meteoric diagenesis of plio pleistocene limestones at enewetak atoll abstract petrographic lithologic mineralogic and minor elemental data from plio pleistocene carbonates at enewetak atoll indicate that these sediments have been calcitized in the near surface meteoric diagenetic environment shallow 100 m mostly pleistocene sediments remain aragonite rich with a few discrete meter scale calcite rich intervals developed either at pleistocene soil zones or in thin 5 m paleophreatic lenses deeper 100 to 180 m mostly pliocene sediments are characterized by pervasive calcitization of aragonite extensive dissolution with well developed moldic porosity and the occurrence of isopachous bladed and pore filling equant calcite cements pervasively calcitized pliocene limestones exhibit a narrow range in 18o value 1 0 53 a wide range in 13c value 1 1 63 and low mean sr 0 02 wt and mg 0 25 wt contents these data are most consistent with diagenetic alteration of plio pleistocene carbonates by meteoric phreatic calcitization although in some intervals calcitization in the mixed meteoric marine environment may have also occurred petrographic data document the superposition of subaerial exposure surfaces a d paleophreatic lenses in the subsurface of enewetak sr isotope data and stratigraphic forward modeling are consistent with the multiphase meteoric phreatic calcitization of pliocene limestones in response to subaerial exposure shortly after sediment deposition and during pleistocene sea level lowstands quantitative water rock interaction modeling of whole rock chemical trends suggests that calcitization occurred in an open diagenetic system at water rock w r volume ratios of 103 to 105 proxy indicators of climate e g retention of aragonite at depth and the development of caliche at subaerial exposure surfaces are consistent with an arid to semi arid plio pleistocene climate moreover a limited supply of meteoric waters in the vadose environment despite vertically and temporally extensive paleovadose environments is consistent with limited calcitization of the pleistocene sequence alternatively previous estimates of the rates of vadose calcitization may be too high to be applicable at enewetak stratigraphic shortening of the plio pleistocene sequence in response to carbonate dissolution during atoll emergence is not consistent with available data

upper mantle anisotropy in se and central brazil from sks splitting evidence of asthenospheric flow around a cratonic keel abstract we present results of upper mantle anisotropy derived from measurements of core refracted shear wave splitting mainly sks phases recorded at 48 stations covering the major tectonic provinces in central and se brazil such as the tocantins province paraguay araguaia and brasilia belts between the amazon and sao francisco cratons the parana intracratonic basin the southern part of the sao francisco craton and the mantiqueira province with the coastal ribeira belt although the fast polarization directions vary across the region consistent orientations are observed over hundreds of kilometers the fast polarization directions tend to be close to the absolute plate motion given by the hot spot reference model hs3 nuvel1a however correlations with geological structures are also observed in the southern brasilia belt and in the ribeira belt respectively located sw and s of the sao francisco craton on the other hand in the northern tocantins province the fast shear wave direction n60 s is oblique to the sw ne trend of the geological units and faults and no anisotropy contribution from lithospheric sources can be clearly identified overall the fast polarization directions show a fan shaped pattern strongly suggesting asthenosperic flow around a thick and stiff keel in the southern part of the sao francisco craton consistent with the high velocity anomaly revealed by recent surface wave tomography the observed nw se directions in the southern part of the brasilia belt may also be interpreted as resulting from asthenospheric flow channeled between the sao francisco craton and a cratonic block beneath the parana basin the largest splitting delays observed in the southern brasilia belt and in the ribeira belt up to 2 4 s suggest contributions from both lithospheric and asthenospheric sources in those two areas our preferred model for the anisotropy causing the observed pattern of sks splitting consists of flow induced deformation in the asthenosphere caused by the absolute plate motion roughly towards w or wsw with flow channeled around a thick cratonic keel in the southern brasilia and in the ribeira fold belts the large delays and the parallelism with geological structures indicate additional anisotropy contribution from lithospheric sources

middle archean continent formation by crustal delamination the processes that created the first large cratonic areas such as the pilbara and the kaapvaal remain poorly understood models based on the uniformitarian extrapolation of present day arc volcanic processes to a hotter early earth have not adequately explained the observations in these terranes here we propose an alternative mechanism for the formation of the earliest continental crust the formation of continental crust may be achieved by delamination of the lower eclogitic part of an oceanic plateau like protocrust such delamination results in uplift extension and the production of tonalite trondhjemite and granodiorite ttg suites as recorded in middle archean cratons the available geologic and geophysical observations in combination with model calculations permit this scenario as an alternative to subduction based hypotheses

mantle flow in regions of complex tectonics insights from indonesia 1 indonesia is arguably one of the tectonically most complex regions on earth today due to its location at the junction of several major tectonic plates and its long history of collision and accretion it is thus an ideal location to study the interaction between subducting plates and mantle convection seismic anisotropy can serve as a diagnostic tool for identifying various subsurface deformational processes such as mantle flow for example here we present novel shear wave splitting results across the indonesian region using three different shear phases local s sks and downgoing s to improve spatial resolution of anisotropic fabrics allows us to distinguish several deformational features for example the block rotation history of borneo is reflected in coast parallel fast directions which we attribute to fossil anisotropy furthermore we are able to unravel the mantle flow pattern in the sulawesi and banda region we detect toroidal flow around the celebes sea slab oblique corner flow in the banda wedge and sub slab mantle flow around the arcuate banda slab we present evidence for deep sub 520 km anisotropy at the java subduction zone in the sumatran backarc we measure trench perpendicular fast orientations which we assume to be due to mantle flow beneath the overriding eurasian plate these observations will allow to test ideas of for example slab mantle coupling in subduction regions

collisional snowbird tectonic zone resurrected growth of laurentia during the 1 9 ga accretionary phase of the hudsonian orogeny the 2800 km long snowbird tectonic zone is one of the most controversial tectonic features of the canadian shield metamorphic and in situ geochronologic data reported here reveal that a 1 9 ga medium to high pressure belt extends along most of this tectonic zone in contrast to recent interpretations a collisional origin is indicated by the length of this metamorphic belt tectonic thickening documented in parts of it geological contrasts across it and subduction type microdiamonds within it this collisional event marks a pre 1 865 ga phase of the hudsonian orogeny involving microcontinent accretion that was fundamental to the growth of laurentia

an oscillating trend in phanerozoic non skeletal carbonate mineralogy textural and compositional data on ancient ooids and cements suggest the existence of an oscillating phanerozoic temporal trend in non skeletal carbonate mineralogy the trend shows good correlation with eustatic climatic oscillations proposed by others the most likely mechanism appears to be plate tectonically influenced oscillations in pco2 the vapour pressure of co2 rather than change in oceanic mg ca ratio

aragonite calcite transformation based on pennsylvanian molluscs a mineralogical microstructural and geochemical approach was used to trace the postdepositional alteration of molluscs and to quantify the biogenic aragonite to calcite transformation in the boggy formation mid desmoinesian of oklahoma the results show that with progressive alteration primary biogenic aragonite was gradually replaced by diagenetic low mg calcite during the initial phase of alteration the nacreous tablets of the shell layers fused and with progressive postdepositional diagenesis they were replaced by either fine or coarsegrained calcite during this diagenetic transformation no significant changes in chemical concentrations were noted for ba cu ni pb and zn whereas mg mn and fe increased by factors of about 14 to 25 relative to their depositional concentrations concurrently na sr 18 o and 13 c concentrations decreased by factors ranging from 2 5 to 10 under similar diagenetic conditions matrix samples were more altered than were contemporaneous fossils and it is postulated that grain size surface area was a major control on the degree of alteration of carbonate grains the matrix of the boggy formation has retained sufficient geochemical information to suggest an original aragonitic limemud precursor of nonmolluscan affinity with the geochemical methods and quantified trends the least altered mollusc samples of the boggy formation consisting of only aragonite can be identified geochemical data of the most altered samples suggest that the diagenetic fluids which affected boggy sediments and fossils were similar in composition to meteoric water moreover the dissolving aragonite through diffusion controlled dissolution determined the elemental as well as isotopic chemistry of the diagenetic calcite in the altered shells it is postulated that similar diagenetic waters and processes affected sedimentary units of the breathitt pennsylvanian kentucky brush creek pennsylvanian pennsylvania ohio lukow jurassic poland bear paw lea park cretaceous western canada and nugssuaq cretaceous greenland formations which contain well preserved aragonitic molluscan faunas

the lithospheric root beneath hudson bay canada from rayleigh wave dispersion no clear seismological distinction between archean and proterozoic mantle abstract we characterise the nature and thickness of the seismological lithosphere beneath hudson bay using dispersion analysis of fundamental mode rayleigh waves along two station paths crossing the region hudson bay is largely underlain by the paleoproterozoic trans hudson orogen and is surrounded by archean cratons this study region thus provides an opportunity to investigate whether fundamental differences exist in the nature of the subcontinental lithosphere between archean and proterozoic terranes dispersion measurements for hudson bay carried out in the 15 220 s period range show seismic phase velocities consistently higher than those derived from global reference models in addition the phase velocities are higher than those of the canadian shield average and of many other cratonic regions worldwide monte carlo modelling is used to estimate 1d shear wave velocity profiles for each of the dispersion curves the models are characterised by a high velocity lid interpreted as the seismological expression of the subcontinental lithosphere beneath which velocities decrease gradually toward global reference values we find lithospheric thicknesses in the range 185 260 km beneath the hudson bay region with maximum lithospheric shear wave velocity perturbations of 4 5 7 3 above global reference values by treating the geotherm as a free parameter the average dispersion curve for hudson bay can be fit using laboratory based mineral physics relations with simple compositional models including constant composition and layered mantle models low mantle temperatures corresponding to surface heat flow values between 37 and 40 mw m 2 are required to match the observed data a layered model based on the archean slave craton is favoured as it provides a lithospheric root that is close to the isopycnic state i e neutrally buoyant for all depths in common with studies of other continental shields particularly fennoscandia and central australia the velocity structure modelled for the hudson bay region suggests that there is no systematic difference between the velocity structure and thickness of archean and proterozoic seismological lithosphere this result may have important implications for models of lithospheric stabilisation and evolution between 2 8 and 1 8 ga

diagenesis and sea level change lessons from moruroa french polynesia an inclined borehole through the margin of moruroa atoll french polynesia in which the ages of units are constrained by radiometric dates provides a unique opportunity to examine the relationship between sea level change and diagenetic history the oldest rocks encountered are dated at 355 840 yr bp and the overlying succession records five cycles of deposition and emergence culminating in the deposition of the holocene sequence successive intervals of emergence provided the driving force for diagenetic change promoting the through flow of chemically aggressive fresh waters that were at times either pervasive or confined within relatively isolated pathways on a millimetre to metre scale these locally varying flows and flow rates defined restricted zones characterized by differences in water chemistry and diagenetic signatures in contrast to some sequences elsewhere the upper part of the moruroa atoll succession is typified by alternations of intervals in which either several generations of cement growth are represented or only one or two cements are present three distinct diagenetic signatures can be recognized as follows i fresh water phreatic zones are reflected in neomorphism and the deposition of cements that occluded porosity thus preventing the deposition of later cements and conferring relative stability ii vadose zones are characterized by more complex diagenetic sequences reflecting multiple phases of dissolution and cement growth attesting to the relative duration of exposure and the conditions of ground water flow and iii deep in the core sequences reflect lowstand deposits that suffered sub sea diagenesis generating pores in which multi layered marine cements apparently formed at more frequent intervals than those subject to subaerial processes all cements appear to be of narrowly constrained ages these results highlight the contentious issue of whether the distributions of former diagenetic environments can be related to specific sea level stands and erosion surfaces it seems that in the moruroa core some cycles of sea level change are reflected in distinctive packages of diagenetic features that have not been overprinted by cement growths during later sea level stands

implications of a simple mantle transition zone beneath cratonic north america many areas of old continental interiors have thick roots that extend to depths of 250 km or more in contrast to the oceans and younger continents whose lithospheric thickness is less than 100 km these cratonic roots might perturb temperatures in their surrounding mantle though the net result could be either cooling or heating both of which may lead to small scale convective flow around the root we show here using new data from a study of the seismic structure of the canadian shield that the relative positions of the 410 km and 660 km seismic discontinuities are unperturbed beneath one of the deepest and broadest cratonic roots on the earth differential arrival times and internal discontinuity structure are remarkably uniform and simple varying by little more than 0 5 s over the root s 3500 km lateral extent this implies that the root has no significant thermal effect on the underlying mantle 50 k and any small scale convection or cold mantle downwelling associated with the large free air gravity anomaly beneath the shield must be confined to the upper mantle our observations are also consistent with phase changes solely in the olivine system mg fe 2sio4 with the caveat that our method is dominantly sensitive to s wave velocity jumps alone

crustal structure beneath hudson bay from ambient noise tomography implications for basin formation summary the hudson bay basin is the least studied of four major phanerozoic intracratonic basins in north america and the mechanism by which it formed remains ambiguous we investigate the crustal structure of hudson bay based on ambient noise tomography using 21 months of continuous recordings from 37 broad band seismograph stations that encircle the bay green s functions that emerge from the cross correlation of these ambient noise recordings are dominated by fundamental mode rayleigh waves in the microseismic period band 5 20 s these signals are most prominently expressed in certain preferred azimuths indicative of stationary coastal source regions in southern alaska and labrador seasonal variations are subtle but consistent with more energetic noise sources during winter months when wave heights in the pacific and north atlantic are larger than in the summer noise emanating from hudson bay does not appear to contribute significantly to the cross correlograms group velocity dispersion curves are obtained by time frequency analysis of cross correlation functions we test and implement a signal to noise ratio snr selection method for producing one sided cross correlograms which yields better defined dispersion ridges than the standard two sided averaging approach tomographic maps and cross sections obtained in the 5 40 s period range reveal significantly lower crustal velocities beneath hudson bay than in the bounding archean superior craton the lowest mid crustal velocities correspond to a previously determined region of maximum lithospheric stretching near the centre of the basin pseudosections extracted from the tomographic inversions along profiles across hudson bay provide the first compelling direct evidence for crustal thinning beneath the basin our results are consistent with a recent estimate of 3 km of crustal thinning but not consistent with a proposed model for basin subsidence triggered by eclogitization of a remnant crustal root

heterogeneous diagenetic patterns in the pleistocene ironshore formation of grand cayman british west indies abstract the ironshore formation on grand cayman consists of six unconformity bounded units a to f that developed in response to repeated transgressive regressive cycles during the middle to late pleistocene corals and matrices in limestones from the rogers wreck point rwp offshore george town gt and western onshore wo areas are characterized by complex diagenetic fabrics that reflect marine bioerosion micrite envelopes internal sediments fibrous high mg calcite acicular aragonite isopachous prismatic calcite cements freshwater phreatic circumgranular cements even random blocky calcite cements and vadose meniscus calcite cements blocky calcite cements diagenesis throughout these limestones the matrices have undergone more meteoric diagenetic alteration than the corals overall however no systematic stratigraphic patterns exist to the distribution of these diagenetic fabrics and it is generally impossible to link the different phases of diagenesis with specific unconformities that cap each unit these heterogenous patterns of diagenetic features can be attributed to many intrinsic and extrinsic factors including the original compositions of the different components in the limestones the porosity and permeability of the substrate the nature of the diagenetic fluids climate and the duration of exposure during each lowstand integration of available data however indicates that maximum diagenesis took place during the lowstands denoted by the unconformities at the top of unit c marine isotope stage 7 and unit d marine isotope stage 5e when long periods of exposure were accompanied by wet climates with high annual rainfalls

the formation of laurentia evidence from shear wave splitting abstract the northern hudson bay region in canada comprises several archean cratonic nuclei assembled by a number of paleoproterozoic orogenies including the trans hudson orogen tho and the rinkian nagssugtoqidian orogen recent debate has focused on the extent to which these orogens have modern analogues such as the himalayan karakoram tibet orogen further the structure of the lithospheric mantle beneath the hudson strait and southern baffin island is potentially indicative of paleoproterozoic underthrusting of the superior plate beneath the churchill collage also in question is whether the laurentian cratonic root is stratified with a fast depleted archean core underlain by a slower younger thermally accreted layer plate scale process that create structures such as these are expected to manifest as measurable fossil seismic anisotropic fabrics we investigate these problems via shear wave splitting and present the most comprehensive study to date of mantle seismic anisotropy in northern laurentia strong evidence is presented for multiple layers of anisotropy beneath archean zones consistent with the episodic development model of stratified cratonic keels we also show that southern baffin island is underlain by dipping anisotropic fabric where underthrusting of the superior plate beneath the churchill has previously been interpreted this provides direct evidence of subduction related deformation at 1 8 ga implying that the tho developed with modern plate tectonic style interactions

the influence of sponge borings on aragonite to calcite inversion in late pleistocene strombus gigas from grand cayman british west indies abstract large robust strombus gigas lived in a variety of habitats in the ironshore lagoon that existed on grand cayman during the late sangamon highstand approximately 125 000 years ago some well preserved shells are still entirely aragonitic whereas others have been transformed completely to calcite permeability of the host rock and sponge boring intensity were the most important controls on the extent of aragonite to calcite inversion sponge borings played a critical role in the inversion process because they provided conduits through which the reactive diagenetic fluids could penetrate the dense gastropod skeleton each boring acted as a nucleus from which inversion fronts could spread into the shell as a result the radial fabric of the neomorphic calcite crystals around th sponge borings contrasts sharply with the mosaic of equant calcite that formed in areas devoid of sponge borings it seems that bioerosion is at least on a local scale as important as other factors in controlling the meteoric alteration of carbonate sediment the 18o values of neomorphic calcite 7 1 to 4 9 approximately 3 more depleted in 18o than the original aragonite confirm the role of meteoric water in the aragonite to calcite transformation 13c values of the diagenetic calcite 0 2 to 4 5 are similar to those in the original aragonite suggesting that the former was inherited from the latter

hydrogeology of grand cayman british west indies a karstic dolostone aquifer abstract on grand cayman freshwater bodies present in the bluff formation are typically small and occur as thin lenses floating on top of dense saline water evaluation of the water resource potential of these freshwater lenses is difficult because of their variable hydrological conditions complex paleohydrogeology and aquifer heterogeneity secondary porosity created by preferential dissolution of aragonitic fossil components is common open fissures and joints developed under tectonic stress and karst development associated with sea level fluctuations are however the two most important causes of porosity and permeability in the aquifers on grand cayman fracture and karst porosity control the lens occurrence by 1 acting as avenues for the intrusion of seawater or upward migration of saline water 2 acting as recharge focal points 3 enhancing hydrodynamic dispersion 4 defining lens geometry 5 facilitating carbonate dissolution along joints and fissures a clear understanding of the hydrological and geological conditions is important in developing small lenses in a setting similar to that on grand cayman this pragmatic approach can help identify the optimum location of the well field and avoid areas particularly susceptible to saline water intrusion

formation of peloids in plant rootlets grand cayman british west indies abstract spherical to elliptical peloids averaging 80 m in length occur inside plant rootlets that penetrate limestones of the pleistocene ironshore formation on grand cayman the peloids which are associated with desiccated fragments of mites are formed of minute calcite platelets and fragments of algal and or fungal filaments bound by organic rich material the calcite platelets are morphologically similar to platelets that occur in the epidermal cells of the surrounding rootlets the peloids probably originated as fecal pellets produced by mites which mined into the softer cores of the roots in so doing they recombined the calcite platelets algal fungal filaments and organic material into coprogeneous material subsequent diagenetic modification produced peloids formed of mic ite that are morphologically identical to peloids found in many caliche units

formation of australian continental margin highlands driven by plate mantle interaction passive margin highlands occur on most continents on earth and play a critical role in the cycle of weathering erosion and atmospheric circulation yet in contrast to the well developed understanding of collisional mountain belts such as the alps and himalayas the origin of less elevated 1 2 km passive margin highlands is still unknown the eastern australian highlands are a prime example of these plateaus but compared to others they have a well documented episodic uplift history spanning 120 million years we use a series of mantle convection models to show that the time dependent interaction of plate motion with mantle downwellings and upwellings accounts for the broad pattern of margin uplift phases initial dynamic uplift of 400 600 m from 120 80 ma was driven by the eastward motion of eastern australia s margin away from the sinking eastern gondwana slab followed by tectonic quiescence to about 60 ma in the south snowy mountains renewed uplift of 700 m in the snowy mountains is propelled by the gradual motion of the margin over the edge of the large pacific mantle upwelling in contrast the northernmost portion of the highlands records continuous uplift from 120 ma to present day totalling about 800 m the northern highlands experienced a continuous history of dynamic uplift first due to the end of subduction to the east of australia then due to moving over a large passive mantle upwelling in contrast the southern highlands started interacting with the edge of the large pacific mantle upwelling 40 50 million years later resulting in a two phase uplift history our results are in agreement with published uplift models derived from river profiles and the cretaceous sediment influx into the ceduna sub basin offshore southeast australia reflecting the fundamental link between dynamic uplift fluvial erosion and depositional pulses in basins distal to passive margin highlands

a uniformly processed data set of sks shear wave splitting measurements a global investigation of upper mantle anisotropy beneath seismic stations anisotropy in the earth s upper mantle is a signature of past and present deformation here we present a new data set of 50 000 uniformly processed sks shear wave splitting measurements that probe upper mantle anisotropy beneath seismic stations in the frequency band 0 02 0 1 hz the data set consists of measurements obtained at 2000 seismic stations from 2000 events we identify several stations characterized by an apparent absence of shear wave splitting so called null stations station averaged measurements are obtained by stacking shear wave splitting error surfaces the stacked data set shows excellent agreement with a compilation of previous sks measurements the average amount of splitting beneath seismic stations after error surface stacking is 0 8 s slightly lower than that found previously by vectorial averaging of non null measurement splitting parameters the data set disagrees however with an azimuthally anisotropic surface wave tomography model dkp2005 suggesting that caution should be exercised when using such models for geodynamic interpretation especially in continental regions studying our data set in detail we find evidence that flow in the asthenosphere exerts partial control over sks splitting in orogenic regions globally in the active orogenic environment of the western usa where we have the densest coverage our data suggest that shallow asthenospheric flow is guided by a wall of thick lithosphere to the east

reactive surface area of skeletal carbonates during dissolution effect of grain size abstract major advances have recently been made in quantifying the chemical parameters which control carbonate mineral reaction kinetics in natural waters one of the primary factors influencing rates of reaction is the amount of surface area per unit mass available for reaction for relatively smooth nonporous carbonates the relationship between reactive and total surface area measured by gas adsorption has proven simple however the dominant carbonates in the marine environment are biogenic and exhibit complex microstructures we have performed experiments to determine dinghy what proportion of the total surface area of these biogenic carbonates is actually available for reaction during dissolution three biogenic carbonates having widely differing microstructures halimeda coral and echinoid were chosen for study and comparison with rhombic calcite the influence of grain size on the relation between total surface area determined from gas adsorption and reactive surface area determined relative to rhombic calcite during dissolution in constant composition solutions was of primary concern unlike rhombic calcites total surface area for texturally complex biogenic grains varied slightly or not at all with increasing grain size however both rhombic calcite and biogenic carbonate dissolution rates were inversely related to grain radius this implies that much of the total surface area of biogenic carbonates is unavailable for reaction with the aqueous solution dissolution rates were a function of both grain size and microstructure we have defined an empirical roughness factor which quantifies the difference in reactive surface area between rhombic calcite and a given biogenic grain of equivalent size the roughness factor increased with grain microstructural complexity but was independent of grain size over the grain size range 51 513 microns studied the maximum roughness factor value observed was 7 for halimeda relative to a value of 1 for rhombic calcite our findings indicate that both microstructure and grain size can play important roles in controlling reactivity of biogenic carbonates during diagenesis the relative significance of grain size versus microstructure for carbonate dissolution rates in natural systems is discus ed

small scale sublithospheric continental mantle deformation constraints from sks splitting observations summary it is generally believed that azimuthal seismic anisotropy is an important property of the subcontinental mantle and that this is a relatively small scale phenomenon compared with anisotropy of the oceanic mantle during recent years shear wave splitting has come to be viewed as the most direct diagnostic of azimuthal anisotropy the magnitude of the splitting delay times can often rule out a crustal source of anisotropy however it is not yet clear whether the source of anisotropy lies within the lithosphere or the sublithosphere it is essential to know the scale of lateral variations and the distribution of anisotropy with depth in order to understand the origin of this phenomenon here we describe a way to constrain the depth of the anisotropy by studying the lateral variation of the splitting parameters at neighbouring seismic stations we use sks and skks phases recorded at the nars nl array a relatively dense network of broad band stations separated by about 50 km fresnel zones at different depths are calculated for these phases the depth of the anisotropy is constrained by the criterion that fresnel zones corresponding to different splitting observations should not overlap variability in the splitting measurements for events with different directions of approach recorded at one station and for single events recorded at various stations provides evidence for a non homogeneously distributed source of anisotropy located below the array at different depths we show that this variability in the measurements cannot be due to effects of crustal scattering our results indicate that the anisotropy is partly located in the sublithospheric mantle at about 400 km depth this suggests that the deformation in this region is subject to significant lateral variations

temporal and spatial variations in the diagenetic fabrics and stable isotopes of pleistocene corals from the ironshore formation of grand cayman british west indies abstract on grand cayman the unconformity bounded units a to f in the ironshore formation represent repeated cycles of deposition and subaerial exposure that were associated with the dramatic oscillations in sea levels that characterized the middle to late pleistocene montastrea annularis and acropora palmata the dominant corals found in units a to f in the rogers wreck point area and offshore george town area reflect these changes with their complex mosaics of marine aragonite cements bioerosion internal sediments and meteoric calcitization of coral skeleton dissolution diagenetic features the distribution of the diagenetic fabrics throughout this sequence however is not predictable the randomness in the degree of skeletal calcitization is shown by the lack of systematic variation in calcitization from units a to f and the fact that the degree of skeletal calcification in m annularis is higher than that in a palmata in the rogers wreck point area but less than in a palmata in the george town cores the stable isotope compositions 13c 18o of the corals from units a to f plot along the same trend line with their positions on that trend reflecting the amount of meteoric diagenesis they have undergone the variable styles and degrees of diagenesis evident in the corals from the ironshore formation reflect 1 the variable impact of intrinsic e g skeletal architecture porosity of coral and extrinsic e g sea level climate factors during each phase of diagenesis and 2 temporal effects associated with the diagenetic overprinting that took place during each successive phase of diagenesis this means that the diagenetic fabrics are not stratigraphically ordered and that it is impossible to correlate specific diagenetic phases with specific sea level lowstands or highstands

geochemical constraints on the origins of calcite cements and their impacts on reservoir heterogeneities a case study on tight oil sandstones of the upper triassic yanchang formation southwestern ordos basin china calcite cementation has been identified as an active process in the upper triassic yanchang formation throughout its burial history and as a major diagenetic factor causing strong reservoir heterogeneities the origins of calcite cements and their relevance to reservoir heterogeneities were investigated using a suite of petrographic and geochemical methods including optical microscopy with fluorescence and cathodoluminescence scanning and backscattered electron microscopy with energy dispersive spectrometry x ray diffraction x ray fluorescence electron probe microanalysis quantitative evaluation of minerals by scanning electron microscopy fluid inclusion analysis and carbon and oxygen stable isotope analyses the sandstones are compositionally immature with relatively high amounts of volcanic rock fragments the two generations of calcite cements are ca i and ca ii the ca i calcites are distributed along the interface of sandstone and mudstone units and were formed during the late triassic to early jurassic at formation temperatures of approximately 90 c the ca ii calcite mainly developed in the lower part of the fining upward sandstone units and was formed in the late jurassic at higher temperatures of approximately 110 c the origins of calcite cements were constrained by geochemical and isotope measurements fluid inclusion homogenization temperature and in situ element analysis the ca i calcite cement originated from dissolution of the lacustrine depositional carbonates in the interbedded mudstones and reprecipitation in the adjacent sandstones the ca ii calcite was mainly related to organic matter decarboxylation with ca2 having been provided internally by volcanic fragment alteration and plagioclase dissolution calcite cementation had caused strong reservoir heterogeneities in the yanchang formation tight sandstones the ca i calcite cementation destroyed reservoir properties along the interface of sandstones and mudstones the lower parts of the fining upward sandstone units were tightly cemented by ca ii calcite although they originally had high porosity and permeability the middle upper parts of the fining upward sandstone units contain less calcite cements and thus have better preserved reservoir pores because of oil emplacement inhibiting the calcite cementation processes

shear wave splitting continental keels and patterns of mantle flow in this study we investigated the origin of seismic anisotropy in the mantle beneath north america in particular we evaluated whether shear wave splitting patterns in eastern north america are better explained by anisotropy caused by lithospheric deformation anisotropy due to mantle flow beneath the lithosphere or a combination of both we examined new measurements of shear wave splitting from the missouri to massachusetts broadband seismometer array moma the north american mantle anisotropy and discontinuity experiment nomad as well as splitting parameters from several previous studies we developed a simple finite difference model that approximates mantle flow around a complex three dimensional continental lithospheric keel to evaluate potential anisotropy from mantle flow beneath the lithosphere in eastern north america we compared shear wave splitting observations to predicted splitting parameters calculated using this mantle flow model our results indicate that a significant portion of observed shear wave splitting in eastern north america can be explained by mantle flow around the continental keel however shear wave splitting patterns in a few regions of eastern north america indicate that a component of lithospheric anisotropy must exist particularly in regions containing the largest keel thicknesses for eastern north america as well as for splitting observations in australia europe and south america we favor a model in which anisotropy is controlled by a combination of both lithospheric deformation and subcontinental mantle flow

lithospheric fabric variations in central north america influence of rifting and archean tectonic styles 1 upper mantle fabric as detected by shear wave splitting measurements records lithospheric strain history that may be related to past tectonic events we present sks core refracted shear wave splitting measurements taken around the western great lakes north america the archean superior province sp and surroundings we analyze 40 sites in the usa and 5 in canada and outline a region of strong anisotropy corresponding to a high velocity zone in tomographic models which we dub the western superior mantle anomaly wsma and interpret to represent a preserved fabric from accretion of the sp to the southwest of the wsma we locate a region of rotated fast direction corresponding to a linear low velocity feature observed by tomography and which may represent a failed branch of the mid continent rift further south within the minnesota river valley terrane the split times drop to near zero suggesting isotropic lithosphere resulting from vertical diapiric rather than horizontal tectonic processes

coupled two dimensional diagenetic and sedimentological modeling of carbonate platform evolution we have used a new coupled two dimensional diagenetic and sedimentological model to investigate the evolution of a simple aggrading carbonate platform subject to glacio eustatic sea level fluctuations the model employs variable hydrological zones internally defined from platform exposure and climate to provide a framework within which diagenetic processes are simulated at specified rates simulations show a clear stacked sequence of diagenetic zones which are readily recognizable because of their lateral continuity and distinct trends in diagenetic evolution from platform interior to margin however spatial associations between unconformity surfaces and underlying diagenetic zones that suggest causality are misleading the interaction between high amplitude sea level fluctuations and subsidence results in substantial overprinting and a complex diagenetic history that could not be unraveled by traditional stratigraphic and sedimentological methods

geological constraints on the origin of the mantle root beneath the canadian shield cratonic north america is composed of a cluster of archaean microcontinents centred on the canadian shield and juvenile proterozoic crust that lies mainly buried beneath the sedimentary cover of the western and southern interior platforms the shield is underlain by an anomalous low temperature mantle root that is absent beneath the platform as there appears to be no systematic difference in crustal thickness or density between the shield and the platform the long lived arching of the shield implies an intrinsic buoyancy imparted by the mantle root that more than offsets its colder temperature isotopic and seismic anisotropy data indicate an archaean age for the mantle root close to the time of formation of the overlying crust the preferential development of the mantle root beneath archaean crust is consistent with an origin by imbrication of partly subducted slabs of highly depleted oceanic lithosphere assuming that buoyant subduction was more common in the archaean formation of the mantle root was not dependent on collisional orogenesis as has been suggested but the archaean cratonic mantle was sufficiently buoyant and refractory to survive later tectonic thickening the mantle root persists beneath archaean crust that was transected by mafic dyke swarms and subjected to short lived episodes of post orogenic crustal melting but the root is reduced at mantle plume initiation sites the partitioning of archaean and proterozic crust between the shield and the platform respectively causes the shield to misrepresent precambrian crust as a whole studies of the shield falsely conclude that a high percentage of precambrian crust formed in the archaean and that the proterozoic was characterized by epicontinental volcanism and sedimentation and crustal reworking furthermore the isotopic ratios of detritus eroded from the craton may tend to overestimate the mean age of continental crust

trans hudson orogen of north america and himalaya karakoram tibetan orogen of asia structural and thermal characteristics of the lower and upper plates 1 the trans hudson orogen tho of north america and the himalaya karakoram tibetan orogen hkto of asia preserve a paleoproterozoic and cenozoic record respectively of continent continent collision that is notably similar in scale duration and character in tho the tectonothermal evolution of the lower plate involves 1 early thin skinned thrusting and barrovian metamorphism 2 out of sequence thrusting and high t metamorphism and 3 fluid localized reequilibration anatexis and leucogranite formation the crustal evolution of the indian lower plate in hkto involves 1 early subduction of continental crust to ultrahigh pressure uhp eclogite depths 2 regional barrovian metamorphism and 3 widespread high t metamorphism anatexis and leucogranite formation the shallow depths of the high t metamorphism in hkto are consistent with early to mid miocene ductile flow of an indian lower plate midcrustal channel from beneath the southern tibetan plateau to the greater himalaya melt weakening of the lower plate in tho is not observed at a similar scale probably due to the paucity of pelitic lithologies tectonothermal events in the upper plate of both orogens include precollisional accretion of crustal blocks emplacement of andean type plutonic suites and high t metamorphism syncollisional to postcollisional events include emplacement of garnet biotite muscovite leucogranites anatectic granites and sporadic metamorphism up to 90 myr following the onset of collision in tho comparing the type and duration of tectonothermal events for tho and hkto supports the notion of tectonic uniformitarianism for at least the later half of dated earth history and highlights the complementary nature of the rock record in an older exhumed orogen compared to one undergoing present day orogenesis

variability and origin of seismic anisotropy across eastern canada evidence from shear wave splitting measurements measurements of seismic anisotropy in continental regions are frequently interpreted with respect to past tectonic processes preserved in the lithosphere as fossil fabrics models of the present day sublithospheric flow often using absolute plate motion as a proxy are also used to explain the observations discriminating between these different sources of seismic anisotropy is particularly challenging beneath shields whose thick 200 km lithospheric roots may record a protracted history of deformation and strongly influence underlying mantle flow eastern canada where the geological record spans 3 ga of earth history is an ideal region to address this issue we use shear wave splitting measurements of core phases such as sks to define upper mantle anisotropy using the orientation of the fast polarization direction and delay time t between fast and slow shear wave arrivals comparison with structural trends in surface geology and aeromagnetic data helps to determine the contribution of fossil lithospheric fabrics to the anisotropy we also assess the influence of sublithospheric mantle flow via flow directions derived from global geodynamic models fast polarization orientations are generally ene wsw to ese wnw across the region but significant lateral variability in splitting parameters on a 100 km scale implies a lithospheric contribution to the results correlations with structural geologic and magnetic trends are not ubiquitous however nor are correlations with geodynamically predicted mantle flow directions we therefore consider that the splitting parameters likely record a combination of the present day mantle flow and older lithospheric fabrics consideration of both sources of anisotropy is critical in shield regions when interpreting splitting observations

malacological characteristics of the middle to upper pleistocene transitional interval mis 7 5 observed in the batajnica locality serbia abstract the batajnica loess section is one of the most complete middle and late pleistocene records known in the vojvodina region serbia the main aim of the malacological investigations was to study the latest middle pleistocene cold interval and to see how the mollusc successions reacted to the climate changes during the cold warm transition the deposits which belong to the upper part of pedocomplex v s2 v l2 loess and partly to the v s1 soil were sampled the penultimate glacial loess v l2 of the batajnica site corresponds to the cold interval marine isotope stage mis 6 the molluscs indicate moderate or sometimes warm climate and dry or moderate steppe like environment composed of adjacent mosaics of drier and humid habitats the mollusc complexes of v s2 soil mis 7c v l2 loess mis 6 and v s1 paleosol mis 5e show almost the same species composition which suggests the existence of a comparatively moderate or warm climate during the late middle pleistocene cold interval in contrast the mollusc complexes mis 6 from western and eastern europe have less species and few individuals compared with the batajnica section comparison with the late middle pleistocene mollusc associations from the northern and southern margins of the fruska gora mountain confirms its environmental originality the northern side of the mountain was characterized by a humid and relatively colder environment than the other areas of the vojvodina region batajnica irig and ruma the mollusc fauna obtained in the batajnica site suggests that this adjacent area represented a refugium for the thermophilous and arid tolerant species where they were able to survive during the cold periods of the pleistocene and thus confirms the hypothesis of p sumegi

calcareous crusts on exposed pleistocene limestones a case study from grand cayman british west indies abstract calcareous crusts which include laminar up to 6 cm thick and non laminar calcretes have developed on ooid grainstones and skeletal wacke pack grain floatstone in the pleistocene ironshore formation exposed at morgan s harbour tarpon springs and rogers wreck point on grand cayman british west indies non laminar calcretes which consist largely of micrite with variably altered skeletal fragments and lithoclasts derived from the host limestone contain peloids in their basal parts in contrast the laminar calcretes are formed of porous laminae and low porosity laminae calcified roots are common in the porous laminae but rare in the low porosity laminae the 18o values of calcretes from 5 22 to 0 16 have a positive correlation with the 13c values that vary from 10 03 to 6 41 at tarpon springs from 11 57 to 5 43 at morgan s harbour and from 9 89 to 8 68 at rogers wreck point the non laminar calcretes reflect progressive micritization of the bedrock with a basal peloidal zone that passes upward into the micrite zone in contrast laminar calcretes are attributed to accretionary build up dictated by microscale processes that were controlled largely by the interplay of plant roots and their associated microorganisms the positive correlation between the 18o and 13c values from the calcareous crusts indicates that evaporation influenced calcite precipitation the 13c values of the laminar calcretes record a change from vegetation dominated by c3 plants that grew in a cool wet climate to a c3 c4 vegetation that grew in a hotter drier climate the cayman calcretes resemble those found on the pleistocene miami limestone in florida and highlight the possibility that these calcareous crusts may reflect regional climates and may therefore enable regional stratigraphic correlation

preserved and modified mid archean crustal blocks in dharwar craton seismological evidence abstract we report significant lateral variability in shear wave velocity and moho depth in the archean crust beneath the dharwar craton using earthquake waveform data recorded over 50 broadband seismographs the craton is a continuously exposed archean continental fragment divided into the west dharwar craton wdc of age 2 7 3 36 ga and the east dharwar craton edc of age dominantly 2 5 ga the craton progressively transitions into the southern granulite terrain sgt with age of metamorphism around 2 6 ga the inversion and modeling of receiver function data reveal significant variation of moho depth viz 38 54 km in the wdc and 40 46 km in the sgt and 32 38 km in the edc the average shear wave velocity vs of crust beneath the wdc is 3 85 km s as compared to 3 6 km s in the edc we infer highly variable thickness 16 30 km of mafic cumulate vs 4 0 km s and vp 7 0 km s beneath the wdc in contrast with a thin one 50 km as compared to felsic to intermediate composition for the edc crust with almost flat moho down to 38 km these results suggest preserved mafic crustal root beneath the middle archean terrain in the wdc that has remained inert since then on the other hand felsic to intermediate composition of crust with a nearly flat moho beneath the late archean edc could be a consequence of regional delamination of lower crust the preserved distinct moho topography across the archean terrains suggests it as compositional boundary considering the surface exposure of 15 20 km crust based on p t condition in the granulite segment of the wdc we speculate a himalaya like crustal thickness 50 70 km beneath the middle archean crust pointing toward a plate tectonic like scenario at 3 0 ga

substrate destruction and sediment production by the boring sponge cliona caribbaea on grand cayman island abstract the boring sponge cliona caribbaea is ubiquitous on the shallow terraces around grand cayman island british west indies occurring as brown sheets covering and growing over both live corals and hard substrate throughout the zone of the radial grooves on the southwest corner of grand cayman c caribbaea covered 5 of the total bottom area between 1 and 10 m values up to 10 occurred commonly lateral extension rates were 4 cm yr the sponge removes an average of 20 up to 45 of the substrate which has an average specific gravity of 1 7 conservative estimation of typical rates of substrate removal yields 8 kg m2 yr a value of about double the typical rates of reef calcification the silt produced by the sponge is rapidly transported downslope

the archean murmac bay group evidence for a giant archean rift in the rae province canada abstract the lower amphibolite to granulite facies archean murmac bay group of the southwestern rae province canada formed during continental extension rifting of an underlying granitoid basement complex that includes the 3060 40 ma lodge bay granite and the 2999 7 ma cornwall bay granite the contact between the murmac bay group and the basement complex is an unconformity that is locally marked by polymictic conglomerate overlying the conglomerate is basal quartzite of the murmac bay group that contains a laterally extensive 10 km intraformational oligomictic quartzite pebble conglomerate that may have formed by uplift during rifting distinctive medium to coarse grained gabbro dykes and sills that cut both the basement complex and the basal quartzite are interpreted as feeders to stratigraphically overlying tholeiitic basalt flows that have geochemical signatures consistent with formation in a continental rift setting the quartzite basalt contact exhibits rare stratigraphic interlayering and is a horizon along which oxide and silicate facies iron formation dolostone and pelite locally occur ultramafic rocks with komatiitic compositions also intrude these lower stratigraphic units the upper murmac bay group is dominated by pelitic to psammitic rocks that may represent deepening of the sedimentary basin during continued extension ancient approximately 3 62 and 3 7 ga grains dominate the detrital zircon population of the murmac bay group sediments but deposition occurred after approximately 2 77 ga the youngest detrital zircon in our study the age of the murmac bay group is thus constrained as younger than approximately 2 77 ga and older than the crosscutting 2631 12 ma dead man and donaldson lake granites the gross lithological and stratigraphic make up of the murmac bay group closely resembles that of the prince albert woodburn lake and mary river groups of the northern rae province all three of which have been interpreted as approximately 2 7 ga continental rift successions we propose that the murmac bay group is correlative with these successions if correct the total length of this neoarchean rift is at least 2000 km extending from baffin island to northwestern saskatchewan canada the supracrustal rocks of the rift record the break up of a large paleo to early neoarchean continent nunavutia which experienced magmatic events at approximately 3 0 3 62 and 3 7 ga

3 d shear wave radially and azimuthally anisotropic velocity model of the north american upper mantle summary using a combination of long period seismic waveforms and sks splitting measurements we have developed a 3 d upper mantle model sawum na2 of north america that includes isotropic shear velocity with a lateral resolution of 250 km as well as radial and azimuthal anisotropy with a lateral resolution of 500 km combining these results we infer several key features of lithosphere and asthenosphere structure a rapid change from thin 70 80 km lithosphere in the western united states wus to thick lithosphere 200 km in the central cratonic part of the continent closely follows the rocky mountain front rmf changes with depth of the fast axis direction of azimuthal anisotropy reveal the presence of two layers in the cratonic lithosphere corresponding to the fast to slow discontinuity found in receiver functions below the lithosphere azimuthal anisotropy manifests a maximum stronger in the wus than under the craton and the fast axis of anisotropy aligns with the absolute plate motion as described in the hotspot reference frame hs3 nuvel 1a in the wus this zone is confined between 70 and 150 km decreasing in strengthwithdepthfromthetop fromthermftothesanandreasfaultsystemandthejuande fuca gorda ridges this result suggests that shear associated with lithosphere asthenosphere couplingdominatesmantledeformationdowntothisdepthinthewesternpartofthecontinent the depth extent of the zone of increased azimuthal anisotropy below the cratonic lithosphere is not well resolved in our study although it is peaked around 270 km a robust result radial anisotropy is such that predominantly 1 where vsh vsv 2 under the continent and its borders down to 200 km with stronger in the bordering oceanic regions across the continent and below 200 km alternating zones of weaker and stronger radial anisotropy with predominantly 1 correlate with zones of small lateral changes in the fast axis direction of anisotropy and faster than average vs below the lab suggesting the presence of small scale convection with a wavelength of 2000 km finally in the western united states complex 3 d patterns of isotropic velocity and anisotropy reflect mantle dynamics associated with the rich tectonic history of the region

sub slab mantle anisotropy beneath south central chile knowledge of mantle flow in convergent margins is crucial to unravelling both the contemporary geodynamics and the past evolution of subduction zones by analysing shear wave splitting in both teleseismic and local arrivals we can determine the relative contribution from different parts of the subduction zone to the total observed sks splitting providing us with a depth constraint on anisotropy we use this methodology to determine the location orientation and strength of seismic anisotropy in the south central chile subduction zone data come from the tipteq network deployed on the forearc during 2004 2005 we obtain 110 teleseismic sks and 116 local good quality shear wave splitting measurements sks average delay times are 1 3 s and local s delay times are only 0 2 s weak shear wave splitting from local phases is consistent with a shape preferred orientation spo source in the upper crust we infer that the bulk of shear wave splitting is sourced either within or below the subducting nazca slab sks splitting measurements exhibit an average north easterly fast direction with a strong degree of variation further investigation suggests a relationship between the measurement s fast direction and the incoming ray s back azimuth finite element geodynamic modelling is used to investigate the strain rate field and predicted lpo characteristics in the subduction zone these models highlight a thick region of high strain rate and strong s wave anisotropy with plunging olivine a axes in the sub slab asthenosphere we forward model the sub slab sourced splitting with a strongly anisotropic layer of thick asthenosphere comprising an olivine a axis oriented parallel to the direction of subduction the subducting lithosphere is not thick enough to cause 1 2 s of splitting therefore our results and subsequent models show that the nazca slab is entraining the underlying asthenosphere its flow causes it to be strongly anisotropic our observation has important implications for the controlling factors on sub slab mantle flow and the movement of asthenospheric material within the earth

forward modeling of bank margin carbonate diagenesis we have applied forward modeling to investigate deposition and early diagenesis resulting from the interaction of high frequency glacioeustatic sea level fluctuation with bank margin carbonates in a subsiding basin the model output yields lithologies predicted by the static model but the relation of these lithologies to any particular subaerial exposure surface is obscured by the complexity of glacioeustatic sea level history the geologic history of our synthetic stratigraphic sequences could not be unraveled by traditional stratigraphic and sedimentologic study methods forward modeling is the preferred approach to analysis of carbonate sequences

precambrian crustal evolution seismic constraints from the canadian shield whether or not plate tectonic processes operated on a younger hotter earth remains ambiguous seismic data from new networks in the hudson bay region of the canadian shield where the precambrian geological record spans more than 2 billion years offer fresh scope to address this problem using receiver function analyses we show that the crust of the rae domain which exhibits ages of paleo to neoarchean 3 9 2 7 ga is likely felsic to intermediate in composition average vp vs 1 73 and seismically transparent with a sharp moho there is little evidence for modern style plate tectonics and based on the simplicity and spatial extent of the felsic crust models favouring vertical tectonic processes such as crustal delamination or plume activity appear better suited to the results data from the hearne domain which exhibits widespread 2 7 ga granite and greenstone geology show a more complex crust with higher vp vs ratios consistent with a greater mafic component the trans hudson orogen tho proposed to be a himalayan scale mountain belt during the paleoproterozoic is thought to have formed during the 1 8 ga collision of the superior and churchill plates results from the quebec baffin island segment of the tho appear to map out the first order shape of the underthrusting superior plate with elevated vp vs ratios likely representing the rifted margin of the superior craton consistently thicker crust is observed beneath central and southern baffin island 43 km coincident with widespread high grade metamorphic surface geology these features can be explained by crustal thickening due to stacking of accreted terranes during continent continent collision analogous to the present day tibetan plateau followed by erosion when reviewed in light of age and compositional constraints from the geological record our seismic observations point towards secular crustal evolution from non plate tectonic during the paleo to mesoarchean evolving towards fully developed modern style plate tectonics during the paleoproterozoic

rare earth element geochemistry of shallow carbonate outcropping strata in saudi arabia application for depositional environments prediction abstract the interpretation of depositional environments provides important information to understand facies distribution and geometry the classical approach to interpret depositional environments principally relies on the analysis of lithofacies biofacies and stratigraphic data among others an alternative method based on geochemical data chemical element data is advantageous because it can simply reproducibly and efficiently interpret and refine the interpretation of the depositional environment of carbonate strata here we geochemically analyze and statistically model carbonate samples n 156 from seven sections of the arab d reservoir outcrop analog of central saudi arabia to determine whether the elemental signatures major trace and rare earth elements rees can be effectively used to predict depositional environments we find that lithofacies associations of the studied outcrop peritidal to open marine depositional environments possess altered ree signatures and that this trend increases stratigraphically from bottom to top which corresponds to an upward shallowing of depositional environments the relationship between rees and major minor and trace elements indicates that contamination by detrital materials is the principal source of rees whereas redox condition marine and diagenetic processes have minimal impact on the relative distribution of rees in the lithofacies in a statistical model factor analysis and logistic regression rees major and trace elements cluster together and serve as markers to differentiate between peritidal and open marine facies and to differentiate between intertidal and subtidal lithofacies within the peritidal facies the results indicate that statistical modelling of the elemental composition of carbonate strata can be used as a quantitative method to predict depositional environments and regional paleogeography the significance of this study lies in offering new assessments of the relationships between lithofacies and geochemical elements by using advanced statistical analysis a method that could be used elsewhere to interpret depositional environment and refine facies models

petrography and geochemistry of the permian triassic boundary interval yangou section south china implications for early griesbachian seawater δ 13 c dic gradient with depth abstract the carbon isotopic composition 13 c carb recorded in shelf carbonates has been widely used as a proxy for the isotopic composition 13 c dic of surface ocean water to establish paleocean chemistry and circulation patterns however 13 c carb values do not necessarily preserve the 13 c dic due to post depositional diagenetic alteration in order to examine the early griesbachian surface to deep 13 c dic gradient with depth the diagenetic features of the permian triassic boundary interval beds 18 to 35 from yangou section located in the yangtze carbonate platform interior south china are delineated to compare with those of the slope gssp meishan section the petrographic and geochemical observations show that the early griesbachian carbonates in the yangou section underwent pervasive dolomitization in its early diagenetic history three types of early replacement dolomites and one type of dolomite cement are present the dolomite crystals display internal zonation with high ca calcian dolomite hcd core being encased successively by calcite and an outermost fe rich hcd cortex the initial dolomitization took place in anoxic seawater and underwent subsequent diagenetic system involved with meteoric water the two most negative 13 c values in claystones of beds 21 3 and 35 are probably related to meteoric diagenesis above and or below the meteorically influenced beds the dolomite and calcite have uniformly positive 13 c values the primary carbon isotopic compositions are probably preserved in the early griesbachian carbonate from the platform yangou section which could probably be related to the poor formation of the outermost fe rich hcd cortex compared to the slope carbonate from the meishan section the platform carbonate from the yangou section has lower primary 13 c carb values it is estimated that the 13 c dic gradient with depth between yangou and meishan is less than the previously suggested the results highlight the need for evaluation of local 13 c carb as record of 13 c dic in paleoseawater and carry important implications for understanding the permian triassic carbonate successions throughout the world

middle pleistocene cold stage climates in the mediterranean new evidence from the glacial record abstract a major reduction in summer temperatures during a middle pleistocene glacial cycle caused the most extensive glaciation recorded in the mediterranean region glaciers in the mountains of greece formed during marine isotope stage mis 12 474 000 427 000 years bp under climatic conditions characterised by mean summer temperatures at least 11 c cooler than today and annual precipitation of 2300 mm at the equilibrium line altitude 1741 m a s l this represents the coldest mean summer temperatures recorded in greece during at least the last 430 000 years later pleistocene glaciations mis 6 and 5d 2 were characterised by warmer summer temperatures and higher annual precipitation

magnetotelluric investigations of the lithosphere beneath the central rae craton mainland nunavut canada new magnetotelluric soundings at 64 locations throughout the central rae craton on mainland nunavut constrain 2 d resistivity models of the crust and lithospheric mantle beneath three regional transects responses determined from colocated broadband and long period magnetotelluric recording instruments enabled resistivity imaging to depths of 300 km strike analysis and distortion decomposition on all data reveal a regional trend of 45 53 but locally the geoelectric strike angle varies laterally and with depth the 2 d models reveal a resistive upper crust to depths of 15 35 km that is underlain by a conductive layer that appears to be discontinuous at or near major mapped geological boundaries surface projections of the conductive layer coincide with areas of high grade archean metasedimentary rocks tectonic burial of these rocks and thickening of the crust occurred during the paleoproterozoic arrowsmith 2 3 ga and trans hudson orogenies 1 85 ga overall the uppermost mantle of the rae craton shows resistivity values that range from 3000 m in the northeast beneath baffin island and the melville peninsula to 10 000 m beneath the central rae craton to 50 000 m in the south near the hearne domain near vertical zones of reduced resistivity are identified within the uppermost mantle lithosphere that may be related to areas affected by mantle melt or metasomatism associated with emplacement of hudsonian granites a regional decrease in resistivities to values of 500 m at depths of 180 220 km increasing to 300 km near the southern margin of the rae craton is interpreted as the lithosphere asthenosphere boundary

petrographic and geochemical features of sinkhole filling deposits associated with an erosional unconformity on grand cayman abstract on grand cayman exposures of dolostones belonging to the cayman formation miocene represent an erosional unconformity that has been developing since the late pliocene 3 6 ma sinkholes that developed during this time have remained open or become partly to fully filled with various combinations of rootcrete breccias loose limestone and dolostone lithoclasts and white red and orange limestones these sinkhole filling deposits have different geochemical attributes to the neogene and pleistocene marine carbonates that form the bedrock of the island the deposits in the sinkholes formed in response to the variations of sea level climate exposure and vegetation that developed during the period when the erosional unconformity was developing the rootcrete oncoids red and orange limestones are terrestrial in origin whereas the limestone and dolostone lithoclasts and white limestones are derived from marine deposits on the erosional unconformity intense root activity led to the formation of rootcrete and terrestrial oncoids but also selectively blackening reworked marine carbonates the red and orange limestone matrices which formed under more arid conditions contrast with the other sinkhole filling deposits that formed during periods when the climate was semi arid to humid the distinctive ree signatures of the sinkhole filling deposits which are different from those of the bedrock limestones and dolostones can probably be attributed to trace amount of terra rossa and or airborne saharan derived dust that are present in those deposits

diagenesis in pleistocene 80 to 500 ka corals from the ironshore formation implications for paleoclimate reconstruction abstract fossil corals provide a unique opportunity for paleoclimate research because they can be dated and they incorporate climate controlled seawater proxies into their aragonitic skeletons during growth coral skeletons however can only produce reliable environmental information if they have not been diagenetically altered an issue that becomes increasingly probable as the age of the corals increase although commonly used analytical techniques such as thin section scanning electron microscopy and x ray diffraction can highlight obvious mineralogical changes subtle changes in the trace element and isotopic composition of the skeletons may not be evident this premise is assessed by examining orbicella annularis and montastrea cavernosa corals from the pleistocene ironshore formation units a f on grand cayman 80 500 ka to determine the diagenetic effects on both physical and chemical properties of corals in the ironshore formation the corals in units a and c were altered in a semi open diagenetic system by seawater diluted by meteoric water which resulted in chemical diagenesis that changed the elemental and isotopic composition of the corals even though their aragonitic skeletons show no obvious signs of change in contrast the limestones in unit b have been pervasively recrystallized in an open diagenetic system by meteoric waters that caused both chemical and physical changes the corals in units d to f however underwent minimal diagenesis and show no evidence of alteration for altered corals from the ironshore formation elevated mg ca and low sr ca ratios provide evidence of subtle chemical diagenetic changes even in corals that have retained their primary aragonitic skeleton thus corals that are formed of 95 wt aragonite show no evidence of cementation and have mg ca ratios 8 0 mmol mol 1 18o values 5 7 and 13c values 3 0 are ideal candidates for paleoclimate reconstruction adoption of these criteria shows that the corals from units d to f can be used to calculate sea surface temperatures sst whereas those from units a to c cannot be used for this purpose the calculated sst and the corresponding temperature profiles derived from the corals in units d to f are consistent with global sst reconstructions during the pleistocene unit d corals record an overall increase in sst which is consistent with elevated temperatures associated with marine isotope stage mis 5e whereas units e and f record overall cooling which is consistent with the end of the peak interglacials of mis 5c and 5a respectively

meteoric diagenesis marine diagenesis and microporosity in pleistocene and oligocene limestones enewetak atoll marshall islands abstract microporosity occurs in pleistocene and oligocene limestones cored on enewetak atoll in cores of pleistocene limestone 10 80 m deep mi microporosity is present in aragonitic skeletal grains that underwent partial intrafabric dissolution chalkification leaving a porous meshwork of aragonite needles microporous aragonite is most abundant in 5 10 m thick intervals below zones of intense diagenesis associated with paleo freshwater lenses extensive microporosity was apparently generated by slow intrafabric dissolution of aragonite in mixing zones below freshwater lenses microporous aragonite is an intermediate step in the calcitization or dissolution of aragonite most calcitized aragonite neomorphic calcite formed by precipitation of sparry calcite in over microporous aragonite aragonitic fossils were not converted to microporous microcrystalline calcite therefore data from the pleistocene of enewetak suggest that widespread microporous microcrystalline calcite does not form directly from pure aragonite at least not in open meteoric systems or mixing zones microporous micrite occurs in oligocene wackestones 930 m deep deposited in a slope environment the microporous micrite is characterized by equant and prismatic crystals of low magnesium calcite lmc equant crystals are anhedral to euhedral and 2 10 m in diameter prismatic calcite crystals are subhedral 5 10 m wide and 20 50 m long stable isotopic and trace element geochemistry suggest that microporous micrite formed by diagenetic alteration of high magnesium calcite hmc micrite in deep seawater undersaturated with respect to hmc but supersaturated with respect to lmc probably near the sediment water interface mineralogy and crystal morphology of the microporous aragonite is very different from microporous shelf limestones in the thamama group cretaceous persian gulf see moshier 1989 equant microcrystals of calcite in the oligocene micrites are similar to those in microporous parts of the thamama group widespread microporosity in lmc limestones could form by alteration of hmc micrite in mixing zones or in seawater undersaturated with respect to hmc but supersaturated with respect to lmc

precambrian plate tectonics seismic evidence from northern hudson bay canada the canadian shield is one of the largest exposures of precambrian rocks on earth it is a mosaic of several archean terranes that were brought together during a series of paleoproterozoic orogens culminating in the so called trans hudson orogen which is thought to have been similar to the himalayan orogen in scale and nature the tectonic evolution and lithospheric subdivisions of this region are poorly understood but new seismic networks in northern hudson bay provide fresh opportunity to place constraints on the precambrian processes that formed and shaped it we show via a study of seismic anisotropy that the lithosphere of the northern hudson bay region retains a strong signature of archean paleoproterozoic tectonics we map the boundary between the upper churchill and lower superior plates that collided ca 1 8 ga and identify back azimuth dependent splitting parameters t on baffin island that indicate complex anisotropy e g dipping fabric beneath the region our results support the view that significant lithospheric deformation occurred during the paleoproterozoic and that modern day plate tectonic processes were thus in operation by at least ca 1 8 ga

deformation and mantle flow beneath the sangihe subduction zone from seismic anisotropy abstract subduction of oceanic lithosphere is the most direct feedback between the earth s surface and deep interior however the detail of its interaction with the broader convecting mantle is still unclear mantle flow around subduction zones can be constrained using seismic anisotropy but despite many such studies a simple global picture is lacking the sangihe subduction zone where the molucca sea microplate is subducting westward beneath the eurasian plate is part of the tectonically complex sulawesi philippine region and an ideal natural laboratory to study complex subduction processes we investigate the anisotropic structure of the sangihe subduction zone with shear wave splitting measurements of local s and sks phases at two stations mni in sulawesi dav in the philippines as well as downgoing s phases at five stations at teleseismic distances combining different phases allows a better vertical resolution of anisotropic fabrics than is possible with a single phase the broad depth distribution of local events 60 630 km allows us to observe a change in splitting behaviour at 380 km depth above fast directions are trench parallel and delay times t are 0 34 0 53 s with no increase with depth we suggest this anisotropy is caused by aligned cracks possibly melt filled beneath the volcanic arc and fossil anisotropy in the overriding plate below 380 km is predominantly trench normal and t are slightly higher 0 53 0 65 s as no correlation is observed with inferred distance travelled inside the slab we attribute this anisotropy to shear layers atop the slab which are coherent from 200 to 400 km depth and perhaps extend into the transition zone sks and source side measurements show larger t 1 53 and 1 33 s respectively and trench parallel since these phases predominantly sample sub slab mantle we consider along strike lateral flow associated with the double sided subduction of the molucca sea microplate to be the most likely explanation we thus infer three dominant regions of anisotropy at the sangihe subduction zone one within the overriding lithosphere one along the slab wedge interface and one below the subducting molucca sea slab the mantle wedge above 200 km depth and the slab itself do not seem to contribute notably to the measured anisotropy this study demonstrates the insight seismic anisotropy can provide into mantle dynamics even in tectonically complex subduction systems

highstands during marine isotope stage 5 evidence from the ironshore formation of grand cayman british west indies abstract the ironshore formation on grand cayman is formed of six unconformity bounded packages units a f units a b c and d known from the subsurface in the northeastern part of grand cayman formed during marine isotope stages mis 11 9 7 and 5e respectively unconformities at the tops of units a b and c are highlighted by terra rossa and or calcrete layers strata in core obtained from wells drilled in george town harbour and exposed on the west part of grand cayman belong to unit d and the newly defined units e and f corals from unit e yielded th u ages of 104 ka whereas conch shells from unit f gave ages of 84 ka unit e equates to mis 5c whereas unit f developed during mis 5a th u dating of corals and conchs from the ironshore formation on the western part of grand cayman shows that unit d formed during the mis 5e highstand whereas units e and f developed in association with highstands at 95 110 ka mis 5c and 73 87 ka mis 5a unit e 5 m thick in the offshore cores is poorly represented in onshore exposures unit f which unconformably overlies unit d at most localities is formed largely of fossil poor cross bedded ooid grainstones the unconformity at the top of unit d a marine erosional surface with up to 2 5 m relief is not characterized by terra rossa or calcrete in the offshore cores or onshore exposures unit d formed with a highstand of 6 m asl whereas units e and f developed when sea level was 2 to 5 asl and 3 to 6 m asl respectively thus the highstands associated with mis 5e 5c and 5a were at similar elevations

moment tensors state of stress and their relation to post glacial rebound in northeastern canada summary in the stable craton of northeastern canada moderate seismicity as well as glacial isostatic adjustment gia has been observed we investigate five earthquakes with moment magnitudes between 3 6 and 4 1 that have occurred in northern hudson bay since 2007 which may be triggered by gia focal mechanisms of the earthquakes are determined using a waveform fitting procedure for surface waves in which the best double couple mechanism is obtained through a grid search over strike dip and rake all events exhibit a thrust fault mechanism in general agreement with mechanisms of previously analysed earthquakes in that area stress inversion results incorporating our new mechanisms as well as previously published results exhibit significant differences from previously calculated stresses from gia models for the first time maximum horizontal stress direction shmax is compared to the modelled stress values for northeastern canada we find that the maximum horizontal stress direction strikes roughly nnw sse in contrast to previously inferred ne sw orientation of shmax our results indicate that the local stress field in northern hudson bay deviates from regional stresses resulting from the existence of a roughly e w oriented zone of faulting which is not included in existing gia models

deep mantle contributions to the surface dynamics of the north american continent abstract the regional and continental scale dynamics of north america and vicinity are explored using a high resolution model of mantle flow the model is constrained by simultaneously inverting global seismic and mantle convection data sets and it includes an explicit treatment of the positive chemical buoyancy of the continental tectosphere moreover it adopts a depth dependent mantle viscosity structure which reconciles both glacial isostatic adjustment gia and convection data the flow model successfully reproduces plate velocities and observations of surface gravity and topography including the continent scale quasi linear depression after corrections for gia and crustal heterogeneity extending from northern alaska to venezuela the predictions also match lithospheric flow and stress fields inferred from local and regional measurements of seismic anisotropy and surface deformation we demonstrate that these signals are largely driven by viscous flow coupled to density anomalies in the deep portions of the upper mantle and within the lower mantle where the latter may be associated with the descent of the ancient kula farallon plate system and an active mantle upwelling below the pacific margin of the north american plate more importantly the flow calculations elucidate how these large scale heterogeneities give rise to regional scale flow and stress patterns below the southwestern u s and below the central u s

isotope signatures associated with early meteoric diagenesis the environments in which carbonate diagenesis proceeds have been documented in previous studies of holocene and late pleistocene sediments and limestones on barbados west indies variations in the carbon and oxygen isotopic composition of limestones produced during early freshwater diagenesis have been observed in this study to occur in specific patterns six potentially useful patterns emerge when one views stable isotope data within a stratigraphic framework 1 the subaerial exposure surface is characterized by strongly 12c enriched limestones 13c compositions of underlying limestones grow progressively heavier with increasing depth 2 the subaerial exposure surface may also be marked by slight 18o enrichment 3 an abrupt shift in 18o values may differentiate sediments above the exposure surface from those below 4 sediments altered in the marine meteoric mixing zone may be characterized by positive covariance between their 18o and 13c compositions 5 the vadose phreatic boundary may be marked by a sharp increase in 13c values in the seaward portions of a fresh groundwater system and 6 samples contemporaneously altered in a single fresh groundwater system within an areally restricted region should display a narrow range of 18o and a wide range of 13c compositions analysis of samples from five palaeozoic and mesozoic formations which contained petrographic evidence of early freshwater diagenesis showed that isotope patterns similar to those observed in barbados limestones have been preserved in rocks as old as mississippian these isotope patterns could prove to be useful for identifying diagenetically induced porosity trends in carbonate rocks they might be used to identify limestones diagenetically altered in meteoric environments to identify mixing zone cements and dolomites and to trace the regional and vertical distributions of early meteoric groundwater systems in ancient carbonate formations

spatial and temporal constraints on sources of seismic anisotropy evidence from the scottish highlands 1 routine sks splitting analyses of seismic anisotropy usually suffer from limitations in station density that hinder attempts to place spatial constraints on anisotropic fabrics data from a 20 km station spacing network in scotland show that splitting parameters vary considerably t 0 45 1 5 s 49 128 over short 10 20 km length scales improved spatial constraints then lead to tighter temporal constraints on the anisotropic sources asthenospheric fabrics due to tertiary rifting and present day plate motions do not strongly influence our results that instead correlate with lithospheric scale trends inferred from surface geology splitting observations track scotland s tectonic history from the precambrian emplacement of crustal basement and through the activation of large scale faulting and thrusting during the caledonian orogeny the shallow lithosphere beneath scotland has preserved a fossil anisotropic signature up to hundreds of millions of years after it was formed

diagenesis in limestone dolostone successions after 1 million years of rapid sea level fluctuations a case study from grand cayman british west indies abstract meteoric diagenesis in young marine carbonate sediments has commonly been linked to fluctuations in quaternary glacio eustatic sea levels the extent to which these sea level changes are recorded in these carbonate successions however remains questionable this is amply demonstrated by the diagenetic record found in the limestones and dolostones of the cayman formation miocene on the cayman islands on the eastern part of grand cayman dolomitization that ceased by 1 million years ago created an architecture whereby the limestones in the central part of the island were surrounded by dolostones in coastal areas of the island since then the upper 90 m of the cayman formation has been repeatedly cycled through many different marine and meteoric diagenetic zones as large rapid eustatic oscillations in sea level affected the island the records of these diagenetic cycles in the dolostones and limestones are however different and impossible to match to the cyclic changes in sea level in the peripheral dolostones post dolomitization diagenetic features are sparse in contrast the limestones in the interior of the island exhibit a wider variety of meteoric diagenetic features including extensive dissolution and calcite cementation the dolostones have low porosity

diagenetically altered sabkha type pleistocene dolomite from the arabian gulf diagenetically altered pleistocene dolomite occurs in the shallow subsurface of the arabian gulf offshore of al jubayl saudi arabia this dolomite accumulated in relatively shallow marine to sabkha depositional environments in contrast with the thin extent of most other quaternary sabkha and sabkha related dolomite deposits these deposits comprise a thick 56 m accumulation additionally this pleistocene dolomite displays a high degree of ordering and has a more nearly ideal stoichiometric composition than the dolomite from the depositionally and diagenetically analogous abu dhabi sabkha complex the pleistocene dolomite also has lower 13 and 18o values than the modern abu dhabi sabkha dolomite and higher values than those commonly reported for analogous dolomite from the ancient rock record the low 18o values in conjunction with the geological setting indicate that the diagenetic waters were meteoric or mixed meteoric and marine in composition thus the degree of ordering stoichiometric and stable isotopic values indicate that this dolomite has undergone diagenetic alteration relative to its presumed holocene precursor

shear stress at the base of shield lithosphere 1 one of the basic unresolved issues regarding plate tectonics is the degree of coupling between surface plates and convecting mantle below are the plates effectively decoupled from the mantle flow field by a low viscosity asthenosphere or are they strongly coupled to mantle flow while these two views are essentially incompatible they both do a reasonably good job of predicting the motions of the surface plates and cannot therefore be distinguished on this basis the significant distinguishing feature for these models is the magnitude of basal shear stress that is applied to the base of the plate while it is difficult to measure this stress directly it is possible in principle to measure the corresponding deformation of the plate through observations of seismic anisotropy and to infer stress here we focus on the canadian shield for which we expect strong plate mantle interaction we show that seismic anisotropy can be used to constrain the magnitude of the stress level applied to the base of the plate and to document the level of interaction between tectonic plates and the mantle below

shear wave splitting observations in the archean craton of western superior shear wave splitting observations in the archean superior province of the canadian shield show moderate to large delay times 1 1 2 1 s with azimuths suggesting upper mantle anisotropy is subparallel to the structural grain of the craton two regions with uniform anisotropy azimuth 62 7 and 87 3 are separated by a transitional zone showing strong dependence of observed azimuth on source direction suggesting that lateral structure is being observed no splitting was observed at stations near the coast of hudson bay continuing a pattern of weak splitting in surrounding proterozoic orogenic belts current absolute plate motion is consistent with the direction of anisotropy but would not explain the regional contrasts instead the anisotropy appears to be related to archean structure and tectonic history

upper mantle anisotropy of the borborema province ne brazil implications for intra plate deformation and sub cratonic asthenospheric flow abstract the geological record of the borborema province northeast brazil documents tectonic events that characterised the precambrian formation and mesozoic breakup of gondwana large scale shear zones and associated granitic plutons that developed during the neoproterozoic brasiliano pan african orogeny and major sedimentary basins of mesozoic age indicate significant deformation across the region however whether or not the shear zones resulted from precambrian terrane accretion or are simply the result of episodes of subsequent intra plate deformation is debated also poorly understood is the effect of the thick sao francisco mantle keel on present day asthenospheric flow to address these issues we have performed a teleseismic shear wave splitting study of mantle seismic anisotropy from a new broadband seismograph network centred on the borborema province shear wave splitting parameters t reveal a lack of plate scale anisotropic fabrics associated within the continental interior perhaps supporting models of formation and evolution of the borborema province involving minimal deformation of the lithospheric mantle delamination of anisotropic lithosphere during the development of cenozoic volcanism that eroded older fossil lithospheric fabrics is unlikely because the widespread cenozoic magmatism required to achieve this is absent in the geological record instead the apparently low levels of seismic anisotropy observed in the interior of the borborema province may simply reflect depth dependent anisotropy nulls low t observations may be the subtractive result of orthogonal fast directions in the lithosphere and asthenosphere towards the brazilian coast t 1 s and fast directions are sub parallel to stretching fabrics formed during the opening of the south atlantic this may imply that the mantle lithosphere was deformed but not completely destroyed during gondwana breakup however a more complete backazimuthal coverage of splitting measurements across the region is needed to confirm or refute these hypotheses unambiguously

insights into sea surface temperatures from the cayman islands from corals over the last 540 years abstract seven large corals orbicella annularis and montastrea cavernosa from grand cayman and cayman brac which grew between 1474 to 1512 ce and 1815 to 2014 ce are characterized by growth banding that reflects bi annual growth periods some of these growth bands however include micro scale bands that reflect sub annual growth phases paleotemperatures derived from the oxygen isotope composition of these corals indicate that sea surface temperature sst between 1474 to 1512 ce and 1815 to 2014 ce ranged from 18 to 34 c and included four main periods of temperature change with smaller scale warm or cool intervals superimposed on these periods cool period 1 1474 to 1512 ce coincided with decreased temperatures that existed in the northern hemisphere during the little ice age lia ca 1450 1850 ce cool period 2 1815 to 1924 ce coincided with a caribbean wide reduction in temperatures that was probably related to the final phase of the lia warm period 1 1924 to 2006 ce is characterized by an overall increase in sst consistent with the pronounced global warming since the 1970 s modern period 1 2006 2014 ce is characterized by temperatures that fluctuate around 28 5 c the current average water temperature for grand cayman the overall 3 c 0 96 c increase in sst since 1815 ce derived from the cayman corals is consistent with the caribbean and global temperature increases since the 1850 s smaller scale deviations in the cayman temperature profiles from the general caribbean global temperature trends probably reflect local factors such as increased storm and or cold air front frequencies a reduction in the size and or intensity of the atlantic warm pool shifts in the phase of the atlantic multidecadal oscillation and or weakening of the cayman basin current

effects of diagenesis on paleoclimate reconstructions from modern and young fossil corals abstract the integrity of coral based reconstructions of past climate variability depends on a comprehensive knowledge of the effects of post depositional alteration on coral skeletal geochemistry here we combine millimeter scale and micro scale coral sr ca data scanning electron microscopy sem images and x ray diffraction with previously published 18o records to investigate the effects of submarine and subaerial diagenesis on paleoclimate reconstructions in modern and young sub fossil corals from the central tropical pacific in a 40 year old modern coral we find secondary aragonite is associated with relatively high coral 18o and sr ca equivalent to sea surface temperature sst artifacts as large as 3 and 5 c respectively secondary aragonite observed in a 350 year old fossil coral is associated with relatively high 18o and sr ca resulting in apparent paleo sst offsets of up to 2 and 4 c respectively secondary ion mass spectrometry sims analyses of secondary aragonite yield sr ca ratios ranging from 10 78 to 12 39 mmol mol significantly higher compared to 9 15 0 37 mmol mol measured in more pristine sections of the same fossil coral widespread dissolution and secondary calcite observed in a 750 year old fossil coral is associated with relatively low 18o and sr ca sims sr ca measurements of the secondary calcite 1 96 9 74 mmol mol are significantly lower and more variable than sr ca values from more pristine portions of the same fossil coral 8 22 0 13 mmol mol our results indicate that while diagenesis has a much larger impact on sr ca based paleoclimate reconstructions than 18o based reconstructions at our site sims analyses of relatively pristine skeletal elements in an altered coral may provide robust estimates of sr ca which can be used to derive paleo ssts

geologic evidence for a mantle superplume event at 1 9 ga 1 both preserved and restored areal distributions of proterozoic marine intracratonic passive margin and platform sediments show a prominent peak at 1 9 ga indicating that shallow marine sediments were widespread on the continents and that sea level was high at this time the chemical index of alteration in shales deposited at this time was high suggesting warm climates possibly due to enhanced co2 levels in the atmosphere high sea level and warms climate may also explain an abundance of black shale banded iron formations and shallow marine phosphate deposits and an increase in the number of occurrences and diversity of stromatolites in general and microdigitate stromatolites at 1 9 ga all of these observations are consistent with a 1 9 ga superplume event the occurrence of only a minor positive carbon isotope shift in marine carbonates at 1 9 ga indicates that the relative rates of burial of organic and oxidized carbon remained about the same as at present slightly low 87sr 86sr isotopic ratios in seawater at 1 9 ga reflect increased mantle input of sr from the proposed superplume event whereas higher ratios at 1 85 1 75 ga may reflect increased input of continental sr from a growing supercontinent the first massive sulfate evaporites in the geologic record at 1 8 1 6 ga follow the possible 1 9 ga superplume event this may reflect an increase in both oxidation state and carbonate deposition in the oceans as plume related volcanism wanes

the great barrier reef a 700 000 year diagenetic history a deep borehole through ribbon reef 5 in the great barrier reef off north eastern australia has identified a variety of cements including epitaxial radial prismatic and spherular aragonite together with blocky prismatic and fibrous calcite these cements are discontinuously arranged within the sequence that consists predominantly of grainstones but locally includes clotted muddy and filamentous textures that may be of microbial origin calcite cements vary in morphology with groups of crystals that include acute scalenohedral rhombohedral and flattened concordant terminations these show varying densities of inclusions that locally define growth zones and in some terminations divide in the manner of split crystals to form fibrous fringes morphological changes in calcite are inferred to reflect changes in water chemistry and crystal growth rates at the time of growth allied to their relationship to the palaeo water table and linked in turn to changes in sea level neomorphism and dissolution are widespread and variations in the severity of both imply response to the degree of undersaturation of pore waters that at times were probably balanced within very narrow limits a total of 10 depositional units are identified those units at the base of the borehole reflect deposition and diagenesis within a marine environment the influence of meteoric waters indicated by stable isotopes is first apparent at the top of unit 1 and in unit 2 184 to 155 m below sea floor petrographic evidence of vadose conditions appears at the tops of unit 3 131 to 99 m below sea floor units 4 to 8 all deposited under marine conditions provide isotopic evidence of meteoric or mixing zone waters and petrographic indicators of vadose conditions typically at the top of the units evidence indicates that in unit 5 the water table was mobile and units 6a 6b 7 and 8 all characterized by ultraviolet fluorescent cements are capped by sub aerial erosion surfaces unit 9 the holocene reflects the recent re establishment of marine conditions the extent of alteration of the entire sequence reflects the substantial and pervasive influence of meteoric waters this effect is interpreted as a result of a greater rainfall and river flow from the mainland during early and late stages of interstadial periods the study reflects progress in the ability to recognize the diagenetic signal generated by sea level change however whereas the isotopic results reflect the changing relationships between vadose and phreatic zones in groundwater systems beneath successive emergent surfaces their correspondence with petrographic features is expressed only weakly and commonly lacks the systematic sequential overprinting implied by the distribution of cathodoluminescent zones of cements in many ancient limestones

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

evaluation of carbonate diagenesis a comparative study of minor elements trace elements and rare earth elements ree y between pleistocene corals and matrices from grand cayman british west indies abstract on grand cayman the pleistocene ironshore formation consists of six unconformity bounded units of limestones that have been partially or completely altered to calcite by post depositional meteoric diagenesis in order to examine the diagenetic history from the perspective of geochemical elements the concentrations of minor element sr na mg trace elements ba fe mn al si and rare earth elements ree and yttrium y were determined for 105 corals and 84 matrices collected from the rogers wreck point rwp western onshore area wo and offshore george town gt areas with the transformation of aragonite to calcite the sr na and ba values decreased but mg increased which are indicative of diagenetic alteration in an open water system due to intrinsic vital effects and the extrinsic diagenetic environment the variations of sr na ba and mg concentrations between acropora and montastrea from the gt area are different to those of their counterparts from rwp and wo the signatures of sr na ba and mg are in good agreement with the diagenetic history as determined from petrographic and stable isotopic criteria the ree y rey concentrations rey are higher in the matrices 0 2 6 9 ppm average 2 6 ppm than in the associated corals 0 1 5 4 ppm average 0 6 ppm shale normalized rey patterns of the pleistocene ironshore formation are similar to those of oxygenated seawater which are characterized by 1 light ree depletion relative to heavy ree average dy sn sm sn 1 7 n 35 2 positive la anomalies average pr pr 1 17 n 53 and 3 negative ce anomalies average ce ce 0 49 n 53 the preserved seawater like rey distribution pattern the lack of correlation between rey and mineralogy and the lack of correlation between rey and diagenesis sensitive stable oxygen isotope 18 o indicate that meteoric diagenesis did not have a major impact on the rey distribution patterns the matrices and corals in the gt area compared to their counterparts from rwp and wo have relatively less depleted lree and lower y in rey distribution pattern suggesting contamination by minor amounts of detrital sediments this is supported by the fact that limestones from the gt area contain abnormally high contents of detrital elements al si the variations in rey in carbonate rocks from locality to locality probably reflect variations that existed in the local depositional environments where these deposits originally formed

diagenesis in coastal carbonates related to pleistocene sea level bermuda platform abstract pleistocene glacioeustatic sea level oscillation on the stable bermuda platform is expressed in a succession of shallow water carbonates interrupted by lowstand unconformities in bermuda the maximum highstands of the last 400 000 yr ranged within 10 m around the present level coastal carbonates of various highstands are exposed along the present shoreline these carbonates were penetrated by meteoric and marine pore waters during lowstands and highstands following on deposition two representative pleistocene shoreline sections were studied to see whether early diagenesis has recorded these pore water changes the sediments of both sections show multiple generations of cement optical and scanning electron microscopy cathodoluminescence microscopy x ray diffraction microprobe studies and stable isotope analyses were used to determine the diagenetic environments involved diagenetic features of multiple pore water changes herein termed early diagenetic oscillation depend on whether substrates were loosely or firmly cemented 1 in loosely cemented beach sands belmont group grape bay truncated layers of mg bearing low mg calcite covered many grain surfaces isopachous high mg calcite cement whose mg content now varies with the degree of meteoric altera ion eventually lithified the sediment there are several types of meteoric low mg calcite cement these sediments have undergone several cycles of deposition cementation erosion and mechanical abrasion of interparticle cements 2 firmly cemented stable substrates cliffs in watch hill park reveal complicated early diagenetic sequences cements with marine mineralogy comprise high mg calcite isopachous blades peloidal aggregates fibrous microstalactites and acicular aragonite freshwater influence is indicated here by leaching neomorphism and low mg calcite cement regardless of the degree of substrate cementation freshwater alteration was mainly vadose whereas marine cementation was either phreatic or vadose or both early diagenetic oscillation is easier recorded in coastal successions than in lagoonal sediments mainly because marine cementation is more active nearshore became the coastal environment is prone to wave destruction the potential for preserving these diagenetic features is usually low data published on tectonically unstable areas enewetak atoll barbados suggest that early diagenetic oscillation may characterize stable coastlines

age and evolution of the lower crust beneath the western churchill province u pb zircon geochronology of kimberlite hosted granulite xenoliths nunavut canada abstract zircon from granulite facies xenoliths can preserve unique geodynamic records of deep and mostly inaccessible levels of the continental crust within archean cratons we present the first such records from beneath two archean crustal blocks that were affected by multiple episodes of regional scale proterozoic metamorphism and are located within the western churchill province nunavut canada zircon u pb geochronology and trace element analysis were carried out on samples that originated in the deep crust at the time of kimberlite eruption as confirmed with raman spectroscopy zircon was recovered from four metabasites grt cpx plg from the rankin inlet area of the chesterfield block and five metabasites grt cpx plg hbl opx and three metatonalite anorthosites plg cpx plg hbl opx from the repulse bay area of the rae craton sem cl imaging reveals several types of growth zoning diagnostic of igneous and metamorphic zircon crystallization the latter are related to local high temperature recrystallization and micro fracturing and are spatially associated with zones of apparent pb loss and or intracrystalline pb variability evidence for the formation of paleoarchean mesoarchean to neoarchean crust is preserved in oscillatory zoned igneous cores having 207pb 206pb ages of ca 3 5 ga and ca 3 0 2 6 ga two periods of paleoproterozoic metamorphism and zircon growth followed with the oldest episode at ca 2 0 1 9 ga in the chesterfield block metamorphic zircon rims from all twelve xenoliths have an age range of ca 1 75 to 1 70 ga the trace element compositions of the metamorphic rims differ from those of older igneous and metamorphic cores with the rims characterized by lower hree contents and low lu gd n suggesting that garnet crystallization occurred syn to pre rim growth at ca 1 75 ga the archean and paleoproterozoic ages correspond broadly to the upper crustal histories of the western churchill province however the prevalence of ca 1 75 1 70 ga metamorphic rims indicates regional scale metamorphic reworking of the lower crust across the region during a period of intracratonic extension and associated magmatism regional magmatic underplating and or remobilization of the lithosphere at ca 1 75 1 70 ga to an extent not widely appreciated represents a terminal stage in western churchill cratonization including stabilization of reworked archean crust relative to underlying diamondiferous mantle

sea level highstands over the last 500 000 years evidence from the ironshore formation on grand cayman british west indies abstract the ironshore formation up to 19 m thick in the rogers wreck point area of grand cayman is divided into the unconformity bounded units a b c and d the unconformities are highlighted by caliches and or terra rossa th u dates from aragonitic corals indicate that unit a formed at 400 ka unit b 346 ka unit c 229 ka and unit d 131 ka these ages correlate with the highstands of the last four interglacial periods and probably represent marine isotope stages 5 unit d 7 unit c 9 unit b and 11 unit a the limestones in units a d which have similar biotic and lithologic characteristics represent deposition in shallow water on a narrow coastal shelf unit a has a high faunal diversity and was deposited in open marine conditions whereas units b c and d were deposited in quieter water lagoons that probably had fringing reefs along their sea ward edges although grand cayman is located on the cayman ridge close to an active spreading center and transform fault the island has undergone little or no vertical movement over the last 500 ky available evidence indicates that sea level at the time of deposition of each unit relative to present sea level was unit a 9 0 to 5 5 m unit b 3 0 to 0 5 m unit c 2 5 to 1 1 m and unit d 2 5 to 6 0 m these data are good estimates of eustatic sea level highstands over the last 500 ky

crustal anisotropy beneath hudson bay from ambient noise tomography evidence for post orogenic lower crustal flow the crust underlying hudson bay canada records a long and complex tectonic history in this study we investigate this region using tomographic inversion based on continuous ambient noise recordings from 37 broadband seismograph stations that encircle hudson bay the ambient noise data were processed to obtain group velocity dispersion measurements from 10 35 s period which were inverted using an algorithm that incorporates the effects of anisotropy this work is among the first in which ambient noise data have been used to investigate azimuthal anisotropy the inversion method uses smoothing and damping to regularize the solution due to the significantly increased number of model parameters relative to the isotropic case we performed a careful analysis for parameter selection to determine whether leakage occurs between isotropic and anisotropic model parameters we observe a robust pattern of anisotropic fast directions in the mid crust that are consistent with large scale tectonic trends based on magnetic anomaly patterns in particular a distinctive double indentor shape for the superior craton is clearly expressed in both data sets this pattern breaks down deeper in the crust suggesting that some degree of lithospheric decoupling in the lower crust such as channel flow occurred during orogenesis given regional evidence for vertically coherent deformation in the crust and underlying mantle we interpret this pattern in the lower crust as a tectonic overprint that post dates the main phase of trans hudson deformation at most levels in the crust we observe a profound change in direction of anisotropic fast direction across an inferred suture beneath hudson bay