eddy formation and interaction in a baroclinic frontal geostrophic model abstract the authors investigate the behavior of buoyancy driven coastal currents in a series of numerical experiments based on a two layer frontal geostrophic model the model focuses on baroclinic instability allows for finite amplitude variations in the upper layer thickness and includes a topographic background vorticity gradient simulations of isolated fronts demonstrate meandering of the frontal outcropping filamentation and the development of both warm core and cold core eddies eddies can merge with each other separate or be reabsorbed into the current despite the assumption of only two layers it is found that growth rates and length scales of the emergent features are in agreement with results of studies based on more sophisticated primitive equation models it is determined that the cross front topographic slope has a significant effect on the instability in particular a bottom that slopes in the same sense as the fluid interface hinders the growth of perturbations simulations with t

transition from arc to intraplate magmatism associated with backarc rifting evolution of the sikhote alin volcanism k ar dates and chemical compositions of lavas in sikhote alin far east russia demonstrate marked secular variation in lava chemistry and suggest a linkage between magmatism and the major tectonic opening of the japan sea backarc basin lavas having typical arc magma chemistry were produced at 40 25 ma in the eastern part of sikhote alin a conspicuous volcanic hiatus is revealed in the region at 25 20 ma which is contemporaneous with the period of the major rifting of the backarc basin subduction related magmatism may have been switched off by an oceanward retreat of the arc trench system in contrast intraplate type lavas with typical hotspot magma compositions have dominated magmatism in both the eastern and western sikhote alin since 20 ma and occupy the eastern margin of the cenozoic intraplate magmatism in the ne asian continent

formation of the japan sea basin reassessment from ar ar ages and nd sr isotopic data of basement basalts of the japan sea and adjacent regions abstract many studies have examined the japan sea basalts recovered during ocean drilling program odp leg127 128 of these the 40ar 39ar dating undertaken is important in constraining the timing of the formation of the japan sea however the implications of their results do not appear to be fully appreciated by the geological community in this paper i reassess the 40ar 39ar age data of the basalts with reference to nd sr isotopic data the 40ar 39ar dating was performed on basalts somewhat enriched in large ion lithophile elements and recovered from odp sites 794 795 and the lower part of 797 yielding the plateau ages of 21 2 17 7 ma these basalts show the nd sr isotopic signature of a moderately depleted mantle source end 0 6 6 9 in contrast the basalts from the upper part of site 797 have yet to be dated due to their low k content although their nd isotopic compositions are similar to that of morb end 8 4 10 4 by analogy to the secular nd sr isotopic trends reported for sikhote alin and northeast japan the age of the upper basalts at site 797 may be inferred to be younger than the lower basalts probably around 16 ma the nd sr isotopic compositions of the japan sea basalts have been interpreted in terms of eastward asthenospheric flow as have the lavas of the sikhote alin and northeastern japan the timing of volcanic activity in the japan sea region i e from 21 2 to 14 86 ma is consistent with the timing of rotational crustal movements inferred from paleomagnetic studies of the japanese islands i e 14 8 4 2 ma for southwest japan and 16 5 14 4 ma for northeast japan

late mesozoic and cenozoic volcanism of the east sikhote alin area russian far east a new synthesis of geological and petrological data abstract the geotectonic framework and the evolutionary history of the east sikhote alin area esa need revision in accordance with various geological and petrological evidence gathered in recent years it has been suggested that the investigated region resulted from the subduction of the izanagi kula and pacific oceanic plates that lasted from late cretaceous to late tertiary however only late cretaceous intermediate and silicic volcanics enriched in lile and depleted in hfse can be interpreted as typical subduction complexes changes in the paleogene paleocene to early miocene magmatic style of the esa area reflect variation in the pacific plate eastern eurasian convergence parameter the termination late cretaceous subduction slab tearing and injection of the oceanic asthenosphere of pacific morb type into subcontinental lithosphere the sea of japan opening began in the eocene with formation of small rift basins in the tatar strait which accumulated coastal facies from eocene to early miocene the extension zone affected by oceanic asthenosphere gradually moved eastward i e to the modern sea of japan the effect of oceanic asthenosphere on the continental margin ended in the late miocene after the sea of japan had opened and new subduction initiated east of the japan islands em i signature of the late miocene pliocene within plate basalts is evidence for eastward asthenospheric flow

paleomagnetic evidence for the miocene counter clockwise rotation of northeast japan rifting process of the japan arc paleomagnetic results have been obtained from more than 700 oriented samples in igneous rocks of cretaceous to miocene age from northeast japan the remanent magnetizations of welded tuffs with age between 32 ma and 21 ma old from 17 widely distributed sampling sites in northeast japan are fairly well grouped with a mean ofd 41 2 i 56 5 95 7 2 the remanent magnetization of welded tuffs of 14 ma and 11 ma from 4 sites shows a northerly direction d 11 7 i 60 5 95 15 3 these data imply that northeast japan was apparently subjected to a counter clockwise rotation through 47 14 about a vertical axis on the representative point of northeast japan 141 e 39 n during the past 21 ma comparing with the period of the clockwise rotation of southwest japan the differential rotation between northeast japan and southwest japan is concluded to have occurred concurrently during the period between 21 ma and 12 ma a rifting model of the japan arc is proposed northeast japan rotated counter clockwise through 50 about a northern pivot at 146 e 44 n while southwest japan rotated clockwise through 54 about a southern pivot at 129 e 34 n the differential rotation about different rotation pivots gives rise to the opening of a diamond shaped back arc basin the japan sea

migration of fluid phases and genesis of basalt magmas in subduction zones dehydration and hydration reactions in both the downgoing lithosphere and the overlying mantle wedge have been examined in order to understand the role of h2o in the production of magmas at convergent plate boundaries the subduction of oceanic lithosphere occurring with increasing pressures and rising temperatures causes liberation of h2o from the slab amphibole which can be stable to the highest pt conditions among hydrous phases in the slab decomposes at around 90 km depth it follows that the subducted lithosphere is essentially anhydrous beneath volcanic arcs lying more than 110 km above the slab and that the supply of slab derived h2o is not a direct trigger for the production of arc magmas instead the h2o released from downgoing lithosphere reacts with the forearc mantle wedge to crystallize hydrous minerals serpentine talc amphibole chlorite and phlogopite this hydrated peridotite is dragged downward on the slab toward higher pt regions and releases h2o to shallower potential magma source regions in the mantle wedge combining experimental data on the stability of serpentine and talc with the thermal structure in the mantle wedge it is concluded that those minerals decompose beneath the forearc region on the other hand high pt experimental and thermodynamic data suggest that dehydration of amphibole and chlorite in the downdragged hydrated peridotite can take place just beneath a volcanic front phlogopite in the peridotite decomposes to release h2o at a deeper level about 200 km h2o liberated from the hydrated peridotite causes partial melting of overlying mantle wedge peridotites along with the migration of h2o through the above processes subduction components especially large ion lithophile elements can be overprinted on the magma source region which governs the geochemical characteristics of arc magmas

on the mid depth circulation in the labrador and irminger seas a numerical circulation model with 1 6 resolution and an accurate topography formulation explains details of the observed circulation in the irminger and labrador seas that were recently revealed by lavender et al 2000 we show that the recirculation pattern is established through a locally wind induced flow controlled by the bottom topography and enhanced through remote baroclinic forcing by the dense plume of denmark strait overflow water the basic circulation is a robust feature in a hierarchy of model setups it exists in the purely barotropic case driven by steady winds and is even maintained when realistic daily forcing is added the narrow recirculation zone is manifested by a sea level depression spanning from the denmark strait across the irminger into the labrador sea

holocene depositional environments and surface level changes at lake fryxell antarctica we report on holocene surface level variations of lake fryxell antarctica as determined from multiproxy analyses of 18 sediment cores during this time accumulating sediments were predominantly aeolian sand with algal and carbonate laminae based on stratigraphy mineralogy and diatom assemblages we suggest some carbonate laminae were deposited when lake level dropped leading to concentration and subsequent precipitation of salts although lake level appears to have remained relatively stable throughout the holocene minor 4 5 m below present lowstands occurred at approximately 6400 4700 3800 and 1600 cal yr bp the stability of lake fryxell during the holocene contrasts with large scale variability at other dry valleys lakes eg lake vanda and with suggestions from chemical diffusion models of a near desiccation at 1200 cal yr bp the reason for the comparative stability of lake fryxell is uncertain but may be the result of basin morphology and the number aspect and proximity of meltwater sources

how does labrador sea water enter the deep western boundary current abstract labrador sea water lsw a dense water mass formed by convection in the subpolar north atlantic is an important constituent of the meridional overturning circulation understanding how the water mass enters the deep western boundary current dwbc one of the primary pathways by which it exits the subpolar gyre can shed light on the continuity between climate conditions in the formation region and their downstream signal using the trajectories of profiling autonomous lagrangian circulation explorer p alace floats operating between 1996 and 2002 three processes are evaluated for their role in the entry of labrador sea water in the dwbc 1 lsw is formed directly in the dwbc 2 eddies flux lsw laterally from the interior labrador sea to the dwbc and 3 a horizontally divergent mean flow advects lsw from the interior to the dwbc a comparison of the heat flux associated with each of these three mechanisms suggests that all three contribute to the transformation of the boundary current a

mechanism of backarc opening in the japan sea role of asthenospheric injection abstract the process of backarc extension in the japan sea includes two modes of spreading the parallel and rotational or fan shape opening data from paleomagnetism for rocks on the northeast japan arc radiometric age determination for dredged volcanic rocks and heat flow in the basin have suggested that the parallel opening of the japan sea took place earlier than the rotational opening at 21 14 ma the mechanism of opening of the japan sea has been examined based on the secular change in the geometry of the subducted slab and in the volcanic activity on the northeast japan arc mechanisms of backarc rifting including mantle diapirism mantle flow induced by the slab subduction or interaction of the over and the underriding plates cannot reasonably explain the process of opening of the japan sea the most plausible mechanism operating in the spreading process of the japan sea is injection of the asthenosphere into the mantle wedge that forcefully pushed the subducted slab to cause both steepening of the slab and the backarc extension the asthenospheric current towards the northeast japan arc trench system may have been associated with the large scale upwelling of the asthenosphere beneath the northeastern china region that caused intra plate volcanism and produced continental rift systems in the region the present mechanism can also explain the active backarc rifting in the okinawa trough in the western pacific and the taupo depression in north island new zealand

aleutian terranes from nd isotopes nd isotope ratios substantiate the identification of oceanic crustal terranes within the continental crustal basement of the aleutian island arc the oceanic terranes are exposed in the westernmost aleutians but to the east they are completely buried by isotopically distinct arc volcanic rocks analogous oceanic terranes may be important components of the terrane collages that comprise the continents

asthenospheric injection and back arc opening isotopic evidence from northeast japan nd and sr isotopic compositions were determined for the tertiary volcanics from the back arc side of the ne japan arc sr isotopes show a linear trend through time from an enriched signature 87sr86sr 0 705437 to a depleted signature 87sr86sr 0 70270 in a complementary fashion nd isotopes start at low value nd 0 80 and show a gradual increase nd 8 3 with decreasing age isotopic change of nd and sr from the enriched signature to the depleted one is synchronous with the opening of the japan sea at 15 ma this synchronism indicates that the opening of the japan sea was initiated by the injection of the asthenosphere during the pre opening stage the mantle wedge was composed of a two layered structure the sub continental lithosphere and the underlying asthenosphere the volcanics of this stage characterized by an enriched isotopic signature were derived from a source with a higher proportion of sub continental lithosphere the sub continental lithosphere of the back arc side was thinned by the injection of the depleted asthenosphere which accelerated the growth of the morb source within the mantle wedge of the back arc side and resulted in magma generation with the depleted isotopic signature of nd and sr at the post opening stage

intra seasonal variability of the dwbc in the western subpolar north atlantic highlights a joint analysis of deep current meter records in the western north atlantic intra seasonal variability dominates the deep boundary current topographic waves near 10d periods trapped over steep topography basin centers are showing longer periods 50d caused by the eddy field observed variability characteristics compared to high resolution model simulation abstract the deep western boundary current dwbc along the western margin of the subpolar north atlantic is an important component of the deep limb of the meridional overturning near its northern origins a network of moored arrays from denmark strait to the tail of the grand banks has been installed for almost two decades to observe the boundary currents and transports of north atlantic deep water as part of an internationally coordinated observatory for the atlantic meridional overturning circulation the dominant variability in all of the moored velocity time series is in the week to month period range while the temporal characteristics of this variability change only gradually between denmark strait and flemish cap a broad band of longer term variability is present farther along the path of the dwbc at the grand banks and in the interior basins labrador and irminger seas the vigorous intra seasonal variability may well mask possible interannual to decadal variability that is typically an order of magnitude smaller than the high frequency fluctuations here the intra seasonal variability is quantified at key positions along the dwbc path using both observations and high resolution model data the results are used to evaluate the model circulation and in turn the model is used to relate the discrete measurements to the overall pattern of the subpolar circulation topographic waves are found to be trapped by the steep topography all around the western basins the labrador and irminger seas in the labrador sea the high intra seasonal variability of the boundary current regime is separated by a region of extremely low variability in narrow recirculation cells from the basin interior there the variability is also on intra seasonal timescales but at much longer periods around 50 days

circulation and deep water export at the western exit of the subpolar north atlantic the current system east of the grand banks was intensely observed by world ocean circulation experiment woce array acm 6 during 1993 95 with eight moorings reaching about 500 km out from the shelf edge and covering the water column from about 400 m depth to the bottom more recently a reduced array by the institut fur meerskunde ifm at kiel germany of four moorings was deployed during 1999 2001 focusing on the deep water flow near the western continental slope both sets of moored time series each about 22 months long are combined here for a mean current boundary section and both arrays are analyzed for the variability of currents and transports a mean hydrographic section is derived from seven ship surveys and is used for geostrophic upper layer extrapolation and isopycnal subdivision of the mean transports into deep water classes the offshore part of the combined section is dominated by the deep reaching north atlantic current nac with currents still at 10 cm s 1 near the bottom and a total northward transport of about 140 sv sv 106 m3 s 1 with the details depending on the method of surface extrapolation used the mean flow along the western boundary was southward with the section mean north atlantic deep water outflow determined to be 12 sv below the 27 74 kg m 3 isopycnal however east of the deep western boundary current dwbc the deep nac carries a transport of 51 sv northward below 27 74 kg m 3 resulting in a large net northward flow in the western part of the basin from watermass signatures it is concluded that the deep nac is not a direct recirculation of dwbc water masses transport time series for the dwbc variability are derived for both arrays the variance is concentrated in the period range from 2 weeks to 2 months but there are also variations at interannual and longer periods with much of the dwbc variability being related to fluctuations and meandering of the nac a significant annual cycle is not recognizable in the combined current and transport time series of both arrays the moored array results are compared with other evidence on deep outflow and recirculation including recent models of different types and complexity

observations inferences and mechanisms of the atlantic meridional overturning circulation a review this is a review about the atlantic meridional overturning circulation amoc its mean structure temporal variability controlling mechanisms and role in the coupled climate system the amoc plays a central role in climate through its heat and freshwater transports northward ocean heat transport achieved by the amoc is responsible for the relative warmth of the northern hemisphere compared to the southern hemisphere and is thought to play a role in setting the mean position of the intertropical convergence zone north of the equator the amoc is a key means by which heat anomalies are sequestered into the ocean s interior and thus modulates the trajectory of climate change fluctuations in the amoc have been linked to low frequency variability of atlantic sea surface temperatures with a host of implications for climate variability over surrounding landmasses on intra annual timescales variability in amoc is large and primarily reflects the response to local wind forcing meridional coherence of anomalies is limited to that of the wind field on interannual to decadal timescales amoc changes are primarily geostrophic and related to buoyancy anomalies on the western boundary a pacemaker region for decadal amoc changes is located in a western transition zone along the boundary between the subtropical and subpolar gyres decadal amoc anomalies are communicated meridionally from this region amoc observations as well as the expanded ocean observational network provided by the argo array and satellite altimetry are inspiring efforts to develop decadal predictability systems using coupled atmosphere ocean models initialized by ocean data

sortable silt records of intermediate depth circulation and sedimentation in the southwest labrador sea since the last glacial maximum abstract the labrador sea is a vital region for the atlantic meridional overturning circulation amoc where overflow waters from the nordic seas mix with locally produced labrador sea water lsw before exiting to the interior of the atlantic ocean the dynamical sedimentary proxy of mean sortable silt size ss can give information on past changes in deep water circulation speed and the strength of amoc we have produced ss records from two core sites at depths between 1500 and 2000 m on the continental slope east of newfoundland to reconstruct changes in intermediate depth water circulation speed including glacial north atlantic intermediate water and labrador sea water over the past 22 000 years increases in ss appear to coincide with much of the deglaciation as well as the mid late holocene end member modeling suggests that ice rafted debris ird is an important factor in interpreting ss during the deglaciation we find that a robust increase in ss is likely unrelated to ird during the past 5 ka and probably reflects increased flow at intermediate depths due to local production of lsw strengthening as nordic seas overflows weakened at this depth our results highlight both the complications of producing ss records in ird rich slope environments and the promise that this proxy nevertheless has for reconstructing dynamical changes in deep ocean currents

near surface circulation in the northern north atlantic as inferred from lagrangian drifters variability from the mesoscale to interannual 1 the near surface circulation of the nordic seas is basically cyclonic and consists of jets and recirculation cells which are tightly linked to the bottom topography variable forcing by the large scale rotation of the wind leads to a modulation in the strength of the gyres and their interconnecting jets this is seen in drifter and altimeter data currents are stronger during winter and during phases of high north atlantic oscillation index the exchanges between the north atlantic and the nordic seas do not seem to be directly affected by this variable forcing the narrow boundary currents and the intergyre jets are subject to instability causing mesoscale current fluctuations which contribute to the stirring and mixing of polar and atlantic water masses

preliminary results from directly measuring middepth circulation in the tropical and south pacific as part of the world ocean circulation experiment 300 autonomous lagrangian circulation explorer floats were deployed in the tropical and south pacific to measure the general circulation at 900 m depth most floats measured average currents over 25 submerged days between ascents to the surface for locating and data relay by the end of 1997 over 12 500 observations spanning 840 float years were obtained accuracy of the average velocity is limited by variability some of which is found to arise from steady flows with spatial variations smaller than averaging scales observed 25 day velocities are serially uncorrelated except near the equator where energetic zonal variability has seasonal timescales and appears to be described by a data assimilating ocean circulation model run by the national centers for environmental prediction space time averages of float velocities disclose a general pattern of middepth flow that resembles the surface circulation with the strongest flows of o 3 cm s in the deep antarctic circumpolar current and east australian current the weaker elements of the flow including an interior subtropical circulation with o millimeters per second flows are described by an objective analysis whose accuracy is limited by sampling noise rather than uncertainty of the analysis parameters the subtropical circulation has a gyres within a gyre structure including boundary currents along australia and east of new zealand and a concentrated equatorial limb with surprising strength in the western basin this current is also involved in a tropical gyre with generally eastward flow along the equator in the western basin at least because of strong seasonal scale variability the equatorial zonal currents are not well measured yet but data are still being accumulated comparison with reid s 1997 absolute circulation shows more areas of agreement than of disagreement

distribution and circulation of labrador sea water abstract labrador sea water is the final product of the cyclonic circulation of subpolar mode water in the open northern north atlantic mccartney and talley 1982 the temperature and salinity of the convectively formed subpolar mode water decrease from 14 7 c 36 08 to 3 4 c 34 88 on account of the cumulative effects of excess precipitation and cooling the coldest mode water is labrador sea water which spreads at mid depths and is found throughout the north atlantic ocean north of 40 n and along its western boundary to 18 n a vertical minimum in potential vorticity is used as the primary tracer for labrador sea water labrador sea water is advected in three main directions out of the labrador sea 1 northeastward into the irminger sea 2 southeastward across the atlantic beneath the north atlantic current and 3 southward past newfoundland with the labrador current and thence westward into the slope water region crossing under the gulf stream off cape hatteras the labrador sea water core is

interaction of the north atlantic current with the deep charlie gibbs fracture zone throughflow the charlie gibbs fracture zone cgfz a passage of 3600 m sill depth through the mid atlantic ridge near 52 n is a known gateway for the passage of deep waters from the northeast atlantic into the western basin during a shipboard survey of august 1997 deep current profiling yielded eastward deep flow through the passage while geostrophy calculated against an intermediate reference level resulted in westward relative deep transport the reason was an unusual and deep reaching northward excursion of the north atlantic current nac inspection of historical data showed that such interference of the nac with the cgfz regime occured occasionally in the past relocation of surface circulation patterns by decadal ocean climate anomalies may thus be of significance also for the deep circulation

across arc variation of lava chemistry in the izu bonin arc identification of subduction components abstract in order to understand the role of the subducted lithosphere in producing the geochemical characteristics of arc magmas major and trace element along with sr and nd isotope compositions have been determined for quaternary volcanic rocks from the izu bonin intra oceanic arc 87 sr 86 sr and 143 nd 144 nd ratios decrease away from the volcanic front of this arc and lie on mixing lines between the assumed isotopic compositions of fluid phases mainly derived from the basalt layer of the subducted lithosphere and upper mantle materials in the sub arc wedge this across arc variation can be explained through a simple sequence of processes involving initial release of fluid phases from the subducted oceanic crust to produce hydrous peridotite at the base of the mantle wedge this hydrous peridotite is dragged downward with the slab and releases a second stage metasomatizing fluid beneath the volcanic arc the higher concentrations of both sr and nd in the fluid beneath the volcanic front than those beneath the back arc side may be a possible cause of the observed across arc variation in sr nd isotopic ratios the difference in compositions of fluid phases is attributed to the different hydrous phases which decompose in the hydrous peridotite layer amphibole beneath the volcanic front and phlogopite beneath the back arc side of the volcanic arc the mineralogically controlled fluid addition may also be responsible for the across arc variation in rb k and rb zr ratios increasing away from the volcanic front

impact of an adiabatic correction technique on the simulation of cfc 12 in a model of the north atlantic ocean 1 a model of the north atlantic ocean is used to simulate the spreading of cfc 12 from the labrador sea deep convection site the standard version of the model fails to capture the local maximum in cfc 12 concentration that is observed along the continental slope of the western boundary hydrographic data are used to apply a simple correction to the model s horizontal momentum equations the corrected model is much more successful at capturing the nearslope maximum in cfc 12 concentration than the uncorrected model and also exhibits a 50 increase of the deep southward export of cfc 12 at 24 n the difference between the two model runs is shown to be a consequence of the different paths taken by the deep western boundary current in the two model versions

ventilation and transformation of labrador sea water and its rapid export in the deep labrador current a model of the subpolar north atlantic ocean is used to study different aspects of ventilation and water mass transformation during a year with moderate convection intensity in the labrador sea the model realistically describes the salient features of the observed hydrographic structure and current system including boundary currents and recirculations ventilation and transformation rates are defined and compared the transformation rate of labrador sea water lsw defined in analogy to several observational studies is 6 3 sv sv 106 m3 s 1 in the model using an idealized ventilation tracer mimicking analyses based on chlorofluorocarbon inventories an lsw ventilation rate of 10 sv is found differences between both rates are particularly significant for those water masses that are partially transformed into denser water masses during winter the main export route of the ventilated lsw is the deep labrador current lc backward calculation of particle trajectories demonstrates that about one half of the lsw leaving the labrador sea within the deep lc originates in the mixed layer during that same year near the offshore flank of the deep lc at about 55 w the transformation of lsw begins in january and is at a maximum in february march while the export of transformed lsw out of the central labrador sea continues for several months lsw generated near the boundary current is exported more rapidly with maximum transport rates during march april within the deep lc

nd and sr isotopic constraints on the origin of igneous rocks resulting from the opening of the japan sea southwestern japan strontium and nd isotopic compositions and trace element abundances were determined for cretaceous to late cenozoic igneous rocks from the japan sea side of southwest japan in order to investigate the effect of the opening of the japan sea on igneous activity the 87sr 86sr ratios for both high and low silica rocks decrease with decreasing age since the middle miocene when the opening occurred similarly 143nd 144nd values for these rocks increase with decreasing age and are negatively correlated with 87sr 86sr ratios a two component mixing process can best account for these isotopic and chemical characteristics one end member is likely the subcontinental lithospheric mantle sclm and its derivative mafic to intermediate materials which had nd values of around 3 the other endmember consists of mafic to intermediate rocks with low nd values e g 8 probably located in the lower crust the mantle upwelling associated with the opening of the japan sea did not supply typical morb or morb source materials to the crust but did provide the heat that caused the melting of lithospheric mantle and lower crust

opening of the sea of japan back arc basin by asthenospheric injection abstract the position of a volcanic front on an arc is governed by dehydration reactions of amphibole chlorite in down dragged hydrated peridotite at the base of the mantle wedge as these reactions are essentially pressure dependent ca 3 5 gpa the paleo position of a volcanic front offers an estimate for the depth of the subducted oceanic lithosphere beneath it this paper evaluates volcanism in the northeast japan arc since the oligocene and discusses the mass transportation in the mantle wedge during the episode of back arc extension in the sea of japan with reference to the migration of the volcanic front the volcanic front during the oligocene ca 30 ma was located along the western coast of the present northeast japan are and migrated about 200 km trenchwards at 23 ma before the opening of the sea of japan back arc basin it follows that the angle of subduction became steeper during the period of 30 23 ma triggered by the injection of the asthenosphere into the mantle wedge the back arc extension of the sea of japan between 21 and 14 ma is a manifestation of the dynamic process of the lithosphere due to the asthenospheric injection the temperature of the injecting asthenospheric mantle was higher than that of the surrounding upper mantle or lithosphere this caused the characteristic volcanism at 23 22 ma 1 high magnesian andesite volcanism took place in the near trench region 2 across arc compositional variation was not observed 3 the volume of volcanic materials increases towards the back arc side of volcanic arc the volcanic front has migrated over a short distance inwards less than 100 km since 15 ma possible as a result of cooling in the mantle wedge because both amphibole and chlorite break down at slightly higher pressures at lower temperatures the upper mantle materials which injected into the mantle wedge and caused back arc spreading are identical to those of the mid oceanic ridge basalt source in sr nd isotopic compositions because volcanic rocks in the back arc region of the northeast japan arc show secular variation in isotopic compositions from enriched to depleted isotopic characteristics during the last 30 ma

sources of eddy kinetic energy in the labrador sea experiments with a suite of north atlantic general circulation models are used to examine the sources of eddy kinetic energy eke in the labrador sea a high resolution model version 112 quantitatively reproduces the observed signature a particular feature of the eke in the labrador sea is its pronounced seasonal cycle with a maximum intensity in early winter as already found in earlier studies based on altimeter data in contrast to a previously advanced hypothesis the seasonally varying eddy field is not related to a forcing by high frequency wind variations but can be explained by a seasonally modulated instability of the west greenland current wgc the main source of eke in the labrador sea is an energy transfer due to reynolds interaction work barotropic instability in a confined region near cape desolation where the wgc adjusts to a change in the topographic slope geostrophic contours tend to converge upstream of cape desolation such that the topographically guided wgc narrows as well and becomes barotropically unstable the eddies spawned from the wgc instability area dominating the eke in the interior labrador sea are predominantly anticyclonic with warm and saline cores in the upper kilometer of the water column while the few cyclones originating as well from the instability area show a more depth independent structure companion experiments with a model exhibit the strength of the wgc influenced by either changes in the wind stress or heat flux forcing as a leading factor determining seasonal to interannual changes of eke in the labrador sea

labrador sea water circulation in the northern north atlantic ocean abstract direct observation of velocities with drifting floats near 1500 m depth in the north atlantic ocean north of about 45 n are combined to derive a new basin wide mean circulation within the layer of labrador sea water lsw inverse methods are used to determine a basin scale circulation constrained by non divergence and conservation of climatological large scale potential vorticity pv with the aim of investigating large scale pv dynamics improving circulation estimates near the basin boundaries and determining the relative amount of recirculation in sub basin gyres with aliased direct velocity data eulerian averages are created from combined datasets and a streamfunction of the non divergent flow is computed from the velocity curl and the imposed boundary conditions unlike deep flow regimes at low latitudes where the mid depth interior circulation is dominated by zonal currents the mid depth subpolar flow is dominated by gyres as suggested by previous estimates of mid depth circulation the lsw layer is rapidly circulating around the periphery of the subpolar basin in northern and western boundary currents recirculation within basins and exchange between the labrador basin and irminger basin is modified by the large scale pv constraints eastward flow across the mid atlantic ridge is concentrated in the charlie gibbs fracture zone with return flow above the reykjanes ridge a weaker subtropical return flow around the azores plateau appears but data are insufficient to map adequately this region

the north atlantic current based on trajectories of satellite tracked buoys and hydrographic sections both dietrich s and worthington s schemes of the north atlantic current are reconsidered ninety satellite tracked buoys have been analyzed in order to distinguish between these schemes neither could be confirmed the north atlantic current appears as a superposition of a broad west wind drift and a frontal jet in the western part of the north atlantic north of 40 n which becomes independent from the subtropical gyre neither a permanent branching suggested by dietrich nor a confinement of the subpolar gyre to the east of newfoundland per worthington can be found the north atlantic current extends over the entire north atlantic carrying more than 20 sverdrups sv toward the north this northward transport is mainly due to the northward flow around flemish cap and a sudden turn of the north atlantic current east of the mid atlantic ridge the buoy tracks have been used to determine speed and direction in the upper layers 100 m we combine these results with hydrographic data in order to obtain an estimate of the mass transport transport across the mid atlantic ridge is based on recent sections fahrbach et al 1985 while transport in the northeastern north atlantic has been derived from the erika dan sections at 53 5 n and 59 5 n according to these two sections the return flow occurs mainly below 1000 m after water mass conversion in the northern areas the results imply that about 7 sv of atlantic water are transformed into deep water in the area between 53 5 n and 60 n

reconciling tracer and float observations of the export pathways of labrador sea water for more than fifty years it has been generally accepted by oceanographers that the deep western boundary current dwbc is the principal conduit of recently convected labrador sea water lsw exported from the high latitude north atlantic to the equator supporting this supposition is observational evidence that the waters of the dwbc have consistently greater equatorward velocities higher concentrations of passive tracers and younger ages compared to ocean interior waters however recent observations and simulations of floats launched in the dwbc in the labrador sea show that most water parcels are quickly ejected from the dwbc and follow instead interior pathways to the subtropics here we show that tracer observations from the last three decades are compatible with the existence of both dwbc and basin interior export pathways from analyses of observational data and model output we find that equatorward transport in the basin interior is consistent with the large scale vorticity balance at mid depth furthermore from the modeling analysis we show that despite higher localized concentrations of tracer and particles in the dwbc only 5 of particles released in the labrador sea are transported from the subpolar to subtropical gyre via a continuous dwbc pathway thus the interior pathway is a significant contributor to lsw export highlights lagrangian observations of labrador sea water match eulerian observations there is deep equatorward flow in the basin interior this interior pathway is significant compared to the pathway along the boundary

interdecadal variability of the meridional overturning circulation as an ocean internal mode the meridional overturning circulation moc in the coupled echam5 mpiom exhibits variability at periods of near 30 years and near 60 years the 30 year variability referred to as interdecadal variability idv exist in an ocean model driven by climatological atmospheric forcing suggesting that it is maintained by ocean dynamics the 60 year variability the multidecadal variability mdv is only observed in the fully coupled model and therefore is interpreted as an atmosphere ocean coupled mode the coexistence of the 30 year idv and the 60 year mdv provides a possible explanation for the widespread time scales observed in climate variables further analyses of the climatologically forced ocean model shows that the idv is related to the interplay between the horizontal temperature dominated density gradients and the ocean circulation temperature anomalies move along the cyclonic subpolar gyre leading to fluctuations in horizontal density gradients and the subsequent weakening and strengthening of the moc this result is consistent with that from less complex models indicating the robustness of the idv we further show that along the north atlantic current path the sea surface temperature anomalies are determined by the slow lsw advection at the intermediate depth

distribution of net collected planktonic cnidarians along the northern mid atlantic ridge and their associations with the main water masses abstract planktonic cnidarians and ctenophores were sampled with a multiple opening closing net multinet as well as a non quantitative plankton net along the northern mid atlantic ridge mar between iceland and the azores sixty four species or genera of planktonic cnidarians 38 siphonophora 21 hydromedusae 5 scyphomedusae and one genus of ctenophore were collected of these leuckartiara adnata and clausophyes laetmata were new records for the area multinet samples collected from depths of 0 100 100 500 500 1000 1000 1500 and 1500 2500 m at 11 stations were compared multivariate analysis of the data indicated that species composition and abundance along the ridge varied with the dominant water masses with changes in the cnidarian zooplankton assemblage observed with regard to geographic location as well as depth the surface waters of the two northernmost stations characterized by modified north atlantic water mnaw as well as the three southernmost stations characterized by north atlantic central water nacw exhibited relatively high abundances 3284 13 915 individuals 1000 m 3 in the upper 100 m no such peak was evident at the middle stations characterized by subarctic intermediate water saiw where the abundances in the upper three depth strata were consistently lower 57 863 individuals 1000 m 3 across the study area the lowest abundances were found in the 1500 2500 m stratum 0 56 ind 1000 m 3 the main divergence in the species composition and abundance of planktonic cnidarians was observed at the subpolar front spf which marked the boundary for the distribution of many species the divergence at the spf was strongest in the upper 500 m but observable down to 1500 m profoundly different epipelagic species assemblages were observed in saiw and nacw on opposite sides of the spf with the distribution of several species of calycophoran siphonophores confined to the southern nacw at mid water depths the species composition north of the spf was possibly influenced by labrador sea water lsw the highest diversity of planktonic cnidarians was observed in the surface waters south of the spf and in the 100 1000 m range north of the spf

diagnosing the influence of mesoscale eddy fluxes on the deep western boundary current in the 1 10 storm ncep simulation abstractusing a 0 1 ocean model this paper establishes a consistent picture of the interaction of mesoscale eddy density fluxes with the geostrophic deep western boundary current dwbc in the at

upper mantle velocity structure estimated from ps converted wave beneath the north eastern japan arc summary the upper boundary of the descending oceanic plate is located by using ps waves converted from p to s at the boundary in the tohoku district the north eastern part of honshu japan the observed ps p time data are well explained by a two layered oceanic plate model composed of a thin low velocity upper layer whose thickness is less than 10 km and a thick high velocity lower layer the upper and lower layers respectively have 6 per cent lower and 6 per cent higher velocity than the overriding mantle the estimated location of the upper boundary is just above the upper seismic plane of the double planed deep seismic zone this result indicates that events in the upper seismic plane at least in the depth range from 60 to 150 km occur within the thin low velocity layer on the surface of the oceanic plate

deciphering the pathways for the deep limb of the meridional overturning circulation for the past 50 years it has been assumed that the principal pathway for the deep limb of the atlantic meridional overturning circulation amoc is the deep western boundary current dwbc however recent observations of lagrangian floats have shown that the dwbc is not necessarily a unique dominant or continuous pathway for these deep waters a significant portion of the deep water export from the subpolar to the subtropical gyres follows a pathway through the interior of the newfoundland and subtropical basins which is constrained by the western boundary and the western flank of the mid atlantic ridge the hypothesis that deep eddy driven recirculation gyres are a mechanism for partitioning the deep limb of the amoc into the dwbc and this interior pathway is investigated here eulerian and lagrangian analyses of the output of ocean general circulation models at eddy resolving eddy permitting and non eddy permitting resolutions are used to test this hypothesis eddy driven recirculation gyres simulated in the eddy resolving and eddy permitting models and similar to recirculations inferred from hydrographic data are shown to shape the export pathways of deep water from the subpolar to the subtropical gyres

lagrangian observations of the middepth and deep velocity fields of the northeastern atlantic ocean the circulation of the northeastern atlantic ocean at intermediate depths is characterized by watermass transformation processes that involve iceland scotland overflow water isow from the northeast labrador sea water lsw from the west and mediterranean water from the south field observations were carried out with 89 eddy resolving floats rafos and marvor types the data coverage achieved is remarkably high and enables a comprehensive study of the eastern basins between iceland and the azores the trajectories show typical pathways of the water masses involved and the role that the complex bottom topography plays in defining them the isow paths tend to lean against the slopes of the reykjanes ridge and rockall plateau westward escapes through multiple gaps in the ridge are possible superimposed on a sustained southward flow in the eastern basin along the mid atlantic ridge lsw pathways leading to the eastern basins are subject to high variability in flow direction and eddy activity in addition to a selection of characteristic trajectories maps of the horizontal distributions of lagrangian eddy kinetic energy and integral time scales are presented these reveal distinct areas of intensified mixing in the iceland basin as well as the sharp contrast between the subpolar and subtropical dynamics a self contained eddy detection scheme is applied to obtain statistics on individual eddy properties and their abundance it is suggested that much of the intensified mixing can be related to cyclonic activity particularly in the subpolar region

the arrival of recently formed labrador sea water in the deep western boundary current at 26 5 n the deep western boundary current dwbc of the north atlantic is a principal conduit between the formation region for labrador sea water lsw and the oceanic interior to the south time series 1985 1997 of hydrographic properties obtained in the dwbc at 26 5 n show that prior to 1994 temperature salinity and transient tracer properties within the lsw density range showed little indication of recently formed parcels properties characteristic of a newer version of lsw cooler fresher and higher tracer concentrations were observed beginning in 1994 and continuing through 1997 longer time series of temperature and salinity developed from a regional data base show both the 1994 and a 1980 1981 event in the abaco region both events are consistent with anomalies in the labrador sea that occurred some 10 years earlier the 10 year transit time from the labrador sea to 26 5 n is less than the 18 year transit time inferred from earlier studies

nd and sr isotopic study of volcanic rocks from japan two older granitic rocks and some selected quaternary volcanic rocks from the japanese islands were analyzed in a reconnaissance study for the purpose of examining the relationships between nd and sr isotopic abundances and the megatectonic structure around the japanese islands model ages of 0 9 ae were determined by the nd and sr methods on a paleozoic gneiss which confirms that a relatively ancient acidic basement exists in the japanese islands the nd and sr isotopic data show that the cretaceous granodiorite is the result of partial melting of older crust the nd of tholeiitic rocks from the izu arc gives e nd ranging from 8 3 to 9 3 and with the corresponding e sr from 14 5 to 18 5 these results are identical to those found for the mariana arc these values are distinctly lower than typical morb by around 1 2 eu this difference in e nd between arcs and morb is attributed to the contribution of oceanic sediments to the partial melts produced during subduction of oceanic crust the hakone volcano is clearly confirmed as belonging to an oceanic source by nd isotopic results e sr e nd values of the volcanics from a section along the fossa magna show a clear indication that they are a blend of oceanic mantle material and continental crustal material the crustal component clearly increases in going from south to north volcanics across the northeast japan arc also show a distinct correlation of e sr e nd related to the position relative to the active subduction zone but with the opposite trend these relationships of the present isotopic pattern and the zonal arrangement relative to the subduction zone suggest the former existence of a local spreading center in the japan sea in general there appear to be regular isotopic relationships between the izu mariana oceanic island arc and the continental island arc of japan which indicates that partially melted or assimilated older continental basement is admixed with young rising oceanic arc magmas

export of labrador sea water from the subpolar north atlantic a lagrangian perspective abstract results from 59 acoustically tracked rafos floats obtained during 2003 2008 are analyzed here to study the spreading pathways of labrador sea water lsw from the subpolar to subtropical north atlantic an earlier paper based on a subset of these float data presented evidence of a potentially important interior pathway for the southward spreading of lsw here those results are reinforced with the full data set and new information on lsw pathways around the grand banks is presented about 70 of the rafos floats were expelled into the ocean interior east of the grand banks and meandered slowly eastward toward the mid atlantic ridge a pathway observed previously with profiling floats less than 10 was advected continuously around the grand banks by the deep western boundary current dwbc a larger fraction 17 drifted into the subtropical interior from the tail of the grand banks suggesting that this pathway is at least if not more important than the dwbc pathway for the export of lsw to the subtropics rafos floats released closest to the continental slope at 700 m depth were more likely to rapidly reach subtropical latitudes mainly because they drifted through flemish pass a 1100 m deep channel between the grand banks and the flemish cap which protects fluid parcels from being swept off the continental slope by meanders of the north atlantic current a statistical comparison of the rafos trajectories with more than 5000 simulated floats obtained from the family of linked atlantic models experiment flame high resolution ocean general circulation model reveals a similar pattern of lsw spreading the rafos and simulated floats are also used to show that contrary to a previous modeling study the isobaric rafos floats do not underestimate the amount of lsw that continuously follows the dwbc around the grand banks compared to isopycnal floats these results have implications for the connectivity of the deep limb of the atlantic meridional overturning circulation between the subpolar and the subtropical north atlantic

middepth spreading in the subpolar north atlantic ocean reconciling cfc 11 and float observations 1 chlorofluorocarbons cfcs enter the middepth layers of the north atlantic ocean from the atmosphere during the formation of dense mode water in the labrador and irminger seas of the subpolar gyre the cfc bearing waters then spread from the convection regions through advection and diffusion of the water masses using recent estimates of the circulation at 1500 m depth from subsurface profiling floats the spreading of cfc 11 is simulated in the subpolar north atlantic with an advective diffusive model several numerical experiments are performed with different stream functions lateral diffusivities and variations in the cfc 11 sources the results are then compared to the observed cfc 11 field during 1996 1998 poor fits are found for diffusivities less than about 500 m 2 s 1 better fits are found for diffusivities ranging from 500 12 000 m 2 s 1 although unrealistically smooth model solutions are produced if the diffusivity exceeds about 3000 m 2 s 1 simulations that include both labrador and irminger sea cfc 11 sources fit the data better than with labrador sources alone none of the model cfc solutions fit the data within the cfc uncertainty over the whole domain the model performs well in the western part of the subpolar gyre but cfc 11 concentrations are consistently too low in the west european basin it is possible that uncertainty in the float based circulation can account for these misfits and a more accurate circulation estimate might be able to fit the observed cfc 11 field alternatively time variations in the flow or deep water formation processes which clearly exist in the real ocean may need to be included

plio pleistocene arc magmatism controlled by two overlapping subducted plates central japan abstract the plio pleistocene volcanoes of the ryohaku hida mountains of central japan have been dated using the k ar method and newly determined ages of 65 lava flows and two pyroclastic flow deposits are presented on the basis of temporal and spatial distribution and the nature of the lavas the volcanism of the area has been split into four stages 1 an early eruptive stage 5 0 2 7 ma of the e w trending kuzuryu volcanic chain of stratovolcanoes 2 the ueno basalts and genetically related rocks which formed monogenetic volcanoes and large scale rhyolite pyroclastic flows during the 2 7 1 6 3 a late eruptive stage 1 5 0 8 ma of the kuzuryu volcanic chain and 4 n s trending andesite dacitic volcanism of the hakusan 0 6 0 ma and the norikura volcanic chains 0 9 0 ma this volcanic evolution was controlled by the relationship between the two overlapping slabs of the philippine sea and the pacific plates which are subducting beneath the central japan arc the kuzuryu volcanic chain with its e w trend is caused by magma generation on the northward subducted slab of the philippine sea plate and the n s trending volcanic chains are caused by magma generated on the westward subducted slab of the pacific plate the change of motional direction of the philippine sea plate from northwards to west northwestwards 4 million years ago caused the disappearance of the subducted slab of the philippine sea plate which had obstructed the ascent of magmas derived from the underlying subducted slab of the pacific plate during the change in direction of movement of the philippine sea plate there was a temporary change in the stress field from compression to extension during which the second stage of volcanism consisting of monogenetic basaltic and rhyolitic volcanoes occurred

lagrangian perspectives of deep water export from the subpolar north atlantic 1 direct observations at the grand banks have raised a quandary concerning the pathways of the lower branch of the meridional overturning circulation in contrast to moored current meters that depict an intense narrow deep western boundary current dwbc observations using different float types failed to show this continuous export path here this issue is addressed by a lagrangian analysis of synthetic particles in an eddy resolving circulation model due to intense eddy activity around the grand banks about 40 of the deep water in the dwbc is diverted into the interior spreading southward along the western flank of the mid atlantic ridge or with the eddying flow field in the basin interior imposing constraints on the vertical displacements of particles similar to those experienced by observational floats further reduces their adherence to the dwbc particularly near the southern tip of the grand banks

effect of a variable eddy transfer coefficient in an eddy permitting model of the subpolar north atlantic ocean abstract the effect of using a variable eddy transfer coefficient for the gent mcwilliams gm parameterization in a 1 3 resolution ocean model of the subpolar north atlantic ocean is investigated results from four experiments with different implementations of this coefficient are compared among themselves as well as with two control experiments a series of improvements have been obtained in all of the experiments that use a low level of explicit horizontal tracer diffusion these include a better representation of the overflow waters originating from the nordic seas leading to a more realistic deep western boundary current and to increased eddy activity in the deep ocean in the eastern north atlantic in the same experiments the gm velocities help the labrador sea water to spread from the deep convection region to the currents that surround it without incurring significant spurious diapycnal mixing thus two classical pathways for the spreading of this water are established moreover the simul

seasonal to interannual variability of the eddy field in the labrador sea from satellite altimetry sea level anomalies measured by the altimeters aboard the topex poseidon and ers satellites for the periods 1993 2001 and 1997 2001 respectively are used to investigate the eddy field in the subpolar north atlantic and in the labrador sea a quadratic correction of the obtained eddy kinetic energy eke with respect to significant wave height is applied that led to an increased correlation between moored and altimetric eke in the central labrador sea the mean eke field shows higher levels associated with the main currents and a strong seasonality in the labrador sea the annual cycle of the eke shows a propagation of west greenland current wgc eke into the central labrador sea with a mean southward propagation speed of about 3 cm s 1 while the eke maximum in the labrador current is well separated from the interior by local eke minima the interannual variability of the eke in the labrador sea shows distinct regional differences in the wgc region strong early winter maxima are found during 1993 and 1997 1999 in the central labrador sea maxima are found during march april 1993 1995 and 1997 variations in the annual cycle of the wgc eke are observed while there is a weak annual cycle in the wgc region during 1994 1996 with more continuous eke generation during 1997 2000 there is a strong seasonal cycle with maximum eke during january and particularly low eke during summer the propagation of wgc eke into the central labrador sea is enhanced during 1997 2000 leading to a long persistence of eke in the central labrador sea during 1993 1995 and 1997 the central labrador sea eke almost instantaneously increased during march april followed in the earlier years by a relatively fast destruction of the winterly generated eke

boundary circulation at the exit of the labrador sea the deep circulation and related transports of the southern labrador sea are determined from direct current observations from ship surveys and a moored current meter array the measurements covered a time span from summer 1997 to 1999 and show a well defined deep boundary current extending approximately out to the 3300 m depth contour and weak reverse currents farther offshore the flow has a strong barotropic component and significant baroclinic flow is only found in the shallow labrador current at the shelf break and associated with a deep core of denmark strait overflow water the total deep water transport below 27 74 kg m 3 was 26 5 sv sv 106 m3 s 1 comprising labrador sea water lsw gibbs fracture zone water gfzw and denmark strait overflow water dsow intraseasonal variability of the flow and transport was high ranging from 15 to 35 sv and the annual means differed by 17 a seasonal cycle is confined to the shallow labrador current in its deeper part where the mean flow is still strong no obvious seasonality could be detected the transport of the interior anticyclonic recirculation was estimated from lowered acoustic doppler current profiler stations and geostrophy yielding about 9 sv thus the net deep water outflow from the labrador sea was about 17 sv the baroclinic transport of gfzw and dsow referenced to the depth of the isopycnal 27 80 kg m 3 is only about one third of the total transport in these layers longer term variations of the total transports are not represented well by the baroclinic contribution

seasonal and interannual changes in the north atlantic subpolar gyre from geosat and topex poseidon altimetry sea surface slopes from the altimetric satellites geosat and topex poseidon are used to calculate eddy kinetic energy of the north atlantic subpolar gyre 40 65 n 60 5 w using two years of data from each satellite december 1986 to december 1988 and october 1992 to september 1994 respectively interannual differences in the north atlantic current nac are revealed in regions of strong currents the eddies are driven by baroclinic instability of the mean flow are not seasonally varying and may therefore be used as a surrogate for the mean flow itself wind stress curl fields for the same periods show that the northward and southward shifts in the current branches across the mid atlantic ridge are related to interannual differences in the winter wind stress curl pattern when the zero in wind stress curl is well defined and wind stress is at a maximum outside the nac the eddies are driven primarily by wind stress indicated by a significant seasonality time series of eddy kinetic energy and wind stress are generated for the 4 years and the magnitude phase and significance of annual and semiannual signals are determined interannual changes in the timing of the maximum eddy kinetic energy are associated with that of the maximum wind stress with each being shifted by a month or two later or earlier throughout much of the northern north atlantic a significant annual signal in eddy kinetic energy is observed lagging the wind stress by on average 6 weeks a negligible lag is found in the region of the east greenland current and irminger sea whereas the greatest lag about 2 months is found in the labrador sea

transportation of beryllium with h2o at high pressures implication for magma genesis in subduction zones the degree of transportation of beryllium with h2o is measured under the upper mantle p t conditions in order to examine the incorporation of 10be in subduction zone magmas 5 4 0 3 of original beryllium in the starting serpentinite is extracted with h2o from the system through the dehydration reaction of serpentine at 850 c and 1 5 gpa it follows that the partial melting of oceanic lithosphere including 10be rich sediment layer is not necessarily required for inclusion of the isotope in island arc lavas we suggest that the dehydration processes in the subducted slab and mantle wedge materials dragged downward by the slab subduction govern the concentration of 10be in arc lavas

strontium isotope composition in volcanic rocks from the northeast japan arc abstract in the northeast japan arc a number of quaternary volcanoes form a long narrow belt parallel to the japan trench 87 sr 86 sr ratios were determined in 52 specimens of volcanic rocks from 27 volcanoes in the northeast japan arc area the results reveal that the ratios change systematically in space decreasing 87 sr 86 sr ratios across the arc were confirmed over a wide area of northeast japan in the same direction increases in both rb and sr contents were also found the regular trends are considered to be a strong constraint for elucidation of subduction originated magma genesis at the eurasia plate vs pacific plate boundary in the northern region of the northeast japan arc 87 sr 86 sr ratios in volcanic rocks along the volcanic front were almost constant 0 7038 0 7045 and slightly higher than those from the izu ogasawara arc 0 7032 0 7038 this suggests that interactions between the eurasia plate and the pacific plate and those between the philippine sea plate and the pacific plate are slightly different the southern region of the northeast japan arc where the direction of the volcanic front bends from southward to westward showed anomalously high 87 sr 86 sr ratios reaching to 0 7077 this region coincides with the triple junction of the eurasia pacific and philippine sea plates suggesting anomalous interaction at the triple junction

the role of bottom pressure torques on the interior pathways of north atlantic deep water abstractfour versions of the same global climate model one with horizontal resolution of 1 8 3 6 and three with 0 2 0 4 are employed to evaluate the role of ocean bottom topography and viscosity on the spatial structure of the deep circulation this study is motivated by several recent observational studies that find that subsurface floats injected near the western boundary of the labrador sea most often do not continuously follow the deep western boundary current dwbc in contrast to the traditional view that the deep water formed in the north atlantic predominantly follows the dwbc it is found that with imposed large viscosity values the model reproduces the traditional view however as viscosity is reduced and the model bathymetry resolution increased much of the north atlantic deep water nadw separates from the western boundary and enters the low latitude atlantic via interior pathways distinct from the dwbc it is shown that bottom pressure torques play an important role in maintai

general circulation of the western subtropical north atlantic observed using profiling floats 1 the general circulation of the subtropical western north atlantic is described using hydrography and subsurface velocity measured simultaneously by 71 profiling floats during the period july 1997 to december 2002 subsurface trajectories of the floats revealed a strong topographic influence and depth dependence of the gulf stream its recirculation gyres and the north atlantic current we discuss eddy like and nonsteady flows in several subregions especially near the major topographic features the seasonal cycle of volume transport estimated using the absolute geostrophic velocity in the upper 900 m was shown to be a maximum in winter and minimum in summer the eddy kinetic energy and eddy diffusivity derived from the float data and mapped over the subtopical gyre show seasonal variability in the vicinity of the gulf stream with a maximum in spring and minimum in winter for both quantities

labrador sea water tracked by profiling floats from the boundary current into the open north atlantic fifteen profiling floats were injected into the deep boundary current off labrador they were ballasted to drift in the core depth of labrador sea water lsw at 1500 m depth and were deployed in two groups during march and july august 1997 initially for about three months the floats were drifting within the boundary current and the flow vectors were used to determine the mean horizontal structure of the deep labrador current which was found to be about 100 km wide with an average core speed of 18 cm s 1 north of flemish cap the boundary current encounters complicated topography around orphan knoll and there the lsw outflow splits up into different routes one obvious lsw path is eastward through the charlie gibbs fracture zone and another route is a narrow recirculation toward the central labrador sea a surprising result was that none of the floats were able to follow the boundary current southward to the grand banks area and exit into the subtropics trajectories and temperature profiles of the eastward drifting floats indicate the importance of the north atlantic current for dispersing the floats even at the level of lsw

why does the deep western boundary current leak around flemish cap abstractthe southward flowing deep limb of the atlantic meridional overturning circulation is comprised of both the deep western boundary current dwbc and interior pathways the latter are fed by

historical position of the north atlantic current the path of the north atlantic current nac within the newfoundland basin is determined from the historical hydrographic database 1910 1988 horizontal probability distributions for dynamical and property front locations on specific volume anomaly surfaces are derived from an analysis of all available hydrographic stations in the basin these probability distributions indicate where the fronts were most likely to have been observed in the past they are observed to exhibit a pattern of stationary meanders with troughs located over the southcast newfoundland rise over the newfoundland seamounts and to the east of the flemish cap the width of the nac s envelope varies from 300 km over the southeast newfoundland rise temporal subsamples of the historical data indicate that the path and meander patterns are persistent features for all seasons and for the four decades between 1950 and 1990

variability in deep western boundary current transports preliminary results from 26 5 n in the atlantic 1 transport fluctuations of the deep limb of the meridional overturning circulation moc near the western boundary are presented from a line of inverted echo sounders bottom pressure sensors and a deep current meter east of abaco island bahamas at 26 5 n from september 2004 through september 2005 the mean southward flow between 800 dbar and 4800 dbar was 39 106 m3 s 1 with a northward recirculation of 28 106 m3 s 1 leaving a net southward flow of 11 106 m3 s 1 as the through flow of the deep western boundary current dwbc the mean southward dwbc flow essentially equals previous values that were measured at the same location by arrays of current meters deployed from 1986 to 1992 dwbc transport spectra indicate that barotropic and baroclinic changes have very similar energy levels at most periods less than 10 days and that barotropic changes dominate at periods of 10 80 days

sr and nd isotopic compositions of the magma source beneath north hokkaido japan comparison with the back arc side in the ne japan arc abstract most middle miocene to quaternary basaltic rocks younger than about 12 ma from north hokkaido show a narrow range in initial 87 sr 86 sr sri and initial 143 nd 144 nd ndi isotope ratios which are slightly undepleted compared to morb sri ranges from 0 70296 to 0 70393 and ndi from 0 51288 to 0 51307 but there is no temporal variation in either sri or ndi an exceptional basalt from ohmu area which is more undepleted in terms of both sri and ndi than other north hokkaido nh basaltic rocks most of the nh basaltic rocks are isotopically similar to basaltic rocks found on the back arc side of the ne japan arc ba basaltic rocks younger than about 15 ma whereas basaltic rocks from the ohmu area are isotopically similar to ba basaltic rocks older than 15 ma the generation of both nh and ba basaltic rocks appears to be closely related to mantle evolution this involves upwelling of asthenosphere into the continental lithosphere beneath north hokkaido and the back arc side of the ne japan arc at the same time as spreading of the kurile back arc basin and japan sea back arc basin the magma source for the ohmu area basalts and the ba basaltic rocks prior to 15 ma may correspond to the subcontinental lithospheric mantle thinned by the upwelling of asthenospheric mantle material from which most of the nh basaltic magmas and the ba basaltic magmas younger than 15 ma were derived this asthenospheric mantle is slightly different from morb source in terms of major and trace element characteristics such as tio 2 k 2 o and zr y as well as sr and nd isotopes which may be due to involvement of em ii like component

deep cross equatorial flow in the atlantic measured with sofar floats neutrally buoyant sofar floats at nominal depths of 800 1800 and 3300 m were tracked for 21 months in the vicinity of tropical boundary currents in the atlantic near 6 n and at several sites near 11 n as well as along the equator trajectories at 1800 m show a swift 50 cm s narrow 100 km wide southward flowing deep western boundary current dwbc extending from 7 n to the equator the average transport per unit depth in the dwbc was estimated to be 13 8 103 m2 s coupling this value with mean velocities measured in the dwbc by current meters gave a volume transport of 15 106 m3 s between depths of 900 m and 2800 m approximately 6 106 m3 s recirculated northward between the dwbc and the mid atlantic ridge leaving 9 106 m3 s as cross equatorial transport no obvious dwbc nor swift equatorial current was observed by the 3300 m floats a low mean velocity at this depth lay between f 11 and higher velocity cores above and below the 1800 m trajectories also suggest that at times february march 1989 the north atlantic deep water in the dwbc turned eastward and flowed along the equator and at other times august september 1990 the dwbc crossed the equator and continued southward the velocity near the equator calculated by grouping floats in a box along the equator was eastward at 4 1 cm s from february 1989 to february 1990 and westward at 4 6 cm s from march 1990 to november 1990 thus the amount of cross equatorial flow in the dwbc appeared to be linked to low frequency variability of the structure of the equatorial current system floats in antarctic intermediate water at 800 m revealed a northwestward western boundary current although flow patterns were complicated three floats that significantly contributed to the northwestward flow looped in anticyclonic eddies that translated up the coast at 8 cm s six 800 m floats drifted eastward along the equator between 5 s and 6 n at a mean velocity of 11 cm s one reached 5 w in the gulf of guinea suggesting that the equatorial currents at this depth extended at least 35 40 along the equator three of these floats reversed direction near the end of the tracking period

secular variation of magma source compositions beneath the northeast japan arc abstract sr and nd isotopic compositions were determined for tertiary volcanic rocks with ages of 22 16 and 8 ma along the volcanic front in the ne japan arc in order to examine the secular variation of magma source compositions in the mantle wedge these rocks show no through time variation in both sr and nd isotope ratios 0 70412 0 704759 and 2 09 4 12 units respectively it follows that the subduction components do not increasingly pollute the magma source region during the continuous subduction of oceanic lithosphere the degree of pollution by the subduction components is controlled by the amount of clinopyroxene and aluminous phases in mantle wedge peridotites because h 2 o which causes partial melting of the mantle wedge beneath a volcanic front is supplied by the breakdown of both amphibole and chlorite in the dragged hydrated peridotite formed by the addition of slab derived fluid beneath the fore arc region

thermal and geochemical evolution of the mantle wedge in the northeast japan arc 1 contribution from experimental petrology melting phase relations at high pressures have been examined for four mg rich basalts from both the trench and the backarc sides of the miocene ne japan arc all these basalts crystallize olivine and orthopyroxene on the liquidus at pressures lower and higher than the point of multiple saturation with those two phases respectively the pressure of multiple saturation is rather constant for all samples 1 0 1 4 gpa suggesting that 1 there was no across arc variation in the depth of magma segregation in the miocene ne japan arc 2 the depth of magma segregation remains constant since 20 ma in the trench side of the volcanic arc whereas it has changed from 30 40 km to 60 km beneath the backarc side of the arc this temporal variation may be caused by increasing thickness of the lithosphere beneath the backarc region associated with cooling of the mantle wedge after the opening of the japan sea backarc basin in the early miocene