the 3d numerical modeling of subduction dynamics plate stagnation and segmentation and crustal advection in the wet mantle transition zone 1 water content in the mantle transition zone mtz has been broadly debated in the earth science community as a key issue for plate dynamics in this study a systematic series of three dimensional 3d numerical simulation are performed in an attempt to verify a hypothesis regarding the behavior of subducted oceanic plate and the role of crust in the mtz under wet conditions this hypothesis is based on the seismic observations of structural variations associated with large scale flattened high velocity anomalies i e stagnant slabs in the mtz in our model weak low viscosity fault zones wfzs which presumably correspond to the fault boundaries of large subduction earthquakes are imposed on the top part of subducting plates the phase transitions of olivine to wadsleyite and ringwoodite to perovskite magnesiowustite under both dry and wet conditions are considered based on recent high pressure experiments the effect of viscosity reduction of wet garnet is also considered for oceanic crust in the mtz results show that there is a substantial difference in the behaviors of subducting plate and the trace of crustal materials between the models under dry and wet conditions under wet conditions the subducting plate tends to stagnate with a maximum lateral extent of over 1000 km the weaker and denser crustal material garnet rich is fed into the wfzs and is advected faster than the plate main body of peridotite olivine rich thus the viscosity and density contrast between the crustal materials and the peridotite permits the plate segmentation under a wet mtz condition

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

implications of modelled radioactivity measurements along coastal odisha eastern india for heavy mineral resources abstract a radioelemental assemblage assessment of two beaches of odisha is performed for the first time the radiation is measured in two ways both on field with the help of a hand held environmental survey meter and in the laboratory where the concentrations of radionuclide s 238 u 232 th and 4 k have been determined with the help of high purity germanium detector hpge mineralogical analysis of selected samples has been performed with the help of x ray fluorescence spectrometry xrf a marked difference between the concentration of uranium 274 bq kg 1 and thorium 2489 bq kg 1 is observed and discussed based on the geology of the area the placer deposits showing an enrichment of thorium can be an important source of nuclear fuel for the thorium based nuclear reactors the ratio of thorium and uranium concentrations gives us an idea about the coastal processes associated with the beach statistical analysis of the data shows a positive correlation between 238 u and 232 th and a strong negative correlation is indicated between 4 k and 238 u 232 th a cross plot between the equivalent thorium and the equivalent uranium and the equivalent thorium and potassium represents the nature of deposition and its association with the heavy mineral along with the radioactive elements heavy minerals exhibit an increasing trend towards northeast southwest along the south eastern coast of india

processes of selective grain transport and the formation of placers on beaches placers are found on many beaches of the oregon coast in the beach that is the focus of this study the principal placer minerals are ilmenite 22 0 epidote 20 0 garnet 18 7 augite 15 5 hypersthene 8 1 hornblende 5 8 and zircon 2 0 the ilmenite fraction actually being partly chromite a series of beach face sand samples was obtained along a profile when sand was being eroded from the upper beach and transported to offshore bars since the processes of grain selective sorting leading to placer formation were probably most active at such a time the landward most sample contained 96 heavy minerals being part of the placer this concentration systematically decreased offshore to a 6 heavy mineral content in the seaward most sample 54 m offshore determinations of individual heavy mineral concentrations revealed that although they all tended to become concentrated within the placer with light quartz and feldspar dominating the offshore transport the sorting processes were m

stability limits of hydrous minerals in sediment and mid ocean ridge basalt compositions implications for water transport in subduction zones phase relationships in hydrated natural sediment and mid ocean ridge basalt morb compositions were investigated experimentally from 6 to 15 gpa and 700 c to 1400 c in order to determine the stability of hydrous phases in subducting slabs and to constrain reactions resulting in the release of water from subduction zones to the mantle phengite and hydrous aluminum silicates topaz oh and phase egg are stable in the sediment composition at depths greater than 150 km and constitute h2o reservoirs similarly lawsonite is an h2o reservoir in morb composition at depths greater than 150 km according to mass balance calculations based on experimental run products the weight proportions of phengite and h2o at 6 gpa in the sediment are about 50 and 2 respectively at pressures from 8 to 12 gpa topaz oh al2sio4 oh 2 appears as the hydrous mineral and the sediment retains about 0 5 0 7 wt h2o at higher pressures phase egg alsio3 oh appears as the hydrous phase with a large stability field the h2o content in the sediment is about 0 5 wt at pressures greater than 12 gpa in natural morb compositions lawsonite is the hydrous phase at pressures lower than 10 gpa about 1 wt h2o in the morb composition is retained by lawsonite at 6 gpa and 800 c above 6 gpa a lawsonite out reaction is controlled by compositional changes of garnet and clinopyroxene if the oceanic crust descends with an overlying sediment layer water can be carried down into the transition zone by the sediment the amount of transported water is controlled by the chemical compositions of the subducting sediments water in the morb layer can be stored to 300 km in lawsonite for a cold subduction zone geotherm water release at depths greater than 200 km through progressive lawsonite breakdown can hydrate the overlying mantle causing the generation of a peridotite region including hydrous phases chondrodite clinohumite phase a and phase e if this hydrous peridotite is dragged down by the descending oceanic crust water can be transported into the transition zone by the subduction process the amount of transported water is controlled by the thermal state of the slab and the chemical composition of morb

p wave anisotropic tomography of the nankai subduction zone in southwest japan 1 the active subduction of the young philippine sea phs plate and the old pacific plate has resulted in significant seismic heterogeneity and anisotropy in southwest sw japan in this work we determined a detailed 3 d p wave anisotropic tomography of the crust and upper mantle beneath sw japan using 540 000 p wave arrival times from 5 249 local earthquakes recorded by 1095 stations the phs slab is imaged clearly as a high velocity high v anomaly which exhibits considerable lateral variations significant low velocity low v anomalies are revealed above and below the phs slab the low v anomalies above the phs slab may reflect the upwelling flow in the mantle wedge and the phs slab dehydration and they form the source zone of the arc volcanoes in sw japan the low v zones under the phs slab may reflect the upwelling flow in the big mantle wedge above the pacific slab the anisotropy in the crust and upper mantle is complex in kyushu the p wave fast velocity direction fvd is generally trench normal in the mantle wedge under the back arc which is consistent with the corner flow driven by the phs slab subduction the fvd is trench parallel in the subducting phs slab under kyushu we think that the intraslab seismicity is a potential indicator to the slab anisotropy that is the phs slab with seismicity has kept its original fossil anisotropy formed at the mid ocean ridge while the aseismic phs slab has reproduced the anisotropy according to its current deformation

the roles of elevation and salinity as primary controls on living foraminiferal distributions cowpen marsh tees estuary uk abstract benthic foraminiferal assemblages in subrecent deposits are commonly used to reconstruct past sea level interpretations are generally made by comparison with either modern dead or total live plus dead assemblages in both cases there will have been post mortem changes that have differentially affected preservation it is therefore important to establish the primary ecological controls by analysis of the living assemblages we have determined the spatial and temporal variability of intertidal benthic foraminifera in the surface 0 1 cm sediments from a time series survey of 31 sampling stations at cowpen marsh for a period of 12 months we counted 112 067 live foraminifera assigned to 28 species the fauna was dominated by two agglutinated species jadammina macrescens and trochammina inflata on the high and middle marshes and three calcareous species elphidium williamsoni haynesina germanica and quinqueloculina spp on the low marsh and tidal flat the standing crop of the whole intertidal zone including the high middle low marsh and tidal flat habitats and the individual species varied both temporally and spatially the standing crop of the intertidal zone as a whole was greatest in the summer months and showed a positive correlation with elevation the standing crops of the high and middle marshes showed similar temporal variation with peaks in summer and autumn and a trough in winter the low marsh showed numerous peaks and troughs of standing crop during the year whereas the tidal flat showed a single peak in summer the standing crops of jadammina macrescens and trochammina inflata on the high and middle marshes peaked from april to may and august to october with troughs in winter these agglutinated species showed a strong correlation with elevation haynesina germanica peaked in may to august and november to january on the low marsh whereas on the tidal flat there was a single peak in july the standing crops of e williamsoni on the low marsh and tidal flat were relatively high in june and may and july respectively quinqueloculina spp peaked in may to july on the low marsh and july on the tidal flat the species was also found in the middle marsh from july to may and high marsh from september to november haynesina germanica showed a strong negative correlation with elevation whereas the other two dominant calcareous species demonstrated weak negative correlations with both elevation and salinity reconstructing former sea level depends primarily on the recognition of high and middle marsh assemblages and in this study these are shown to be strongly controlled by elevation rather than salinity caution may be needed in interpreting low marsh and tidal flat data as salinity plays a more important role here

precise determination of the phase boundary between coesite and stishovite in sio2 abstract the phase boundary between coesite and stishovite in sio2 was determined over the range of 1200 1700 k using a multi anvil high pressure apparatus combined with the synchrotron x ray diffraction technique the stability of each phase was determined by observing the powdered x ray diffraction data the transition boundary between the coesite and stishovite phases was found to occur at p gpa 4 7 0 0031 t k the transition pressure determined in this study was in general agreement with what was observed in previous high pressure experiments and heat capacity measurements however the slope of the transition dp dt determined in our study was higher than that estimated in a previous study utilized with the in situ x ray method in contrast the slope calculated by the heat capacity data was in good agreement with that determined in our study the phase transition determined in our study occurs at around 10 gpa at the normal mantle geotherm coinciding with the seismic discontinuity around 300 km depth known as the x discontinuity

decomposition of phase d in the lower mantle and the fate of dense hydrous silicates in subducting slabs abstracthigh pressure high temperature quench type experiments were carried out on serpentine to pressures of 53 gpa andtemperatures between 800 1800oc x ray analyses show that recovered phase assemblages varied considerably for thedifferent high pressure and high temperature studies the dense hydrous magnesium silicate phases decompose sequentiallyas the pressure increases and appear to serve as reservoirs for h 2 o in the mantle eventually releasing it at the highestpressure superhydrous phase b is stable up to pressure equivalent to the boundary between the transition zone and lowermantle phase d decomposes at pressures of 44 gpa equivalent to about 1250 km depth this may de ne the lower depthlimit for the presence of dense hydrous magnesium silicates beyond this pressure only nominally anhydrous phases havebeen recovered in high pressure experiments this may indicate a lack of stoichiometric hydrogen bearing silicate phasesat higher pressure 1998 elsevier science b v all rights reserved keywords x ray diffraction analysis lower mantle serpentine high pressure

ecological zonation of benthic foraminifera in the lower guadiana estuary southeastern portugal abstract the present study analyzes the spatial distribution and seasonal distribution of live benthic foraminiferal communities in the estuary of guadiana the fourth largest river on the iberian peninsula and establishes through statistical analysis their relationships with a series of environmental parameters forty four superficial sediment samples were collected along distance to sea and elevation gradients in the winter and summer of 2010 fifty three foraminifera species were identified along the intertidal margins of the estuary foraminiferal distribution reflected seasonal variation of environmental factors whose relative importance varied according to species tolerances elevation in relation to mean sea level appeared to be the most important parameter controlling foraminiferal distribution probably because it combines the effects of a series of other variables i e organic matter sediment texture ph and temperature in the highest marsh areas where environmental conditions approach survival thresholds only some agglutinated species are able to survive in the lower intertidal zone where subaerial exposure is diminished and environmental conditions are generally less variable more diverse faunas mainly composed of calcareous species prevail during winter when fluvial discharge peaks agglutinated species represent more than 80 of the total individuals in summer when marine conditions prevail calcareous species become more competitive increase their densities and expand into higher marsh zones and estuarine upper reaches in the estuary three different foraminiferal assemblages are distinguished i miliammina fusca assemblage which dominates in unvegetated areas of the lower marsh and tidal flats of the mid upper estuary ii jadammina macrescens assemblage which dominates in the highest marsh areas in the lower estuary and iii ammonia aberdoveyensis assemblage which dominates the areas of low marsh and tidal flats of the lower estuary

seismic signature of the collision between the east tibetan escape flow and the sichuan basin abstract gps displacement vectors show that the crust in east tibet is being squeezed in an easterly direction by the northward motion of the indian plate and the sichuan basin is resisting this stream and redirecting it mainly towards indochina the longmen shan containing the steepest rise to the high plateau anywhere in tibet results from the strong interaction between the east tibetan escape flow and the rigid yangtze block sichuan basin but the kinematics and dynamics of this interaction are still the subject of some debates we herein present results from a dense passive source seismic profile from the sichuan basin into eastern tibet in order to study the deep structure of this collision zone using p and s receiver function images we observe a sudden rise of the lithosphere asthenosphere boundary lab from 120 to 150 km beneath the sichuan basin and from 70 to 80 km beneath eastern tibet in contrast the depth of the crust mantle boundary moho increases from 36 to 40 km beneath the sichuan basin and from 55 to 60 km beneath eastern tibet the 410 km discontinuity is depressed below eastern tibet by about 30 km although the 660 remains at nearly the same depth throughout the lms from these observations we conclude that the mode of collision that occurs between tibet and the sichuan basin is very different to that found between india and tibet in southern tibet we observe in essence the subduction of the indian plate which penetrates northwards for several hundred kilometers under central tibet the very thin mantle part of the lithosphere beneath eastern tibet may indicate delamination or removal of the bottom of the lithosphere by hot asthenospheric escape flow this process leads to the exceptionally steep topography at the eastern tibetan margin as a result of gravitational buoyancy this view is supported by the very unusual depression of the 410 km discontinuity beneath eastern tibet which could be caused by the dynamics of the sub vertical downward asthenospheric flow

paleoenvironmental evolution of the guadiana estuary portugal during the holocene a modern foraminifera analog approach in this paper we reconstruct the holocene paleoenvironmental evolution of the guadiana estuary southwestern iberian peninsula two previously studied boreholes cm3 and cm5 were revisited and an

the 10a phase a high pressure expandable sheet silicate stable during subduction of hydrated lithosphere h2o storage and release in deep subducting lithosphere is controlled by complex reaction suites involving a variety of hydrous phases as a result of its relatively large thermal stability and intermediate composition the 10a o phase mg3si4o10 oh 2wnh2o has been regarded as a relevant h2o reservoir in a wide range of rock compositions and mineral assemblages high pressure syntheses of the 10a o phase were carried out at 6 7 gpa and 650 c under fluidsaturated conditions in a walker type multi anvil apparatus from 5 min to 430 h x ray powder diffraction of large platy hexagonal crystals of the 10a o phase up to 100 wm were indexed on the basis of a trioctahedral type structure long term run products s 110 h reveal sensitivity of the 10a o phase to treatment with acetone leading to the appearance of diffractions at greater d spacings 10 2 11 6 a o with respect to the basal peak of the 10a o phase 9 64 10 07 a o this swelling behavior is strongly related to synthesis run duration the raman spectrum of the 10a o phase at frequencies less than 800 cm 31 shows a strong similarity to talc in the si o stretching region 800 1100 cm 31 the 10a o phase exhibits three modes 909 992 and 1058 cm 31 as compared to two in talc the bending mode of water x2 i s found at 1593 cm 31 in the oh stretching region peaks at 3593 3622 and 3668 cm 31 were observed the acetone treated sample shows a c h stretching mode at 2923 cm 31 while the double bond cno signal is absent the swelling behavior of the 10a o phase is interpreted as due to intercalation of acetone with pre existing interlayer water the efficiency of this process is dependent on the amount of the interlayer water which in turn depends on run duration the relation between the response to acetone treatment and run duration is therefore interpreted as a time dependent hydration of the 10a o phase the fractions transformed from non expandable to expandable fractions was fitted to the avrami empirical law which suggests that kinetics are mainly controlled by diffusion rather than phase boundary reactions the ability to accommodate variable amounts of h2o makes the 10a o phase a major h2o sink whenever a hydrous phase such as chlorite and serpentine breaks down during prograde transformations in the subducted lithosphere under h2o saturated conditions a fully hydrated 10a o phase occurs when h2o undersaturated conditions prevail a h2o deficient 10a o phase incorporates the volatile component available the exchange capacity of interlayer molecules in the 10a o phase structure opens new scenarios on the control of fluid compositions escaping from subducted

behaviour and taphonomic loss in foraminiferal assemblages of mangrove swamps of french guiana abstract fifty three surface sediment samples were collected in january 1999 july 2000 and december 2000 on mud flats and in mangrove swamps of french guiana sampling stations were selected to provide information on different environmental conditions on the mud flats and in the mangrove forest thirteen samples were also collected at a depth of about 8 12 cm in the sediment additional samples were collected from three long cores in the channel of the kaw river the main parameter controlling the distribution of recent foraminiferal assemblages is the seasonally variable relative influence of coastal water of reduced salinity and low ph low ca river water areas under the influence of coastal water are dominated by calcareous species mainly ammonia tepida and cribroelphidium spp even in mangrove swamps during the dry season river water influences almost all the study area during the rainy season and leads to the dominance of agglutinated species with the nature of the assemblages depending on local environmental characteristics on the mud flats tests are typically well preserved in the 10 cm below the surface that is the most important taphonomically active zone for foraminifera however owing to intense bioturbation the taphonomic processes in the mangrove swamp may result in the total destruction of the microfauna including agglutinated tests this study shows that the preservation of thanatocoenoses in mangrove swamp environments of french guiana only occurs in exceptional circumstances when they are preserved the remaining assemblages even if modified by taphonomic processes give valuable information for paleoenvironmental reconstructions in this environment

the ad 1755 tsunami deposits onshore and offshore of algarve south portugal sediment transport interpretations based on the study of foraminifera assemblages abstract tsunamis are amongst the most catastrophic natural hazards affecting worldwide coastal areas their impacts are recorded in the geological archives and its study provides crucial insights into the dynamics and intensity of these events the accurate establishment of return periods can also be derived using sedimentological data onshore tsunami deposits have been extensively described over recent years through the application of a wide range of sedimentological proxies including micropalaeontological analysis in this work the identification of palaeotsunami evidence in both the onshore and offshore sedimentary record of south portugal was performed relying mostly on the study of foraminifera assemblages three onshore trenches and one offshore core were analyzed and imprints of the ad 1755 tsunami were observed based in the identification of allochthonous foraminiferal species increase in the number of broken abraded tests and its correlation with coarser mean grain size this was possible after micropalaeontological environmental characterization of both low energy onshore and offshore present day sedimentation regimes in the onshore record three distinct sediment transport phases were differentiated within the tsunamigenic unit the basal phase portraying the initial inundation represented by the dominance of marine foraminifera species in a highly diverse assemblage the higher energetic character of this phase was also reflected in the identification of the highest number of broken abraded tests the intermediate phase reflects an unspecified number of inundation and backwash pulses that were recognized based in the well balanced presence of estuarine brackish and marine species the sharp decrease in the diversity index and in the number of broken abraded tests the final phase recorded in the tsunamigenic unit registers the imprint of the last relevant backwash which was identified based on the dominance of estuarine brackish fauna the overlying unit is dominated by tidal marsh species demonstrating the start of the coastal system recovery phase in the offshore record it was possible to identify 2 distinct historical tsunami events ad 1755 and ad 1969 this was achieved through the identification of significant increases in coastal foraminifera species coupled with the presence of coarser layers this interpretation was sustained by indirect age estimation methods radiocarbon and 210 pb that ascribed compatible ages to the historical tsunami events the results obtained in this study stress the relevance and innovative character of this onshore and offshore multiproxy approach in the recognition and understanding of tsunami events

wet plume atop of the flattening slab insight into intraplate volcanism in east asia abstract geophysical observations imply the intraplate volcanism in east asia is related to dehydration of slab stagnating in the transition zone to better understand the dynamics of such process a thermochemical mantle convection model is constructed to simulate numerically the thermal evolution of slab and the transportation of water in the process of slab downgoing flattening and stagnation equation of water transfer is included and water effects on density and viscosity are considered model results indicate the warming of slab by surrounding mantle is rather slow water could be successfully dragged into the transition zone if the reference viscosity of the hydrous layer with initial water of 2 wt is higher than 1017 pa s and that of mantle is 1021 pa s wet plumes could then originate in the flat lying part of the slab relatively far from the trench generally the viscosity of the hydrous layer governs the initiation of wet plume whereas the viscosity of the overlying mantle wedge controls the activity of the ascending wet plumes they are more active in the weaker wedge the complex fluid flow superposed by corner flow and free thermal convection influences greatly the water transport pattern in the upper mantle modeling results together with previous modeling infer three stages of water circulation in the big mantle wedge 1 water is brought into the mantle transition zone by downward subducting slab under some specific thermo rheological conditions otherwise water is released at shallow depth near wedge tip 2 wet plume generates from surface of the flattening slab warmed by surrounding mantle and 3 water spreads over the big mantle wedge wet plume from the flattening pacific plate arrives at the lithospheric base and induces melting which can explain the intraplate cenozoic volcanoes in east asia

radiometric analysis of quaternary deposits from the southeastern brazilian coast natural gamma radiation measurements of the beach sand deposits were performed with the aim of understanding the provenance and transport processes of sediments along the coastal zone of three brazilian states sao paulo sp rio de janeiro rj and espirito santo es the method employs thorium uranium and potassium as tracers of the mineralogical properties of beach sand minerals which reflect the geological history of transport and sorting processes a cross plot of eth eu and eth k shows a considerable positive correlation with the geological evolution of the quaternary coastal deposits from 35 areas of the southeastern brazilian coast additionally the distribution of heavy minerals along this coastal zone was mapped

natural radioactivity measurements in beach sand along southern coast of orissa eastern india the beach placer deposits in the southern coastal orissa india may have significant levels of radiation due to the presence of th and u bearing minerals such as monazite and zircon in this study gopalpur and rushikulya beach regions were selected to study the ambient radiation environment the average activity concentrations of radioactive elements such as 232th 238u and 40k of beach sand samples were measured by ray spectrometry using a hpge detector and found to be much higher than the internationally accepted values the cross plots of eth eu and eth k inferred that the sand samples of gopalpur and rushikulya beach placer deposit were deposited in leached uranium and an oxidising environment the absorbed dose rate levels of the study areas are similar to other monazite sand bearing hbras of southern and southwestern coastal regions of india and of world different radiation hazard indices were estimated for the present study area and were found to be much higher than the internationally accepted values hence gopalpur and rushikulya beach placer region can be considered as a high background radiation area and a potential zone for radiogenic heavy mineral exploration

evolution and facies architecture of the mesotidal guadiana river delta s w spain portugal abstract the guadiana river estuary and delta is one of the most important mesotidal fluvio marine systems of the southwestern iberian peninsula the river mouth was formed as a narrow channel excavated by fluvial incision during the pleistocene lowstand and then flooded 6500 yr ago during the flandrian transgression holocene the estuary is in an advanced state of sediment infilling in its proximal part due to its narrow morphology which prevents passage of sediment through to the open coast consequently sediment is accumulating in the river mouth causing progradation as response of the interaction between coastal and fluvial processes sedimentation is controlled by the interaction of sediment load delivered by the guadiana river and the active longshore current which also supplies sand eroded from barrier islands and friable cliffs in the portuguese algarve to the west tidal range is mesotidal 2 0 m and mean significant wave height is 0 6 m with an average period of 3 6 s morphological differences exist between the two sides of the river mouth the western side has a large spit separated from the mainland by a marsh and the eastern side shows an array of old barrier islands separated by salt marsh areas a vibracore study allows us to distinguish 12 different depositional facies in the system fluvial delta tidal inlet flood tidal delta washover beach dune estuarine channel lagoon tidal creek channel margin tidal flat and salt marsh furthermore a different architectural scheme of these facies is present on each side to the east there is a longitudinal repetition of the sandy facies corresponding to the barrier islands on the west side there is a transverse growth of a single spit containing eolian dunes these differences result from asymmetric interaction between the tidal and fluvial currents at the river s mouth and the waves and tidal currents of the open coastal zone

kinetics of the olivine spinel transformation in subducting lithosphere experimental constraints and implications for deep slab processes abstract the persistence of metastable olivine to depths greater than 400 km in subducting slabs has implications for the generation of deep focus earthquakes the magnitude of buoyancy forces driving plate motion and the state of stress in the slab the depth to which metastable olivine can survive in a subduction zone and the depth interval over which transformation to or mg fe 2sio4 occurs have been evaluated from experimental kinetic data for the mg2geo4 and ni2sio4 transformations and the mg2sio4 transformation the data were extrapolated using a kinetic model for grain boundary nucleation and interface controlled growth under non isothermal and non isobaric conditions the results predict that metastable mg1 8fe0 2sio4 olivine survives to depths greater than 500 km in the cold interior of rapidly subducting slabs of old lithosphere the onset of transformation to mg fe 2sio4 spinel depends only on growth kinetics and coincides with the 550 50 c isotherm including the effects of latent heat production causes the transformation to occur by a runaway process over a very narrow depth interval at the onset of transformation high nucleation rates and low growth rates are consistent with the formation of very fine grained reaction products which are required for the transformational faulting mechanism of deep focus earthquakes when olivine crosses the equilibrium boundaries at higher temperatures e g higher than 700 c at 400 km depth transformation to or occurs much closer to equilibrium at a depth controlled by nucleation kinetics in this case the effect of latent heat production on the transformation kinetics is small and microstructural evolution is unlikely to result in transformational faulting below the depth of olivine breakdown cold slabs are likely to have a complex rheological structure owing to temperature dependent microstructural evolution during the phase transformation

stability of dense hydrous magnesium silicate phases and water storage capacity in the transition zone and lower mantle abstract we evaluated the water storage capacity in the mantle based on the phase relations of silicate systems containing water we conducted high pressure and temperature synthesis experiments on some silicate systems such as the superhydrous phase b phase c and phase g phase d and f compositions in the mgo sio2 h2o system and the cmas pyrolite 2 wt h2o system up to the uppermost part of the lower mantle superhydrous phase b is stable at temperatures below 1200 c at 18 gpa and below 1300 c at 20 gpa the following reaction sequences were observed with increasing temperature from 1200 to 1600 c in the pressure range from 18 to 20 gpa superhydrous phase b phase b liquid wadsleyite periclase liquid anhydrous phase b periclase liquid phase g is stable at temperatures below 1000 c at 18 gpa below 1100 c at 20 22 gpa and below 1200 c at 25 gpa wadsleyite stishovite liquid are stable above 1000 c at 18 gpa ringwoodite or phase e stishovite liquid above 1100 c at 22 gpa ilmenite stishovite liquid above 1100 c at 23 5 gpa and perovskite stishovite liquid above 1200 c at 25 gpa superhydrous phase b is stable at 1200 c at 18 5 and 25 gpa in cmas pyrolite 2 wt h2o composition whereas it decomposes at around 1400 c at 25 gpa in the composition there may be a layered structure in the mantle in its water storage potential solubility of water per unit mass i e the upper and lower mantles have relatively small water storage potentials whereas the transition zone has a larger potential the water content in the transition zone might be large because the primordial water trapped in the lower mantle and the recycled water transported by the slab subduction might have been released upwards and stored in the transition zone during geological time

evidence for h 2 o bearing fluids in the lower mantle from diamond inclusion abstract in this study we report the first direct evidence for water bearing fluids in the uppermost lower mantle from natural ferropericlase crystal contained within a diamond from sao luiz brazil the ferropericlase exhibits exsolution of magnesioferrite which places the origin of this assemblage in the uppermost part of the lower mantle the presence of brucite mg oh 2 precipitates in the ferropericlase crystal reflects the later stage quenching of h 2 o bearing fluid likely in the transition zone which has been trapped during the inclusion process in the lower mantle dehydration melting may be one of the key processes involved in transporting water across the boundary between the upper and lower mantle

detailed structure of mantle transition zone beneath southeastern china and its implications for thinning of the continental lithosphere abstract this paper presents a new study on 3 d structure temperature and water content estimation of the mantle transition zone mtz by p wave receiver functions common conversion point ccp method on broadband array with 267 temporary stations and 203 permanent stations in southeastern china the results show different structural characteristics of the mtz to the north and south of the 29 n line in the northern part the 660 km discontinuity is generally deeper than globe models and there are two independent local depressions with characteristics of high velocity low temperature and water shortage we interpret them as two previous subducting plates stagnated in mtz possibly related to two separate subduction events at different times while in the southern part the 410 km discontinuity is locally depressed resulting in thinning of the mtz particularly in the cathaysian block south of the 29 n line based on the mtz structure and the estimation of temperature and water content we conside dehydration of the mtz as one of the important mechanisms of lithospheric thinning through interaction with the asthenosphere we then propose a dynamic model that combines paleo pacific flat slab subduction and upper mantle convection to explain the lithospheric extension and thinning magmatic rock distribution weak seismic activity and basin and range geomorphology this model differs from that north of the 29 n line in the north china block

reconstructing relative sea level change using uk salt marsh foraminifera abstract the position of past sea levels can be reconstructed using a series of sea level index points that possess information on age location altitude and a quantified vertical relationship with a former tidal frame the indicative meaning whilst these points fix the altitude of relative sea level at one instant in time they provide no information on its variation between them here a foraminiferal transfer function is used to reconstruct changes in water depth from fossil assemblages preserved within salt marsh sediments the transfer function performs reliably in the high marsh zone where agglutinated foraminiferal species are dominant in lower marsh environments where calcareous species are prevalent postmortem test dissolution alters the fossil assemblages and leaves them without modern analogues to circumvent this problem a new transfer function based on agglutinated foraminifera and utilising preserved test linings is developed this new transfer function significantly reduces the number of samples without modern analogues but at the expense of diminished sensitivity in the low marsh environment additionally the transfer function is used to establish a series of sea level index points that place observed lateral shifts in depositional environment into a vertical and temporal context the combination of age altitude and high resolution water level reconstructions offers the potential to obtain more detailed and reliable records of relative sea level change from salt marsh sediments

experimental constraints on the depth of olivine metastability in subducting lithosphere the hypothesis that metastable olivine persists in some subducting slabs into the transition zone has wide implications for mantle dynamics and rheology in order to evaluate this possibility we derive new thermo kinetic subduction zone models to predict the extent of olivine metastability within the stability fields of its high pressure polymorphs wadsleyite and ringwoodite our updated models improve on previous work by incorporating experimental kinetic data on realistic mantle compositions mg fe 2sio4 rather than analogue systems furthermore latent heat due to the transformation is fed back into both the kinetics and the thermal model we also consider the effects of transformation stress on growth kinetics and the possibility of an intracrystalline transformation mechanism previously thought to be important only at high shear stresses our models predict significantly smaller wedges of metastable olivine than previous work in the case of tonga for example where high values of lithospheric age 100 140 million years and convergence rate 14 cm per year are most favorable for metastability models considering only grain boundary nucleation and interface controlled growth predict olivine metastability to 600 km depth in contrast to 660 km predicted previously by kirby et al rev geophys 34 1996 261 when intracrystalline transformation is considered the depth of metastability is further reduced by as much as 100 km due to the large increase in the density of nucleation sites inhibition of growth by transformation stress can increase the depth interval over which the transformation takes place but is unlikely to be a dominant factor especially if the intracrystalline mechanism operates these results indicate that the existence of metastable olivine at depths corresponding to those of the deepest earthquakes 680 km requires subduction of old lithosphere 100 million years and a high vertical subduction velocity 15 cm per year such conditions currently may only be achieved in the very northern part of the tonga subduction zone yet earthquakes occur at depths down to 680 km in other subduction zones in which lithospheric age and or subduction velocity are relatively low e g indonesia and the marianas therefore mechanisms other than transformational faulting in metastable olivine must operate to cause the deepest earthquakes

environmental constraints of foraminiferal assemblages distribution across a brackish tidal marsh caminha nw portugal abstract foraminiferal living assemblages from a brackish tidal marsh were studied across three sampling transects under spring and autumn conditions foraminiferal analysis was also extended to the dead assemblages of spring samples altimetric control of each sampling point was acquired through the connection to the national datum by gps differential positioning combined with a regional geoid model the influence of the geochemical characteristics on the assemblages including chemical composition of the estuarine and marsh surface sediments in addition to salinity ph and calcite saturation of estuarine and marsh interstitial waters was evaluated a high affinity was found between spring and autumn living assemblages probably related to the low number of representative species the affinity also found between living and dead assemblage was seen as a consequence of low ph and calcite undersaturation that prevent the accumulation effect of several generations of foraminifera salinity appears as a clear constraint to the foraminiferal assemblages composition that is enhanced by calcite undersaturation in brackish marshes interstitial waters and sediments generally abundant marsh species like trochammina inflata jadammina macrescens and the calcareous foraminifera are limited to an episodic presence foraminiferal assemblages distribution across brackish tidal marsh can be assumed as an elevation proxy the high marsh ib sub zone is dominated by haplophragmoides sp and haplophragmoides manilaensis low marsh zone ii is characterised by the dominance of miliammina fusca but can be subdivided in an iia upper part where is followed by pseudothurammina limnetis and in a iib lower part defined by a subsidiary presence of psammosphaera sp haplophragmoides manilaensis dominance between 1 55 m and 1 67 m at living as well as dead assemblages suggests this species as a potential sea level indicator for the brackish tidal marshes

sedimentary characterization of bed types along the guadiana estuary sw europe before the construction of the alqueva dam abstract the guadiana river estuary is one of the most important mesotidal fluvio marine systems of the south western iberian peninsula the river mouth was formed as a narrow channel that was excavated by fluvial incision during the pleistocene lowstand and was then flooded 6500 years ago the estuary is in an advanced state of sediment infilling in its proximal part due to its narrow morphology which prevents passage of sediment through to the open coast consequently sediment is accumulating in the river mouth which causes progradation the analysis of the sediments into the estuarine channel grain size organic carbon content and bedform distribution side scan sonar allows a distinction between two types of sedimentation to be made a fluvial marine sedimentation favoured by tidal action and an autochthonous sedimentation that is related to water mixing the net transport of sediments is towards the sea because of the tidal current asymmetry the autochthonous deposits and the sediments from the extreme fluvial floods that tides are unable to rework are preserved on the meandering convex margins under low tidal velocities these areas acquire a lateral tidal bar morphology with cohesive beds because the narrow geometry of the estuary inhibits the presence of longitudinal tidal bars as may be expected in tide dominated systems the higher energy zones deeper zones of the channel become bypassing channels where the flocculated material cannot be settled on the bottom this part of the channel then develops sandy beds with mesoforms as ebb oriented two dimensional and three dimensional dunes sand waves and megaripples

seismic discontinuities and subduction zones abstract the seismic discontinuities at 410 and 660 km depth in the mantle are generally believed to be due to phase transformations in its major component olivine an increasing observational base of short period regional seismic network data supports this view here we analyse cross sections of discontinuity topography in the northwest pacific subduction zones to infer the thermal and chemical state of the mantle in a subduction environment most of the data is from the southern part of the izu bonin subduction zone penetration of the lower mantle by the steeply dipping subducting slab is clearly indicated by depression of the 660 km discontinuity elevation of the 410 km discontinuity in this region by up to 60 km implies a thermal anomaly of approximately 900 1000 c relative to the ambient mantle observation of the elevated 410 in the seismically active part of the slab indicates that the olivine wadsleyite phase transformation occurs under essentially equilibrium conditions there which argues against the transformational faulting hypothesis as the mechanism for deep earthquakes upgoing p wave reflections from the 410 km discontinuity indicate that it is sharp in the mantle far from subducting slabs reflection coefficient variability in and near slabs indicates either a chemical effect or the consequences of topographic focusing

the effects of deep water cycling on planetary thermal evolution 1 we use a parameterized convection model to investigate the effects of deep water cycling on the thermal evolution of an earth like planet the model incorporates two water reservoirs a surface and an interior mantle reservoir exchange between the two is calculated using a mantle convection parameterization that allows for temperature and water dependent mantle viscosity together with internally self consistent degassing and regassing parameterizations the balance between degassing and regassing depends on the average spreading rate of tectonic plates the amount of water partitioned into melt the thickness of a mantle melt zone and of a hydrated layer at the top of subducting plates degassing scales with melt zone thickness such that an early period of extensive melting would create a drier and more viscous mantle shifting the solidus line in a direction that would reduce the melt zone thickness and the rate of mantle heat loss coupling a hydrated zone thickness dependent regassing factor to the model to mimic water delivery to the mantle via a serpentinized layer allows for the potential of a reversing point where the overall water flow direction switches from degassing to regassing as the mantle cools the water effect on viscosity creates a negative feedback that tends to regulate the final amount of water in the mantle so it is not strongly dependent on the initial amount of planetary water the final amount of water in the surface reservoir is then determined by this feedback effect together with the initial water budget of the entire planet this implies that if the initial water budget of a planet can be estimated from planetary formation models then the volume of surface water can be used to estimate the volume of water in the mantle of an earth like planet applying this methodology to the earth leads to predictions for water concentration in the earth s mantle that are in line with geochemical and petrological constraints

stability of hydrous melt at the base of the earth s upper mantle seismological observations have revealed the existence of low velocity and high attenuation zones above the discontinuity at 410 km depth at the base of the earth s upper mantle1 2 it has been suggested that a small amount of melt could be responsible for such anomalies3 4 5 the density of silicate melt under dry conditions has been measured at high pressure4 6 7 8 9 10 11 12 13 and found to be denser than the surrounding solid thereby allowing the melt to remain at depth but no experimental investigation of the density of hydrous melt has yet been carried out here we present data constraining the density of hydrous basaltic melt under pressure to examine the stability of melt above the 410 km discontinuity we infer that hydrous magma formed by partial melting above the 410 km discontinuity may indeed be gravitationally stable thereby supporting the idea that low velocity or high attentuation regions just above the mantle transition zone may result from the presence of melt

uk intertidal foraminiferal distributions implications for sea level studies foraminiferal assemblages have been collected from ten intertidal study areas situated on the east south and west coasts of the uk the assemblages display a vertical zonation which indicates that the distribution of foraminifera in these intertidal environments is usually the direct function of altitude with the duration and frequency of intertidal exposure the most important factors multivariate analyses separate foraminiferal assemblages into two faunal zones a high and middle marsh zone consisting of differing abundances of jadammina macrescens trochammina inflata and miliammina fusca and a low marsh and tidal flat zone dominated by calcareous foraminiferal species notably elphidium williamsoni haynesina germanica and quinqueloculina spp these faunal zones are similar to those in other mid latitude cool temperate intertidal environments although there are spatial and temporal variations between areas the altitudinal ranges of the faunal zones are employed to identify the vertical relationship of the local environment in which the assemblage accumulated to a reference tide level

water solubility in mg perovskites and water storage capacity in the lower mantle the water storage capacity of the major constituent of the lower mantle mg perovskite is a matter of debate here we report water solubility of mg perovskites with different compositions observed in peridotite and morb systems ir spectra of pure mgsio3 perovskite show bands at 3397 3423 3448 and 3482 cm 1 and suggest about 100 ppm h2o the h2o content in al mg perovskite 4 7 wt al2o3 mg 100 is 1000 1500 ppm major band at 3448 cm 1 whereas al fe mg perovskite in morb al2o3 13 17 wt mg 58 61 contains 40 110 ppm h2o major band at 3397 cm 1 the h2o content in al fe mg perovskite observed in peridotite al2o3 5 6 wt mg 88 90 is 1400 1800 ppm major band at 3397 cm 1 al fe mg perovskite from the morb system has a high fe3 content fe3 fe 0 6 determined by electron energy loss spectroscopy measurements water can enter into the perovskite structure with oxygen vacancies originating from the substitution of si by al and fe3 oxygen vacancy incorporation is favored for aluminous perovskite synthesized from the mgo rich peridotite system the substitution of si4 mg2 2 al fe 3 prevails however in the al fe mg perovskite from the morb system mgo poor al and fe rich explaining its restricted water solubility the maximum amount of water stored in the lower mantle is estimated to be 3 42 1021 kg which is 2 5 times the present ocean mass comparison of the phase relations in hydrous pyrolite and hydrous morb indicates that pyrolite is more important as water container and water carrier in the mantle pyrolite contains 1 dense hydrous magnesium silicates existing under conditions of subducting slabs and 2 hydrous wadsleyite hydrous ringwoodite and water bearing perovskite under the normal mantle and hotter conditions distribution of water to the morb is restricted at the conditions of the transition zone and lower mantle

intertidal and subtidal benthic foraminifera in flooded caves implications for reconstructing coastal karst aquifers and cave paleoenvironments abstract coastal karst aquifers have an upper meteoric water mass stratified from a saline groundwater mass below that increasingly mix and discharge towards the ocean preserved sediment in caves and sinkholes provide an opportunity to evaluate the long term hydrodynamics in the local coastal karst aquifer e g salinity oxygenation vertical displacement in response to sea level forcing here we evaluate the response of shallow 45 m versus 63 m mesh sieve fractions these modern analog results can be used to interpret subfossil benthic foraminiferal records from coastal karst aquifers especially during early cave inundation by concomitant groundwater and sea level rise

seismic evidence for water transport out of the mantle transition zone beneath the european alps abstract the mantle transition zone has been considered a major water reservoir in the deep earth mass transfer across the transition zone boundaries may transport water rich minerals from the transition zone into the water poor upper or lower mantle water release in the mantle surrounding the transition zone could cause dehydration melting and produce seismic low velocity anomalies if some conditions are met therefore seismic observations of low velocity layers surrounding the transition zone could provide clues of water circulation at mid mantle depths below the alpine orogen a depressed 660 km discontinuity has been imaged clearly using seismic tomography and receiver functions suggesting downwellings of materials from the transition zone multitaper correlation receiver functions show prominent 0 5 1 5 velocity reductions at 750 800 km depths possibly caused by partial melting in the upper part of lower mantle the gap between the depressed 660 km discontinuity and the low velocity layers is consistent with metallic iron as a minor phase in the topmost lower mantle reported by laboratory studies velocity drops atop the 410 km discontinuity are observed surrounding the alpine orogeny suggesting upwelling of water rich rock from the transition zone in response to the downwelled materials below the orogeny our results provide evidence that convective penetration of the mantle transition zone pushes hydrated minerals both upward and downward to add hydrogen to the surrounding mantle

salt marsh foraminifera from the great marshes massachusetts environmental controls abstract by using sites in the great marshes at barnstable massachusetts usa this study examines the effects of a set of environmental parameters on the foraminiferal distribution the studied parameters are elevation above mean high water salinity of the porewater various sediment characteristics vegetation and food source relations between the environmental parameters and foraminiferal properties frequencies densities and diversities are quantified with correlation coefficients for the first time siphotrochammina lobata and balticammina pseudomacrescens are documented in the new england region the following species show a significant correlation with one or more of the studied parameters and are designated as key species haplophragmoides manilaensis jadammina macrescens balticammina pseudomacrescens miliammina fusca and tiphotrocha comprimata based on cluster analysis and the presence absence or dominance of the key species characteristic associations are distinguished the distribution of three associations is indicative of specific marsh environments the marsh fringe the middle marsh and the marsh edge these three marsh units are separated by their own salinity regime flooding and sediment characteristics the marsh fringe is typified by the h manilaensis association and experiences freshwater input seepage surface runoff and rainwater and only slight marine influence resulting in low salinity values 2 5 20 the width of the marsh fringe is variable dependent on the amount of seepage which in turn is controlled by the permeability of the basement and the peat the j macrescens association characterizes the middle marsh where salinities are controlled by infiltration of sea and rainwater and by evaporation salinity values are higher than 20 while temporarily salinity can reach extreme high values during periods without flooding and high evaporation rates e g 44 the fully marine m fusca association occupies the daily flooded marsh edge where the salinities have the same values as cape cod bay water ca 28 unlike many other salt marshes the distribution of foraminiferal assemblages in the great marshes does not show a vertical zonation with respect to mean high water this shows that a worldwide applicable model for paleoenvironmental studies in salt marshes based on foraminifera is not feasible each salt marsh has its own characteristics regional factors such as climate play an important role in the salinity regime while the local upland characteristics determine if seepage takes place thus each marsh has its own foraminiferal fingerprint showing the opportunistic behaviour of the salt marsh agglutinants a surface study is an indispensable first step in assessing the value of foraminifera as paleo ecological indicators

water transport into the deep mantle and formation of a hydrous transition zone abstract the phase relations in the peridotite water system to the top of the lower mantle reveal that a certain amount of water can be transported and stored in the transition zone by various hydrous minerals resulting in formation of a hydrous transition zone results of preliminary experiments to clarify the effect of water on kinetics of the olivine wadsleyite transformation are given and show that water enhances the transformation kinetics significantly there are potential dehydration sites in the deep mantle four sites along the descending slabs depending on their temperature profiles i e the mantle wedge of the subduction zone mantle transition zone top of the lower mantle where hydrous ringwoodite and superhydrous phase b decompose and deep in the lower mantle around 1200 1500 km depth where decomposition of phase d g occurs we also expect one site of dehydration melting at the base of the upper mantle where the hot mantle plume hydrated in the transition zone can be dehydrated the transition zone can serve as a water absorber for the mantle scale water circulation system some of the recent seismic works including the p wave tomography images reveal the existence of low velocity anomalies in the transition zone suggesting possibly a wet transition zone elevation of the 410 km discontinuity and lack of clear evidence for a metastable olivine wedge in cold slabs may be consistent with the equilibrium phase transformation from olivine to wadsleyite in cold and wet slabs on the basis of our kinetic study of the transformation under wet conditions

chronology of the sedimentary processes during the postglacial sea level rise in two estuaries of the algarve coast southern portugal abstract four profiles of estuarine sediments obtained from boreholes drilled in the algarve southern portugal were studied in order to reconstruct the process of sediment accumulation driven by the postglacial sea level rise in addition to the sedimentological analysis the foraminifera index of marine influence fimi permitted assessment of the nature and organization of sedimentary facies in the beliche guadiana and gilao almargem estuaries the beliche guadiana cm5 and almargem g2 profiles accumulated in a sheltered environment with the former presenting an almost continuous record of the sea level rise since ca 13 000 cal yr bp the g1 and g3 profiles from the gilao almargem area represent a more discontinuous record of the last 8000 years which accumulated in the more dynamic environment of an outer estuary the integration of all radiocarbon ages of dated levels led to an estimate of sediment accumulation rates assuming a constant position of the sediment surface with respect to the tidal range and a negligible compaction of sediment the sea level rose at the rate of 7 mm yr 1 in the period from 13 000 to 7500 cal yr bp this process slowed down to ca 0 9 mm yr 1 from 7500 cal yr bp until the present the marked historical change in the rate of sediment accumulation in these estuaries also occurred with the accumulation of organic matter and is therefore important data for global biogeochemical models of carbon the main obstacle to obtain higher temporal resolution of the sedimentary processes was the intense anaerobic respiration of organic matter via sulphate reduction which did not allow any accumulation of peat and furthermore led to erasure of the palaeontological record by acid formed from the subsequent oxidation of sulphides

is phase a relevant to the earth s mantle abstract dense hydrous magnesium silicates dhms such as phase a mg 7 si 2 o 8 oh 6 are potential hosts for water in the earth s mantle previous work has delineated the stability field of phase a pha coexisting with clinoenstatite cen and forsterite fo in the system mgo sio 2 h 2 o msh in this study the effects of the presence of magnesite mc diopside di and pyrope py on the stability of coexisting pha cen fo have been determined in msh the breakdown reaction pha cen fo h 2 o was bracketed between 720 and 740 c at 7 gpa between 800 and 820 c at 8 gpa and between 860 and 880 c at 9 gpa at 10 gpa pha cen transforms to a clinohumite ch bearing assemblage between 920 and 940 c which in turn dehydrates between 940 and 960 c to fo v at higher pressures pha becomes unstable relative to phase e another dhms kanzaki 1991 addition of carbonate to msh changes the breakdown reaction to pha cen mc fo v where v is an h 2 o co 2 fluid at 8 gpa this reaction occurs between 800 and 850 c indistinguishable from the position of the co 2 free reaction in msh in the presence of diopside in the cao mgo sio 2 h 2 o system the reaction pha en ss fo di ss h 2 o occurs between 700 and 730 c at 8 gpa where en ss and di ss are ca poor and ca rich pyroxene solid solutions respectively in the pyrope containing assemblage in the mgo al 2 o 3 sio 2 h 2 o system the pha bearing assemblage reacts to form ch between 740 and 760 c at 8 gpa the ch bearing assemblage dehydrates to fo py v between 800 and 830 c at 8 gpa in the system mgo feo sio 2 h 2 o with molar mg mg fe 0 90 the reaction pha cen ss ch v occurs at ss ch fo v reaction occurs between 850 and 900 c in no case did the presence of additional components such as cao feo al 2 o 3 or co 2 or the additional phases diopside magnesite or pyrope stabilize the hydrous phases pha or ch to significantly higher temperatures therefore pha will not be a stable phase in lherzolite assemblages under normal geothermal gradients in the mantle in the interior of subducting slabs temperatures may be sufficiently low that assemblages containing pha may be stable to pressures greater than 10 gpa allowing pha to transport h 2 o to the deeper mantle it is more likely that the higher pressure phase e will be stabilized instead

can the confinement index calculated on the basis of foraminiferal populations be used in the study of coastal evolution during the quaternary abstract one of the major present preoccupations of ecologists is to foresee the effect of the predicted sea level change on the coastline and therefore on the biosphere prospective studies first require a thorough knowledge of present conditions and of past evolution paralic environments are very sensitive to coastline variations and provide one of the most sensitive tools for studying sea level change one of their main characteristics is the degree of marine influence named confinement that depends on climatic conditions and on the coastline morphology and evolution however these environments are under the influence of complex interacting parameters and their study requires a set of complementary tools a confinement index ic calculated on the basis of the main species of foraminifers has been worked out in the paralic environments of west africa it provides a numerical value that can be calculated rapidly and gives an immediate objective indication of slight changes in the environment with a tendency toward more restricted ic increases or less restricted ic decreases conditions this tendency might be in relation with sea level change thus ic is proposed as a preliminary tool for quantifying changes in the paralic ecosystems used together with a set of complementary indicators sedimentology geochemistry and so on it can be used for the detection of sea level change

implications of seismic waveforms complex physical properties associated with stagnant slab 1 anomalously broadened p waveforms of deep focus events were observed at some regional stations after sampling flattened high velocity zones the source processes of these events are short and the p waveforms at other stations as well as the corresponding sh waves do not show such anomaly the rays of the anomalous waves propagated in the vicinity of those whose waveforms can be synthesized with a relatively simple model m3 11 or m2 0 for a flattened cold slab in the transition zone we suggest that the p waveform broadening is caused by sv converted waves at the heterogeneities within a narrow zone near the 660 km discontinuity or slab boundary the creation of such heterogeneities may involve in the complex structure of phase transformation with water and cold temperature anomaly associated with stagnant slab

a sea level rise curve from guilford connecticut usa abstract high resolution stratigraphic studies based on sediment chemistry lithology macroflora and benthic foraminiferal assemblages in three peat cores from coastal salt marshes at guilford connecticut show that coastal marshes are ephemeral environments marsh wide environmental variations were common and century long episodes of relative submergence alternated with emergence despite about 2 m of relative sea level rise rslr in connecticut over the last 1500 years the marshes have expanded both landwards and seawards and marsh accretion has been outpaced only marginally by rslr we used radiocarbon dating and the level of anthropogenic markers metal pollution as dated with 210 pb in the cores for age control for most of the last 1000 years the rate of rslr was between 1 3 and 1 8 mm yr but over the last 300 400 years it increased to 2 9 3 3 mm yr and has been faster than the accretion rate especially in the middle marsh the net submergence rate or submergence index ratio of the rate of rslr and marsh accretion rate averaged about 1 15 over the last 1000 years and increased to about 1 5 over the last 200 years the rate of rslr was very sluggish during the early part of the little ice age but we found a slightly higher rate during the little climate optimum this excursion is close to the noise level however the most significant observation is that rslr increased strongly around a d 1650 the onset of this acceleration falls in the middle to end of the little ice age and thus preceded the period of modern global warming that started late last century and that has been tentatively correlated with anthropogenic emissions of greenhouse gases

elastic properties of hydrous ringwoodite γ phase in mg2sio4 abstract single crystal elastic properties of hydrous ringwoodite are reported for samples that were synthesized at 19 gpa and 1300 c the mg si ratio is determined to be 1 95 by epma which is slightly lower than that of ideal anhydrous ringwoodite the h 2 o content determined by sims is 2 2 wt resulting in the stoichiometry mg 1 89 si 0 97 o 4 h 0 33 the lattice parameter a 8 0786 4 a and elastic constants were determined from a single crystal with x ray diffractometry and brillouin scattering spectroscopy the crystal structure is cubic fd3m with a unit cell volume which is 0 51 larger than that of the anhydrous ringwoodite the adiabatic single crystal elastic moduli of hydrous ringwoodite in gpa are c 11 281 6 c 44 117 4 c 12 92 5 the isotropic properties of hydrous ringwoodite are k vrh 155 4 and g vrh 107 3 which are about 16 and 10 smaller than those of anhydrous ringwoodite thus hydrous ringwoodite if present drastically reduces the seismic velocity in the mantle transition zone relative to anhydrous ringwoodite in addition our results show that the anisotropy is enhanced relative to anhydrous ringwoodite using the present results we discuss the 520 km seismic discontinuity in a wet mantle transition zone

hydration adjacent to a deeply subducting slab the roles of nominally anhydrous minerals and migrating fluids subduction related transport of water into the mantle has significant dynamical and geochemical implications dehydration of hydrous phases within the slab can introduce water into the transition zone and lower mantle potentially hydrating nominally anhydrous minerals nam and impacting the viscosity and density structure of the mantle over a wide area we present models of fluid transport and mantle hydration in the vicinity of a deeply subducting slab focusing on the fate of water released by deep dehydration reaction in the subducted serpentinized mantle a sharp decrease in water storage capacity across the lower boundary of the transition zone may produce secondary dehydration of hydrated nam leading to precipitation of a hydrous fluid and heterogeneous hydration of the transition zone rapid fluid migration relative to the solid flow field can lead to a broad region of diffuse hydration within the upper mantle wedge and the potential for localized melt regions at the top of the transition zone coincident with fluid pathways slower fluid migration instead implies that the fluid phase can be transported deep into the lower mantle water stabilized in nam and as a free fluid can initiate upwelling within and above the transition zone a less abrupt change in water storage capacity across the base of the transition zone leads to high nam water contents in a channel adjacent to the slab where viscosity is reduced however seismic and electromagnetic observations of hydration in the transition zone are most compatible with a sudden drop of water storage capacity

complete miscibility between silicate melts and hydrous fluids in the upper mantle experimental evidence and geochemical implications the phase relationships between silicate melts and hydrous fluids were studied by direct visual observation in an externally heated diamond anvil cell complete miscibility of silicate melt and water was observed for a wide range of melt compositions including nepheline jadeite dacite haplogranite and ca bearing granite some evidence for complete miscibility was also observed in the system basalt h2o the critical temperatures in all systems decrease rapidly with pressure at 15 kbar the critical temperature for nepheline is around 550 c for jadeite around 800 c and for granitic compositions it is close to 900 c in general the critical temperatures appear to increase with silica content in the system our results suggest that there is complete miscibility between silicate melts and water in most of the upper mantle except at very shallow depths this means that a water saturated solidus cannot be defined any more in the deeper parts of the upper mantle very silica rich melt inclusions found in spinel lherzolites associated with fluid inclusion are probably the result of the unmixing of a supercritical fluid containing comparable amounts of water and silicate components the decomposition of amphibole in a subducted slab occurs at conditions where the miscibility gap between fluid and silicate melt is not yet closed while the decomposition of phengite and lawsonite occurs far beyond the critical curve accordingly the fluids released by the breakdown of these minerals should have very different properties highly mobile hydrous fluids containing little dissolved silicate should be produced by amphibole breakdown while the decomposition of lawsonite and phengite will lead to much more silicate rich and less mobile fluid phases

postglacial sea level rise and sedimentary response in the guadiana estuary portugal spain border abstract the study of sedimentological features foraminifera and molluscan fauna in samples recovered from four rotary boreholes permitted after 14 c dating the first assessment of the post glacial transgression in guadiana river estuary the holocene sequence is underlain by coarse sediments from delta fan enclosing continental swamp deposits dated 16 980 year bp the holocene sedimentary sequence was deposited either directly on this delta fan gravels or on top of fluvial sands deposited probably between 11 000 and 10 000 yr bp an accelerated phase of the estuary infilling by clayey sediments containing the salt marsh foraminifer trochammina began ca 9800 year bp when sea level was about 39 m below present between 7500 and 7000 year bp the central part of the estuary started to accommodate coarser sediments partially introduced from the continental shelf the first phase of holocene sea level rise at a rate of 0 85 m century terminated ca 6500 year bp since then lagoonal sediments in the vicinity of the estuary have been enclosed behind sand spits and predominantly sandy sedimentation was initiated within the estuary after a second phase of slower rise at the rate of 0 3 m century which lasted until ca 5000 year bp the sea approached the present level

saltmarsh foraminifera in the subarctic white sea thrive in summer endure in winter abstract abundance and diversity of intertidal benthic foraminifera decreases towards the poles well studied in the temperate zone saltmarsh foraminiferal assemblages have only recently received attention at higher latitudes we report modern foraminifera including the taxonomy abundance and zonation from a pristine saltmarsh in the subarctic white sea northern europe we sampled 10 stations surface sediment two replicates from two high tide flats covered with lush halophytic plants nine foraminiferal species found exhibited distinct vertical zonation confined to certain elevation levels the high marsh assemblage comprised arenaceous balticammina pseudomacrescens trochammina inflata jadammina macrescens and monothalamous ovammina opaca this is the northernmost location where the first two species have been encountered alive the low marsh assemblage was dominated by miliammina fusca and elphidium williamsoni summer abundances of live foraminifera were surprisingly higher than on other subarctic saltmarshes studied mostly 100 500 and up to 3000 per 10 cm in winter under a 40 cm of ice the abundances were only 15 20 ind 10 cm but the species richness was the same unlike summer specimens of calcareous foraminifera which are always brightly colored winter elphidium williamsoni all had bleak colorless cytoplasm suggesting they discard kleptoplasts and abandon algal diet we conclude that the insulating blanket of fast ice which hardly moves on saltmarshes prevents elimination of foraminifera in the harshness of winter whereas the intense continental heating in summer lets them thrive on white sea saltmarshes the composition of foraminiferal assemblages under local climatic conditions may thus deviate substantially from the latitudinal trend

recycling of crustal materials through study of ultrahigh pressure minerals in collisional orogens ophiolites and mantle xenoliths a review abstract newly recognized occurrences of ultrahigh pressure uh p minerals including diamonds in ultrahigh temperature uh t felsic granulites of orogenic belts in chromitites associated with ophiolitic complexes and in mantle xenoliths suggest the recycling of crustal materials through deep subduction mantle upwelling and return to the earth s surface this circulation process is supported by crust derived mineral inclusions in deep seated zircons chromites and diamonds from collision type orogens from eclogitic xenoliths in kimberlites and from chromitities of several alpine himalayan and polar ural ophiolites some of these minerals contain low atomic number elements typified by crustal isotopic signatures ophiolite type diamonds in placer deposits and as inclusions in chromitites together with numerous highly reduced minerals and alloys appear to have formed near the mantle transition zone in addition to ringwoodite and inferred stishovite a number of nanometric minerals have been identified as inclusions employing state of the art analytical tools reconstitution of now exsolved precursor uh p phases and recognition of subtle decompression microstructures produced during exhumation reflect earlier uh p conditions for example tibetan chromites containing exsolution lamellae of coesite diopside suggest that the original chromitites formed at p 9 10 gpa at depths of 250 300 km the precursor phase most likely had a ca ferrite or a ca titanite structure both are polymorphs of chromite and at 2000 c would have formed at minimum pressures of p 12 5 or 20 gpa respectively some podiform chromitites and host peridotites contain rare minerals of undoubted crustal origin including zircon feldspars garnet kyanite andalusite quartz and rutile the zircons possess much older u pb ages than the time of ophiolite formation these uh p mineral bearing chromitite hosts evidently had a deep seated evolution prior to extensional mantle upwelling and partial melting at shallow depths to form the overlying ophiolite complexes these new findings together with stable isotopic and inclusion characteristics of diamonds provide compelling evidence for profound underflow of both oceanic and continental lithosphere recycling of surface organic carbon into the lower mantle and ascent to the earth s surface through mantle upwelling intensified study of uh p granulite facies lower crustal basement and ophiolitic chromitites should allow a better understanding of the geodynamics of subduction and crustal cycling

revised 1 d mantle electrical conductivity structure beneath the north pacific summary the 1 d electrical conductivity model in the mid mantle beneath the northern pacific was revised in order to discuss the mean state of the mantle and to obtain a credible starting model for 3 d inversions semi global geomagnetic depth sounding gds responses obtained at 13 stations and submarine cable magnetotelluric mt responses for eight cables in a period range of 1 7 113 d were used to obtain the revised structure it is well known that surface conductivity heterogeneity due to the ocean land conductivity contrast has a large influence on the responses up to a period of 20 d the effect of surface ocean land distribution was removed by performing 3 d electromagnetic induction modelling including a surface conductivity heterogeneity distribution laid above a 1 d mantle structure the corrected responses were averaged in the logarithmic space and the resulting responses quasi 1 d response were inverted to obtain 1 d conductivity models correction of the surface heterogeneity effect requires a 1 d conductivity model and the 1 d model must be similar to the resultant 1 d model for a consistent correction we attained this by making iterations of the surface layer correction and 1 d inversion synthetic tests revealed that the iterative scheme could recover the supposed structure even for a 3 d heterogeneous mantle since electromagnetic sounding is not sensitive to the presence of a sharp discontinuity we examined three different cases of conductivity variation with depth 1 a model with no discontinuity 2 a model with two jumps at 400 and 650 km depths and 3 a model with three jumps at 400 500 and 650 km depths the conductivity of the two jump model in the transition zone is higher than the experimentally determined conductivity of dry wadsleyite and ringwoodite if the difference is entirely due to the effect of water in the transition zone then the conductivity is consistent with a water content in the region of 0 5 wt per cent however if an additional discontinuity of electrical conductivity is allowed at the 500 km depth the obtained conductivity for the upper 100 km of the transition zone is lower and can be explained by a less water content 0 1 wt per cent in wadsleyite

experimentally based water budgets for dehydrating slabs and consequences for arc magma generation phase diagrams of hydrous mid ocean ridge mor basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones a synthesis of our experimentally determined phase relations of chlorite lawsonite epidote zoisite amphibole paragonite chloritoid talc and phengite in basalts and of phase relations from the literature of serpentine talc chlorite amphibole and phase a in ultramafics permits calculation of h2o contents in hydrous phase assemblages that occur in natural compositions this yields the information necessary to calculate water budgets for descending slabs starting from low grade blueschist conditions 10 20 km depth with h2o contents between 5 and 6 wt for hydrated oceanic crust complete dehydration is achieved between 70 and 300 km depth as a function of individual slab geotherms hydrous phases which decompose at depth below volcanic arcs are lawsonite zoisite chloritoid and talc phengite in mafic compositions and chlorite and serpentine in peridotite approximately 15 35 of the initially subducted h2o are released below volcanic arcs the contribution of amphibole dehydration to the water budget is small 5 20 and occurs at relatively shallow depth 65 90 km in any predicted thermal structure dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab such a dehydration characteristic is incompatible with single phase dehydration models which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts as a consequence of continuously progressing dehydration water ascending from the slab will be generally available to depth of ca 150 200 km the fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge we propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas thermal models show that such an isotherm ca 1300oc locates below volcanic fronts slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts 1998 elsevier science b v all rights reserved

mid mantle seismic anisotropy beneath southwestern pacific subduction systems and implications for mid mantle deformation abstract observations of seismic anisotropy can offer relatively direct constraints on patterns of mantle deformation but most studies have focused on the upper mantle while much of the lower mantle is thought to be isotropic several recent studies have found evidence for anisotropy in the transition zone and uppermost lower mantle the mid mantle particularly in the vicinity of subducting slabs here we investigate anisotropy at mid mantle depths in the tonga kermadec sumatra new britain new hebrides and philippines subduction zones using the source side shear wave splitting technique we measure splitting of direct teleseismic s phases originating from deep events 300 km that have been corrected for the effect of upper mantle anisotropy beneath the seismic stations we find evidence for considerable anisotropy at mid mantle depths in all subduction systems studied with delay times averaging 1 0 1 5 s several measurements originating from depths greater than 600 km exhibit delay times greater than 1 s suggesting a significant contribution from anisotropy in the uppermost lower mantle we combine our results with those documented in previous studies into a quasi global set of source side shear wave splitting measurements that reflect mid mantle anisotropy we document significant variability in the dominant fast directions both within and among individual subduction systems suggesting different deformation geometries among different subduction systems as further constraints on the elasticity and deformation of mid mantle minerals become available our dataset can be used to constrain patterns of mid mantle flow associated with subduction

effect of water in depleted mantle on post spinel transition and implication for 660 km seismic discontinuity abstract we have determined the post spinel transition boundary in anhydrous and hydrous mg2sio4 in a temperature range from 1173 to 2023 k at 19 3 25 4 gpa using synchrotron in situ x ray diffraction measurements the phase boundary in mg2sio4 is located at 22 gpa and 1800 k and 22 1 gpa and 1500 k which is slightly lower 0 3 0 5 gpa than that determined in the previous in situ measurements using the same pressure scale e g katsura et al 2003 post spinel transition in mg2sio4 determined by high p t in situ x ray diffractometry phys earth planet inter 136 11 24 the clapeyron slope of mg2sio4 was found to be gentle i e between 0 4 and 0 7 mpa k which is also consistent with previous in situ measurements but inconsistent with diamond anvil cell experiments and theoretical estimations the phase boundary in mg2sio4 2 wt h2o which is relevant to fe free depleted harzburgitic composition is located between 23 4 and 23 6 gpa and 1500 k which shifts the hydrous boundary to the higher pressures relative to anhydrous mg2sio4 from 1 3 to 1 0 gpa the result for hydrous mg2sio4 shows steeper clapeyron slope between 3 2 and 3 1 mpa k compared with anhydrous mg2sio4 and hydrous pyrolite system the present data suggest that water has a strong influence on 660 km discontinuity and the depressions observed at this boundary in several regions especially related to subduction zones can be explained by the presence of water in depleted harzburgite component

variable seismic structure near the 660 km discontinuity associated with stagnant slabs and geochemical implications abstract this study reports a compilation of reflectivity synthetic modeling for the structure of the upper mantle transition zone with high velocity anomalies hva s associated with the northwestern pacific subduction zones here the employed method of reflectivity synthetics effectively determines the structure of flattened hva i e stagnant slab as triplicated regional body waves are very sensitive to the velocity discontinuities in the transition zone results show a distribution of hva s with or without a depression of the 660 km discontinuity depth which indicates a possible variation of geochemical properties at the bottom of the upper mantle a hypothesis is proposed for this implication i e that the structural variation may represent the contrast between a hydrous garnet rich layer subducted crust and bulk peridotite associated with a stagnant slab which is supported by the results of recent laboratory experiments the garnet rich layer can flow and descend faster than bulk peridotite as hydrous garnet is weaker and denser than peridotite in the transition zone given that the clapeyron slope for hydrous garnet perovskite is positive at 660 km two zones of hva with and without a depression in the discontinuity depth may exist next to each other at the bottom of the transition zone this variation of the discontinuity depths coincides with the segmentation identified at deeper depths 629 km in a p wave travel time tomography model although the resolution of the tomography inversion is limited to elaborate the discontinuity structure

slab mantle interactions 1 sheared and refertilised garnet peridotite xenoliths samples of wadati benioff zones abstract most sheared and deformed garnet peridotite facies xenoliths found in kimberlites worldwide have been formed from initially depleted peridotite protoliths which have subsequently experienced varying degrees of refertilisation ti fe ca al na p zr and y have been introduced to various degrees by silicate melts and the protoliths thereby refertilised these melts were not kimberlitic they possessed basaltic 100 mg mg fe mole ratios of 65 70 and contained several wt tio2 and were often capable of precipitating ilmenite equilibration temperatures recorded by sheared refertilised garnet periodtites are usually above those of a steady state shield geotherm it is proposed that these rocks acquired their high equilibration temperatures refertilised character and highly deformed fabrics along wadati benioff zones in regions of active subduction beneath cratons bodies of former serpentinite in the deeper layers of the basaltic crust of the subducted lithosphere or in underlying harzburgite may transport water to depths of several hundred kilometers water released by the breakdown of subducted former serpentinite migrates upwards into hotter regions of the eclogitic former oceanic crust causing partial melting to occur particularly in the depth interval between 150 and 300 km these slab derived melts are episodically injected into peridotite immediately overlying the former oceanic crust hybridisation occurs in this boundary layer with ti fe ca al na p zr and y being transferred from the slab into this region thereby rendering it fertile episodic and repeated injection of small volumes of slab derived melts into adjacent pre stressed peridotite would also contribute to local weakening accompanied by catastrophic shear failure at high strain rates the deformed fabrics of most sheared peridotites may have been produced in this manner local transient heating may also accompany displacements of this kind thus accounting for the seemingly elevated equilibration temperatures recorded by these rocks

elasticity of single crystal superhydrous phase b at simultaneous high pressure temperature conditions we investigated the combined effect of pressure and temperature on the elasticity of single crystal superhydrous phase b shy b using brillouin scattering and x ray diffraction up to 12 gpa and 700 k using the obtained elasticity we modeled the anisotropy of shy b along slab geotherms showing that shy b has a low anisotropy and cannot be the major cause of the observed anisotropy in the region modeled velocities of shy b show that shy b will be shown as positive velocity anomalies at the bottom transition zone once shy b is transported to the topmost lower mantle it will exhibit a seismic signature of lower velocities than topmost lower mantle we speculate that an accumulation of hydrous phases such as shy b at the topmost lower mantle with a weight percentage of 17 26 in the peridotite layer as subduction progresses could help explain the observed 2 3 low shear velocity anomalies in the region

hydrogen site analysis of hydrous ringwoodite in mantle transition zone by pulsed neutron diffraction hydrogen site positions and occupancies in the crystal structure of synthetic hydrous ringwoodite have been determined for the first time by high resolution neutron powder diffraction conducted at the pulsed neutron source at japan proton accelerator research complex j parc it is demonstrated that hydrogen exchanges not only with mg or fe but also with si cations to form hydroxyl and hydrogen bonding both within the relevant octahedra or tetrahedra created by their surrounding oxygen anions the hydrated octahedra shrink whereas the hydrated tetrahedra expand the occurrence of simultaneous hydration of octahedra and tetraheda is thus unambiguously confirmed whereby the latter plays an especially unique role in reducing seismic velocity and for enhancing the electrical conductivity of ringwoodite at high pressures and temperatures these results provide distinctive implications for discussing the physical properties of hydrous ringwoodite occurring in the mantle transition zone and evaluating the actual water content in the zone

evolution of the continental crust in the proterozoic eastern ghats belt india and new constraints for rodinia reconstruction implications from sm nd rb sr and pb pb isotopes abstract for this study nd sr and pb isotope compositions were analyzed for ortho and paragneisses from the eastern ghats belt of india in order to determine its crust formation and crustal evolution this belt represents a proterozoic orogen that extends along the east coast of peninsular india and forms part of the mobile belts in east gondwana and rodinia the eastern ghats belt was affected by mesoproterozoic granulite facies metamorphism in the western segment western charnockite zone and a grenvillian regional scale high grade event in the central and eastern segments western khondalite zone charnockite migmatite zone and eastern khondalite zone as well as a local pan african overprint the results of the isotope studies are used for the large scale reconstruction of the indo antarctic part of the rodinia supercontinent based on nd model ages and pb isotope ratios from leached feldspars four crustal domains can be distinguished in the eastern ghats belt these domains can in part be correlated with the lithological division of the belt 1 the western charnockite zone south of the godavari graben is characterized by nd model ages between 2 3 and 2 5 ga for orthogneisses and 2 6 and 2 8 ga for metasediments domain 1 the pb isotopes are primitive indicating reworking of dominantly archean and mixing with minor proterozoic material 2 north of godavari graben nd model ages for orthogneisses are significantly higher with values ranging from 3 2 to 3 9 ga the pb isotopes are strongly retarded 3 the north eastern parts of the charnockite migmatite zone and western khondalite zone form a distinct and almost homogeneous crustal domain domain 3 with nd model ages between 1 8 and 2 2 ga 4 between the isotopically homogeneous terranes stretches a broad transition zone domain 2 enclosing parts of the western khondalite zone charnockite migmatite zone and eastern khondalite zone the nd model ages for metasediments 2 1 2 5 ga are younger than paragneiss ages of the adjoining western charnockite zone the nd model ages for orthogneisses 1 8 3 2 ga display a large spread which is consistent with the pb isotope signatures that indicate mixing of archean with proterozoic material the border zone between domain 3 and the archean eastern indian craton forms a second transition zone domain 4 characterized by metasediments with nd model ages between 2 2 and 2 8 ga and orthogneisses with model ages around 3 2 ga reworking of archean crustal material is most intense along the border zones of the belt and juvenile material is more dominant away from the orogenic front this scenario is indicative of an active continental margin setting for the two proterozoic episodes of orogenesis in the eastern ghats belt a correlation of domain 3 with the rayner complex and the prydz bay region antarctica for the early crustal evolution is supported by the similarity of the isotope signatures the napier complex is very different to the eastern ghats belt and an early joint evolution of these terranes is ruled out on the basis of the pb pb and sm nd systematics

foraminiferal response to holocene environmental changes of a tidal estuary in victoria southeastern australia abstract the holocene strata in the anderson inlet area in victoria can be stratigraphically divided into four units unit i unit ii unit iii and unit iv unit i and unit iv lack fossils and were deposited in non marine probably fluvial environments unit ii and unit iii contain abundant foraminifera with molluscs ostracods and bryozoans foraminiferal analysis suggests that unit iii was formed in a partially sheltered marine environment while the high plankton content and relatively high diversity of benthic species in unit ii indicate that this unit was deposited in an open bay at water depths possibly less than 5 m the foraminiferal data are integrated with radiocarbon dates to arrive at the following holocene palaeoenvironmental history in this area 1 low alluvial plain stage 10 000 7000 yr b p 2 open bay environment stage 7000 5500 yr b p 3 partially sheltered marine environment stage 5500 4500 yr b p 4 alluvial plain and coastal lagoon environment stage since about 4500 yr b p the foraminiferal fauna show a clear response to these palaeoenvironmental changes globigerina bulloides can be used as an indicator for cold water marine environments the high concentration of this species in these middle holocene sediments shows a strong cold water influence on the coastal environments which reduced the effect of regional warm currents during this period the holocene palaeoenvironmental changes in the area were controlled by the holocene sea level fluctuations associated with the deglaciation history during this period similar integrated studies of shallow to marginal marine strata in southern africa america and new zealand will lead to a better understanding of holocene relative sea level change and the interplay between holocene cold and warm water regimes in the southern hemisphere

the influence of hydrous phases on the microstructure and seismic properties of a hydrated mantle rock abstract to better understand the microstructural evolution of a serpentinized mantle rock and the influence of various hydrous phases on the seismic properties of the mantle wedge we have conducted the detailed microstructural analyses of a sample of tremolite chlorite antigorite schist collected from the moses rock dike central part of the colorado plateau we performed differential effective media dem modelling to study the effect of three hydrous phases forming two phase aggregates with olivine considering the crystallographic preferred orientation cpo of each phase and the shape ratio of the hydrous phases we have demonstrated that in a partially serpentinized peridotite the olivine cpo characteristic of 100 010 dislocation glide is still preserved and the high temperature asthenospheric flow is preserved with a foliation normal to that of antigorite schist the transformation of olivine into antigorite occurs predominantly 75 by the relationship 100 ol 001 atg with 001 ol 010 atg with the 010 ol 001 atg and 001 ol 010 atg relationship observed in areas of weak antigorite cpo chlorite results from the phase transformation of olivine in a relatively static environment as shown by the correlation between the olivine chlorite cpos with 100 ol 100 ch 010 ol 001 ch and 001 ol 010 ch the fluid percolation that caused the localized metasomatism and partial hydration of the mantle occurred possibly along trans lithospheric shear zones the presence of chlorite induces the most important drop on the p wave velocities and may help to explain some local low velocities in the fore arc mantle wedges but is unlikely to be of global importance due to its very high vp vs ratio 1 9 on the other hand antigorite is the only phase that causes important modification on the propagation directions of p and s waves and the only phase to explain the polarization of the fastest shear waves parallel to the subduction trench

in situ x ray diffraction study of post spinel transformation in a peridotite mantle implication for the 660 km discontinuity abstract we present the phase relations in anhydrous cao mgo feo al2o3 sio2 pyrolite to examine the influence of compositional difference between pyrolite and mg2sio4 on the post spinel phase transformation it is shown that in the pyrolite system the transformation occurs at about 0 5 gpa lower pressure relative to mg2sio4 we have carried out several in situ x ray diffraction experiments on ringwoodite to mg perovskite ferropericlase and backward transformations and found that post spinel transformation boundary can be expressed as p gpa 0 0005 t k 23 54 using the gold equation of state by tsuchiya t tsuchiya first principles prediction of the p v t equation of state of gold and the 660 km discontinuity in earth s mantle j geophys res 108 2003 doi 10 1029 2003jb002446 the interval for coexisting ringwoodite and mg perovskite was found to be 0 1 0 5 gpa the discrepancy between our data and the depth of the seismic discontinuity global average 654 km is about 20 km at 1850 k based on the results of in situ measurements we confirmed that the difference in chemical composition between pyrolite and mg2sio4 cannot modify the clapeyron slope of the post spinel transformation using experimental data and assuming the average mantle temperature 1850 k at 660 km we can account for only a half of variations in the depth of the 660 km discontinuity in subduction zones and at hot spots an additional explanation for the observed seismological variations at the 660 km discontinuity is required and may reflect influence of other minor components or volatiles

comparison between the dead and living benthic foraminiferal assemblages in aveiro lagoon portugal abstract this work provides a comparison between living las and dead das benthic foraminiferal assemblages in 53 sampled sites located in intertidal and subtidal areas of ria de aveiro portugal the results of multivariate statistical analysis provide evidence for the main factors that control the distribution of das and las which seems to correspond to different gradients of marine continental influence conditioned by differences in the hydrodynamics of tidal currents in addition to these factors the las are clearly influenced by the total organic carbon biopolymers and pollutants the main factor that however drives the high level of agreement or the degree of disorder between the das and the las seems to be the hydrodynamics the highest level of agreement between the das and the las is found in sheltered inner lagoon areas out of the tidal currents strongest influence this study shows an interesting application of advanced statistical analysis to study the similarity between las and das aiming to identify the zones or conditions of a coastal system in which the das can potentially be a better record of las through similar studies conducted in coastal systems it is possible to select the most suitable sites to collect cores in view of the development of paleoenvironmental studies

high hydrogen solubility in al rich stishovite and water transport in the lower mantle abstract stishovite is an important phase in subducting oceanic crust the post garnet assemblage from a precursor eclogite lithology contains up to 25 stishovite at pressures above 25 gpa this stishovite may contain up to 5 wt al 2 o 3 we measured the hydrogen contents of stishovite samples synthesized at 20 25 gpa and 1200 1800 c from several starting materials containing up to 10 wt al 2 o 3 ftir spectra of stishovite show major bands at 3111 3134 cm 1 with the frequencies increasing as h 2 o and al 2 o 3 contents increase and several minor bands at 2659 2667 3240 3261 3312 3334 and 3351 cm 1 the h 2 o content of al free stishovite is in the range of 16 30 wt ppm the maximum h 2 o content of al bearing stishovite 4 4 wt al 2 o 3 synthesized at 20 gpa and 1400 c is 3010 300 wt ppm most hydrogen in stishovite is associated with al 3 substitutional defects on the octahedral si 4 site the hydrogen can occupy 40 of vacancies created by incorporation of al 3 at 20 gpa this observation along with some anomalies in the ftir spectra may indicate an alternative mechanism of al 3 incorporation into stishovite via the formation of oxygen vacancies or interstitial al i defects we report the highest h 2 o concentrations in al stishovite to date and argue that it is the most important carrier of water into the lower mantle post garnet eclogitic assemblages

olivine flotation in mantle melt abstract molten komatiite and peridotite have been compressed in an octahedral multi anvil device up to 10 gpa densities of the melts were measured at pressure intervals in the range 7 to 10 gpa by observing sinking and floating san carlos olivines and synthetic forsterite marker spheres the multi anvil results for komatiite when combined with piston cylinder measurements done at 4 to 6 gpa and a calculated reference density at 105 pa yield a birch murnaghan isothermal bulk modulus of k1900c 26 gpa and pressure derivativek 4 25 the pressure of neutral buoyancy for olivine in komatiite is confirmed to be near 8 gpa as predicted in earlier work olivine flotation in the experimental komatiite commences at a pressure close to where the liquidus phase changes from olivine to denser garnet leading to the possibility of density driven crystal sorting during fractionation molten peridotite klb 1 shows an isothermal compression 2000 c of 0 065 g cm 3 gpa 1 in the interval 105 pa to 8 2 gpa the olivine liquid peridotite density crossover is predicted to lie between 9 and 11 gpa indicating that olivine flotation can operate at depths of 300 500 km in a molten peridotitic mantle

imaging the subducting slabs and mantle upwelling under the japan islands summary a high resolution p wave tomography of the crust and mantle down to 700 km depth beneath the japan islands is determined using a large number of high quality arrival time data from local earthquakes and teleseismic events simultaneously the tomography shows that the philippine sea slab is subducting aseismically down to 430 km depth under southwest japan though the seismicity within the slab ends at 180 km depth a low velocity low v zone in the mantle wedge under tohoku and kyushu is found to extend westward from the volcanic front to the backarc under the japan sea and east china sea significant low v anomalies are revealed in the deep portion of the mantle wedge 400 500 km depth above the pacific slab under southwest japan which may reflect hot mantle upwelling associated with fluids from the deep dehydration of the pacific slab low v anomalies appear at 420 700 km depths beneath the pacific slab under eastern japan which may reflect hot mantle upwelling associated with the deep subduction of the pacific slab and its collapsing down to the lower mantle

foraminiferal microhabitats in a high marsh consequences for reconstructing past sea levels abstract the foraminiferal faunas of the high marsh of plougoumelen in the western part of the gulf of morbihan on the french atlantic coast have been studied in three successive sample sets taken in weekly intervals in april 2001 each sampling day four cores have been taken in different marsh environments with or without vegetation in order to determine the microhabitat of the major species and the eventual influence of a vegetation cover physico chemical parameters have been surveyed during each sampling day the foraminiferal faunas are similar to other faunas described from temperate salt marshes we have distinguished three groups of taxa with different microhabitats a jadammina macrescens is found in comparable numbers down to about 9 cm depth b shallow infaunal taxa such as miliammina fusca and trochammina inflata and c deep infaunal taxa such as haplophragmoides wilberti and arenoparrella mexicana although these deep infaunal taxa have to cope with low oxygen concentrations or even anoxia life at depth in the sediment appears to provide ecological advantages our survey of the physico chemical parameters shows an extreme variability at the surface in comparison with much more stable conditions deeper in the sediment these more stable conditions demanding smaller tolerance ranges for several parameters temperature salinity water content appear to be advantageous to deep infaunal taxa for the species j macrescens the microhabitat depth appears to be controlled by the water content of the sediment that strongly fluctuates in function of the meteorological conditions during dry periods when the water content of the sediment is lower j macrescens displays a much shallower microhabitat than during more humid periods at the sediment surface the presence of vegetation appears to have a positive influence on the density of the foraminiferal faunas by providing a shelter to desiccation during low tide or more prolonged dry periods important differences exist between the live faunas found in the uppermost centimetre of the sediment and those of the total 9 cm deep sediment cores therefore data inventories of living faunas based on the superficial centimetre alone can not be used as a basis for paleo environmental reconstructions without correction for these microhabitat effects

phase relations and melt compositions in cmas pyrolite h2o system up to 25 gpa abstract phase relations and melt compositions in the cao mgo al 2 o 3 sio 2 pyrolite under hydrous 2 of h 2 o and anhydrous conditions have been determined at 10 25 gpa and temperatures from 1400 to 2400 c at the pressure of 16 and 25 gpa the apparent solidus temperatures for the hydrous system are about 200 and 100 c lower than the solidus temperatures for the dry system respectively the apparent solidus temperature changes drastically from 1600 c at 13 5 gpa to about 1850 c at 16 gpa majorite is a liquidus phase in the hydrous pyrolite from 10 to 25 gpa the liquidus phases in the dry pyrolite are olivine at 10 14 gpa majorite at 14 20 gpa and periclase at pressures above 20 gpa we observed expansion of the stability field of anhydrous phase b and its complex relations with the other phases in the cmas pyrolite system we suppose it is formed not only by incongruent melting of wadsleyite but also by the subsolidus reaction of wadsleyite and majorite in the dry condition compositions of partial melts formed by low degree of melting 10 20 have al 2 o 3 depleted and cao rich compositions up to 22 gpa and enriched in al 2 o 3 5 7 wt and cao at 25 gpa at 10 gpa dry and hydrous melts formed by low degree of melting have high sio 2 48 51 wt and relatively high al 2 o 3 3 5 5 2 wt their compositions are generally consistent with those of aluminum depleted komatiite at 13 22 gpa dry and hydrous melts have low sio 2 44 49 wt and al 2 o 3 1 7 3 0 wt and high cao contents 9 15 wt an essential depression of the apparent solidus temperature in the hydrous deep upper mantle compared to that in the transition zone provides the dehydration melting of the hydrous plume the present results support a model of komatiite genesis by dehydration melting of rising wet plumes at the base of the upper mantle

modeling the tonga slab can travel time data resolve a metastable olivine wedge we present the results of detailed modeling of the tonga slab with the goals of determining whether high resolution travel time data 1 can be fit by simple thermal and petrological slab models and 2 can resolve a metastable olivine wedge at depths greater than the equilibrium olivine spinel phase boundary we model arrival times recorded by a 1000 km line of 23 ocean bottom seismometers obs and island broadband seismic stations extending from the lau backarc basin across the tonga trench and onto the pacific plate the data consist of 388 local p wave travel times from 17 deep and 3 intermediate earthquakes recorded during the 3 month obs deployment in late 1994 we locate the events using both local and teleseismic arrival times and apply a relocation operator to the theoretical travel times to simulate the biases introduced in the data by locating the events with a reference earth model the modeling consists of grid searches using a three dimensional finite difference algorithm to compute local first arriving travel times for equilibrium and metastable p wave velocity models constructed from thermal mineralogical and morphological constraints the travel time anomalies are well fit by standard slab thermal models and p velocity temperature derivatives of 0 4 to 0 3 ms 1 c 1 forward calculations indicate that the presence of a metastable olivine wedge has a subtle effect on the travel times due to the tendency of first arriving waves to avoid the low velocity region wedge velocity models provide a slightly better fit to the data than equilibrium models but f tests indicate the improvement is not significant at the 95 level our results suggest that providing direct seismological evidence of a wedge of metastable olivine in subduction zones will require either waveform modeling or the observation of later arriving phases created by the depressed phase boundary

does elevation matter living foraminiferal distribution in a hyper tidal salt marsh canche estuary northern france abstract in the present study we investigate the ecology and distribution of living benthic foraminifera to test the effect of hyper tidal exposure and their suitability as sea level indicators within a salt marsh area along the canche estuary northern france four transects were sampled to see the effects of maximal tidal constraints shore transects and minimal tidal constraints alongshore transects multivariate analyses have been performed to determine the correlations between biotic foraminiferal absolute abundances and abiotic factors elevation grain size toc and total sulphur for each of the principal benthic foraminiferal species the tolerance to subaerial exposure have been estimated as well two distinctive foraminiferal zones have been identified along the vertical tidal gradient a zone i in the higher part of the salt marsh dominated by agglutinated and porcelaneous taxa and a zone ii in the lower one dominated by hyaline specimens hyper tidal exposure constraints the foraminiferal vertical zonation in accordance with the tidal frame however it does not constitute a threshold parameter able by itself to explain all the faunal variations in the canche estuary for sea level indicators foraminifera should be considered relative to tidal subaerial exposure rather than to absolute altitude

absence of density crossover between basalt and peridotite in the cold slabs passing through 660 km discontinuity 1 interaction between subducting slabs and surrounding mantle near 660 km discontinuity is one of the key issues in the dynamic of deep earth distinguishing phase transformations occur in the both major parts of the slab basaltic crust and underlying peridotite post spinel transformation in peridotite takes place at 660 km whereas post garnet transformation in basalt is considered to occur at 720 800 km creating density crossover in this paper we determined the post garnet phase boundary in mid ocean ridge basalt representative for oceanic crust by in situ x ray diffraction studies the phase boundary can be expressed as p gpa 0 0041 t k 17 92 the present results imply that the basaltic component of the cold slab transformed to perovskite assembly at lower pressure than the surrounding mantle and provide direct evidence for penetration of the oceanic crust into the lower mantle without gravitational separation from the peridotite body of the slab

water partitioning between nominally anhydrous minerals in the mgo sio2 h2o system up to 24 gpa implications for the distribution of water in the earth s mantle multi anvil experiments have been conducted in the mgo sio2 h2o system at pressures of 15 24 gpa and temperatures of 1200 1600 c to investigate the partitioning of water between mantle phases the water contents were obtained from fourier transform infrared spectroscopy on doubly polished and clear crystals most of the mineral phases coexisted with a hydrous melt the results show that among all magnesium silicate phases 1 mgsio3 perovskite is the only phase which does not dissolve a detectable amount of water and 2 the high pressure polymorphs of mg2sio4 wadsleyite and ringwoodite dissolve about one order of magnitude more water than the polymorphs of mgsio3 clinoenstatite majorite and akimotoite the following partition coefficients of water were measured dwadsleyite clinoenstatite 3 8 at 15 gpa and 1300 c dringwoodite akimotoite 21 at 19 gpa and 1300 c dakimotoite perovskite 1 at 24 gpa and 1600 c and dpericlase perovskite 1 at 24 gpa and 1500 c these results suggest that most of the water in the earth s mantle is partitioned in the transition zone

depth extent of the lau back arc spreading center and its relation to subduction processes seismic tomography and wave form inversion revealed that very slow velocity anomalies 5 to 7 percent beneath the active lau spreading center extend to 100 kilometer depth and are connected to moderately slow anomalies 2 to 4 percent in the mantle wedge to 400 kilometer depth these results indicate that geodynamic systems associated with back arc spreading are related to deep processes such as the convective circulation in the mantle wedge and deep dehydration reactions in the subducting slab the slow regions associated with the tonga arc and the lau back arc are separated at shallow levels but merge at depths greater than 100 kilometers suggesting that slab components of back arc magmas occur through mixing at these depths

water induced convection in the earth s mantle transition zone 1 water enters the earth s mantle by subduction of oceanic lithosphere most of this water immediately returns to the atmosphere through arc volcanism but a part of it is expected as deep as the mantle transition zone 410 660 km depth there slabs can be deflected and linger before sinking into the lower mantle because it lowers the density and viscosity of the transition zone minerals i e wadsleyite and ringwoodite water is likely to affect the dynamics of the transition zone mantle overlying stagnant slabs the consequences of water exchange between a floating slab and the transition zone are investigated in particular we focus on the possible onset of small scale convection despite the adverse thermal gradient i e mantle is cooled from below by the slab the competition between thermal and hydrous effects on the density and thus on the convective stability of the top layer of the slab is examined numerically including water dependent density and viscosity and temperature dependent water solubility for plausible initial water content in a slab 0 5 wt an episode of convection is likely to occur after a relatively short time delay 5 20 ma after the slab enters the transition zone however water induced rheological weakening is seen to be a controlling parameter for the onset time of convection moreover small scale convection above a stagnant slab greatly enhances the rate of slab dehydration small scale convection also facilitates heating of the slab which in itself may prolong the residence time of the slab in the transition zone

water and partial melting in mantle plumes inferences from the dissolved h2o concentrations of hawaiian basaltic magmas knowledge of dissolved h2o in primary kilauean magmas provides a constraint on h2o abundance in the deep mantle region that feeds the upwelling plume beneath hawaii given an h2o k2o mass ratio of 1 3 for basaltic glasses and melt inclusions from kilauea the mantle source is estimated to contain 450 190 ppm h2o this value is 3 times greater than that estimated for the mantle source for mid ocean ridge basalts consideration of oh solubility in olivine suggests that water undersaturated melting of the upwelling hawaiian plume probably begins at a depth of 250 km thus during plume ascent through most of the upper mantle water is partitioned between nominally anhydrous silicates and a small mass fraction of entrained hydrous silicate melt because water strongly influences the viscosity of olivine aggregates partial melting and melt extraction will have important effects on the rheology of the upwelling plume beneath hawaii

density of basaltic melt at high pressure and stability of the melt at the base of the lower mantle abstract we made density measurements of model morb and picritic melts up to about 15 gpa using the floating diamond method the compression behavior of the melts is fitted by the vinet equation of state and extrapolated to higher pressure a density crossover between the morb melt and the mantle peridotite is suggested at the base of the lower mantle thus the ultra low velocity zone in the d layer may be explained by the existence of gravitationally stabilized basaltic melt formed by melting of the accumulated slab materials this region might become unstable due to progressive melting of slab materials and or supply of volatiles

metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere earth s deepest earthquakes occur as a population in subducting or previously subducted lithosphere at depths ranging from about 325 to 690 km this depth interval closely brackets the mantle transition zone characterized by rapid seismic velocity increases resulting from the transformation of upper mantle minerals to higher pressure phases deep earthquakes thus provide the primary direct evidence for subduction of the lithosphere to these depths and allow us to investigate the deep thermal thermodynamic and mechanical ferment inside slabs numerical simulations of reaction rates show that the olivine spinel transformation should be kinetically hindered in old cold slabs descending into the transition zone thus wedge shaped zones of metastable peridotite probably persist to depths of more than 600 km laboratory deformation experiments on some metastable minerals display a shear instability called transformational faulting this instability involves sudden failure by localized superplasticity in thin shear zones where the metastable host mineral transforms to a denser finer grained phase hence in cold slabs such faulting is expected for the polymorphic reactions in which olivine transforms to the spinel structure and clinoenstatite transforms to ilmenite it is thus natural to hypothesize that deep earthquakes result from transformational faulting in metastable peridotite wedges within cold slabs this consideration of the mineralogical states of slabs augments the traditional largely thermal view of slab processes and explains some previously enigmatic slab features it explains why deep seismicity occurs only in the approximate depth range of the mantle transition zone where minerals in downgoing slabs should transform to spinel and ilmenite structures the onset of deep shocks at about 325 km is consistent with the onset of metastability near the equilibrium phase boundary in the slab even if a slab penetrates into the lower mantle earthquakes should cease at depths near 700 km because the seismogenic phase transformations in the slab are completed or can no longer occur substantial metastability is expected only in old cold slabs consistent with the observed restriction of deep earthquakes to those settings earthquakes should be restricted to the cold cores of slabs as in any model in which the seismicity is temperature controlled via the distribution of metastability however the geometries of recent large deep earthquakes pose a challenge for any such models transformational faulting may give insight into why deep shocks lack appreciable aftershocks and why their source characteristics including focal mechanisms indicating localized shear failure rather than implosive deformation are so similar to those of shallow earthquakes finally metastable phase changes in slabs would produce an internal source of stress in addition to those due to the weight of the sinking slab such internal stresses may explain the occurrence of earthquakes in portions of lithosphere which have foundered to the bottom of the transition zone and or are detached from subducting slabs metastability in downgoing slabs could have considerable geodynamic significance metastable wedges would reduce the negative buoyancy of slabs decrease the driving force for subduction and influence the state of stress in slabs heat released by metastable phase changes would raise temperatures within slabs and facilitate the transformation of spinel to the lower mantle mineral assemblage causing slabs to equilibrate more rapidly with the ambient mantle and thus contribute to the cessation of deep seismicity because wedge formation should occur only for fast subducting slabs it may act as a parachute and contribute to regulating plate speeds wedge formation would also have consequences for mantle evolution because the density of a slab stagnated near the bottom of the transition zone would increase as it heats up and the wedge transforms to denser spinel favoring the subsequent sinking of the slab into the lower mantle

using foraminiferal transfer functions to produce high resolution sea level records from salt marsh deposits maine usa quantitative analyses of foraminiferal distributions along the coast of maine are used to provide transfer functions for reconstruction of sea level changes from fossil foraminiferal assemblages the indicative meaning of fossil assemblages is most accurately predicted by a training set that incorporates live and dead foraminifera and that uses flooding duration not height as the predictor variable sea level records are produced for two sites in maine usa a middle marsh site in scarboro and a high marsh site in machiasport these records offer temporal resolution of five to seven years and span the past century they are compared with instrumental data from a nearby tidal station to assess their accuracy strong correlations are found between the geologic records and the observational data in particular between the foraminiferal record from machiasport and the tide gauge record from eastport r2 0 83 p 0 005 thus a foraminifera based transfer function approach offers great potential for reso