high precision isotopic characterization of usgs reference materials by tims and mc icp ms the pacific centre for isotopic and geochemical research pcigr at the university of british columbia has undertaken a systematic analysis of the isotopic sr nd and pb compositions and concentrations of a broad compositional range of u s geological survey usgs reference materials including basalt bcr 1 2 bhvo 1 2 andesite agv 1 2 rhyolite rgm 1 2 syenite stm 1 2 granodiorite gsp 2 and granite g 2 3 usgs rock reference materials are geochemically well characterized but there is neither a systematic methodology nor a database for radiogenic isotopic compositions even for the widely used bcr 1 this investigation represents the first comprehensive systematic analysis of the isotopic composition and concentration of usgs reference materials and provides an important database for the isotopic community in addition the range of equipment at the pcigr including a nu instruments plasma mc icp ms a thermo finnigan triton tims and a thermo finnigan element2 hr icp ms permits an assessment and comparison of the precision and accuracy of isotopic analyses determined by both the tims and mc icp ms methods e g nd isotopic compositions for each of the reference materials 5 to 10 complete replicate analyses provide coherent isotopic results all with external precision below 30 ppm 2 sd for sr and nd isotopic compositions 27 and 24 ppm for tims and mc icp ms respectively our results also show that the first and second generation usgs reference materials have homogeneous sr and nd isotopic compositions nd isotopic compositions by mc icp ms and tims agree to within 15 ppm for all reference materials interlaboratory mc icp ms comparisons show excellent agreement for pb isotopic compositions however the reproducibility is not as good as for sr and nd a careful sequential leaching experiment of three first and second generation reference materials bcr bhvo agv indicates that the heterogeneity in pb isotopic compositions and concentrations could be directly related to contamination by the steel mortar pestle used to process the materials contamination also accounts for the high concentrations of certain other trace elements e g li mo cd sn sb w in various usgs reference materials

the u series toolbox for paleoceanography the geochemistry of marine sediments is a major source of information about the past environment of the many measurements that provide such information those of the u series nuclides are unusual in that they inform us about the rate and timescales of processes oceanic processes such as sedimentation productivity and circulation typically occur on timescales too short to be assessed using parent daughter isotope systems such as rb sr or sm nd so the only radioactive clocks that we can turn to are those provided by cosmogenic nuclides principally 14c or the u series nuclides this makes the u series nuclides powerful allies in the quest to understand the past ocean climate system and has led to their widespread application over the last decade as in other applications of the u series those in paleoceanography rely on fractionation of the nuclides away from secular equilibrium in the oceanic setting this fractionation is generally due to differences in the solubility of the various nuclides the behavior of the u series nuclides in the ocean environment was widely researched in the middle decades of the twentieth century this work established knowledge of the concentrations of the nuclides in the various compartments of the ocean system and of their fluxes between these compartments such understanding was comprehensively summarized in the ivanovich and harmon u series volume 1992 particularly by cochran 1992 understanding of the behavior of the u series nuclides has not advanced very dramatically in the decade since that summary but a major theme of research has been the use of this geochemical understanding to develop u series tools to assess the past environment table 1 view this table table 1 summary of seawater data for u series nuclides with paleoceanographic applications s soluble i insoluble full descriptions of the paleoceanographic uses and references are provided in the text further details of the half

toroidal mantle flow through the western u s slab window the circular pattern of anisotropic fast axis orientations of split sks arrivals observed in the western u s cannot be attributed reasonably to either preexisting lithospheric fabric or to asthenospheric strain related to global scale plate motion a plume origin for this pattern accounts more successfully for the anisotropy field but little evidence exists for an active plume beneath central nevada we suggest that mantle flow around the edge of the sinking gorda juan de fuca slab is responsible for creating the observed anisotropy seismic images and kinematic reconstructions of gorda juan de fuca plate subduction have the southern edge of this plate extending from the mendocino triple junction to beneath central nevada and flow models of narrow subducted slabs produce a strong toroidal flow field around the edge of the slab consistent with the observed pattern of anisotropy this flow may enhance uplift extension and magmatism of the northern basin and range while inhibiting extension of the southern basin and range

rare earth elements in the permian phosphoria formation paleo proxies of ocean geochemistry abstract the geochemistry of deposition of the meade peak member of the phosphoria formation mpm in southeast idaho usa a world class sedimentary phosphate deposit of permian age that extends over 300 000 km2 is ascertained from its rare earth element ree composition ratios of ree al2o3 suggest two sources seawater and terrigenous debris the seawater derived marine fraction identifies bottom water in the phosphoria sea as o2 depleted denitrifying suboxic most of the time and seldom sulfate reducing anoxic this interpretation is supported by earlier research that showed progressively greater ratios in the marine sediment fraction of cr ni v ni mo ni relative to their ratios in seawater for which marine cr v and mo can have a dominantly o2 depleted bottom water source and ni a photic zone largely algal source the water chemistry was maintained by a balance between bacterial oxidation of organic matter settling through the water column determined largely by primary productivity in the photic zone and the flux of oxidants into the bottom water via advection of seawater from the open ocean samples strongly enriched in carbonate fluorapatite the dominant ree host mineral have variable er sm tm sm and yb sm ratios their distribution may represent greater advection of seawater between the phosphoria sea and open ocean during deposition of two ore zones than a center waste and greater upwelling of nutrient enriched water into the photic zone however the mean rate of deposition of marine ni a trace nutrient of algae and po43 a limiting nutrient indicate that primary productivity was probably high throughout the depositional history an alternative interpretation of the variable enrichments of er tm and yb relative to sm is that they may reflect temporally variable carbonate alkalinity of open ocean seawater in permian time a more strongly negative ce anomaly for all phosphatic units than the ce anomaly of modern pelletal phosphate is further indicative of an elevated o2 concentration in the permo carboniferous open ocean as proposed by others in contrast to the depletion of o2 in the bottom water of the phosphoria sea itself the oceanographic conditions under which the deposit accumulated were likely similar to conditions under which many sedimentary phosphate deposits have accumulated and to conditions under which many black shales that are commonly phosphate poor have accumulated a shortcoming of several earlier studies of these deposits has resulted from a failure to examine the marine fraction of elements separate from the terrigenous fraction

opposite subduction polarity in adjacent plate segments the work of m p m f and j v is part of the projects alpimed pie csic 201530e082 and mite cgl2014 59516 p and partly financed by the generalitat de catalunya 2014 sgr 1471 and 2017 sgr 847 the work of s z is funded by the spanish ministry of economy and competitiveness through grant cicyt dpi2017 85139 c2 2 r and generalitat de catalunya 2017 sgr 1278

toward 1 mgal accuracy in global marine gravity from cryosat 2 envisat and jason 1 more than 60 of the earth s land and shallow marine areas are covered by 2 km of sediments and sedimentary rocks with the thickest accumulations on rifted continental margins figure 1 free air marine gravity anomalies derived from geosat and ers 1 satellite altimetry fairhead et al 2001 sandwell and smith 2009 andersen et al 2009 outline most of these major basins with remarkable precision moreover gravity and bathymetry data derived from altimetry are used to identify current and paleo submarine canyons faults and local recent uplifts these geomorphic features provide clues to where to look for large deposits of sediments while current altimeter data delineate large offshore basins and major structures they do not resolve some of the smaller geomorphic features and basins yale et al 1998 fairhead et al 2001 improved accuracy and resolution is desirable to facilitate comparisons between continental margins as an exploration tool and to permit extrapolation of known structu

plutonium 241am and 137cs ratios inventories and vertical profiles in washington and oregon continental shelf sediments abstract continental shelf sediments from nine locations off washington and oregon have 239 240 pu inventories which average 8 0 2 6 mci km 2 the columbia river and seawaters advecting over the shelf supply pu which is removed to underlying sediments principally through scavenging by inorganic paniculate matter mass balance calculations argue that less than 20 percent of the advected pu need be scavenged from the water column to balance river input and total shelf sediment inventories the percentage of the pu removed through scavenging is consistent with observed participate concentrations in shelf waters and published sediment water distribution coefficients no marked separation of pu from 137 cs is observed with depth in pacific shelf sediments as has been reported in atlantic coastal sediments this interocean distinctness can be explained by differences in particle mixing and downward diffusion of cs in sediments of varying porosities the transuranic inventories and pu cs ratios in the pacific sediments do not support the hypothesis of livingston and bowen that pu is remobilized within the sediment column by complexone formation with principally organic substances excess 210 pb 239 240 pu inventory ratios in eight representative cores from the washington shelf average 100 19 even though absolute values of both inventories vary by much larger factors this reasonably constant ratio for a given water depth permits estimation of total pu inventories and prediction of sites of unusual pu accumulation from data on the more easily measured natural radionuclide

variation in bioturbation with water depth on marine slopes a study on the little bahamas bank abstract reconstructing the paleoceanography of intermediate depth waters is dependent on sedimentary records preserved on marine slopes but knowledge of bioturbation processes in such slope environments is much poorer than for the deep sea in this study 210 pb profiles were measured for the upper 20 cm of sediment from five box cores taken on the slopes of the bahamas in water depths ranging from 400 to 1500 m these were compared with data from the bahamas bank tops and from elsewhere to assess the rate and depth of mixing in the slope environment excess 210 pb inventories are too high to explain by water column decay of 226 ra and are derived largely from atmospheric fallout down core 210 pb xs profiles show an upper zone where 210 pb xs activities decrease exponentially with depth suggesting uniform mixing and a lower zone with patchy 210 pb values reflecting isolated burrows in contrast to bank top sediment which shows 210 pb xs mixing to depths of 40 cm and mixing rates of 1000 cm2 kyr mixing of 210 pb on the slopes penetrates to only 8 cm with mixing rates of 210 pb derived mixing activity in ocean margin sediments diffusive versus nonlocal mixing j mar res 54 1207 1227 a simple model is used to assess the effects of mixing on typical bahamian slope sediments which have deposition rates and compositions which vary dramatically in response to sea level change proxy records such as 18 o are expected to be displaced by up to 0 8 kyr in bulk sediment and individual sediment constituents may be moved by up to 2 kyr relative to one another these mixing effects are reasonably small and may be safely ignored for some studies but for high resolution records or studies where precise age control is important bioturbation of slope sediments should be considered

using geochemistry to map mantle flow beneath the lau basin geochemical tracers can be used to map mantle flow beneath the lau basin tonga region over the past 6 m y and constrain the extent and means by which different components contribute to the magmas erupted there helium isotope data track the samoan plume to show that it infiltrated into the northwestern part of the lau basin at 40 mm yr 1 through an opening tear in the pacific plate in contrast to other recent studies our analysis indicates that this plume does not contribute to the arc lavas a louisville plume component is present in the northern tonga arc lavas but this was added from volcaniclastic sediments transported into the arc on the subducting slab and does not require the presence of plume material within the mantle wedge the louisville tracer has been used to infer that the mantle wedge beneath the island arc is downwelling at the rate of 20 40 mm yr 1 owing to viscous drag against the downgoing pacific plate a third geochemical tracer provided by the distinction between indian and pacific mantle indicates that indian mantle has migrated eastward at rates of 45 65 mm yr 1 the inferred rates of migration indicate that the translation of geochemical signatures occurs by mantle flow rather than by the order of magnitude faster movement of partial melts except for the flow in the mantle wedge the directions of mantle flow in this region are unrelated to the overlying plate motions

effects of benthic transport processes on abrupt climatic changes recorded in deep sea sediments a time dependent modeling approach 1 a one dimensional time dependent model is developed to characterize qualitatively and quantitatively the combined effects of bioturbation and burial on sedimentary signals related to abrupt climatic changes this model is based on the widely used advection diffusion equation and simulates rapid climatic transitions by representing the deposition flux at the sediment water interface by a steep ramp function of time this model is applied to both planktonic foraminifera abundance curves and 18o distributions output signals obtained at the bottom of the bioturbated layer for these rapid transitions have been generated for a broad range of sedimentation rates 5 50 cm kyr 1 biodiffusion coefficients 10 3 1 cm2 yr 1 and mixed layer thicknesses 2 15 cm numerical results clearly demonstrate the importance of a dynamic description of sediment mixed layers properties of the sedimentary response to fast transitions appear to be highly and nonlinearly dependent upon the sedimentation rate and the relative values of the original transition length and the time required to bury the signal below the mixed layer moreover for low sedimentation rates 10 cm kyr 1 we show that signals occurring over a 50 to a 250 year timescale are indistinguishable to quantify the distortion of sedimentary signals we determined the transition length of the bioturbated simulated signals and compare it to the initial transition length for given sets of bioturbation and sedimentation rates and mixed layer thicknesses comparison between model predictions and experimental results reveals a good agreement showing that transition timescales are compatible with ice core derived transitions 100 years for environments in which bioturbation can be approximated by a diffusive process tests performed on 18o distributions have illustrated the effect of bioturbation on the stratigraphic offsets between isotopic compositions of cold and warm species of foraminifera during climatic transitions

acid sulfate alteration of fluorapatite basaltic glass and olivine by hydrothermal vapors and fluids implications for fumarolic activity and secondary phosphate phases in sulfate rich paso robles soil at gusev crater mars 1 phosphate rich rocks and a nearby phosphate rich soil paso robles were analyzed in gusev crater mars by the mars exploration rover spirit and interpreted to be highly altered possibly by hydrothermal or fumarolic alteration of primary phosphate rich material to test mineral phases resulting from such alteration we performed hydrothermal acid vapor and acid fluid experiments on olivine ol fluorapatite ap and basaltic glass gl as single phases and a mixture of phases minerals formed include ca al fe and mg sulfates with different hydration states anhydrite bassanite gypsum alunogen hexahydrite and pentahydrite phosphate bearing minerals formed included monocalcium phosphate monohydrate mcp acid vapor and acid fluid alteration of fluorapatite only and ferrian giniite acid fluid alteration of the ol gl ap mixture mcp is likely present in paso robles if primary ca phosphate minerals reacted with sulfuric acid with little transport of phosphate under fluid rock ratios allowing transport of phosphate a ferric phosphate phase such as ferrian giniite might form instead mossbauer measurements of ferrian giniite bearing alteration products and synthetic ferrian giniite are consistent with spirit s mossbauer measurements of the ferric bearing phase in paso robes soil but are also consistent with ferric sulfate phases in the low p soil arad samra therefore mossbauer data alone do not constrain the fluid rock ratio however the excess iron hematite in paso robles soil which implies aqueous transport combined with our laboratory experiments suggest acid sulfate alteration in a hydrothermal fumarolic environment at fluid rock ratios sufficient to allow dissolution transport and precipitation of secondary chemical components including a ferric phosphate such as ferrian giniite

accumulation and biological mixing of peru margin sediments abstract rates of sediment accumulation and biological particle mixing in eight cores from the peru shelf have been estimated by application of uranium series disequilibrium techniques and radiocarbon dating activities of 210pb 226ra 234th and 238u have been determined in closely spaced intervals in each sediment core biological particle mixing coefficients were determined via a two box advection diffusion steady state mixing model based on the distribution profiles of excess 210pb and in some cases excess 234th activities the sedimentation rates estimated were in the order of a few millimeters yr except for cores enriched in phosphatic components which displayed rates a few orders of magnitude slower bioturbation was significant in most cores studied with the exception of one core collected from within the middle of the oxygen minimum zone estimated biological mixing coefficients were in the order of 100 101 cm2 yr 1 for the surface mixed layer and 10 1 100 cm2 yr 1 for the layer below sediment slumping appears to have affected the 210pb distribution of two or three of our cores

recent sedimentation and mass accumulation rates based on 210pb along the peru chile continental margin abstract the concentrations of the natural radionuclide 210 pb were measured in sediment samples collected with a multicorer boxcorer and gravity corers along the chilean continental margin 20 45 s in order to estimate sedimentation rate sr and mass accumulation rate this database was compared with previously obtained results from central peru callao 210 pb activities decreased exponentially with core depth and profiles showed a deepening of the 210 pb in excess unsupported in the sediments from north to south 6 18 cm sediment depth off peru and northern chile vs 8 35 cm off central and southern chile an onshore offshore decrease of surface activities was observed off central peru whereas the reverse was true for surface activities along the chilean margin srs increased from north 0 04 0 17 cm yr 1 to south 0 09 0 29 cm yr 1 across the shelf higher rates were observed at deeper sites especially at those close to submarine canyons except off central peru where srs showed similar values through the slope area off northern chile and central peru srs seem to reflect the export of particles over the shelf and redistribution processes such as slumps and lateral transport in contrast srs in central and southern chile seem to be controlled by several additional factors that modulate the settling particles such as onshore precipitation and river runoff in addition the more complex topography of the southern chile margin allows for settling or winnowing areas

effect of three dimensional slab geometry on deformation in the mantle wedge implications for shear wave anisotropy shear wave splitting observations from many subduction zones show complex patterns of seismic anisotropy that commonly have trench parallel fast directions three dimensional flow may give rise to trench parallel stretching and provide an explanation for these patterns of seismic anisotropy along strike variations in slab geometry produce trench parallel pressure gradients and are therefore a possible mechanism for three dimensional flow in this study we quantify the effects of variable slab dip curved slabs oblique subduction and slab edges on flow geometry and finite strain in the mantle wedge of subduction zones temperature dynamic pressure velocity and strain are calculated with high resolution three dimensional finite element models these models include temperature and stress dependent rheology and parameterized slab and trench geometry thick layers 20 60 km with strong trench parallel stretching are observed in the mantle wedge when slab geometry involves a transition to slab dip less than 15 or strong curvature in the slab in these cases strong trench parallel stretching develops when flow lines have an oblique to trench normal orientation this suggests that trench parallel seismically fast directions may not indicate trench parallel flow lines in systems with large along strike variations an oblique component of stretching is confined to a 20 30 km layer above the slab in systems with oblique subduction the effects of slab edges include strong toroidal flow and focusing in the mantle near slab edges and trench parallel flow that extends 50 100 km into the core of the mantle wedge

lillooet glacier basalts southwestern british columbia canada products of quaternary glaciovolcanism the retreat of lillooet glacier lg has exposed a succession of pillow basalt and subordinate amounts of breccia and hyaloclastite the lithofacies and physiographic setting suggest that the deposits have a glaciovolcanic origin and represent a partially dissected basaltic pillow dominated tindar chemically the lg volcanic rocks are basalt to basaltic andesite and as a group they represent the highest silica quaternary mafic products in the garibaldi volcanic belt gvb like other northern gvb alkaline basalts they lack the nb ta depletion signature typically associated with subduction related products geochemical and petrologic analysis indicates that the lg basalts are comagmatic and that chemical variations within the suite are consistent with sorting of the observed phenocryst assemblage olivine plagioclase thermodynamic modeling establishes shallow crustal pre eruptive storage conditions at 2 kbar 1 kbar 100 mpa or 7 5 km and an h2o content of 0 5 1 wt we estimate that the

bioturbation short lived radioisotopes and the tracer dependence of biodiffusion coefficients bioturbation refers to the mixing of sediment particles resulting from benthic faunal activity it is the dominant particle mixing process in most marine sediments and exerts an important control on diagenetic processes in models bioturbation is usually treated as a diffusive process where the biodiffusion coefficient db characterizes the biological mixing intensity biodiffusion coefficients are classically computed by fitting a diffusive model to vertical profiles of particle bound radioisotopes one peculiar observation is tracer dependence db values from short lived tracers tend to be larger than those obtained from long lived tracers from the same site recent theoretical work based on random walk theory and lattice automaton bioturbation simulations labs has suggested that this tracer dependence is simply a model artifact and has concluded that the biodiffusion model is not applicable to the short observational time scales associated with short lived radioisotopes here we have compiled a global dataset of db values obtained from different radiotracers to assess tracer dependence from a data perspective tracer dependence is significant in low mixing environments like slope and deep sea sediments but is not present in intensely mixed coastal areas tracer dependence is absent when the number of mixing events is larger than 20 or the potential length scale is greater than 0 5 cm roughly this comes down to tracer derived db values greater than 2 cm2 yr 1 this condition is met for 68 50 and 8 of published db values obtained from coastal continental slope and abyssal environments respectively these results show that short lived radioisotopes are suitable to quantify biodiffusion mixing in sedimentary environments featuring intense bioturbation

trench parallel flow beneath the nazca plate from seismic anisotropy shear wave splitting of s and sks phases reveals the anisotropy and strain field of the mantle beneath the subducting nazca plate cocos plate and the caribbean region these observations can be used to test models of mantle flow two dimensional entrained mantle flow beneath the subducting nazca slab is not consistent with the data rather there is evidence for horizontal trench parallel flow in the mantle beneath the nazca plate along much of the andean subduction zone trench parallel flow is attributale utable to retrograde motion of the slab the decoupling of the slab and underlying mantle and a partial barrier to flow at depth resulting in lateral mantle flow beneath the slab such flow facilitates the transfer of material from the shrinking mantle reservoir beneath the pacific basin to the growing mantle reservoir beneath the atlantic basin trenchparallel flow may explain the eastward motions of the caribbean and scotia sea plates the anomalously shallow bathymetry of the eastern nazca plate the long wavelength geoid high over western south america and it may contribute to the high elevation and intense deformation of the central andes

petrology and major element geochemistry of peru margin phosphorites and associated diagenetic minerals authigenesis in modern organic rich sediments abstract new petrographic and major element geochemical data from modern peru margin upper slope outer shelf phosphorites are presented which provide insight into their origin and paragenetic relationship with other authigenic minerals glauconite pyrite and dolomite occurring in organic rich sediments glauconites are precipitated relatively early following the partial reduction of ferric iron and following this process phosphate pyrite and then dolomite precipitation take place at progressively deeper levels in the sediment in association with microbial reduction of sulfate as in many ancient economic phosphorite deposits the phosphatic facies here consist of nodules crusts coatings and strata composed of phosphatic pelletal grains ooids structureless grains intraclasts clumps and biogenic grains in association with organic rich biosiliceous sediments all are considered to have formed within a few centimeters or within a few tens of centimeters below the sediment water interface important factors that influence which morphology will tend to develop include the amount of available pore space the presence of suitable nucleation sites the amount and size of siliciclastic detritus incorporated as inclusions and the relative solution chemistries of the precipitating solutions bacterial mediation may play an important but as yet unspecified role in the precipitation process textural data and factor analysis of chemical data suggest that structureless pellets are relatively inclusion free na f mg co3 substituted pore water precipitates whereas ooids are inclusion rich pore water precipitates poor in lattice substituted components variations in nodular cement birefringence and crystallinity are suggested to have been produced by similar lattice substitutions that directly reflect pore water carbonate ion concentrations and thus relative degrees of organic matter degradation phosphate and dolomite are intimately mixed yet mineralogically distinct phases in phosphatized dolomicrites depth stratified threshold carbonate ion concentrations may control the lower limit at which phosphatic minerals may precipitate below depths of a few centimeters excessive carbonate ion concentrations and diminished reactive iron and sulfate concentrations favor the development of dolomite while precluding further development of phosphatic minerals and pyrite periodic sediment reorganization bioturbation current winnowing and erosion mass wasting etc plays an important role in both concentrating pelletal grains and maintaining nodules and crusts at critical depth levels in the sediment as well as mixing ordered mineral parageneses into complicated sequences

peruvian sediments as recorders of an evolving hiatus for the last 22 thousand years highlights review of sediment archives from the peruvian margin since the lgm focus on the evolutionary feature of the hiatus found in archives modern analogue for current dominated environments for paleo reconstructions new results for erosional potential of the non linear internal waves nliws abstract the peruvian continental margin is characterized by the presence of one of the strongest and most distinct oxygen minimum zones omzs in today s oceans therefore it has long been in the focus of oceanographic and geological investigations observations indicate that omzs are expanding in relation with currently changing climate to advance understanding of the temporal evolution of omzs and climate change complete paleoceanographic and palaeoclimatological reconstructions are needed however the development of paleoenvironmental scenarios for the period since the last glacial maximum at this region was hampered by a ubiquitous hiatus and short term interruptions of the stratigraphical record in the present study we combined the stratigraphical information from 31 sediment cores from the peruvian margin located between 3 and 18 s and water depths of 90 to 1300 m within and below today s omz in order to determine the extent of the hiatus and assess the responsible mechanisms a widespread unconformity and related erosional features omission surfaces and phosphorites were observed in sediment cores from the area south of 7 s depicting a prograding feature on the continental slope from south to north during the deglaciation combining recent oceanographic and sedimentological observations it is inferred that tide topography interaction and resulting non linear internal waves nliws shape the slope by erosion carry sediments upslope or downslope and leave widespread phosphoritic lag sediments while the peru chile undercurrent pcuc transports the resuspended sediments southward causing non deposition this exceptional sedimentary regime makes the peruvian margin a modern analogue for such environments overall our compilation of downcore records showed that enhanced bottom currents due to tide topography interaction were progressively evolving and affected a wider area with the onset of the last deglaciation elevated tidal amplitudes and variability of mid depth water masses i e density changes and hydrodynamics in relation with changing climate were potential reasons of this evolving feature of erosion and reworking additionally erosion and non deposition was observed widest and even was encountered on the continental shelf during the early holocene potentially indicating a strong phase of the pcuc mirroring today s el nino like conditions

the supply and accumulation of silica in the marine environment rivers and submarine hydrothermal emanations supply 6 1 1014g sio2 yr to the marine environment approximately two thirds of the silica supplied to the marine environment can be accounted for in continental margin and deep sea deposits siliceous deep sea sediments located beneath the antarctic polar front convergence account for over a fourth 1 6 1014g sio2 yr of the silica supplied to the oceans deep sea sediment accumulation rates beneath the polar front are highest in the south atlantic with values as large as 53cm kyr during the last 18 000 yr siliceous sediments in the bering sea sea of okhotsk and subarctic north pacific accumulate 0 6 1014g sio2 yr or 10 of the dissolved silica input to the oceans the accumulation of biogenic silica in estuarine deposits removes a maximum of 0 8 1014g sio2 yr although estuarine silica versus salinity plots indicate extensive removal of riverine silica from surface waters the removal rates must be considered as maximum values because of dissolution of siliceous material in estuarine sediments and bottom waters siliceous sediments from continental margin upwelling areas e g gulf of california walvis bay or peru chile coast have the highest biogenic silica accumulation rates in the marine environment 69 g sio2 cm2 kyr despite the rapid accumulation of biogenic silica upwelling areas account for less than 5 of the silica supplied to the marine environment because they are confined laterally to such small areas

crustal plumbing system of post rift magmatism in the northern margin of south china sea new insights from integrated seismology abstract the origin and evolution of magmatism in rifting margins are a fundamental geological subject and remain the focus of intense studies unlike the classical magma poor and magma rich rifting margins the northern margin of the south china sea scs exhibits a strong post rift magmatism but the evolution and mechanism of this behavior are still poorly understood in this work we investigate the detailed structural features of the post rift magmatism in the crust of the scs northern margin our analysis is based on wide angle and multichannel reflection seismic data combined with previous seismological results our results show a wide distribution of shallow igneous sills and intrusions these features are spatially and tectonically linked to the lower crustal high velocity bodies hvbs and submarine volcanism indicating a causative connection between the three features considering the existence of an obvious low velocity branch of the hainan plume in the northern scs and uplift of the lithosphere asthenosphere boundary we propose that the hvbs reflect consolidated mafic intrusions which formed the lower crustal reservoirs feeding the overlying igneous sills and intrusions in the scs northern margin where abundant extensional and detachment faults can act as magmatic channels such a dynamic process might cause post rift volcanism our results suggest that the crustal magmatic system of post rift volcanism has a multilevel upward migration mode and the hvbs in the lower crust could be the product of post rift magmatism further indicating that the scs northern margin had a magma poor property at the rifting phase but has undergone a strong magmatic rejuvenation by the subsequent mantle plume

crustal structure and inferred rifting processes in the northeast south china sea abstract taiger project deep penetration seismic reflection profiles acquired in the northeastern south china sea scs provide a detailed view of the crustal structure of a very wide rifted continental margin these profiles document a failed rift zone proximal to the shelf a zone of thicker crust 150 km from the shelf and gradually thinning crust toward the cob spanning a total distance of 250 300 km such an expanse of extended continental crust is not unique but it is uncommon for continental margins we use the high quality images from this data set to identify the styles of upper and lower crustal structure and how they have thinned in response to extension and in turn what rheological variations are predicted that allow for protracted crustal extension upper crustal thinning is greatest at the failed rift uc 7 5 but is limited farther seaward uc 1 2 we interpret that the lower crust has discordantly thinned from an original 15 17 km to possibly less than 2 3 km thick beneath the central thick crust zone and more distal areas this extreme lower crustal thinning indicates that it acted as a weak layer allowing decoupling between the upper crust and the mantle lithosphere the observed upper crustal thickness variations and implied rheology lower crustal flow are consistent with large scale boudinage of continental crust during protracted extension

the organic geochemistry of peru margin surface sediments i a comparison of the c37 alkenone and historical el nino records abstract the alkenone u k 37 paleothermometer has potential as a sedimentary marker for el nino southern oscillation enso events in the peru upwelling regime we assessed this potential by comparing the historical enso record with detailed u k 37 profiles for 210 pb dated cores from the peru margin oxygen minimum zone omz sediments with the greatest potential for holding a sedimentary u k 37 record of el nino events are sediments from the center of the omz sectioned at intervals the yearly sedimentation rate the u k 37 signals of individual el nino events were substantially attenuated in the sediments we examined and periods of frequent enso activity e g 1870 1891 were more readily identified than isolated enso events in periods of less frequent enso activity detailed depth profiles of the c 37 alkenones in a core from 253 m o 2 k 37 values for the five 2 mm sections from 0 1 cm suggests that the u k 37 may be unaffected by the alkenone loss correlation between the c 37 alkenone concentration profiles in two cores from 15 s collected nine years apart are consistent with the use of these compounds for molecular stratigraphy

distribution texture and composition of modern phosphate pellets in peru shelf muds abstract textural analysis including estimates of concentrations of authigenic phosphate pellets were made for eight sediment cores from the peru continental margin phosphatic pellets separated from these modern organic rich sediments are black spherical ovoidal in shape and in thin section often display a concentric structure around a nucleus consisting of inorganic mineral grains some pellets have a gray white exterior coating which appears to be secondary diagenetic calcite phosphatic pellets account for upwards of 80 of the sediment mass in some cores pellets concentrate in specific size classes generally between 125 and 500 m in diameter and occur within a poorly sorted sediment

phosphate regeneration from sediments of the peru continental margin by dissolution of fish debris generally oxidative regeneration of phosphate from anoxic sediments is by microbially mediated sulfate reduction processes stoichiometric modelling of such reactions takes into consideration varying proportions of decomposable organically bound p to account for the ratios among nutrients in depth concentration profiles of near surface sediments new results of interstitial water composition from sediments underlying the water masses influenced by coastal upwelling of the eastern boundary current system off peru indicate that dissolution of phosphatic fish debris represents a mechanism for remineralization of phosphate comparable to or larger in magnitude than that by oxidative regeneration of organically bound p dissolved interstitial phosphate from fish debris is revealed by an excess amount of phosphate over that predicted from a simple stoichiometric oxidative regeneration model and by anomalously high dissolved interstitial fluoride concentrations phosphate flux estimates based on diffusion from the sediment suggest that this mechanism may generate up to 10 of the nutrient pool in the waters of the peru undercurrent partitioning of p among the two sources reveals further that fish debris phosphate is about four times more important than organically bound p in nutrient generation from sediments of the peru continental margin not only does this mechanism of regeneration affect the nutrient cycling but may also control widespread phosphorite formation in this area

recent movements of the juan de fuca plate system analysis of the magnetic anomalies of the juan de fuca plate system allows instantaneous poles of rotation relative to the pacific plate to be calculated from 7 ma to the present by combining these with global solutions for pacific america and absolute relative to hot spot motions a plate motion sequence can be constructed this sequence shows that both absolute motions and motions relative to america are characterized by slower velocities where younger and more buoyant material enters the convergence zone pivoting subduction the resistance provided by the youngest portion of the juan de fuca plate apparently resulted in its detachment at 4 ma as the independent explorer plate in relation to the hot spot framework this plate almost immediately began to rotate clockwise around a pole close to itself such that its translational movement into the mantle virtually ceased after 4 ma the remainder of the juan de fuca plate adjusted its motion in response to the fact that the youngest material entering the subduction zone was now to the south differences in seismicity and recent uplift between northern and southern vancouver island may reflect a distinction in tectonic style between the normal subduction of the juan de fuca plate to the south and a complex underplating occurring as the explorer plate is overridden by the continent the history of the explorer plate may exemplify the conditions under which the self driving forces of small subducting plates are overcome by the influence of larger adjacent plates the recent rapid migration of the absolute pole of rotation of the juan de fuca plate toward the plate suggests that it too may be nearing this condition

is burial velocity a master parameter for bioturbation abstract the intensity of biological mixing in a marine sediment is widely believed to be related to the flux of labile organic matter at the same time this flux has been shown to be strongly correlated to the burial velocity one would then expect a significant correlation between biological mixing coefficient d b and burial velocity a nonlinear regression of existing data with a power law eqns 2 and 3 of the text produces a statistically significant correlation the statistical significance of the regression in no way implies causation and in fact it accounts for only about 22 of the total variance in the dataset thus reducing its physical significance a number of additional processes contribute to the large observed variance including timing of mixing events temporal and spatial heterogeneity differences in temporal scales sampling artifacts and specific particle associations with due care the d b relationship may prove useful in defining the likely range of d b values in a particular environment a similar analysis of the tracer identified surface mixed depth shows that it is essentially independent of burial velocity with a worldwide mean of 9 8 4 5 cm depth of mixing is probably more closely related to the energetic costs of deep burrowing

geochemistry of a tertiary sedimentary phosphate deposit baja california sur mexico abstract the san gregorio formation in baja california sur a phosphate enriched sedimentary unit of late oligocene to early miocene age has been analyzed in two areas la purisima and san hilario for its chemical composition major oxides cu cd cr co v and rare earth elements ree and isotopic composition 18 o and 13 c a detrital and a marine component were determined from major oxides the detrital component consists of an unaltered volcanic ash fraction and a terrigenous clay silt fraction the marine component which accumulated initially as biogenic and hydrogenous material is now present as opal a opal ct caco 3 organic matter and an authigenic phosphate fraction mostly pelletal and composed of the carbonate fluorapatite mineral francolite the minor elements have been partitioned into these components by assuming a constant composition for the two detrital fractions the composition of the marine component of minor elements can then be interpreted by assuming that the stoichiometry of the original accumulating organic matter was equal to that of modern plankton the cu and cd contents in the marine component of all rocks require that the seawater derived fractions of these two metals were supplied to the seafloor solely by organic matter enrichments of cr and v at both sites required an additional marine input on the basis of their geochemistry in the modern ocean cr and v could have precipitated or been adsorbed onto settling particles from an o 2 minimum zone in which the o 2 content was low enough to promote denitrification rather than oxygen respiration an enrichment of the ree now within the apatite fraction resulted from their adsorption onto particulates also in the o 2 minimum zone and to the dissolution and alteration of biogenic phases predominantly silica within the sediment co and fe 2 o 3 show no enrichment above a detrital contribution the 18 o values of apatites from the la purisima site are heavier than those of apatites from the san hilario site whereas the 13 c values show the opposite trend one possible interpretation of these variations is that 18 o reflects seawater values and 13 c sediment pore water values this interpretation suggests that upwelling rates and primary productivity within the water column were greater at la purisima an interpretation that is corroborated by a greater abundance of apatite measured in outcrop at la purisima the ce anomalies of the phosphate enriched samples also differ between the two sites indicating that they also recorded water masses similar to the 18 o values

particle mixing rates in sediments of the eastern equatorial pacific evidence from 210pb 239 240pu and 137cs distributions at manop sites abstract particle mixing rates db calculated from excess 210pb gradients in sediments of the east equatorial pacific range from 0 04 to 0 5 cm2 y with variation of a factor of 3 4 at a single site diffusion of the 236ra daughter 222rn may affect 210pb distributions under conditions of slow mixing and low 210pb flux to the seafloor as shown by a siliceous ooze clay core which contained the fallout radionuclides 239 240pu and 137cs but no excess 210pb relative to 226ra there is no clear relationship between 210pbderived mixing rates and sediment type accumulation rate or organic carbon flux to the sediments comparison of 210pb mixing rates with those calculated from 239 240pu and 137cs distributions reveals better agreement for a pulse input of the fallout radionuclides db 0 03 0 4 cm2 y than for continuous input at a constant rate db 0 1 1 6 cm2 y although the pu and 137cs data are better fit by the latter model the agreement may be fortuitous because 239 240pu and 137cs appear significantly deeper than 210pb in at least one core tracer separation could be caused by particle size selective mixing by the benthic fauna or by chemical mobilization if the fallout radionuclides are scavenged from surface waters by large organic rich particles such as fecal pellets their release and migration may result from decomposition of the carrier in surface sediments either a relatively unreactive form of pu e g oxidized pu has been released by this process or a one dimensional model is inadequate to explain its observed penetration into the sediments activity ratios of 239 240 pu 137 cs in the sediments decrease with increasing north latitude and the trend reflects higher fluxes of 239 240pu near the weapons test site at christmas island 2 n the 239 240 pu 137 cs ratios and fluxes to the sediment assuming constant input at the siliceous ooze red clay site are consistent with published sediment trap data from a nearby site thus if fallout radionuclide fluxes to the sea floor were higher in the past both 239 240pu and 137cs have been released from sinking particles

porewater ph and authigenic phases formed in the uppermost sediments of the santa barbara basin abstract in this paper porewater and solid phase analyses are used in combination with in situ o 2 and ph microelectrode measurements to characterize early diagenetic processes in the uppermost sediments of the santa barbara basin california rapid reduction of dissolved oxygen nitrate solid phase manganese and iron and dissolved sulfate is observed between sediment depths of 0 and 2 cm reductive solubilization of ferric iron phases releases fe 2 adsorbed phosphate and fluoride to the porewaters and contributes to a sharp increase in porewater ph between 2 and 4 cm sulfate reduction rates peak ph levels off and acid volatile sulfides and pyrite become the dominant forms of solid phase iron saturation state calculations which depend largely on ph indicate that the porewaters of the santa barbara basin become saturated with respect to carbonate fluorapatite and calcite within the first 0 25 mm of the sediment and are highly supersaturated by and below 2 cm in spite of this result porewater evidence of phosphate and fluoride removal into a solid phase is observed only in the first 5 cm of some cores whereas dissolved ca profiles suggest dispersed calcite precipitation throughout the sediment column this finding is interpreted as an indication of the nonsteady state nature of the surface reactions that may given sufficient nucleation sites and time lead to carbonate fluorapatite genesis in anoxic sediments finally microelectrode ph profiles from two other basins in the california borderlands are presented these demonstrate that the porewaters of the santa barbara basin are more alkaline than those of other basins this outcome is attributed to the lack of particle mixing and a unique interplay between fe liberation and fes precipitation reactions in the santa barbara basin

rare earth elements in the phosphatic enriched sediment of the peru shelf abstract apatite enriched materials from the peru shelf have been analyzed for their major oxide and rare earth element ree concentrations the samples consist of 1 the fine fraction of sediment mostly clay material 2 phosphatic pellets and fish debris which are dispersed throughout the fine grained sediment 3 tabular shaped phosphatic crusts which occur within the uppermost few centimeters of sediment and 4 phosphatic nodules which occur on the seafloor the bulk ree concentrations of the concretions suggest that these elements are partitioned between the enclosed detrital material and the apatite fraction analysis of the fine grained sediment with which the samples are associated suggested that this detrital fraction in the concretions should have shale ree values the analysis of the fish debris suggested that the apatite fraction might have seawater values the seawater contribution of ree s is negligible in the nodules and crust in which the apatite occurs as a fine grained interstitial cement that is the concentration of ree s and the ree patterns are predominantly a function of the amount of enclosed fine grained sediment by contrast the ree pattern of the pelletal apatite suggests a seawater source and the absolute ree concentrations are relatively high the ree p 2 o 5 ratios of the apatite fraction of these samples thus vary from approximately zero in the case of the crust and nodules to as much as approximately 1 2 10 3 in the case of the pellets the range of this ratio suggests that rather subtle variations in the depositional environment might cause a significant variation in the ree content of this authigenic fraction of the sediment pelletal glauconite was also recovered from one sediment core its ree concentrations closely resemble those of the fish debris

226ra in phosphate nodules from the peru chile seafloor abstract 226ra and other uranium series radionuclides have been measured in a suite of marine phosphorite samples from the upwelling area off peru chile by gamma ray spectrometry and radiochemical techniques our results lead to the following conclusions 1 phosphorite nodules typically show unidirectional growth at rates of 1 to 10 mm kyr 2 very young samples less than a few thousand years contain slight excess amounts of 226ra probably derived from pore fluids and 3 slow but persistent leakage of 226ra out of phosphate nodules occurs resulting in systematically lower 226ra ages compared to 230th ages for samples older than about twenty thousand years radium fluxes from these phosphate nodules appear to be 1 to 3 orders of magnitude less than those calculated for deep sea sediments and ferromanganese nodules

pore water fluoride in peru continental margin sediments uptake from seawater abstract fluoride analyses display downward decreasing pore water gradients in peru shelf phosphatic muds that require diffusion from the overlying seawater into the sediment column and removal by reaction within the upper few tens of centimeters presumably by incorporation into carbonate fluorapatite the profiles can be modeled as first order f removal with rate constants of 3 yr 1 and asymptotic f concentrations deep in the cores of 35 45 m almost one half the seawater value the integrated flux of fluoride from seawater into organic rich shelf sediments in coastal upwelling zones phosphatic muds yields a contemporaneous global f burial of 0 54 10 10 mol f yr 1 about one fifth the burial in other sinks mostly carbonates and opal the associated burial flux of phosphorus in shelf phosphorites is about 1 6 10 10 mol p yr 1 comparable to p burial in the deep sea with organic matter 1 4 10 10 mol yr 1 and biogenic carbonates 1 4 10 10 mol yr 1 thus phosphorite formation on the peru shelf is a significant contemporaneous process

opal zn si ratios as a nearshore geochemical proxy in coastal antarctica during the last 50 years the antarctic peninsula has experienced rapid warming with associated retreat of 87 of marine and tidewater glacier fronts accelerated glacial retreat and iceberg calving may have a significant impact on the freshwater and nutrient supply to the phytoplankton communities of the highly productive coastal regions however commonly used biogenic carbonate proxies for nutrient and salinity conditions are not preserved in sediments from coastal antarctica here we describe a method for the measurement of zinc to silicon ratios in diatom opal zn si opal which is a potential archive in antarctic marine sediments a core top calibration from the west antarctic peninsula shows zn si opal is a proxy for mixed layer salinity we present down core zn si opal paleosalinity records from two rapidly accumulating sites taken from nearshore environments off the west antarctic peninsula which show an increase in meltwater input in recent decades our records show that the recent melting in this region is unprecedented for over 120 years

arc parallel flow within the mantle wedge evidence from the accreted talkeetna arc south central alaska 1 residual mantle exposures in the accreted talkeetna arc alaska provide the first rock analog for the arc parallel flow that is inferred from seismic anisotropy at several modern arcs the peridotites exposed at the base of the jurassic talkeetna arc have a moho parallel foliation and indicate dislocation creep of olivine at temperatures of 1000 to 1100 c slip occurred chiefly on the 001 100 slip system which has only rarely been observed to be the dominant slip system in olivine stretching lineations and olivine 100 slip directions are subparallel to the long axis of the talkeetna arc for over 200 km indicating that mantle flow was parallel to the arc axis the alignment of the olivine 100 axes yields a calculated s wave anisotropy with the fast polarization direction parallel to the arc thus 1 the fast polarization directions observed parallel to some modern arcs now have an exposed geological analog 2 arc parallel fast polarization directions can be caused by anisotropic peridotites and do not require the presence of fracture zones fluid filled pockets or glide on the 010 001 h2o induced slip system 3 seismic anisotropy beneath modern arcs may be due to slip on 001 100 with a horizontal foliation rather than slip on 010 100 with a vertical foliation and 4 the observed dominance of the 001 100 slip system may be due to high h2o concentrations suggesting that strain in the oceanic upper mantle may be accommodated dominantly by 001 100 olivine slip

a marine biogeochemical perspective on black shale deposition abstract deposition of marine black shales has commonly been interpreted as having involved a high level of marine phytoplankton production that promoted high settling rates of organic matter through the water column and high burial fluxes on the seafloor or anoxic sulfidic water column conditions that led to high levels of preservation of deposited organic matter or a combination of the two processes here we review the hydrography and the budgets of trace metals and phytoplankton nutrients in two modern marine basins that have permanently anoxic bottom waters this information is then used to hindcast the hydrography and biogeochemical conditions of deposition of a black shale of late jurassic age the kimmeridge clay formation yorkshire england from its trace metal and organic carbon content comparison of the modern and jurassic sediment compositions reveals that the rate of photic zone primary productivity in the kimmeridge sea based on the accumulation rate of the marine fraction of ni was as high as 840 g organic carbon m 2 yr 1 this high level was possibly tied to the maximum rise of sea level during the late jurassic that flooded this and other continents sufficiently to allow major open ocean boundary currents to penetrate into epeiric seas sites of intense upwelling of nutrient enriched seawater would have been transferred from the continental margins their present location onto the continents this global flooding event was likely responsible for deposition of organic matter enriched sediments in other marine basins of this age several of which today host major petroleum source rocks bottom water redox conditions in the kimmeridge sea deduced from the v mo ratio in the marine fraction of the kimmeridge clay formation varied from oxic to anoxic but were predominantly suboxic or denitrifying a high settling flux of organic matter a result of the high primary productivity supported a high rate of bacterial respiration that led to the depletion of o2 in the bottom water a high rate of burial of labile organic matter albeit a low percentage of primary productivity in turn promoted anoxic conditions in the sediment pore waters that enhanced retention of trace metals deposited from the water column

relation between sedimentation rate and burial of organic phosphorus and organic carbon in marine sediments organic carbon and organic phosphorus concentrations in marine sediment cores with known sedimentation rates were determined and collected from the literature with the exception of organic rich sediments from upwelling areas organic phosphorus concentrations in marine sediments do not correlate linearly with organic carbon concentrations the c p ratios of organic matter buried in marine sediments vary systematically with sedimentation rate over the range studied 0 0001 to 2 5 cm y low c p ratios 200 atomic occur in sediments with sedimentation rates both less than 0 002 cm y and greater than 1 cm y higher c p ratios up to 600 are found at intermediate sedimentation rates a simple steady state model is developed for the early diagenesis of organic carbon and organic phosphorus in marine sediments the processes included are sediment burial pore water diffusion and bacterial decomposition of organic matter by oxic respiration and sulfate reduction preferential regeneration of phosphorus relative to carbon is assumed to take place during oxic respiration the modeling shows that both organic matter preservation and the degree of elemental fractionation between phosphorus and carbon depend strongly on the overall sedimentation rate very high sedimentation rates coastal deltaic environment result in a good preservation of deposited organic matter with an initial c p ratio close to the redfield value at very slow sedimentation rates pelagic environment the oxidation of the sedimentary organic matter is nearly complete and produces a low c p residual organic material this residual organic matter may represent either refractory detrital organic compounds enriched in phosphorus or in situ synthesized low c p bacterial remains the high c p ratios at intermediate sedimentation rates slope and shelf environment reflect the preferential regeneration of phosphorus during the incomplete decomposition of the metabolizable organic matter

uranium series and ams 14c studies of modern phosphatic pellets from peru shelf muds recent sediments and separated phosphate pellets 125 500 m in diameter from the peru shelf have been analyzed for uranium decay series isotopes and 14c in order to determine age relationships and mineralization rates uranium series ages of pellets separated from one box core are significantly higher than ams radiocarbon ages determined for the same pellets these differences appear to be a consequence of mixing of an older generation of pellets with ones which are more recently formed postdepositional adsorption of reactive elements such as thorium and protactinium onto pellet surfaces may also contribute to the observed discordancy with radiocarbon ages sediment radiocarbon and 210pb sediment results as well as some trends in the uranium series data suggest that high concentrations of phosphate pellets have accumulated in some peru shelf sediments without extensive reworking individual pellets apparently form very quickly on time scales of a few years estimated authigenic uptake rates of phosphorus into pellets ranges from 0 5 to 9 40 mol p cm 2 yr 1 somewhat higher than rates measured for nodules from the same area this is consistent with observations that pelletal morphologies predominate over nodular forms within ancient phosphorite deposits

cross stratigraphies from a seismically active mud lens off peru indicate horizontal extensions of laminae missing sequences and a need for multiple cores for high resolution records abstract marine laminated sediments in dysoxic areas of the ocean floor are an excellent archive for high resolution climate reconstructions while the existence of discontinuities produced by natural events such as underwater landslides slumps strong bottom currents and or bioturbation is usually acknowledged for long records the extent of their influence on high resolution sequences is usually not considered in the present work we show strong evidence for multiple stratigraphic discontinuities in different gravity and box cores retrieved off pisco peru covering the last 600 years chronostratigraphies are largely based on cross correlation of distinct sedimentary structures determined by x ray image analysis and validated using 210 pb 241 am and 14 c profiles as well as proxy records the cross correlation of distinct stratigraphic layers allows for chronostratigraphic tie points and clearly shows that some sedimentary sequences are continuous across scales of tens of kilometers indicating that regional processes often determine laminae formation some differences in laminae thickness were found among cores which could be explained by different sedimentation rates spatially variable deposition of diatom blooms changes in silica dissolution and partial deposition erosion caused by bottom currents using multiple stratigraphic tie points provides clear evidence for laminated sequences present in some cores to be missing in other cores moreover instantaneous depositions from upslope were identified in all the cores disrupting the continuity of the sediment records these discontinuities instantaneous deposits and missing sequences may be due to slumps possibly triggered by earthquakes and or erosion by strong bottom currents in spite of the missing sequences in some cores a continuous composite record of the last six centuries was reconstructed from spliced sequences of the different cores which provides a well constrained temporal framework to develop further high resolution proxies in this region the present work shows that paleoreconstructions developed from single cores particularly in areas with strong seismic activity and or strong bottom currents are subject to both temporal gaps and instantaneous depositions from upslope both of which could be misinterpreted as abrupt climate changes or anomalous climate events we stress the need for multiple cores to determine the stratigraphic continuity and chronologies for high resolution records

significant geometric variation of the subducted plate beneath the northernmost cascadia subduction zone and its tectonic implications as revealed by the 2014 mw 6 4 earthquake sequence abstract at the northernmost extent of the cascadia subduction zone the explorer plate subducts at approximately 2 cm yr less than half the rate of the juan de fuca plate to the south the boundary between these two plates is known as the nootka fault zone which is one of the focuses of the seafloor earthquake array japan canada cascadia experiment seajade during this survey an m w 6 4 earthquake occurred on 24 april 2014 this event and the subsequent aftershocks referred to as the nootka sequence reveal an approximately 40 km long subducted fault within the explorer plate to the north of the nootka fault zone we infer that the fault is a subducted conjugate fault because of its nearly identical orientation to those seaward of the subduction front within the nootka fault zone the depth distribution and focal mechanisms of the aftershocks indicate significant margin parallel deformation of the subducting plate the subduction interface at the nootka sequence fault has been deflected downward to the northwest from a depth of approximately 15 25 km over a distance of 25 km we propose two possible scenarios that are modified from previously suggested slab tear model with induced margin parallel mantle flow to explain the significant deformation of the young warm subducting explorer plate to the northwest of this change in slab geometry a lack of seismic activity above the plate interface indicates that the explorer plate has partially decoupled from the overriding north america plate we conclude that the geometric variation separating the southern explorer plate from the north along with decoupling and a possible intraslab tear may be a significant combination to resist the propagation of a megathrust rupture across this boundary

rifting and magmatism in the northeastern south china sea from wide angle tomography and seismic reflection imaging we present a new travel time tomography velocity model and seismic reflection images that delineate the rift architecture and magmatic features of the rifted margin in the northeastern south china sea these data reveal moderately stretched crust 25 km thick along the continental shelf and thin but laterally variable crustal thickness in the distal margin along the continental slope crust rapidly thins to 4 km in a basin characterized by tilted fault blocks that sole into a low angle detachment strain was localized to a degree within the highly stretched basin but failed to progress to breakup and seafloor spreading crust in the distal margin is 12 15 km thick few extensional structures are apparent in the distal margin but seismic velocities are suggestive of highly thinned and magmatically intruded continental crust the magmatic features we interpret include volcanic zones at the top of the basement that deform or disrupt overlying postrift strata sills intruded into the postrift sedimentary section and a high velocity 6 9 7 5 km s lower crustal layer that we take to be magmatic underplating or pervasive lower crustal intrusions these features primarily occur in the distal margin and may have been emplaced during postrift seafloor spreading the postrift magmatism may have been induced by convective removal of continental lithosphere following breakup and the onset of seafloor spreading in the south china sea

geochemical characteristics of phosphorite and carbonate nodules from the miocene funakawa formation western margin of the yokote basin northeast japan abstract phosphorite nodules were discovered in diatomite and diatomaceous mudstones of the late miocene funakawa formation in dewa height on the western margin of the yokote basin northeast japan they occur as small nodules cemented burrows and small trace fossils in diatom rich sediment that was deposited just above the contact between underlying siliceous shales the underlying shales are essentially devoid of fossils and the overlying bioturbated diatomaceous mudstones are fossil rich the underlying shale is rich in manganese mn calcite nodules and the overlying mudstone is rich in magnesian mg calcite nodules some phosphorite nodules exhibit a zonal structure caused by two phases of mg calcite cementation with or without mn calcite within the pore space of the nodule the phosphorus p content of these nodules exceeded 25 wt in p2o5 the principle phosphate mineral composing the nodules is fluor f carbonate apatite crystals which are a few m in length and cocoon shaped their aggregate includes a small amount of silica aluminum and water si al and h2o respectively the rare earth element ree composition of the phosphorite nodules is characterized by a positive europium eu anomaly and no cerium ce anomaly the biomarker distribution of the phosphorite nodules is characterized by low cpi low carbon number n alkane and high hopenes type hopane and isoprenoid ketone phosphorite nodules originate from the phosphate contained in diatoms the mineralization occurs in the bacterial sulphate reduction zone and bacteria appear to have played an important role in the concentration of p and precipitation of apatite

estimates of particle flux and reworking at the deep sea floor using234th 238u disequilibrium abstract because of high specific activities of excess234th t1 2 24 1 days on suspended particles in the deep sea this nuclide is potentially an extremely sensitive indicator of particle inputs and dynamics at the seafloor measurements were made at two deep sea sites in order to examine this potential inventories of excess234th at a low current hemipelagic mud site 3990 m in the panama basin were 1 5 september 81 and 2 5 june 82 dpm cm2 the steady state fluxes to the seafloor calculated from these inventories are in rough agreement with radionuclide fluxes measured in sediment traps small scale 100m spatial variability in inventories implies biologically significant heterogeneity in particle inputs sediment from the continental rise site in the northwest atlantic 2800 m a site with higher current velocities than the panama basin had an inventory of 1 9dpm cm2 this inventory is also in rough agreement with predictions made on the basis of nearby sediment trap data particle mixing coefficients of 30cm2 yr calculated at the pacific and atlantic sites are similar to those in shallow water deposits but could reflect disturbance during handling based on210pb data from the panama basin sediment from below 6cm is mixed at a rate 10 slower than the near surface sediment to a depth of at least 20 cm agreement between234th predicted mixing rates at the panama basin site with210pb profiles and in situ experiments with glass bead tracers implies that these rates are real although the diffusion of dissolved234th into deep sea sediments complicates interpretations 234thxs distributions in bottom sediments offer a useful adjunct to sediment traps for investigation of particle dynamics near the deep sea floor

nannostructure and genesis of phosphorites from odp leg 112 the peru margin abstract a sem based petrographic study of miocene to quaternary phosphorites recovered during odp leg 112 shows that they contain a variety of apatitic nannostructures in porous types apatite appears as ovoid dumbbell like botryoid micron sized and inframicron apatite particles and flakes these structures are probably due to microbially mediated apatite precipitation in compact varieties diagenesis reduced the porosity obscuring the original apatite microparticles macroscopic aspects of phosphorites colour brightness hardness density are essentially linked to the porosity of apatitic structures but independent of the apatite particles and structures generally they reflect diagenetic stages in evolution the variety and distribution of the apatite particles and structures suggest that phosphorites mainly formed in three different ways phosphatization of 1 fish excrements forming copronodules 2 diatom muds producing nodules and 3 faecal pellets resulting in rounded grains these phosphorite particles formed in low energy conditions many of them were subsequently reworked

mantle flow in subduction systems the mantle wedge flow field and implications for wedge processes 1 the mantle wedge above subducting slabs is associated with many important processes including the transport of melt and volatiles our understanding of mantle wedge dynamics is incomplete as the mantle flow field above subducting slabs remains poorly understood because seismic anisotropy is a consequence of deformation measurements of shear wave splitting can constrain the geometry of mantle flow in order to identify processes that make first order contributions to the pattern of wedge flow we have compiled a data set of local s splitting measurements from mantle wedges worldwide there is a large amount of variability in splitting parameters with average delay times ranging from 0 1 to 0 3 s up to 1 0 1 5 s and large variations in fast directions we tested for relationships between splitting parameters and a variety of parameters related to subduction processes we also explicitly tested the predictions made by 10 different models that have been proposed to explain splitting patterns in the mantle wedge we find that no simple model can explain all of the trends observed in the global data set mantle wedge flow is likely controlled by a combination of downdip motion of the slab trench migration ambient mantle flow small scale convection proximity to slab edges and slab morphology with the relative contributions of these in any given subduction system controlled by the subduction kinematics and mantle rheology there is also a likely contribution from b type olivine and or serpentinite fabric in many subduction zones governed by the local thermal structure and volatile distribution

tectonic implications of early miocene oib magmatism in a near trench setting the outer zone of sw japan and the northernmost ryukyu islands abstract the outer zone of the sw japan and northernmost ryukyu arcs was affected by intense igneous activity during the miocene characterized by morb like basalts alkaline basalts and s type with subordinate i type felsic to intermediate volcano plutonic complexes these igneous rocks are inferred to be the products of near trench magmatism early miocene 18 ma alkaline basalt dikes from the shingu otoyo area in central northern shikoku and an alkaline lamprophyre dike from tanegashima one of the northernmost ryukyu islands pre date the middle miocene felsic to intermediate igneous rocks the basalts and lamprophyre have compositions of basanite basalt trachybasalt and phonotephrite they are characterized by elevated large ion lithophile elements liles e g sr ba and th and high concentrations of high field strength elements hfses e g tio 2 nb and zr the geochemical signatures of the basalts and lamprophyre suggest an ocean island basalt type oib type mantle source the occurrence of alkaline basalts and lamprophyre with oib type intraplate geochemical signatures in a near trench setting is unusual with regard to plate tectonic processes we propose that trench ward motion of the overriding plate during the period around the early miocene resulted in a shallowly dipping slab and interplate coupling between the subducting philippine sea plate psp and the overlying crust beneath most of the outer zone in the western part of sw japan and the northernmost ryukyu islands the oib type magmatism in the near trench environment is most plausibly explained by the upwelling of asthenospheric material from beneath the subducting slab which migrated through fractures and or tears in the slab we envisage two possible scenarios for the formation of these fractures or tears 1 the shallowing dip angle of the subducted psp resulted in concave upwards flexure of the slab generating fractures in the flexed region and 2 differential motion within the subducting slab induced slab tearing and segmentation

mantle flow and dynamic topography associated with slab window opening insights from laboratory models abstract we present dynamically self consistent mantle scale laboratory models that have been conducted to improve our understanding of the influence of slab window opening on subduction dynamics mantle flow and associated dynamic topography over geological time scales the adopted setup consists of a two layer linearly viscous system simulating the subduction of a fixed plate of silicone lithosphere under negative buoyancy in a viscous layer of glucose syrup mantle our experimental setting is also characterized by a constant width rectangular window located at the center of a laterally confined slab modeling the case of the interaction of a trench parallel spreading ridge with a wide subduction zone we found that the opening of a slab window does not produce consistent changes of the geometry and the kinematics of the slab on the contrary slab induced mantle circulation quantified both in the vertical and horizontal sections using the feature tracking image analysis technique is strongly modified in particular rollback subduction and the opening of the slab window generate a complex mantle circulation pattern characterized by the presence of poloidal and toroidal components with the importance of each evolving according to kinematic stages mantle coming from the oceanic domain floods through the slab window indenting the supra slab mantle zone and producing its deformation without any mixing between mantle portions the opening of the slab window and the upwelling of sub slab mantle produce a regional scale non isostatic topographic uplift of the overriding plate that would correspond to values ranging between ca 1 and 5 km in nature assuming that our modeling results can be representative of the natural behavior of subduction zones we compared them to the tectonics and volcanism of the patagonian subduction zone we found that the anomalous backarc volcanism that has been developing since the middle miocene could result from the lateral flow of sub slab mantle and that part of the patagonian uplift could be dynamically supported

sedimentology geochemistry and origin of phosphatic chalks the upper cretaceous deposits of nw europe santonian lower campanian and lower maastrichtian phosphatic chalks in northern france southern england and belgium are europe s largest sedimentary phosphatc deposits the stratigraphy sediment ology petrography mineralogy and geochemistry of the lithofacies are reviewed and new data presented depositional and diagenetic models for phosphatic chalk deposits are developed using published experimental work and from observations of modern high and low productivity phosphogenic systems it is concluded that phosphatic chalks were deposited in well oxygenated current swept environments phosphatization required a delicate balance to be maintained between moderate organic carbon and carbonate sedimentation rates reduced bulk sediment accumulation rate and an enhanced rate of bioturbation precipitation of carbonate fluorapatite francolite accompanied the bacterially mediated decomposition of organic matter occurring within centimetres of the sediment seawater interface and taking place preferentially within microbial bodies and coatings in addition to the organically derived component pore water phosphate levels were enhanced by phosphate absorbed on ferric oxyhydroxides which was liberated during iron reduction mineralization was probably a dominantly post oxic process but occurred in a thick sediment mixed layer in which marine organic matter was undergoing intense mixed aerobic and anaerobic microbial degradation phosphogenesis occurred predominantly on the ne margins of the anglo paris basin where shallower sea floors and suitable palaeoceanographic conditions prevailed phosphogenic episodes were limited by sea level fluctuations which controlled the effectiveness of the erosional currents that formed and maintained the phosphatic basins and may have stimulated local productivity

particle mixing rates in deep sea sediments determined from excess 210pb and 32si profiles particle mixing rates have been determined for 5 south atlantic antarctic and 3 equatorial pacific deep sea cores using excess 210 pb and 32 si measurements radionuclide profiles from these siliceous calcareous and clay rich sediments have been evaluated using a steady state vertical advection diffusion model in antarctic siliceous sediments 210 pb mixing coefficients 0 04 0 16 cm 2 y are in reasonable agreement with the 32 si mixing coefficient 0 2 or 0 4 cm 2 y depending on 32 si half life in an equatorial pacific sediment core however the 210 pb mixing coefficient 0 22 cm 2 y is 3 7 times greater than the 32 si mixing coefficient 0 03 or 0 07 cm 2 y the difference in 210 pb and 32 si mixing rates in the pacific sediments results from 1 non steady state mixing and differences in characteristic time and depth scales of the two radionuclides 2 preferential mixing of fine grained clay particles containing most of the 210 pb activity relative to coarser particles large radiolaria containing the 32 si activity or 3 the supply of 222 rn from the bottom of manganese nodules which increases the measured excess 210 pb activity relative to 226 ra at depth and artificially increases the 210 pb mixing coefficient based on 32 si data and pore water silica profiles dissolution of biogenic silica in the sediment column appears to have a minor effect on the 32 si profile in the mixed layer deep sea particle mixing rates reported in this study and the literature do not correlate with sediment type sediment accumulation rate or surface productivity based on differences in mixing rate among three antarctic cores collected within 50 km of each other local variability in the intensity of deep sea mixing appears to be as important as regional differences in sediment properties

the volcanic margins of the northern labrador sea insights to the rifting process 1 a new interpretation of seismic reflection data on the continental margin of northern labrador shows basement structures similar to those observed on other volcanic rifted margins seaward dipping reflections inner flows volcanic plateaus and lava deltas are observed on these data magnetic chron 27n 61 ma is coincident with the volcanic plateaus connecting these features to the paleocene volcanism farther north in davis strait therefore we are able to extend the region of volcanism about 500 km south along the margin similar structures are also observed on the conjugate west greenland margin below the volcanic plateaus gravity and wide angle seismic data show thick igneous crust which is fairly symmetrically distributed across the conjugate margins however the geometry of the thinned continental crust is not symmetrical a narrower zone of thinning is observed off northern labrador the thick igneous crust lies seaward of thinned continental crust which exhibits little or no underplating by the magmatic event this observation is compatible with recent models for the formation of nonvolcanic margins followed by a magmatic event late in the rift evolution in mesozoic time rifting of the cold thick cratonic lithosphere occurred between greenland and north america with minimal volcanic activity then in late cretaceous time the zone of rifting narrowed and localized seaward of the shelf creating a relatively narrow zone of lower lithospheric thinning and mantle upwelling into which the paleocene magmas were eventually channeled this scenario is supported by studies of mesozoic igneous rocks on land in coastal west greenland and labrador

basalt geochemistry of the explorer ridge area northeast pacific ocean forty two fragments of young fresh basalts dredged from the explorer ridge paul revere ridge fracture zone and dellwood knolls have been analysed for 34 major minor and trace elements and 87sr 86sr ratios have been determined in seven of the fragments the explorer ridge basalts have major element compositions similar to most mid ocean ridge basalts morb particularly to the iron rich basalts of the southern juan de fuca ridge the basalts exhibit variability in trace element content largely attributable to crystal fractionation at low pressure with respect to morb the incompatible minor and trace elements are weakly to strongly enriched in the explorer samples and are most strongly enriched in basalts from explorer deep adjacent ridge segments erupt basalts with variable incompatible element concentrations and ratios this could be an indicator of a chemically heterogeneous mantle source or may be the result of intermittent injection of enriched magmas from a hotspot beneath explorer deep

kinematics of subduction and subduction induced flow in the upper mantle 1 results of fluid dynamical experiments are presented to model the kinematics of lithospheric subduction in the upper mantle the experiments model a dense high viscosity plate subducting lithosphere overlying a less dense low viscosity layer upper mantle the overriding lithosphere is not incorporated several important features of slab behavior were investigated including the temporal variability of hinge line migration the kinematic behavior of the slab and the subduction induced upper mantle flow both fixed and free trailing edge boundary conditions of the subducting plate were investigated results show that hinge line retreat is a natural consequence of subduction of a negatively buoyant slab the migration rate increases until the slab approaches the upper lower mantle discontinuity resulting in a decrease in migration rate followed by a renewed increase and finally approaching a steady state slab retreat results in mantle flow with material initially located underneath the slab flowing around the lateral slab edges toward the mantle wedge experimental results indicate that all rollback induced flow occurs around the lateral slab edges forcing the hinge line to attain a convex shape toward the direction of retreat no signs for poloidal flow underneath the slab tip have been detected only a small component of toroidal type flow was observed underneath slanting slab tips for a fixed trailing edge the slab does not sink vertically downward but sinks at an angle in a regressive manner for a free trailing edge slab sinking is oriented more vertically while the surface part of the subducting plate is pulled into the subduction zone

hf nd isotope variation in mariana trough basalts the importance of ambient mantle in the interpretation of subduction zone magmas in the study of geochemical mass balances at subduction zones the composition of the mantle wedge prior to additions from the slab is a critically important yet poorly constrained parameter deconvolving the influence of ancient versus modern enrichments is particularly difficult especially when considering elements that are highly mobile here we provide an alternative approach using less mobile elements and a filter to remove the effects of recent slab additions we provide new hf isotope data for 30 mariana trough mt backarc basin lavas once filtered hf and nd isotope ratios are highly correlated of indian mid oceanic ridge basalt character and display variations similar to ocean ridges of comparable lengths the isotopic variability observed in this ambient mantle provides a new paradigm for the interpretation of the varied volcanic products of the arc thus shoshonites associated with the northern termination of the backarc basin rift axis reflect the interaction of a subducted sediment melt with an isotopically enriched mantle source in contrast the large volcanoes of the central island province have a consistent offset in nd isotope compositions from the mt array resulting from fluid addition existing data for smaller edifices in the submarine portion of the arc have larger variations resulting from fluid addition on a more local scale we suggest that the similar characterization of ambient mantle elsewhere may help to resolve many conflicting geochemical observations in arc lavas worldwide

temporal and spatial evolution of northern cascade arc magmatism revealed by la icp ms u pb zircon dating we present thirty new laser ablation inductively coupled plasma mass spectrometry u pb zircon dates for intermediate to silicic plutons of the northern cascade arc with emphasis on the chilliwack batholith mount baker magmatic focus located in southwestern british columbia and northern washington chilliwack magmatism commenced at 35 ma in southwestern british columbia and the most voluminous plutons define a cluster at 32 29 ma documenting an early flare up during the same interval the index squire creek and cascade pass intrusions were emplaced south of the chilliwack batholith north of the chilliwack maximum pluton ages become progressively younger northward tracking the northerly migration of the edge of the farallon juan de fuca explorer plate system relative to north america chilliwack magmatism continued from 29 ma to 22 ma at a slightly reduced flux followed by a lull from 22 to 11 ma during which magmatism shifted north to the mount barr batholith 18 ma chilliwack magmatism res

partial melting of slab window margins genesis of adakitic and non adakitic magmas abstract when a mid ocean spreading ridge subducts it typically splits apart at depth to form two tapered slab edges separated by asthenospheric mantle within a slab window we examine the fate of the slab edges using simplified slab window geometries specific thermal parameters and assumptions regarding shear stress and slab hydration six fundamental zones of slab anatexis are identified and classified according to expected melt and restite compositions non adakitic melts of granodioritic to tonalitic composition are generated along the plate edges at depths of 5 65 km whereas adakitic melts form proximal to the edges at depths of 25 90 km as anatexis proceeds the subducted crust is converted to a migmatite of slab melt and eclogitic restite much of the migmatite may shear away from the slab and become incorporated into the mantle the melts will rise and leave behind fragments of restite within mantle peridotite if the peridotite is part of the overriding plate then the restite fragments may become long term residents of the continental lithospheric mantle however if the restite becomes entrained in the asthenosphere it may undergo upwelling and melting or flow away as ductile streaks to become long term mantle heterogeneities slab windows are thereby identified as important sites for slab melting and geochemical replenishment of the mantle

variations in bioturbation across the oxygen minimum zone in the northwest arabian sea abstract oxygen minimum zones are expected to alter substantially the nature rates and depths of bioturbation along continental margins yet these effects remain poorly studied using excess 210 pb profiles sediment x radiography and box core samples for macrofauna we examined bioturbation processes at six stations 400 700 850 1000 1250 and 3400 m deep along a transect across the oxygen minimum zone omz on the oman margin bottom water oxygen concentrations ranged from 0 13 ml l 1 at 400 m to 2 99 ml l 1 at 3400 m 210 pb mixed layer depth and bioturbation intensity db exhibited high within station variance and means did not differ significantly among stations however the mean mixed layer depth 4 6 cm for pooled omz stations 400 1000 m depths 0 13 0 27 ml l 1 bottom water oxygen was half that for stations from similar water depths along well oxygenated atlantic and pacific slopes 11 1 cm suggesting that oxygen stress reduced 210 pb mixing depth on the oman margin modal burrow diameter and the diversity of burrow types at a station were highly correlated with bottom water oxygen concentration from the edge to the core of the oman omz spearman s rho 0 89 p 0 02 suggesting that these parameters are useful proxies for bottom water oxygen concentrations under dysaerobic conditions in contrast neither the maximum diameter and nor the maximum penetration depth of open burrows exhibited oxygen related patterns along the transect reduced 210 pb mixing depth within the oman margin omz appeared to result from a predominance of surface deposit feeders and tube builders within this zone rather than from simple changes in horizontal or vertical distributions of macrofaunal abundance or biomass the number of burrow types per station was highly correlated with macrofaunal species diversity suggesting that burrow diversity may be a good proxy for species diversity in paleo dysaerobic assemblages we conclude that bottom water oxygen concentrations of 0 13 0 27 ml l 1 substantially alter a number of bioturbation parameters of importance to diagenetic and biofacies models for continental margins

fish scales in sediments from off callao central peru abstract we study fish scales as a proxy of fish abundance and preservation biases together with phosphorus from fish remains p fish in a sediment core retrieved off callao peru 12 1 s 77 42 w water depth 179 m core length 52 cm we interpret our results as a function of changing redox conditions based on ratios of redox sensitive trace elements cu al mo al ni al zn al v al terrigenous indicators fe in clays ti al and biogenic proxies caco 3 biogenic opal total nitrogen organic carbon barite ba the core covers roughly 700 years of deposition based on 210 pb activities extrapolated downcore and 14 c dating at selected intervals our fish scale record is dominated by anchovy engraulis ringens scales followed by hake merluccius gayii scales the core presented an abrupt lithological change at 17 cm corresponding to the early 19th century above that depth it was laminated and was more organic rich 10 15 organic carbon than below where the core was partly laminated and less organic rich fish p total ratios were also observed within the upper 17 cm of the core the behavior of biogenic proxies and redox sensitive trace elements was similar more reduced conditions corresponded to higher contents of caco 3 c org total nitrogen and fish scales suggesting that these proxies might convey an important preservation signal

cocos nazca slab window beneath central america abstract integration of petrologic and tectonic data favours a model of slab window formation beneath central america in the pliocene pleistocene central america has been the site of voluminous cenozoic arc volcanism the cocos and nazca plates which are subducting beneath central america are diverging along the east trending cocos nazca spreading ridge since 25 ma the americas have advanced about 1800 km west over the ridge transform system since at least 8 ma plate integrity and the ridge transform configuration have been preserved during convergence resulting in subduction of the spreading ridge and development of a slab window the panama fracture zone an active transform fault is the part of the ridge transform system currently being subducted the ridge transform system formerly adjoining the northern end of the panama fracture zone is likely to have been left stepping we use present day plate motions to design a slab window to fit known variations in igneous composition hypocentre distribution and mantle anisotropy the modeling demonstrates that subduction of ridge segments and resultant slab window development began between 6 and 10 ma cessation of ridge subduction occurred between 1 and 3 ma when subduction of the panama fracture zone is considered to have begun the slab window is continuing to expand and migrate northeastward below the central american volcanic arc the absence of a wadati benioff zone from southeastern costa rica through panama corresponds to the position of the slab window within this region dacitic and rhyolitic volcanic rocks have adakitic compositions and are thought to result from anatexis of the young buoyant crust which forms the trailing edges of the slabs bounding the window basalts in this area were derived from an enriched ocean island type mantle source whereas basalts from the rest of the arc in nicaragua el salvador and guatemala are mainly derived from slab modified depleted mantle characteristic of volcanic arcs the presence of ocean island type mantle beneath southern costa rica and panama is explained by eastward flow of enriched asthenosphere from the galapagos plume head through the slab window and into the volcano source region eastward transfer of asthenosphere is consistent with global plate motion studies and seismic anisotropy in the asthenosphere beneath the nazca and caribbean plates the flow of peridotite is a consequence of progressive shrinkage of the pacific mantle reservoir and concurrent expansion of the atlantic mantle reservoir

morphology of the explorer juan de fuca slab edge in northern cascadia imaging plate capture at a ridge trench transform triple junction the explorer plate is a young oceanic microplate that accommodates relative motion between the pacific juan de fuca and north america plates near northern vancouver island canada the northern limit of explorer plate juan de fuca subduction and the fate of the slab in northern cascadia are poorly understood we use passive teleseismic recordings from an array of polaris broadband seismic stations to image crustal and upper mantle structure beneath northern vancouver island into the interior of british columbia a clear signature of subducted material extends northeast from the brooks peninsula at crustal levels beneath georgia strait and the mainland deep into the mantle to 300 km depth complexity in slab morphology results from juan de fuca ridge subduction and toroidal flow around the slab edge in agreement with geophysical and geological data we propose a tectonic model for the explorer plate in which its separation from the juan de fuca plate is caused by the thermomechanical erosion of the slab edge and slab thinning at shallow levels both of which slow convergence with north america and lead eventually to plate capture

transition from arc to oceanic magmatism at the kamchatka aleutian junction spatial geochemical variations of quaternary lavas erupted along the northern segment of the kamchatka arc are used to trace changes in magma generation across the subducting pacific slab edge the late pleistocene holocene lavas of the northern end of the central kamchatka depression north of the pacific slab edge show strong enrichment in high field strength elements and light rare earth elements relatively unradiogenic strontium and lead but radiogenic neodymium isotope ratios and oxygen isotope compositions similar to those of mid oceanic ridge basalts these geochemical characteristics are distinct from the southern central kamchatka depression volcanoes located above the subducting pacific slab extensive fluid triggered mantle melting dominates magma genesis beneath the largest kamchatka volcanoes in the south whereas low degree decompression melting of the pacific asthenospheric mantle is the major magma generation process north of the pacific slab edge quaternary detachment of the subducted pacific plate fragment resulted in the influx of fertile mantle beneath kamchatka we propose that upwelling and southward flow of this hotter more fertile mantle is the main reason for recent magmatism in northern kamchatka and for the exceptional productivity of the central kamchatka depression volcanoes klyuchevskoy and sheveluch the most active arc volcanoes on earth

evidence for trench parallel mantle flow in the northern cascade arc from basalt geochemistry a b s t r a c t geochemical data for basalts from the garibaldi volcanic belt northern segment of the cascade arc define arc parallel gradients in trace elements and isotope ratios that extend at least 150 km into the arc from the northern margin of the subducting juan de fuca plate southerly increases in zr nb ba nb th la pb ce 208 pb 204 pb and 176 hf 177 hf indicate greater mantle depletion and higher slab derived contributions with distance from the slab edge temperatures and pressures of mantle melt segregation also decrease to the south the gradients are most plausibly explained as a consequence of slab rollback induced toroidal flow at the northern slab edge nootka fault zone whereby enriched oib type ne pacific asthenospheric mantle from beneath the slab is drawn into the mantle wedge in a trench parallel southerly flow pattern melts of the enriched asthenosphere are progressively diluted to the south by melts of the slab modified depleted mantle wedge arc parallel changes in slab thermal conditions cannot account for these gradients trench parallel geochemical gradients in the northern cascade arc are consistent with shear wave splitting data numerical modeling and experimental studies showing that trench parallel mantle flow may be a common phenomenon near slab edges and slab gaps

volcano growth mechanisms and the role of sub volcanic intrusions insights from 2d seismic reflection data abstract temporal and spatial changes in volcano morphology and internal architecture can determine eruption style and location however the relationship between the external and internal characteristics of volcanoes and sub volcanic intrusions is often difficult to observe at outcrop or interpret uniquely from geophysical and geodetic data we use high quality 2d seismic reflection data from the ceduna sub basin offshore southern australia to quantitatively analyse 56 pristinely preserved eocene age volcanogenic mounds and a genetically related network of sub volcanic sills and laccoliths detailed seismic mapping has allowed the 3d geometry of each mound to be reconstructed and distinct seismic facies within them to be recognised forty six continental basaltic shield volcanoes have been identified that have average flank dips of 3 parallel seismic reflections within the shield volcanoes are interpreted to represent interbedded volcanic and clastic material suggesting that a series of temporally separate eruptions emanated from a central vent the shield volcanoes typically overlie the lateral tips of sills and we suggest that the intermittent eruption phases correspond to the incremental emplacement of discrete magma pulses within the laterally extensive sill complex eight volcanogenic hydrothermal vents which are also associated with the lateral tips of sills were also recognised and these appear to have formed from the seepage of intrusion related hydrothermal fluids onto the seafloor via emplacement induced fractures this work highlights that deformation patterns preceding volcanic eruptions may i be offset from the eruption site ii attributed to intrusions with complex morphologies and or iii reflect magma movement along pre existing fracture systems these complexities should therefore be considered in eruption forecasting models that link pre eruption ground deformation to subterranean magma emplacement depth and volume more generally our study highlights the key role that seismic reflection data can play in understanding the geometry distribution and evolution of ancient and modern volcanic systems

an upwelling model for the phosphoria sea a permian ocean margin sea in the northwest united states the permian phosphoria formation a petroleum source rock and world class phosphate deposit was deposited in an epicratonic successor basin on the western margin of north america we calculate the seawater circulation in the basin during deposition of the lower ore zone in the meade peak member from the accumulation rates of carbonate fluorapatite and trace elements the model gives the exchange rate of water between the phosphoria sea and the open ocean to the west in terms of an upwelling rate 84 m yr 1 and residence time 4 2 yr of seawater in the basin these hydrographic properties supported a mean rate of primary productivity of 0 87 g m 2 d 1 of carbon in the uppermost few tens of meters of the water column the photic zone and denitrifying redox conditions in the bottom water below approximately 150 m depth high rain rates onto the sea floor of the organic matter that hosted the phosphate and several trace elements contributed to the accumulation of phosphorite chert and black shales and mudstones evaporation in the goose egg basin to the east of the phosphoria basin ensured the import of surface seawater from the phosphoria sea budgets of water salt phosphate and oxygen plus the minor accumulation of the biomarker gammacerane show that exchange of water between the two basins was limited possibly by the shallow carbonate platform that separated the two basins

early diagenesis of organic matter in peru continental margin sediments phosphorite precipitation abstract pore water chemistry total dissolved co 2 nh 4 no 3 no 2 po 4 si oh 4 ca mg fe mn so 4 h 2 s and f and titration alkalinity solid phase chemistry c org p org c tot n tot f si opal and s ii and sediment characteristics porosity dry bulk density and formation factors were determined on a centimeter scale spacing in the upper 20 40 cm of sediments under intense upwelling areas on the peru continental shelf these data demonstrate that carbonate fluorapatite cfa is precipitating from pore waters in the upper few centimeters of a gelatinous mud with high organic carbon content up to 20 c org very high porosity 0 96 ml cm 3 and very low dry bulk density 3 dissolved phosphate concentrations at the sediment water interface range from 20 to 100 m orders of magnitude higher than bottom water concentrations and much higher than predicted from regeneration of organic matter the mechanism of this interfacial phosphate release is unclear but is apparently uncoupled from carbon and nitrogen metabolism and thus may be linked either to dissolution of fish debris or to the presence of a microbial mat in surficial sediments fluoride is incorporated into cfa by diffusion from the overlying seawater and carbonate ions are provided from pore water alkalinity magnesium concentrations in this reaction zone are not significantly different from those of seawater suggesting that magnesium depletion is not a necessary prerequisite for cfa precipitation the environment of precipitation is interface linked rather than driven by organic diagenesis of phosphorus deeper in the sediment most of the cores display a wide range of diagenetic characteristics below the immediate interfacial region but almost all show the precipitation signature near the interface this interface linked early diagenetic porewater environment for the precipitation of cfa explains many of the geochemical characteristics of phosphorites and provides a testable model to compare the modern phosphogenic analog with ancient phosphorite deposits two of the cores display very high solid phase phosphorus and fluoride contents reflecting the presence of apparently modern pelletal apatites