale rich in illite and organic matter increases in the methylphenanthrenes ratio index and solid bitumen reflectance correlate with decrease of the bulk rock k al ratio upward providing evidence for hot fluid migration above the nearly impermeable shale clay from the upper part of the formation is enriched in 18 o but not in d relative to clay from the lower parts indicating progressive 18 o enrichment of hydrothermal fluids that percolated upward and laterally through permeable 18 o rich carbonates a maximum hydrothermal alteration temperature of 200 c is estimated from a calibration curve for illitization during burial diagenesis but given that the hydrothermal activity probably occurred in short pulses the temperature could have been much higher k ar ages are consistent across different size fractions of fine illite from member 4 shale 430 ma and from a k bentonite bed near the base of member 2 in the jiuqunao section 325 ma 25 km from jiulongwan and huajipo these age values show that the diagenetic illite of the doushantuo formation is a product of either deep burial diagenesis overprinted by spatially limited hydrothermal activity or of two localized hydrothermal events patterns of carbonate 13 c and 18 o depletion in the basal doushantuo formation are similar to chloritization trends and 18 o variation in diagenetic clay minerals given independent evidence for 13 c depletion of hydrothermal fluids these trends indicate carbonate fluid isotope exchange commensurate with the degree of post sedimentary alteration supporting a model of lithologically controlled differential diagenesis induced by hydrothermal fluids as the main control on c and o isotope variability in this stratigraphic interval this model could potentially explain other notable 13 c excursions higher up in member 3

magnetic and thermal effects of dike intrusions in iceland oriented core samples were collected from 18 lava dike contacts in iceland in locations of low to moderate 100 c regional hydrothermal alteration alternating field demagnetization of these samples shows that they generally carry a strong and stable remanence magnetic directions in the lavas follow a simple pattern demonstrating that the remanence of the contact zone has only been reset within very short distances from the contact surface on average this distance may be only one tenth of the thickness of a typical dike 4 m other magnetic properties remanence intensity susceptibility curie point of the contact zone are affected to a lesser or a less regular degree than the remanence direction it is concluded that for the crustal depth of this study thermal conduction models do not apply to temperature anomalies set up by dike intrusions or fissure eruptions in iceland presumably because of rapid advection of groundwater into the active fissures these results also indicate that the magnetic anomalies observed over iceland are only to a small extent caused by dikes

a linear model for velocity anomalies variations of seismic interval velocities within the cable length cause anomalies in the stacking velocity analyses utilizing the approximation of rectilinear ray propagation i e supposing that the velocity changes cause time delays only it is shown that the stacking velocity anomalies are linearly related to the interval velocity variations in particular the stacking velocity anomaly is calculated when the interval velocity of an intermediate layer undergoes a stepwise variation the amplitude of the anomaly increases with the ratio between horizon depth and cable length from the forward model a program for the inversion is derived in order to identify lateral changes of interval velocities from unsmoothed stacking velocity analyses some examples of the application of this technique to synthetic and real data are presented

the cenozoic subduction history of the pacific margin of the antarctic peninsula ridge crest trench interactions new magnetic anomaly identifications w of the shackleton fracture zone show 5 spreading sections separated by fracture zones in the 2 most southerly the ridge crest collided with a trench at the margin of the antarctic peninsula only 6 5 and 4 ma ago the latest of a series of collisions starting at the base of the peninsula so ma ago following each collision spreading and subduction both stopped opposite the south shetland trench and actively extending bransfield strait spreading also stopped 4 ma ago but before the spreading centre reached the trench the tendency of the subducting plate to continue sinking probably initiated the opening of the bransfield strait the parallelism between fracture zone orientation on the descending plate and convergence direction previously thought responsible for the tectonic segmentation of the peninsula was effective for only 10 30 ma before collision before that either the fracture zones did not extend to the trench or the subducting ocean floor formed at the farallon phoenix nazca aluk boundary with fracture zones oblique to the subduction direction the aleutian arc kula ridge model in which arc magmatism virtually ceases when ocean floor younger than 25 30 ma occurs beneath the arc fits the distribution of antarctic peninsula radiometric dates explaining a 50 60 ma gap between collisions and youngest ages in the s and possibly the migration of the youngest activity towards the trench thus gaps in the geologic record of arc magmatism need not imply cessation of subduction the progressive steepening of the peninsular margin towards the 4 ma collision site suggests tectonic erosion of the fore arc as the ridge crest approached

paired carbonate and organic carbon isotope variations of the ediacaran doushantuo formation from an upper slope section at siduping south china abstract intensive carbon isotope analyses from the doushantuo formation ca 635 551 ma in south china reveal decoupled carbonate and organic carbon isotopes 13ccarb and 13corg that have been interpreted as recording a large dissolved organic carbon doc reservoir in the ediacaran ocean or chemoautotrophic methanotrophic recycling modification of organic matter in a redox stratified ocean existing paired 13ccarb 13corg data however are mostly from sections of an intrashelf lagoon near the yangtze gorges area where basin restriction may have resulted in local isotope departure from the open ocean to better understand the spatial and temporal variations of the ediacaran 13c excursions we have conducted high resolution paired 13ccarb 13corg analyses of the doushantuo formation from an upper slope section at siduping hunan province south china the data reveal three negative 13ccarb excursions at the basal middle and upper doushantuo formation two prominent negative 13corg anomalies are also observed in black shales and shaly dolostones at the base and top of the doushantuo formation for the majority of the doushantuo formation however 13ccarb and 13corg values are decoupled similar to those observed in the yangtze gorges area both 13ccarb and 13corg show large up to 10 variations among shallow and deep water sections that are consistent with a large carbon isotope gradient in a stratified ediacaran ocean the large 13ccarb gradient between shallow and deep water sections implies the availability of sulfate in deep water environments where the addition of authigenic carbonate films cements into surface ocean derived carbonate particles in anoxic water column or pore water led to more negative 13ccarb values significant spatial 13corg variations suggest that the decoupled 13ccarb 13corg may not have been resulted from direct buffering of a large doc reservoir but from recycling of organic matter by chemoautotrophs and methanotrophs compilation of available paired 13ccarb 13corg data from the yangtze platform reveals alternating intervals of coupled and decoupled 13ccarb 13corg values up to the early cambrian 520 ma intervals with coupled 13ccarb and 13corg coincide with evidence for ocean oxygenation events this phenomenon may be explained by episodic growth and oxidation of a moderate doc reservoir 6 12 times of the modern ocean the upper doushantuo 13ccarb anomaly temporally crosses a major stratigraphic unconformity indicating that part of the upper doushantuo shuram 13ccarb excursion was formed during sea level regression large spatial and temporal variations of the upper doushantuo 13ccarb excursion may record a mixed signature of open ocean seawater overprinted by facies dependent isotope variation stratigraphic truncation and authigenic diagenetic modification along the unconformity the real primary 13ccarb excursion may be shorter lived than previously thought

volcaniclastic rocks of the reydarfjordur drill hole eastern iceland 1 primary features a 1918 m deep bore hole in a late miocene shield basalt central volcano mixed sequence contains 58 stratigraphically distinct volcaniclastic interbeds constituting 8 8 of the extrusive part of the section textural mineral chemical and bulk chemical criteria are applied to identify the original nature composition and origin of these rocks which have been metamorphosed by hydrothermal alteration between 100 c top to ca 300 c base of the section units range from a few centimeters to 18 m excluding a 30 m thick ignimbrite with 75 of the units being less than 1 m thick formerly vitric to tachylitic shards and lapilli lithic fragments and crystals are the main components most thin volcaniclastic units are basaltic vitric to tachylitic tuffs thin felsic air fall tuffs a few millimeters to a few centimeters thick occur as separate layers and interbedded with basaltic tuffs nearly all thicker units are made up dominantly of felsic vitric to lithic clasts and crystals phenocrysts common only in felsic air fall tuffs are clinopyroxene ranging from augite to ferrohedenbergite and plagioclase ranging from an80 20 both of which span the entire range of compositions previously known from iceland bulk chemical analyses of major and 11 trace elements of 65 samples allow texturally indistinct rocks to be compositionally identified even though chemical changes due to mixing and alteration are pronounced in many rocks the rhyolitic ignimbrite cooling unit is composed of four flow units the upper most being slightly more mafic in composition because of admixture of a basaltic component the two central flow units show extensive high temperature devitrification with lensoid tridymite rimmed cavities now laumontite filled representing collapsed pumice lapilli the composition of the clastic units corresponds in a general way to that of the interbedded lava flows these data and evidence for weathering and reworking in some units are used to reconstruct the volcanic history of the area phase a 1918 1306 m dominantly mafic shield basalts contains only few and mostly basaltic tuffs except for sparse rhyolitic air fall tuff from distant central volcanoes phase b 1306 653 m composed dominantly of evolved basalts and icelandites culminating with the rhyolitic ignimbrite 920 950 m probably represents the eastern flank of a nearby central volcanic complex whose center may be situated less than 10 km westward downdip the upper extrusive 500 m of the section phase c are basalt lavas of average tholeiitic composition containing abundant chiefly felsic and reworked clastic units in the interval between 286 and 410m these are tentatively interpreted to reflect a phase of structural basin forming not compensated by extrusive volcanic activity allanite phenocrysts were found for the first time in icelandic rocks and the host pumice tuffs may constitute an important stratigraphic marker horizon that may help to tie the drilled section to the subaerial volcanic series in this part of eastern iceland the alteration of the clastic layers is discussed in a companion paper

estimation of gas hydrate concentration and free gas saturation from log and seismic data when no direct measurements are available detailed knowledge of the compressional and or shear wave velocity fields is essential for quantitative estimation of gas hydrate and free gas in bottom simulating reflectors bsr discrepancies between experimental velocity profiles and the velocity for water filled sediments reveal the presence of gas hydrate positive anomalies and free gas negative anomalies

on the significance of δ13c correlations in ancient sediments a graphical analysis of the correlations between c and etoc was introduced by rothman et al 2003 to obtain estimates of the carbon isotopic composition of inputs to the oceans and the organic carbon burial fraction applied to cenozoic data the method agrees with independent estimates but with neoproterozoic data the method yields results that cannot be accommodated with standard models of sedimentary carbon isotope mass balance we explore the sensitivity of the graphical correlation method and find that the variance ratio between c and o is an important control on the correlation of c and e if the variance ratio c o 1 highly correlated arrays very similar to those obtained from the data are produced from independent random variables the neoproterozoic data shows such variance patterns and the regression parameters for the neoproterozoic data are statistically indistinguishable from the randomized model at the 95 confidence interval the projection of the data into c e space cannot distinguish between signal and noise such as post depositional alteration under these circumstances there appears to be no need to invoke unusual carbon cycle dynamics to explain the neoproterozoic c e array the cenozoic data have c o 1 and the c vs e correlation is probably geologically significant but the analyzed sample size is too small to yield statistically significant results

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

forearc tectonic evolution of the south shetland margin antarctic peninsula the main provinces of the south shetland margin antarctic peninsula characterized on the basis of multichannel seismic long range side scan sonar and swath bathymetry data include from northwest to southeast 1 the oceanic crust of the former phoenix plate flexed down toward the margin and affected by normal faulting in the upper crust 2 a narrow trench with a horizontally layered sediment fill which onlaps the thin sedimentary cover on the oceanic crust and shows incipient deformation near its landward edge 3 an accretionary prism with a complex internal structure the toe of which is overthrust above the youngest trench deposits 4 a midslope forearc basin with an asymmetric synformal structure and 5 the continental shelf which includes two distinct tectonic provinces calculations of the late cenozoic convergence history at the trench indicate a rapid decrease in convergence rate after 6 7 ma from about 60 mm yr resulting from the slowing and eventual cessation of spreading on the antarctic phoenix ridge once spreading had completely ceased 3 5 2 4 ma the convergence rate at the trench equalled the rate of extension in bransfield strait which was probably less than 10 mm yr at first but may have increased since 1 3 ma and this in turn would imply a corresponding increase in convergence rate above the basal detachment the toe of the accretionary prism is composed of a stack of thrust fault bounded wedges laterally and vertically segmented by normal faults most of the trench fill sediments are overthrust by the toe of the accretionary prism and subducted beneath it for as far as they can be traced on the seismic profiles this suggests that there may be tectonic erosion of the forearc since a large part of the trench sediment appears to be derived from erosion on the forearc slope the forearc basin records a long history of subsidence during which the depocenter migrated landward paralleling retrogradational erosion of the distal margin of the basin subduction of the hero fracture zone at approximately the same location since 10 ma may have enhanced tectonic erosion of the accretionary prism in the forearc basin a synform is coincident with a bathymetric reentrant but its axis is oblique to the trends of both the basin and the fracture zone the origin of this synform and the final phase of uplift of the blueschist facies terrane on which smith island stands are probably related to their position above the subducting fracture zone ridge the present elevation of smith island is thought to result from uplift which followed a ridge crest trench collision directly southwest of the hero fracture zone at 5 5 ma the northwestern edge of the mesozoic to early cenozoic arc batholith trending northeast southwest on the shelf is proposed to have acted as a backstop to the accretionary prism during cenozoic subduction

the origin of decoupled carbonate and organic carbon isotope signatures in the early cambrian ca 542 520 ma yangtze platform abstract the early cambrian ca 542 520 ma strata in south china record two prominent negative carbonate carbon isotope 13ccarb excursions of early nemakit daldynian n d and early tommotian ages across each of these excursions carbonate and organic carbon isotopes 13ccarb and 13corg are strongly decoupled regional correlation across a shelf to basin transect shows lateral heterogeneity of 13corg during the early middle n d but more homogenized 13corg values across the basin during the late n d and tommotian the temporal and lateral variations in 13corg suggest that decoupled 13ccarb and 13corg across the n d 13ccarb excursion were possibly caused by diagenetic alteration of organic matter and or amplification of detrital organic carbon isotope signature in low toc carbonates in contrast decoupled 13ccarb and 13corg of the upper n d and tommotian were likely resulted from chemoautotrophic methanotrophic biomass contribution to toc in organic rich black shale and carbonates the decoupled 13ccarb 13corg pattern from the lower n d strata ca 542 ma shows striking similarities with those from the basal ca 635 ma and upper ca 551 ma doushantuo formation in all three cases decoupled 13ccarb 13corg are seen in organic poor carbonates toc 0 1 and coupled 13ccarb 13corg occur in organic rich black shale and carbonates at the end of the negative 13ccarb excursion these similarities suggest that the shift from decoupled to coupled 13ccarb 13corg has no causal link with the terminal oxidation of a large oceanic doc reservoir given the pervasive anoxia euxinia in ediacaran early cambrian oceans local doc rich environments may have been common but a large oceanic doc reservoir capable of buffering the 13c of marine organic matter requires independent evidence

the mode of emplacement of neogene flood basalts in eastern iceland the plagioclase ultraphyric basalts in the graenavatn group abstract plagioclase ultraphyric basalt lava with high fraction of solids have a mode of emplacement that is poorly understood in this study we conduct detailed mapping of a pub group in eastern iceland namely the graenavatn group and assess the group architecture flow morphology and internal structure with additional constraints from petrography petrology and crystal size distribution to derive information on emplacement dynamics of plagioclase ultraphyric basalts we also derive information on the plumbing system of the group with reference to the source of the macrocysts the group is exposed in steep glacially carved fjords and can be traced for more than 70 km along strike the flows have mixed architecture of simple and compound flows individual flow lobes have thicknesses in the range of 1 24 m and many reach widths and lengths exceeding 1000 m the flows vary from rubbly to slabby pahoehoe but are predominantly of pahoehoe type the aspect ratio of the group and the nature of the flows indicate fissure fed eruptions the plagioclase macrocrysts 5 30 mm are an rich exhibit bimodal size distribution and the modal proportions within the group varies from 15 40 clinopyroxene macrocrysts are also present ranging from 1 6 the lowermost flow is thickest and carries the greatest crystal cargo load the morphology of the lava flows suggests low viscous behavior at odds with the high crystal content the very calcic plagioclase macrocrysts an80 85 are in disequilibrium with the groundmass and plagioclase microlaths therein an50 70 meaning that the crystal laden magmas quickly ascended from deeper crustal levels to the surface the flows with highest crystal content may have maintained high temperatures by heat exchange with the primitive macrocrysts in the flows and developed non newtonian behavior such as shear thinning such conditions would have enabled the flows to advance rapidly during episodes with high effusion rates forming the simple flows and subsequently maintained by insulated lobe by lobe emplacement occasionally surges disrupted the crust to form rubbly and slabby pahoehoe a herschel bulkley model is applied to the flows yielding effusion rates in the order of 103 104 m 3 s for the largest eruptions variation in crystal proportions within the group and individual flows is considered to be the result of magmas penetrating a crystal mush followed by disaggregation and mobilization sorting by flotation mixing of magma batches and transportation of heterogeneous crystal laden magmas to the surface with opportunities for staging en route

assessment of gas hydrate and free gas distribution on the south shetland margin antarctica based on multichannel seismic reflection data summary processing and interpretation of a grid of intermediate resolution multichannel seismic reflection profiles collected on the ne sector of the south shetland continental margin allowed us to map the lateral extent of a bottom simulating reflector bsr the margin an accretionary wedge located off the northern tip of the antarctic peninsula consists of two distinct and superimposed tectonic regimes an older regime is related to mesozoic middle cenozoic subduction related tectonism a younger one is associated with a mainly extensional tectonic phase and related to the oligocene development of the western scotia sea the occurrence of the bsr appears to be controlled by the geological structure of the margin the bsr lacks continuity near basement structures main geological discontinuities and faults on the other hand the amplitude and continuity of the bsr are not affected by the presence of folded structures and undeformed sedimentary layering we found that the bsr is mostly confined to the ne sector of the south shetland margin where propagation of faulting associated with the shackleton fracture zone may have driven migration of natural gas towards the surface and created the conditions for a bsr to appear the application of reflection tomography techniques allowed us to reconstruct the averaged seismic velocity field between the seafloor and bsr in order to map the depth of bsr by averaging the observed velocity structure above and below the bsr and applying a theoretical model of elastic wave propagation in porous media we attempted as rigorous a quantitative assessment as possible of the natural gas present as gas hydrate above the bsr and as free gas between the bsr and the base of gas reflector bgr

origin of bottom simulating reflectors geophysical evidence from the cascadia accretionary prism vertical seismic profile vsp data from two drill sites on the cascadia margin show low velocity zones indicative of free gas be neath a bottom simulating reflector bsr offshore oregon at ocean drilling program odp site 892 velocities drop from an average of 1750 m s above the bsr to less than 1250 m s below it sonic logs confirm that seismic velocity in the sediments adjacent to the borehole is less than that of water for at least 50 m beneath the depth of the bsr at this site similarly at odp site 889 offshore vancouver velocities range from 1700 to 1900 m s in the 100 m above the bsr and drop abruptly to 1520 m s in the 15 m just beneath it the low velocities observed beneath the bsr are strong evidence for the presence of 1 5 free gas by volume the bsr at these two sites results from the contact between gas free sediments containing a small quantity of hydrate above the bsr and a low velocity free gas zone beneath it although the bsr is associated with the base of the hydrate stability field hydrate appears to account for relatively little of the velocity contrast that produces the bsr velocity above the bsr at site 889 is only about 100 m s greater than that expected for sediments of similar porosity sediments above the bsr at site 892 appear to have normal velocity for their porosity and may contain little hydrate

magnetostratigrahy of the exposed lava section east of the irdp drill hole in reydarfjordur eastern iceland magnetic polarity directions vgp of 154 exposed miocene basalt lavas from reydarfjordur eastern iceland can be used successfully for correlation with the irdp drill core three partly synchronous lava sections east of the drill site are predominantly normally magnetized and are stratigraphically of the same age as lavas dated k ar at about 9 to 10 5 m y mcdougall et al 1976 the age range and magnetic properties of these lavas the landward equivalents to marine magnetic anomaly 5 epoch 9 suggest that several reversely magnetized lavas in one of the investigated sections were remagnetized by a 10 m thick dyke the data presented suggest that in fresh lavas with low curie temperatures relatively thick dykes about 10 m may cause thermal overprinting of the host rocks to a distance from the dyke equal to its thickness these results are in contrast with the generally accepted thesis that dykes affect host rocks only up to about 1 5 of their thickness no systematic decrease in magnetic intensity with depth was observed for a 0 9 km thick vertical lava section in holmatindur about 13 5 km east of the drill site it is suggested that the heterogeneity of the lava pile in terms of magnetic intensity reflects primarily the distinct geochemistry and or grain size of magnetic phases of individual lavas

the iceland research drilling project synthesis of results and implications for the nature of icelandic and oceanic crust study of a 3 km thick section of tertiary basalts in eastern iceland has resulted in the formulation of a model for crustal construction based on accumulation of lava flows on the flanks of large central volcanoes and has led to the development of alteration and physical property profiles in a crustal segment formed at the mid atlantic ridge deep drilling at reydarfjordur revealed the existence of subaerial lava flows and volcaniclastic rocks to a crustal depth of 3 5 km three major stratigraphic units composed largely of basalts are recognized each characterized by distinctive major and trace element patterns each of these units is believed to be the product of a separate episode of mantle melting and fractionation and probably reflects accumulation on the flanks of a central volcano extensive fractionation in shallow magma chambers led to intermediate and silicic lavas which mark the cores of the central volcanic complexes dike density does not increase downward below about 1 5 km crustal depth and the sampled dikes appear to have been intruded laterally along rifts on the flanks of the central volcanoes secondary mineral assemblages are dominated by smectite and zeolites near the top of the section and by epidote and quartz near the base in general the temperature of alteration increases downward but the distribution of individual minerals is irregular locally earlier secondary mineral assemblages are overprinted by contact metamorphism along dike margins the upper surface of greenschist facies metamorphism crops out at the surface in thingmuli volcano but it lies at an estimated crustal depth of about 4 km in the reydarfjordur borehole located only 3 km east of the edge of thingmuli thus the metamorphic zones clearly crosscut the lava stratigraphy physical property profiles correlate well with variations in secondary mineralogy and the top of layer 3 velocities corresponds closely with the projected upper boundary of amphibolite facies rocks at about 6 km crustal depth magnetic susceptibility decreases with depth and is extrapolated to zero at about 4 3 km crustal depth close to the estimated upper limit of greenschist facies metabasalts a much greater crustal thickness leading to the emplacement of high level magma chambers within the extrusive part of the crust is thought to account for many of the differences in crustal structure and evolution between iceland and the rest of the mid atlantic ridge

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

seawater carbon and strontium isotope variations through the late ediacaran to late cambrian in the tarim basin abstract the radiation of early animals during the cambrian explosion was accompanied by widespread perturbations in oceanic and atmospheric conditions however the cause effect relationships between evolutionary innovations carbon c cycling and continental weathering are still a matter of debate further paleoenvironmental reconstructions of poorly studied paleo continents covering the precambrian cambrian pc c transition may improve the correlation of the events and their impact on the earth system on a global scale the tarim basin was presumably located proximal to the well studied south china craton during the ediacaran cambrian boundary and is expected to contribute equally to our paleoenvironmental reconstructions here we apply carbon and strontium sr isotope records 13c and 87sr 86sr obtained from well preserved marine carbonates spanning the late ediacaran to the late cambrian from the keping region in the northwestern tarim basin nw china by comparison to other carbonate successions from various paleo continents our obtained seawater 87sr 86sr curve in this study shows a stepwise first order increase to more radiogenic values superimposed by second order presumably regional variations our obtained 13c variations exhibit four prominent negative and three positive excursions identical to the global 13c record based on existing paleontological evidence and our new tarim 13c and 87sr 86sr curves we propose a stratigraphic correlation from the late ediacaran to the late cambrian between the tarim basin and other paleo continents besides our paleoenvironmental reconstruction reveals both regional and global sea level changes that may have controlled the influx of radiogenic sr and presumably the influx of nutrients controlling primary production and the rate of organic carbon burial in the tarim basin at the time of deposition these feedbacks may have ultimately influenced the rapid diversification of metazoans and the oxygenation of the cambrian ocean and atmosphere approaching present po2 levels

quantitative detection of methane hydrate through high resolution seismic velocity analysis a laterally extensive high resolution travel time velocity analysis and acoustic wave form inversion were used to quantitatively determine methane hydrate content in deep water sediments of the blake ridge off the southeast u s coast the interval acoustic velocity vp analyses were performed in the p domain by interactively picking the p trajectories of prominent reflections in each of 50 plane wave decomposed common midpoint gathers the reflections correspond to seismic stratigraphic boundaries so that lateral vp changes due to lithology changes are mitigated and vp changes due to changing hydrate content are enhanced two separate interval vp analyses were performed one with thick 0 4 km layers which yielded lower uncertainty but also lower resolution and one with thinner layers 0 1 km yielding higher resolution but slightly larger uncertainties results show no correlation between low sediment reflectivity and vp however in the areas exhibiting a bottom simulating reflector bsr a high vp interval 2 0 km s and 0 15 km thick is seen immediately above the bsr where the bsr is strongest a 256 layer least squares acoustic wave form inversion reveals the bsr to be caused by a vp decrease from 2 0 to 1 5 km s with little or no change in density the inversion also reveals a thin 0 025 km layer of anomalously low vp lying immediately below the bsr two models of methane hydrate distribution are tested each indicating that the volume of methane hydrate in the intervals of elevated vp is up to 25 of the total volume

stratigraphy and correlation of the region surrounding the irdp drill hole 1978 reydarfjordur eastern iceland the 1 9 km thick sequence intersected by the irdp 1978 drill hole at reydarfjordur eastern iceland is correlated with the westward dipping exposed volcanic pile on the basis of extensive field mapping and paleomagnetic studies the base of the landward extension of sea floor magnetic anomaly 5 epoch 9 can be located at the top of the grjota olivine tholeiite formation intersected at about 1 5 km in the core and exposed at sea level about 9 km east of the drill site other exposed marker formations can also be identified in the core the drill site is located within a relatively dense dyke swarm 10 crustal dilation previously interpreted as the feeder of the overlying volcanic formations field observations including vertically discontinuous dykes and the lack of lava feeder relationships suggest that the dykes were intruded by lateral injection from the south and are considerably younger than and thus could not have fed the exposed lava pile in the vicinity of the drill site it is proposed that about 10 3 m y ago roughly coinciding with the lower boundary of anomaly 5 the reydarfjordur volcanic center ceased to be active and volcanic activity shifted about 20 km to the west where the breiddalur and thingmuli volcanic centers later developed the exposed stratigraphic succession between thingmuli and reydarfjordur volcanic centers is composed of lavas deposited in flank areas relatively removed from a paleovolcanic zone which now probably lies buried to the west of the irdp drill site the lowest 450 m of the core may represent lavas related to activity in the reydarfjordur volcanic center 13 5 km to the east

testing the preservation potential of early diagenetic dolomites as geochemical archives early marine diagenetic dolomite is a rather thermodynamically stable carbonate phase and has potential to act as an archive of marine porewater properties however the variety of early to late diagenetic dolomite phases that can coexist within a single sample can result in extensive complexity here the archive potential of early marine dolomites exposed to extreme post depositional processes is tested using various types of analyses including petrography fluid inclusion data stable 13c and 18o isotopes 87sr 86sr ratios and u pb age dating of various dolomite phases in this example a triassic carbonate platform was dissected and overprinted diagenetic temperatures of 50 to 430 c in a strike slip zone in southern spain eight episodes of dolomitization a dolostone cataclasite and late stage meteoric vadose cementation were recognized the following processes were found to be diagenetically relevant i protolith deposition and fabric preservation and marine dolomitization of precursor aragonite and calcite during the middle late triassic ii intermediate burial and formation of zebra saddle dolomite and precipitation of various dolomite cements in a proto atlantic opening stress regime t ca 250 c during the early middle jurassic iii dolomite cement precipitation during early alpine tectonism rapid burial to ca 15 km and high grade anchizone overprint during alpine tectonic evolution in the early eocene to early miocene iv brecciation of dolostones to cataclasite during the onset of the carboneras fault zone activity during the middle miocene and v late stage regression and subsequent meteoric overprint data shown here document that under favourable conditions early diagenetic marine dolomites and their archive data may resist petrographic and geochemical resetting over time intervals of 108 or more years evidence for this preservation includes preserved late triassic seawater 13cdic values and primary fluid inclusion data data also indicate that oversimplified statements based on bulk data from other petrographically complex dolomite archives must be considered with caution

isotope stratigraphy of the ediacarian neoproterozoic iii of the adelaide rift complex australia and the overprint of water column stratification abstract this study establishes a high resolution 13c chemostratigraphic framework for the type ediacarian terminal proterozoic section in the adelaide rift complex a framework that contributes to the emerging global chronostratigraphic synthesis of a period that saw momentous changes in the earth s surface environment only 15 m of the 3 5 km thick type section consists of shallow water carbonates of the kind that have formed the basis of most previous studies of neoproterozoic chemostratigraphy however the organic 13corg record reveals a surprising richness of information and shows excellent utility in correlation between sections in the adelaide rift complex lateral variations in 13corg profiles correspond to expected diachronous patterns of basin fill with the addition of 87sr 86sr data correlation can also be made to the officer and amadeus basins consistent with known lithostratigraphic and sequence stratigraphic constraints sublittoral micritic carbonates of the upper nuccaleena formation cap dolostone and lower and middle wonoka formation show strong basinwide 13c depletion 13ccarb 3 to 10 despite compelling evidence for preservation of depositional 13ccarb such as good textural preservation evidence for sea floor lithification and in the wonoka formation pristine 87sr 86sr and low mn sr associated with the depleted carbonates are rare subaqueous evaporite relatively heavy 13corg and anomalously 13c depleted organic carbon of benthic microbial mat origin and a common cause salinity stratification of the water column is postulated the indicated palaeogeographic setting an evaporative mediterranean like basin tenuously connected to the world ocean is entirely consistent with previous suggestions of one or more messinian type desiccation events to explain very deep 1 km incised valleys or canyons in the adelaide rift complex and officer basin at horizons within the wonoka formation and correlates positive excursions in 13corg during basin stratification may in part be due to 12c depletion of surface waters by photosynthetic carbon fixation despite the proposed semi isolation of australian ediacarian basins from the world ocean major 13c excursions which can to some extent be differentiated by the monotonic rise in ediacarian marine 87sr 86sr appear to be correlatable to the upper windermere supergroup northwest canada

depth dependent δ 13 c trends in platform and slope settings of the campbellrand malmani carbonate platform and possible implications for early earth oxygenation highlights carbon cycle of neoarchean carbonate platform and potential oxygen oasis carbon isotopes reveal a shift to aerobic biosphere and increasing oxidation state rare earth element patterns reveal decrease in open ocean water influx rimmed margin architecture was crucial for evolution of aerobic ecosystems abstract the evolution of oxygenic photosynthesis is widely seen as the major biological factor for the profound shift from reducing to slightly oxidizing conditions in earth s atmosphere during the archean proterozoic transition period the delay from the first biogenic production of oxygen and the permanent oxidation of earth s atmosphere during the early paleoproteorozoic great oxidation event goe indicates that significant environmental modifications were necessary for an effective accumulation of metabolically produced oxygen here we report a distinct temporal shift to heavier carbon isotope signatures in lagoonal and intertidal carbonates 13ccarb from 1 6 to 0 2 relative to vpdb and organic matter 13corg from about 40 to 25 relative to vpdb from the 2 58 2 50 gy old shallow marine campbellrand malmani carbonate platform south africa this indicates an increase in the burial rate of organic matter caused by enhanced primary production as well as a change from an anaerobic to an aerobic ecosystem trace element data indicate limited influx of reducing species from deep open ocean water into the platform and an increased supply of nutrients from the continent both supporting primary production and an increasing oxidation state of the platform interior these restricted conditions allowed that the dissolved inorganic carbon dic pool in the platform interior developed differently than the open ocean this is supported by coeval carbonates from the marginal slope setting which had a higher interaction with open ocean water and do not record a comparable shift in 13ccarb throughout the sequence we propose that the emergence of stable shallow water carbonate platforms in the neoarchean provided ideal conditions for the evolution of early aerobic ecosystems which finally led to the full oxidation of earth s atmosphere during the goe

velocity structure of a gas hydrate reflector at ocean drilling program site 889 from a global seismic waveform inversion strong subhorizontal reflectors in the upper few hundred meters of oceanic sediments have been imaged by seismic reflection profiling adjacent to many continental margins these reflectors are known as bottom simulating reflectors bsrs as they run roughly parallel to the seabed bsrs generally have been interpreted as representing the base of a methane gas hydrate stability field a series of boreholes was drilled in october november 1992 during leg 146 of the ocean drilling program to examine fluid discharge and its relationship to the bsr in the cascadia accretionary wedge independently we performed a global waveform inversion of seismic reflection data collected near site 889 we use a multistep strategy for global waveform inversion of seismic reflection data the strategy is based on the bayesian inference theory in which the a posteriori probability distribution is determined by the a priori probability distribution and the likelihood function relating the data with the model three different likelihood functions have been defined relating different parts of the data with different wavelengths of model parameter variation first global nonlinear search algorithms are used to estimate the large scale features of one dimensional velocity variations then a nonlinear iterative search is used to retrieve the velocity structure in fine detail at each stage bounds on the model parameters are estimated the strategy has been implemented in the slowness and intercept time domain we find that the bsr corresponds to a 15 20 m zone where the velocity drops from about 1 8 km s to a minimum of 1 4 km s suggesting the presence of free methane gas in the low velocity zone and a 15 20 m transition zone where the velocity gradually increases to a velocity of the surrounding sediments thus the total thickness of the low velocity zone is about 30 40 m a small or negligible velocity increase just above the bsr suggests that the hydrate fills less than 10 of the pore space in the sediments our result was confirmed by the drilling the velocity derived using a vertical seismic profile and sonic log from the drill hole agree closely with our result waveform inversion can recover detailed and well constrained velocity variations from a high amplitude target such as a bsr combined with the detailed interpretation of these velocity variations which the results from site 889 will provide the inversion approach is a powerful tool for analysis of bsrs elsewhere

investigations on geomagnetic reversals in icelandic lavas 1953 78 recent reviews of the development of palaeomagnetic techniques in geology have shown that palaeomagnetism has contributed significantly to the acceptance of new concepts in global tectonics the present paper extends frankel s 1987 review of palaeomagnetic work on the basalt lava sequences of iceland in the early 1950s these lavas provide some of the most complete series of reliable spot readings of the geomagnetic field available it is shown that research on icelandic basalts has continued to be of importance in improving our understanding of palaeomagnetism and its applications

neogene growth of the sedimentary organic carbon reservoir we develop a recycling model using 13c 12c mass balance for net growth loss of the sedimentary organic carbon corg reservoir and apply it to the neogene bulk marine carbonate 13c record the model allows for variations in photosynthetic fractionation factors carbon cycling rates and the isotopic composition of riverine carbon inputs to the oceans the sign of the net flux term is controlled by the difference between fractional corg burial and fractional corg weathering independent of any variations in carbon cycling rate these terms are in turn estimated from the carbon isotope mass balance of newly deposited and weathered sediments respectively the magnitude of the net flux is sensitive to the global carbon cycling erosion deposition rate which may be estimated by the use of the records of radiogenic isotopic variations nd sr in paleoseawater a key observation and input to the model is that photosynthetic carbon isotope fractionation by both marine algae and terrestrial plants has decreased during the cenozoic incorporating time dependent carbon isotope fractionation into the model shows that the sedimentary corg reservoir has grown throughout most of the neogene even as marine 13c values fell after 14 ma a similar result is obtained if the variation in the marine 13c record is largely caused by changes in the carbon isotopic composition of river fluxes to the oceans rather than changes in the organic inorganic ratio of output to the burial sink the growth of the sedimentary organic carbon reservoir requires that the neogene sedimentary carbon cycle was a net source of o2 and a net sink of co2 to the atmosphere at least until the plio pleistocene as a consequence neogene co2 consumption by silicate weathering cannot be balanced by oxidation of sedimentary corg placing a significant constraint on global carbon balance models a related prediction of our model is that atmospheric o2 levels rose during the neogene

isotopic composition of organic and inorganic carbon from the mesoproterozoic jixian group north china implications for biological and oceanic evolution a b s t r a c t analyses of marine carbon isotope profiles have provided much of our current understanding of the evolution of earth surface environments particularly in the latter portion of the proterozoic eon earlier mesoproterozoic successions however have received comparatively little attention due to the relatively subdued nature of carbon isotope variation in this study we present high resolution isotopic profiles from three sections in the yanshan basin north china craton that combined comprise the entirety of the early mesoproterozoic 1600 1400 ma calymmian period jixian group high resolution profiles of both carbonate and organic carbon provide critical data for global comparison and permit us to better con strain both the pattern and origin of isotopic variation in the mesoproterozoic marine carbonate rocks of the jixian group show generally muted isotopic variation with average values near 0 consistent with previous observations from the early mesoproterozoic data furthermore record an increase in isotopic variation through the succession that is interpreted to reflect a long term decrease in pco2 and conse quently in the isotopic buffering capacity of marine dissolved inorganic carbon dic by contrast the isotopic composition of marine organic matter suggests facies dependent differences in carbon cycling organic carbon compositions suggest a dominance of autotrophic carbon fixation and aerobic decom position in shallow water environments and increased remineralization by anaerobic heterotrophs in deeper water environments correlation between organic carbon composition and depositional environ ment are interpreted to reflect differences in carbon cycling within benthic microbial mats under low oxygen conditions and dynamically maintained stratification of marine waters

chemostratigraphy of the shaler supergroup victoria island nw canada a record of ocean composition prior to the cryogenian glaciations a b s t r a c t a new 13 ccarb curve combined with 13 corg values is presented for the upper shaler supergroup 900 to 720 ma amundsen basin northwestern canada the dataset fills gaps in the existing stratigraphic record and makes correlations with adjacent basins more robust there is a pronounced negative 13 c excursion in the wynniatt formation that can be correlated with a putative worldwide negative car bon isotope excursion namely the bitter springs stage however in the amundsen basin the 13 ccarb excursion drops to anomalously negative values 14 which we attribute to local overprints wherein isotopically light carbon in pore waters released by oxidation of methane and organic matter during sulphate and iron reduction was incorporated into authigenic carbonate cement we document basin euxinia and anoxia during the same time interval using a multi proxy approach specifically fe speciation and redox sensitive trace metal data patterns of pronounced enrichment in mo v and u concentrations in euxinic black shales suggest that the bitter springs stage was a transitional period in earth s redox evo lution from the more reduced global oceans during the mid proterozoic to the more oxygenated oceans during the phanerozoic 2015 elsevier b v all rights reserved

variable dispersion of neogene geomagnetic field directions in iceland abstract analysis of the primary paleomagnetic directions in 4000 lava flows in iceland reveals that the magnitude of total virtual geomagnetic pole dispersion has decreased during the last 15 ma this dispersion which is independent of the polarity of the field is in part caused by the occurrence of erratically varying paleofield directions during intermittent periods possibly as long as 100 ka

distribution of methane hydrate bsrs and its implication for the prism growth in the nankai trough abstract detailed mapping of a bottom simulating reflector bsr which marks the phase transition from the methane hydrate layer above the reflector to free gas below was conducted in the nankai accretionary prism off shikoku and tokai bsrs are widely distributed in the prism slope from the toe region to the forearc basin bsr positions provide us with the regional heat flow variations using pressure temperature conditions for methane hydrate stability estimated heat flows generally show constant values about 50 mw m2 shallower than the middle slope of the prism and gradually increase seaward in the lower prism slope occurrences of bsrs are regarded as accumulation of free gas beneath the base of a gas hydrate stability field bghs and or concentration of methane hydrate above the bghs these conditions can be accomplished by updip migration of methane gas because it is unlikely that such methane concentration is completed by in situ biogenic methanogenesis within sediments including low total organic carbon moreover sedimentation uplifting and sediment stacking by thrust faulting cause upward migration of the bghs and migration of methane from the dissociated hydrate to new bghs such recycling of methane gas may have actively occurred in accretionary prisms in contrast there are five regions of no bsrs the nankai trough floor prism toe slope basin steep slope and deep sea canyon the trough floor the prism toe and the slope basin are characterized by young sediments with low production of methane gas and sub horizontal strata unsuitable for migration of gases and fluids erosion at the steep slope and the canyon causes removal of hydrated sediments and downward movement of the bghs bsr distribution and thermal structure estimated from bsr positions offer information about active processes occurring in accretionary prisms

marine organic matter cycling during the ediacaran shuram excursion ediacaran ca 635 541 ma marine carbonates capture a global 13 c carbon isotope excursion to extremely negative values 12 known as the shuram excursion se that cannot be explained by conventional mass balance scenarios furthermore the carbon isotopic variation of bulk organic matter om does not mirror that of carbonate through the excursion suggesting that the om reflects a mixture of different sources to evaluate this hypothesis we investigated thermally immature marine sedimentary rocks that record the se from the sultanate of oman compound specific carbon isotopic analyses of the extractable hydrocarbons reveal low 13 c values of long chain c 20 n alkanes and mid chain monomethyl alkanes as low as 40 such light signatures are rare in marine rocks of any age and provide evidence that the se reflects a primary carbon cycle perturbation the magnitude of the se recorded in these organic phases is smaller than observed in carbonate and implies that the primary perturbation to dissolved inorganic carbon dic was at least 5 7 and more likely 7 12 in magnitude when correcting for end member source mixing due to isotopic differences in stratigraphic patterns of the different organic compounds we propose that bulk organic carbon both bitumen and kerogen reflects source mixing between two distinct pools that previously masked the excursion in bulk 13 c org measurements om sources were derived both from autotrophs fixing 13 c depleted dic and from a less 13 c depleted heterotrophic microbial biomass feeding on a marine om pool sustained by petroleum expelled from older sedimentary om expulsion of these sedimentary fluids also helps explain both the duration and magnitude of the se

was the ediacaran shuram excursion a globally synchronized early diagenetic event insights from methane derived authigenic carbonates in the uppermost doushantuo formation south china abstract the ediacaran period is characterized by the most profound negative carbon isotope 13c excursion in earth history the shuram excursion various hypotheses including the massive oxidation of dissolved organic carbon doc in the oceans the weathering of terrestrial organic carbon or the release and oxidation of methane hydrates and or expelled petroleum from the subsurface have been proposed as sources of the 13c depleted carbon more recently it has been suggested that global scale precipitation of early authigenic carbonates driven by anaerobic microbial metabolism in unconsolidated sediments may have caused the shuram excursion but empirical evidence is lacking here we present a comprehensive analysis of a shuram associated interval from the uppermost doushantuo formation in south china our study reveals petrographic evidence of methane derived authigenic calcite formed as early diagenetic cements and nodules that are remarkably depleted in 13c suggesting a buildup of alkalinity in pore fluids through the anaerobic oxidation of methane aom and systematically depleted in 18o relative to co occurring dolomite early authigenesis of these minerals is likely to be driven by increased microbial sulfate reduction triggered by enhanced continental weathering in the context of a marked rise in atmospheric oxygen levels in light of the finding of methane derived authigenic carbonates at zhongling and based on our basin scale stratigraphic correlation we hypothesize that the marked 13c and 18o depletion including their co variation noted worldwide in the shuram excursion may reflect an episode of authigenesis occurring within a sulfate methane transition zone smtz if true the shuram excursion was then a global biogeochemical response to enhanced seawater sulfate concentration in the ediacaran ocean driven by the neoproterozoic oxidation of surface environments this paleo oceanographic transition may have therefore paved the way for subsequent evolution and diversification of animals our study highlights the significance of an integrated approach that combines petrography mineralogy and texture specific micro drilling geochemistry in chemostratigraphic studies

efficient velocity model building for prestack depth migration prestack depth migration is capable of imaging subsurface reflectors in the presence of severe lateral velocity gradients however to produce accurate images of such structures using prestack depth migration it is critical to build an accurate velocity model the velocity model building should be data driven and based upon offset consistency that is images from different offsets must be identical to one another if the velocity model is correct velocity model building is typically iterative and requires many runs of computationally intensive prestack depth migration

precise dating of the holmatindur cooling event in eastern iceland evidence for mid miocene bipolar glaciation a succession of basaltic lavas volcaniclastic sediments and lignite beds in the holmatindur region of eastern iceland provides the means for wing magnetic reversal stratigraphy and a record of major north atlantic cooling to a precise radiometric timescale a prominent part of the section is the holmatindur clastic bed which is up to 66 m thick and can be traced 80 km along strike the geological significance of this unit lies in its composition and associated thin lignite seams the volcaniclastic composition is basaltic in contrast to the common rhyolitic dacitic composition of tuff units in the area and hyaloclastic material is dominant hydroexplosive volcanism most likely produced the hyaloclastite through subglacial eruptions within the paleovolcanic zone pollen assemblages from the lignite beds indicate a dramatic climatic deterioration from subtropical to cool temperate conditions through these sediments feldspar crystals separated from the holmatindur clastic bed were determined to be from 10 72 0 16 ma from 4oar 39ar incremental heating experiments we correlate the holmatindur cooling event with a relative maximum in benthic foraminiferal bso zone mi6 of miller et al 1991 which has been interpreted as an episode of ice accumulation within the mid miocene cooling period as well as with pulses in ice rafted debris supplied to deep water sites in the north atlantic ocean a global sea level drop of 70 m and submarine canyon cutting all of which occurred within the early part of magnetic anomaly 5 normal c5n while strong evidence exists for mid miocene ice sheets in antarctica northern hemisphere glaciation is thought to have started much later the iceland sites studied here demonstrate that significant ice accumulation occurred albeit intermittently as early as late middle miocene time in the north atlantic ocean from radiometric dating of tertiary lavas that lie below and above the well established lower boundary c5n we conclude that the onset of this long normal polarity period occurred at 10 94 0 16 ma the new data are compatible with the cande and kent 1995 interpolated estimate for this boundary 10 95 ma but are significantly older or more precise than previous direct dating by conventional k ar methods

constraining dolomitization by mg isotopes a case study from partially dolomitized limestones of the middle cambrian xuzhuang formation north china the dolomite problem refers to the rare dolomite formation in modern oceans that is in sharp contrast to the widespread ancient dolostone in rock record as well as failure of laboratory inorganic dolomite precipitation at near earth surface temperature novel mg isotope systematics provides a promising tool in resolving the dolomite problem here we develop a protocol to place constraints on the dolomitization process by using mg isotopes in this study we measured mg isotopic compositions 26mg of two batches of partially dolomitized limestone samples from the middle cambrian xuzhuang formation in north china 26mg varies between 0 55 and 3 18 and shows a negative linear correlation with 1 mg suggesting that 26mg can be described by a binary mixing between the calcite and dolomite components mg isotopic composition of the dolomite component 26mgdol for the lower sample set that is collected from a 4 m stratigraphic interval containing three high frequency ribbon rock packstone cycles is 1 6 while 26mgdol for the upper sample set from a thick sequence of ribbon rock is significantly higher 0 3 however neither mineralogical and elemental compositions carbon and oxygen isotopes nor crystal morphologies of dolomite provides diagnostic criteria to differentiate these two batches of samples 26mgdol of the xuzhuang limestone is simulated by the advective flow af and the diffusion advection reaction dar models the af model assumes that mg is transported by advective fluid flows while the dar model simulates a contemporaneous seawater dolomitization process in which mg is delivered by diffusion the af modeling result indicates that 26mg of the dolomitization fluid is 0 4 and 1 7 for the lower and upper sample sets respectively these values are significantly higher than modern and cenozoic seawater mg isotopic composition suggesting that the dolomitization fluid is not contemporaneous seawater the af model also predicts spatially heterogeneous 26mgdol with progressive enrichment in 26mg along the fluid flow pathway in the dar model both dolomite content and 26mgdol of the lower sample set can be simulated by using seawater mg isotopic composition of 0 75 thus contemporaneous seawater dolomitization may explain 26mgdol of the xuzhuang limestone furthermore the dar model demonstrates spatially homogeneous 26mgdol to differentiate the af and dar models samples from multiple sections are required nevertheless this study implies that mg isotope might be a useful tool in the study of dolomitization

ambiguity in analysis of velocity and depth velocity analysis methods generally involve an analysis of the traveltime moveout of seismic events to investigate ambiguities for such methods we can start with a simple single layer model and investigate the uncertainty in the estimation of velocity and reflector depths this analysis will show that the velocity depth ambiguity is dependent on the offset depth ratio and on the accuracy of traveltime picks

methane hydrate and free gas on the blake ridge from vertical seismic profiling seismic velocities measured in three drill holes through a gas hydrate deposit on the blake ridge offshore south carolina indicate that substantial free gas exists to at least 250 meters beneath the bottom simulating reflection bsr both methane hydrate and free gas exist even where a clear bsr is absent the low reflectance or blanking above the bsr is caused by lithologic homogeneity of the sediments rather than by hydrate cementation the average methane hydrate saturation above the bsr is relatively low 5 to 7 percent of porosity which suggests that earlier global estimates of methane in hydrates may be too high by as much as a factor of 3

wide but not ubiquitous distribution of glendonite in the doushantuo formation south china implications for ediacaran climate abstract glendonites are pseudomorphs of syndepositional or early authigenic ikaite caco3 6h2o that often forms at near freezing temperatures silicified glendonite has been reported from inner shelf deposits of the lower ediacaran doushantuo formation at a single stratigraphic section in south china where they are stratigraphically associated with the positive 13c excursion ep1 but predate the negative 13c excursion en3 shuram excursion indicating a period of cool climate somewhere between 609 ma and 551 ma this interpretation predicts a wider geographic distribution of ediacaran glendonites in equivalent strata in the yangtze block of south china to test this prediction we conducted a regional survey of doushantuo formation and found that although glendonites are not universally present they occur in expected stratigraphic intervals and in association with ep1 at two new sections representing inner shelf and intrashelf basin facies the wide but not ubiquitous distribution of doushantuo glendonites indicates that ikaite precipitation and glendonite formation was controlled by both regional climatic and local geochemical factors glendonites at the new localities are stellate clusters pseudomorphed by calcite spar and sometimes are rimmed with silica the calcite spar is characterized by highly variable and mostly negative 13c values as low as 37 indicating that diagenetic transformation of precursor ikaite to calcitic glendonite may be related to anaerobic oxidation of organic matter or methane in sediment the new data suggest an early ediacaran cold period prior to en3 or the shuram excursion and facilitation of glendonite ikaite formation by both climatic and local geochemical conditions such as redox conditions and phosphate concentrations

the mode of emplacement of neogene flood basalts in eastern iceland facies architecture and structure of simple aphyric basalt groups simple flows tabular in the neogene flood basalt sections of iceland are described and their mode of emplacement assessed the flows belong to three aphyric basalt groups the kumlafell group the holmatindur group and the hjalmadalur group the groups can be traced over 50 km and originate in the brei dalur thingmuli volcanic zone the groups have flow fields that display mixed volcanic facies architecture and can be classified after dominating type morphology the kumlafell and the holmatindur groups have predominantly simple flows of p hoehoe and rubbly p hoehoe morphologies with minor compound or lobate p hoehoe flows the hjalmadalur group has simple flows of rubbly p hoehoe but also includes minor compound or lobate flows of rubble and a simple flows are most common in the distal and medial areas from the vents while more lobate flows in proximal areas the simple flows are formed by extensive sheet lobes that are several kilometers long with plane parallel contacts some reaching thicknesses of 40 m aspect ratios 0 01 they have overlapping contacts and are free of tubes and inflation structures their internal structure consists generally of a simple upper vesicular crust a dense core and a thin basal vesicular zone the brecciated flow top is formed by clinker and crustal rubble the clinker often welded or agglutinated the simple flows erupted from seemingly short lived fissures and have the characteristics of cooling limited flows we estimate the effusion rates to be 105 m3 s for the simple flows of the kumlafell and holmatindur groups and 104 m3 s for the hjalmadalur group the longest flows advanced 15 20 km from the fissures with lava streams of fast propagating flows inducing tearing and brecciation of the chilled crust compound or lobate areas appear to reflect areas of low effusion rates or the interaction of the lava with topographic barriers or wetlands resulting in chaotic flowage slowing lobes with brecciated flow tops developed into a flows the groups interdigitated with lava groups from the rey arfjor ur volcanic zone to the east and the exhumed brei dalur thingmuli volcanic zone which appear to have formed as a flank lineament parallel the main rift zone flood basalt volcanism in flank areas may support a mantle anomaly more pronounced and or perhaps more widespread in the neogene of iceland than today eruptions of simple flows have not been observed in modern times and are significant for models of crustal accretion in iceland and other large igneous provinces

the distribution of gold in volcanic rocks of eastern iceland abstract gold contents are reported for basaltic lavas and dykes from the tertiary succession of eastern iceland in unsaturated olivine normative tholeiites au decreases in concentration with degree of differentiation however in saturated quartz normative and more differentiated varieties au shows a high degree of positive correlation with incompatible elements such as y and zr this behaviour suggests that in the latter rocks au is not held in early crystallized phases but is enriched in the residual melt so that it follows the trend of the incompatible elements thus basaltic andesites and silica rich icelandites contain twice as much au x 4 1 ng g 1 as basalts this general trend appears to follow an erratic but concomitant enrichment of s in the rocks oxidation and zeolite alteration have led to mobilization and to enrichment of au in volcaniclastic units and in the porous top of a lava flow it is suggested that in older volcanic piles of icelandic type the most suitable source rocks for au mineralization could be in the more evolved quartz normative tholeiites ferrobasalts and basaltic andesites and icelandites particularly dykes and volcaniclastic units loosely held au is readily mobilized from volcanic glass and sulfides due to weathering burial metamorphism and hydrothermal alteration therefore only the freshest glassy rocks should be expected to retain their primary au concentrations

an analysis of a seismic reflection from the base of a gas hydrate zone offshore peru seismic reflection data recorded near odp site 688 offshore peru exhibit a persistent bottom simulating reflector bsr from a depth corresponding to the theoretical base of the gas hydrate stability field although gas hydrate has been recovered in cores from scientific drilling in both the pacific and the atlantic drilling in areas exhibiting a bsr has usually been avoided because of the perceived risk of releasing free gas trapped beneath the hydrated sediments to carry out a quantitative analysis of the bsr the seismic data were reprocessed using signature deconvolution and true amplitude recovery techniques synthetic seismograms that were created using acoustic parameters extracted from the seismic data and physical properties from site 688 borehole measurement were compared to the observed seismic data to estimate the thickness of the free gas zone results indicate the bsr is discontinuous laterally where the bsr is of high amplitude free gas in a zone 5 5 17 m thick beneath the hydrated sediments provides the observed waveform where the bsr amplitude is low the free gas zone is much thinner than 5 5 m or is entirely absent

a multilayered water column in the ediacaran yangtze platform insights from carbonate and organic matter paired δ13c abstract large carbon isotope fluctuations have been systematically reported for ediacaran carbonate rocks the meaning of which remains controversial in order to better understand the mechanisms behind such variations we present new paired 13c data on carbonates 13ccarb and their associated organic matter 13corg from a shelf margin section yangjiaping of the doushantuo formation in this section the doushantuo formation starts with cap dolostones presenting 13ccarb values around 5 and 13corg values between 30 3 and 27 6 up section phased variations in 13ccarb and 13corg describe positive and negative excursions while their difference 13cdol org 13ccarb 13corg remains around 29 2 these new data allow the first reconstruction of lateral variations of 13ccarb 13corg and 13ccarb org for a shelf to basin cross section of the yangtze platform south china after integration with results reported previously for two other sections across the yangtze platform the isotope signals reveal strong lateral heterogeneities with complex variations of 13ccarb and 13cdol org in the inner shelf section phased variations in the shelf margin section with positive 13ccarb and 13cdol org close to 29 and dominantly negative 13ccarb with 13corg as low as 35 in the basin these variations are incompatible with the idea that the 13ccarb can systematically be used as a proxy of ocean surface waters assuming that 13ccarb are acquired in bottom waters and or upper sediments we show that the heterogeneous 13ccarb and 13cdol org are compatible with a stratified water column composed of up to three layers i an oxic surface layer where dissolved inorganic carbon dic is probably in isotope equilibrium with the atmosphere ii an intermediate euxinic layer with a lower 13cdic due to organic matter oxidation by ongoing sulphate reduction iii a bottom euxinic layer that seems to be restricted to the inner shelf lagoonal facies lacking sulphate containing methane and with a higher 13cdic due to dic production by methanogenesis if our model holds true it suggests that not only negative but also positive ediacaran carbon isotope excursions may reflect ocean stratification the positive excursion possibly recording a sulphate free methanogenic layer at the bottom of restricted basins

dynamic redox conditions control late ediacaran metazoan ecosystems in the nama group namibia the first appearance of skeletal metazoans in the late ediacaran 550 million years ago ma has been linked to the widespread development of oxygenated oceanic conditions but a precise spatial and temporal reconstruction of their evolution has not been resolved here we consider the evolution of ocean chemistry from 550 to 541 ma across shelf to basin transects in the zaris and witputs sub basins of the nama group namibia new carbon isotope data capture the final stages of the shuram wonoka deep negative c isotope excursion and these are complemented with a reconstruction of water column redox dynamics utilising fe s c systematics and the distribution of skeletal and soft bodied metazoans combined these inter basinal datasets provide insight into the potential role of ocean redox chemistry during this pivotal interval of major biological innovation the strongly negative 13c values in the lower parts of the sections reflect both a secular global change in the c isotopic composition of ediacaran seawater as well as the influence of local basinal effects as shown by the most negative 13c values occurring in the transition from distal to proximal ramp settings critical though is that the transition to positive 13c values postdates the appearance of calcified metazoans indicating that the onset of biomineralization did not occur under post excursion conditions significantly we find that anoxic and ferruginous deeper water column conditions were prevalent during and after the transition to positive 13c that marks the end of the shuram wonoka excursion thus if the c isotope trend reflects the transition to global scale oxygenation in the aftermath of the oxidation of a large scale isotopically light organic carbon pool it was not sufficient to fully oxygenate the deep ocean both sub basins reveal highly dynamic redox structures where shallow inner ramp settings experienced transient oxygenation anoxic conditions were caused either by episodic upwelling of deeper anoxic waters or higher rates of productivity these settings supported short lived and monospecific skeletal metazoan communities by contrast microbial thrombolite reefs found in deeper inner and mid ramp settings supported more biodiverse communities with complex ecologies and large skeletal metazoans these long lived reef communities as well as ediacaran soft bodied biotas are found particularly within transgressive systems where oxygenation was persistent we suggest that a mid ramp position enabled physical ventilation mechanisms for shallow water column oxygenation to operate during flooding and transgressive sea level rise our data support a prominent role for oxygen and for stable oxygenated conditions in particular in controlling both the distribution and ecology of ediacaran skeletal metazoan communities

palynological evidence for miocene climatic cooling in eastern iceland about 9 8 myr ago the processes which led to the pleistocene glaciations in the northern hemisphere are presently known only from fragmentary evidence of major tertiary climatic cooling events1 2 although there is good evidence3 for glacial intervals in alaska 10 9 myr most terrestrial records4 5 which suggest a corresponding high latitude climatic cooling in the north east atlantic are poorly dated and north atlantic palaeoceanographic records contain large erosional hiatuses2 6 we have obtained new palynological data from eastern iceland from clastic units between lava formations dated by k ar and palaeomagnetic methods7 8 we now report that these data record a floral succession which suggests that a temperature decrease of 10 c occurred in iceland between 10 and 9 5 myr

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

diagenetic overprint on negative δ13c excursions across the permian triassic boundary a case study from meishan section china the meishan section d from china is the global stratotype section and point gssp of the permian triassic boundary ptb the succession has been divided into 115 beds with the ptb being placed in the middle of bed 27 in this section and the laterally equivalent section a the diagenetic features of beds 24 to 62 are examined in this study to determine if the negative shift in 13ccarb across the permian triassic boundary is diagenetic in origin or not bed 24 is formed of skeletal packstones whereas beds 25 to 62 are formed of claystones or argillaceous dolomitic limestones the dolomite crystals are typically 30 m long subhedral to euhedral and held in a very fine groundmass formed of calcite and clay minerals the cathodoluminescent features and backscattered images show that the dolomite crystals are zoned with a non luminescent low calcium calcian dolomite lcd core being encased successively by an irregular dull orange calcite zone a euhedral bright orange high calcium calcian dolomite hcd cortex and an outermost ferroan hcd zone available evidence indicates that the zoned dolomite crystals formed episodically the dolomite crystals following their initial syndepositional formation with euhedral lcd cores and hcd cortices were subsequently modified by calcitization mediated by meteoric water and overgrowth of fe rich hcd during burial the limestones in bed 24 display 13c values ranging from 1 72 to 2 92 18o values from 7 74 to 4 84 in beds 25 to 62 the bulk samples have 13c values from 0 76 to 1 28 and 18o values from 8 58 to 3 99 there is a positive correlation between these isotopic compositions and dolomite content it is estimated that the first formed dolomite and the precursor limestones have 13c value of 2 19 and 18o value of 1 14 the dramatic negative 13c excursions of calcite in beds 26 and 28 are related to meteoric diagenesis whereas the negative 13c excursions of bulk samples in the dolomite bearing beds are ascribed to the diagenetic modifications that post dated the initial formation of dolomite the results show that the 13c signals recorded in the gssp that spans the ptb are a mixture of original and diagenetic signals for the dolomite bearing bulk samples diagenesis contributes 1 7 to the negative shift in the 13c hence diagenetic alteration needs to be considered before using 13ccarb to estimate the dissolved inorganic carbon in the ancient oceans and to calculate the global carbon cycle

seismic velocities for hydrate bearing sediments using weighted equation a weighted equation based on the three phase time average and wood equations is applied to derive a relationship between the compressional wave p wave velocity and the amount of hydrates filling the pore space the proposed theory predicts accurate p wave velocities of marine sediments in the porosity range of 40 80 and provides a practical means of estimating the amount of in situ hydrate using seismic velocity the shear s wave velocity is derived under the assumption that the p to s wave velocity ratio of the hydrated sediments is proportional to the weighted average of the p to s wave velocity ratios of the constituent components of the sediment in the case that all constituent components are known a weighted equation using multiphase time average and wood equations is possible however this study showed that a three phase equation with modified matrix velocity compensated for the clay content is sufficient to accurately predict the compressional wave velocities for the marine sediments this theory was applied to the laboratory measurements of the p and s wave velocities in permafrost samples to infer the amount of ice in the unconsolidated sediment the results are comparable to the results obtained by repeatedly applying the two phase wave scattering theory the theory predicts that the poisson s ratio of the hydrated sediments decreases as the hydrate concentration increases and the porosity decreases in consequence the amplitude versus offset avo data for the bottom simulating reflections may reveal positive negative or no avo anomalies depending on the concentration of hydrates in the sediments

integrated bio chemostratigraphical correlations and climatic evolution across the danian selandian boundary at low latitudes abstract the global boundary stratotype section and point for the base of the selandian stage is defined in the zumaia section spain at an abrupt change in lithology base of itzurun formation which coincides with the onset of a negative carbonate carbon isotope shift however this lithological change is not always very well expressed in other sections in order to document the stratigraphic position of the danian selandian boundary dsb on a more global scale we have investigated three sections across the dsb the zumaia reference section gssp the loubieng section auxiliary dsb reference section france and the sidi nasseur section tunisia the danian selandian boundary interval is subdivided and correlated throughout low latitudes from the altlantic bay of biscay to the southern tethys on the basis of seven calcareous nannofossil and planktonic foraminiferal events e events the base of the selandian is proved to coincide with the end of the braarudosphaera acme which correlates with the lowest consistent occurrence lcso of lithoptychius aff bitectus fasciculithus janii sensu steurbaut and sztrakos 2008 event e4 but which is slightly posterior to the second radiation of the fasciculiths up to now considered to represent the primary correlation tool of the dsb a short term 13 c org negative excursion associated with an increase in p co 2 is recorded at the very base of the selandian it is interpreted as a short period of global warming hyperthermal the duration of which is estimated at 30 kyr it is followed in all the three studied sections by a long term decoupled carbon isotope event marked by increasing 13 c org and decreasing 13 c carb values it may reflect a period of climatic cooling of a few 100 kyr interpreted as a possible precursor of the global cooling event marking the late paleocene in the north atlantic realm the integration of the biostratigraphic and the isotope data indicates major differences in sedimentation rates during the early selandian in the studied sections but there is no evidence of substantial breaks in sedimentation in any of the sections during this interval the lithological shift at the base of the selandian points to an abrupt palaeoenvironmental reorganisation although our integrated bio chemostratigraphical investigation does not allow for estimating its duration nor the presence of a hiatus at that time

applying carbon isotope stratigraphy using well cuttings for high resolution chemostratigraphic correlation of the subsurface basin scale correlations in the subsurface generally rely on lithostratigraphic information synthesized from wireline logs and in some cases well cuttings and cores however lithostratigraphic boundaries are often diachronous and as such the correlations based upon them may not provide reliable timelines in this paper we use data from well cuttings and a core to generate chronostratigraphic logs of late ordovician strata spanning the black river group trenton group and utica shale across the subsurface of new york state although particular values may be impacted by primary variability in local dissolved inorganic carbon reservoirs and or secondary diagenetic alteration it is possible to identify spatially and stratigraphically coherent patterns in which can be used to effectively correlate time equivalent strata on a basin wide or even global scale including across lithologies e g between limestone and calcareous shale the present study emphasizes the use of well cuttings as these are commonly collected during drilling and can provide the maximum lateral resolution for subsurface correlation parallel geochemical percent carbonate and total organic carbon and isotopic and data are used to understand the origin of stratigraphic and spatial variability in the signal and to identify diagenetic alteration stratigraphically coherent trends across new york were used to identify six isotopically distinct packages of time equivalent strata within these formations pairing chemostratigraphic and lithostratigraphic data improves our ability to document the diachronous nature of lithologic contacts including the base of the utica shale which is progressively younger moving west through new york