understanding the effects of dissolution on the mg ca paleothermometer in planktic foraminifera evidence from a novel individual foraminifera method

processes influencing tidal mixing in the region of sills 1 a non hydrostatic model in cross sectional form with idealized topography representing a sill and forced by a barotropic tidal flow is used to examine the role of a hydraulic transition and internal lee waves in determining mixing in the sill region calculations using smooth topography show that unsteady lee waves are generated on the sill slope during flood tide these waves propagate toward the sill when the tide reverses leading to enhanced mixing in the sill region the addition of small scale topography on the leeside of the sill changes the lee wave distribution with an associated increase in mixing depending upon wavelength and amplitude of topography calculations show the importance of the hydraulic transition lee waves and small scale topography in determining mixing in sill regions compared to smooth large scale topography

late glacial diatom accumulation at 9 s in the indian ocean a continuous 10 m long section consisting of roughly two thirds ethmodiscus rex a diatom and one third mixed planktonic foraminifera was identified in a core from 3800 m depth at 9 s on the indian ocean s 90 e ridge radiocarbon dates place the onset of deposition of this layer at 30 000 years b p and its termination at close to 11 000 years b p however precise dating of the foraminifera from the ethmodiscus layer itself proved to be impossible owing to the presence of secondary calcite presumably precipitated from the pore waters during the holocene high calcium carbonate content ooze free of diatoms was deposited at this locale as the site currently lies beneath the pathway taken by upper ocean waters entering the indian ocean from the pacific via the indonesian straits it appears that during glacial time thermocline waters moving along this same path provided the silica and other nutrients required by these diatoms

an s coordinate density evolving model of the northwest european continental shelf 1 model description and density structure the proudman oceanographic laboratory three dimensional baroclinic b grid model pol3db has been developed to incorporate features suitable for the modeling of baroclinic processes on the shelf at the shelf slope and in ocean regions to allow long term coupled ocean shelf simulations we test the model on the northwest european continental shelf in the period november 1988 to october 1989 against satellite sea surface temperature measurements against ctd sections both on and off the shelf and in summary against the whole of the north sea project ctd data set the model accurately reproduces the seasonal cycle in the shelf wide spatial temperature structure seen in the observations spatial correlations range from r 0 92 in december to 0 79 in july and the overall rms errors range from 0 8 c in april to 1 6 c in july we demonstrate that the increase in the errors during the summer is due to uncertainties in modeling the vertical temperature structure compared with a climatology the large scale sea surface salinity structure is also well modeled r 0 80 but there is a tendency for the model to overestimate the salinity in the norwegian trench and underestimate it in the kattegat the s coordinates a modified sigma coordinate system allow the formation of a seasonal mixed layer across the shelf break and into the northeast atlantic with a modest vertical resolution the accuracy to which this deep water region is modeled is limited by the initial and boundary conditions and by the extent of the model domain

age of initiation of the india asia collision in the east central himalaya abstract we document the stratigraphy and provenance of the lower tertiary terrigenous sections in the zhepure shan region of the tethyan himalaya southern tibet using petrographic and geochemical whole rock and single grain techniques the cretaceous early tertiary shelf deposits of shallow marine carbonates and siliciclastics of the former indian passive margin near the western end of the zhepure shan are conformably overlain by lower tertiary clastic rocks sandstones in the jidula formation paleocene mostly contain monocrystalline quartz grains of cratonic origin in contrast significant amounts of immature framework grains with a distinct ophiolitic and volcanic arc influence are present in the youxia early eocene and shenkeza post early eocene formations major trace and rare earth element concentrations in both sandstones and shales complement the petrographic data and indicate that the source of the jidula formation consisted primarily of quartzose basement rocks probably of indian con

microstructural and geochemical perspectives on planktic foraminiferal preservation glassy versus frosty in recent years it has become apparent that the cool tropic paradox of paleogene and cretaceous greenhouse climates arises because of the diagenetic alteration of tropical planktic foraminiferal calcite near the seafloor yielding artificially high 18o values because the mg ca compositions of foraminiferal and inorganic calcite are thought to be quite different mg ca measurements should be a sensitive way of monitoring diagenetic alteration here we examine the extent of diagenetic alteration of eocene planktic foraminiferal calcite using scanning electron microscope imaging of foraminiferal test microstructures and geochemical 18o and mg ca analyses we compare microstructural and geochemical characteristics between given species exhibiting two contrasting states of preservation those that appear frosty under reflected light and those that appear glassy microstructural evidence reveals extensive diagenetic alteration of frosty foraminiferal tests at the micron scale while 18o analyses document consistently higher 18o and therefore lower paleotemperatures in this material yet we find that 18o offsets between species in these frosty foraminiferal assemblages appear to be generally preserved suggesting that frosty foraminifera remain valuable for generating relatively short approximately 1 myr paleoceanographic time series that do not demand absolute estimates of paleotemperature we also find that the observed increase in mg ca for planktic foraminifera exhibiting diagenetic alteration compared to glassy taphonomies is far smaller than would be expected from the addition of inorganic calcite based on laboratory derived mg2 partition coefficients our findings imply that a much lower mg2 partition coefficient controls inorganic calcite formation in deep sea sedimentary sections in accordance with the findings of baker et al 1982

a three dimensional baroclinic model of the irish sea formation of the thermal fronts and associated circulation a high resolution three dimensional baroclinic shelf sea model is developed and applied to the determination of the various processes influencing the generation and position of thermal fronts and the associated circulation in the irish sea the model has a horizontal grid resolution of approximately 3 6 km by 3 0 km and can resolve the thermal fronts in the region in particular in the western irish sea although the grid is not sufficiently fine to resolve small scale features along these fronts produced by baroclinic instabilities using meteorological forcing from dublin airport located close to the center of the region and tidal forcing a series of numerical experiments were performed to examine the processes influencing the location and dynamics of thermal fronts and associated circulation in the irish sea the results show a reasonable qualitative agreement with observations of the thermal stratification and the associated circulation in the western irish sea the ability of the model to reproduce the main features of the frontal structures in the region having been established it is used to examine the interannual variability of the density flow field calculations using meteorological forcing from different years show that the western irish sea fronts and the associated cyclonic circulation are persistent features although exact details of the fronts and their times of formation and breakdown show a large interannual variability model results also reveal a patch of thermal stratification in the northern part of the eastern irish sea with northward thermal density driven currents in this region

anomalous radiocarbon ages for foraminifera shells the causes for discordant radiocarbon results on multiple species of planktonic foraminifera from high sedimentation rate marine sediments are investigated we have documented two causes for these anomalous results one is the addition of secondary radiocarbon for which we have to date only one firm example it involves an opal rich sediment the other is the incorporation of reworked material again we have to date only one firm example it involves a rapidly deposited ocean margin sediment however we have three other examples where reworking is the most likely explanation on the basis of this study it is our conclusion that where precise radiocarbon dating of high deposition rate marine sediment is required a prerequisite is to demonstrate that concordant ages can be obtained on pairs of fragile and robust planktic shells for sediment rich in opal it is advisable to check for secondary calcite by comparing ages obtained on acid leached samples with those on unleached samples

persistently well ventilated intermediate depth ocean through the last deglaciation during the last deglaciation 18 11 thousand years ago existing radiocarbon 14c reconstructions of intermediate waters in the mid to low latitude oceans show widely diverging trends with some broadly tracking the atmosphere and others suggesting extreme depletions these discrepancies cloud our understanding of the deglacial carbon cycle because of the diversity of hypotheses needed to explain these diverging records for example injections of 14c dead geological carbon mixing of extremely isolated waters from the abyssal ocean or changes in sites of deep water ventilation here we present absolutely dated deglacial deep sea coral 14c records of intermediate waters from the galapagos platform close to the largest reported deglacial 14c depletions together with data from the low latitude atlantic our records indicate coherent well equilibrated intermediate water 14c ventilation in both oceans relative to the atmosphere throughout the deglaciation the observed overall trend towards 14c enriched signatures in our records is largely due to enhanced air sea carbon isotope exchange efficiency under increasing atmospheric p rm co 2 these results suggest that the 14c depleted signatures from foraminifera are likely sedimentary rather than water mass features and provide tight 14c constraints for modelling changes in circulation and carbon cycle during the last deglaciation intermediate depth waters in the atlantic and pacific oceans were well equilibrated with the atmosphere through the last deglaciation according to radiocarbon data from deep sea corals

stable warm tropical climate through the eocene epoch earth s climate cooled from a period of extreme warmth in the early eocene epoch ca 50 ma to the early oligocene ca 33 ma when a large ice cap first appeared on antarctica evidence from the planktonic foraminifer oxygen isotope record in deep sea cores has suggested that tropical sea surface temperatures declined by 5 10 degrees over this interval eventually becoming much cooler than modern temperatures here we present paleotemperature estimates from foraminifer isotopes and the membrane lipids of marine crenarcheota from new drill cores in tanzania that indicate a warm and generally stable tropical climate over this period we reinterpret the previously published isotope records in the light of comparative textural analysis of the deep sea foraminifer shells which shows that in contrast to the tanzanian material they have been diagenetically recrystallized we suggest that increasingly severe alteration of the deep sea plankton shells through the eocene produced a diagenetic overprint on their oxygen isotope ratios that imparts the false appearance of a tropical sea surface cooling trend this implies that the long term eocene climatic cooling trend occurred mainly at the poles and had little effect at lower latitudes

rapid transition to long lived deep crustal magmatic maturation and the formation of giant porphyry related mineralization yanacocha peru abstract the yanacocha magmatic field northern peru hosts the largest high sulfidation gold deposit on earth mineralization is associated with porphyritic intrusions distributed along a ne trending magmatic structural corridor eight of these intrusions investigated in this study range in age from 12 4 to 8 4 ma and show systematic chemical and isotopic changes through time they are interpreted to derive from hydrous mafic magmas evolving through amphibole clinopyroxene garnet fractionation and lower crust melting leaving a garnet residue at deeper levels which led to variably strong adakite like signatures and through plagioclase amphibole fractionation at shallower levels both accompanied by crustal assimilation and recharge recharge assimilation fractional crystallization rafc processes systematic geochemical and isotopic changes with intrusion ages coupled with plagioclase zoning and amphibole geobarometry suggest that the evolution of the magmatic system occurred through interaction of mantle derived melts with an increasing length of the crustal column and propagation from deep towards shallower crustal levels through time this was probably the result of a steadily increasing compression that has progressively slowed down magma ascent forcing magmas to evolve at a series of intermediate level chambers between the lower and upper crust increased compression might have been related to the onset of subduction of the buoyant inca oceanic plateau estimated to occur at 12 ma i e the same time of the onset of the rapid transition from normal to adakite like signatures the giant yanacocha ore system developed in coincidence with the 3 6 4 0 ma long intrusion of the adakite like magmas 12 4 12 0 8 4 ma formed by the above processes into a small upper crustal volume and peaked during the last 2 4 ma 10 8 8 4 ma of magmatic activity after a 1 4 ma long 12 4 11 0 ma maturation of magmas at deep crustal levels further investigation is needed to understand whether the association of adakite like magmas with ore which is typical of other giant porphyry systems is the result of the build up of incompatible volatiles and metals in oxidized magmas that evolve under high pressure conditions of recycling of lower crustal sulfide rich cumulates and or of a long lived focused transfer of magmas from deep to shallow crustal levels

comparison between radiocarbon ages obtained on coexisting planktonic foraminifera as a test of the reliability of paleocean ventilation rates reconstructed from radiocarbon age differences between planktonic and benthic foraminifera measurements have been made on coexisting species of planktonic foraminifera while ideally no differences should exist we do find them in this paper we discuss the possible causes for these differences and attempt to evaluate their impact on the interpretation of benthic planktonic age differences

zircon u pb age and sr nd hf o isotope geochemistry of the paleocene eocene igneous rocks in western gangdese evidence for the timing of neo tethyan slab breakoff northward neo tethyan oceanic lithosphere subduction beneath southern tibet in the mesozoic early cenozoic produced continental arc magmas in the 1600 km long gangdese belt the most voluminous magmatism occurred in the paleocene eocene and is characterized by extensive i type calc alkaline to high k calc alkaline linzizong volcanic rocks and coeval plutons these rocks have been extensively studied in the eastern gangdese belt east of 89 e but few data exist from the western gangdese belt new data for eleven samples of these rocks combined with existing data from the literature show that they are similar to the eastern gangdese belt rocks with relative depletions in nb ta p and ti and enrichments in rb ba th u k pb zr and hf on a primitive mantle normalized trace element diagrams typical of continental arc related igneous rocks however compared to the east western gangdese igneous rocks range to higher k2o contents up to 6 1 wt higher 87sr 86sr i ratios up to 0 7151 and lower endi values down to 8 1 suggesting that an evolved crustal source was involved in arc magmatism the gangdese arc magmatism lasted to 80 ma and has a gap or quiescent period afterwards starting at 69 ma the arc magmatism initiated and shifted southward from 30 5 n to 29 5 n in southern tibet with an abrupt change of india asia convergence rate the magmas through the whole gangdese belt at 69 53 ma are characterized by intermediate endi values 0 6 to 4 0 positive ehfi values 3 8 to 7 1 intermediate 18o values 5 0 to 6 5 and low th y and la yb ratios 20 these magmas were likely derived from the mantle with crustal contamination by mash process in response to neo tethyan slab rollback as proposed by previous studies the sr nd hf o isotopic compositions of linzizong volcanic rocks and coeval intrusions are relatively homogeneous during 69 53 ma but show a wide range at 53 50 ma zircon temperature values from these rocks are distinctively high during the latter period and whole rock th y and la yb ratios increased significantly after 50 ma we suggest that the high temperature and heterogeneous geochemistry of these magmas at 53 50 ma reflect neo tethyan slab breakoff during india asia collision which triggered asthenospheric mantle upwelling mantle derived magmas resulted in extensive heating induced crustal melting and generated magmas with variable isotopic compositions tibetan crust was significantly thickened after 50 ma

incorporation and preservation of mg in globigerinoides sacculifer implications for reconstructing the temperature and 18o 16o of seawater using bathymetric transects of surface sediments underlying similar sea surface temperatures but exposed to increasing dissolution we examined the processes which affect the relationship between foraminiferal mg ca and 18o we found that globigerinoides saccculifer calcifies over a relatively large range of water depth and that this is apparent in their mg content on the seafloor foraminiferal mg ca is substantially altered by dissolution with the degree of alteration increasing with water depth selective dissolution of the chamber calcite formed in surface waters shifts the shell s bulk mg ca and 18o toward the chemistries of the secondary crust acquired in colder thermocline waters the magnitude of this shift depends on both the range of temperatures over which the shell calcified and the degree to which it is subsequently dissolved in spite of this shift the initial relationship between mg ca and 18o determined by their temperature dependence is maintained we conclude that paired measurements of 18o and mg ca can be used for reconstructing 18owater though care must be taken to determine where in the water column the reconstruction applies

the influence of caco3 dissolution on core top radiocarbon ages for deep sea sediments radiocarbon ages on caco3 from deep sea cores offer constraints on the nature of the caco3 dissolution process the idea is that the toll taken by dissolution on grains within the core top bioturbation zone should be in proportion to their time of residence in this zone if so dissolution would shift the mass distribution in favor of younger grains thereby reducing the mean radiocarbon age for the grain ensemble we have searched in vain for evidence supporting the existence of such an age reduction instead we find that for water depths of more than 4 km in the tropical pacific the radiocarbon age increases with the extent of dissolution we can find no satisfactory steady state explanation and are forced to conclude that this increase must be the result of chemical erosion the idea is that during the holocene the rate of dissolution of caco3 has exceeded the rain rate of caco3 in this circumstance bioturbation exhumes caco3 from the underlying glacial sediment and mixes it with caco3 raining from the sea surface

extreme deepening of the atlantic overturning circulation during deglaciation glacial terminations during the late pleistocene epoch are associated with changes in insolation they are also punctuated by millennial scale climate shifts characterized by a weakening and subsequent strengthening of the atlantic meridional overturning circulation this ubiquitous association suggests that these oscillations may be a necessary component of deglaciation model simulations have suggested that the period of weakened circulation during these terminal oscillations would be followed by an overshoot of the circulation on its resumption but this phenomenon has not yet been observed here we use radiocarbon measurements of benthic foraminifera and carbonate preservation indices to reconstruct ventilation changes in the deep south atlantic ocean over the past 40 000 years we find evidence for a particularly deep expansion of the atlantic overturning cell directly following the weak mode associated with heinrich stadial 1 our analysis of an ocean general circulation model simulation suggests that north atlantic deep water export during the expansion was greater than that of interglacial conditions we find a similar deep expansion during dansgaard oeschger interstadial event 8 38 000 years ago which followed heinrich stadial 4 we conclude that the rise in atmospheric co2 concentrations and resultant warming associated with an especially weak overturning circulation are sufficient to trigger a switch to a vigorous circulation but a full transition to interglacial conditions requires additional forcing at an orbital scale

abyssal water carbon 14 distribution and the age of the world oceans the carbon 14 distribution in the abyssal waters of the world oceans indicates replacement times for pacific indian and atlantic ocean deep waters more than 1500 meters deep of approximately 510 250 and 275 years respectively the deep waters of the entire world ocean are replaced on average every 500 years

origin of late oligocene adakitic intrusives in the southeastern lhasa terrane evidence from in situ zircon u pb dating hf o isotopes and whole rock geochemistry abstract we describe the whole rock geochemistry biotite ar ar and zircon u pb geochronology and in situ hf o isotopic compositions of the linzhi two mica granitoid in the southeastern lhasa terrane the granitoids are calc alkaline to high k calc alkaline in composition and they have geochemical affinities with adakites the whole rock geochemistry with high sio 2 and al 2 o 3 contents low mgo cr and ni contents low mg values negative e nd t 6 2 to 2 9 and low 87 sr 86 sr i 0 7059 0 7074 together with the in situ zircon hf o isotopic data e hf t 2 7 to 2 7 18 o 7 2 8 8 indicates that the granitoid was derived mainly from the partial melting of a rejuvenated lower crust underneath the lhasa terrane this lower crust was dominated by ancient crustal material with the minor involvement of juvenile mantle inputs 40 ar 39 ar ages for biotites and the presence of magmatic epidote suggest the granitoid was intruded at mid crustal depths indicating that the eastern lhasa terrane experienced a pulse of rapid uplift in the interval from 26 to 22 ma the geochemical features of the linzhi adakitic rocks are significantly different from those of eocene adakitic rocks from the north himalayan belt and this suggests that we can rule out the possibility of crustal thickening in the eastern part of lhasa terrane caused by impingement of the indian lower crust post collisional 26 10 ma adakitic rocks occur within an e w trending magmatic belt that runs parallel to the yarlung tsangpo suture in the lhasa terrane these rocks have distinctive variations in geochemistry along the strike of the lhasa terrane indicating that the lower crust is isotopically heterogeneous from west to east along the terrane these geochemical variations probably result from variable degrees of input along strike of juvenile mantle derived melts into an old lower crust the middle part of the lhasa terrane is marked by a juvenile lower crust that contains large amounts of mantle derived melt underplated during neo tethyan oceanic crust subduction in contrast the lower crust below the western and eastern parts of the terrane is relatively ancient therefore crustal thickening caused by underplating of mantle derived magmas could play an important role in the middle segment of the terrane whereas crustal compression and shortening as a result of the collision of the indian and asian plates could be the major cause for crustal thickening in the western and eastern segments

assessing the effect of dissolution on planktonic foraminiferal mg ca ratios evidence from caribbean core tops in order to assess the dissolution effect on foraminiferal mg ca ratios we analyzed mg ca of seven planktonic foraminiferal species and four of their varieties from caribbean core tops from 900 4700 m water depth depending on the foraminiferal species and variety mg ca start to decline linearly below co32 levels of 18 26 mol kg by 0 04 0 11 mmol mol per 1 mol kg co32 similar to decreases of 0 5 0 8 mmol mol per kilometer below 2500 3000 m water depth above these species specific critical levels mg ca remains stable with higher intraspecific mg ca variability than below we developed routines to correct mg ca from below these critical thresholds for dissolution effects which reduce the overall intraspecific variability by 24 64 and provide dissolution corrected mg ca appropriate to calculate holocene paleotemperatures when taking into account only dissolution unaffected mg ca from 2000 m the systematic succession of foraminiferal species according to their mg ca reflects expected calcification depths

the gangdese magmatic constraints on a latest cretaceous lithospheric delamination of the lhasa terrane southern tibet abstract this paper presents a systematic study of late cretaceous to early tertiary magmatism in the eastern gangdese batholith lhasa nyingchi segment the southern lhasa terrane southern tibet based on zircon u pb age rock association and geochemistry the magmatism can be divided into three stages 95 86 ma stage 1 85 73 ma stage 2 and 68 60 ma stage 3 with a short magmatic quiescence or gap of 73 69 ma the stage 1 is composed mainly of intermediate mafic rocks including norite gabbro diorite charnockite and subordinate granodiorite whereas the stage 2 consists largely of felsic rocks including tonalite granodiorite monzogranite and granite in contrast the stage 3 comprises a wide range of rock types from gabbro to granite all these samples show i type geochemical characteristics with high zircon hf isotope ratios e hf t 7 1 13 0 medium k calc alkaline and metaluminous to weakly peraluminous features stage 1 granitic rocks show low sr y 31 43 and la yb n 6 3 8 3 ratios in contrast to stage 2 that show gradually increasing sr y 47 450 and la yb n 6 38 ratios their magma temperatures constrained by titanium in zircon thermometer decrease from 740 719 c stage 1 to 685 628 c stage 2 these are consistent with an interpretation of progressive magma source deepening coupled with crustal thickening stage 3 granitoids in turn show reduced sr y 3 40 and la yb n 3 5 17 6 ratios and elevated magma temperatures 755 676 c suggesting a high temperature low pressure melting condition and thus a shallow magma source which may imply the presence of thinner crust the contrasting temperature pressure conditions between stage 2 and stage 3 magmatism suggest an event of crustal thinning which we interpret through a synthesis of other lines of geologic evidence to have resulted from a lithospheric delamination of the lhasa terrane in the latest cretaceous in conclusion the lhasa terrane underwent continuous crustal thickening in the late cretaceous and lithospheric delamination in the latest cretaceous this implies that the overthickened crust of southern tibet is generated in the cenozoic

natural variability of the arctic ocean sea ice during the present interglacial the impact of the ongoing anthropogenic warming on the arctic ocean sea ice is ascertained and closely monitored however its long term fate remains an open question as its natural variability on centennial to millennial timescales is not well documented here we use marine sedimentary records to reconstruct arctic sea ice fluctuations cores collected along the lomonosov ridge that extends across the arctic ocean from northern greenland to the laptev sea were radiocarbon dated and analyzed for their micropaleontological and palynological contents both bearing information on the past sea ice cover results demonstrate that multiyear pack ice remained a robust feature of the western and central lomonosov ridge and that perennial sea ice remained present throughout the present interglacial even during the climate optimum of the middle holocene that globally peaked 6 500 y ago in contradistinction the southeastern lomonosov ridge area experienced seasonally sea ice free conditions at least sporadically until about 4 000 y ago they were marked by relatively high phytoplanktonic productivity and organic carbon fluxes at the seafloor resulting in low biogenic carbonate preservation these results point to contrasted west east surface ocean conditions in the arctic ocean not unlike those of the arctic dipole linked to the recent loss of arctic sea ice hence our data suggest that seasonally ice free conditions in the southeastern arctic ocean with a dominant arctic dipolar pattern may be a recurrent feature under warm world climate

remnants of a cretaceous intra oceanic subduction system within the yarlung zangbo suture southern tibet extensive field investigations along the yarlung zangbo suture zone in southern tibet reveal the presence of now fragmented remnants of a south facing intra oceanic subduction system this system developed within tethys during the cretaceous the associated arc forearc ophiolite and subduction complex were emplaced onto the leading edge of india at the end of the cretaceous rapid sedimentation in oblique slip basins and disruption of water saturated sediments into melange was widespread and concomitant with ophiolite emplacement we describe the tectonic entities that developed during this previously unrecognized phase of tethys tibet evolution and present a new model for the evolution of this portion of tibet

the generation of large amplitude unsteady lee waves by subinertial k1 tidal flow a possible vertical mixing mechanism in the kuril straits numerical experiments with a two dimensional nonhydrostatic model are performed to investigate tidally generated internal waves in the kuril straits and their effect on vertical mixing the results show that sill scale internal waves at the k1 tidal frequency are confined to the sill slopes because the k1 tide is subinertial in the kuril straits in contrast to previous theories the authors show that intense short internal waves generated at the sill breaks by the subinertial k1 tidal current can propagate upstream as the tidal current slackens theoretical considerations identify these short waves as unsteady lee waves which tend to be trapped at the generation region and grow into large amplitude waves eventually inducing vigorous mixing along their ray paths in particular superposition of a propagating unsteady lee wave and a newly generated lee wave over a sill causes significant wave breaking leading to a maximum vertical diffusivity of 103 cm2 s21 this quite intense mixing reaches down to the density layer of the north pacific intermediate water npiw in contrast the m2 tidal current does not cause such strong vertical mixing because most of generated internal waves propagate away as first mode internal tides and because the barotropic flow amplitude is small the authors therefore suggest the possibility that generation of lee waves through interactions between the k1 current and the bottom topography of the kuril straits contributes to the observed modification of the okhotsk sea water required in the formation of the npiw

a time dependent three dimensional model of the delaware bay and river system part 2 three dimensional flow fields and residual circulation abstract the three dimensional model of delaware bay river and adjacent continental shelf was described in part 1 here part 2 of this two part paper demonstrates that the model is capable of realistic simulation of current and salinity distributions tidal cycle variability events of strong mixing caused by high winds and rapid salinity changes due to high river runoff the 25 h average subtidal circulation strongly depends on the wind forcing monthly residual currents and salinity distributions demonstrate a classical two layer estuarine circulation wherein relatively low salinity water flows out at the surface and compensating high salinity water from the shelf flows at the bottom the salinity intrusion is most vigorous along deep channels in the bay winds can generate salinity fronts inside and outside the bay and enhance or weaken the two layer circulation pattern since the portion of the continental shelf included in the model is limited the model shelf circulation is locally wind driven and excludes such effects as coastally trapped waves and interaction with gulf stream rings nevertheless a significant portion of the coastal elevation variability is hindcast by the model also inclusion of the shelf improves simulation of salinity inside the bay compared with simulations where the salinity boundary condition is specified at the mouth of the bay

determination of eocene porphyritic monzogranite intrusions in tinggong ore district of tibet and its geological significance the exploration of the tinggong porphyry copper deposit pcd has been always focused on a suite of porphyritic intrusions which have been thought to be the miocene in age but there is no precise dating work on them and the genesis of these intrusions is still enigmatic zircon u pb sims dating of porphyritic monzogranite in this study indicates that the intrusions intensively exposed in the tinggong ore district are eocene 48 88 0 53 ma instead of miocene in age and constitute a part of the gangdise batholiths high precision in situ ion microprobe analysis of zircon hf o isotopes from porphyritic monzogranite shows relatively stable ranges ehf t 4 4 8 4 and 18o 5 81 6 62 close to data of the member of depleted mantle dm in the plot of 18o versus ehf t such isotopic features indicate that abundant mantle derived materials up to 70 90 were involved in the formation of porphyritic monzogranite further studies indicate that such mineralization unrelated porphyritic intrusions occur not only in tinggong but also in such ore deposits as chongjiang bairong and gangjiang in the ore concentration area thus the relationship between the porphyritic intrusions and cu mo mineralization should be adequately considered in the upcoming exploration in this area

a time dependent three dimensional model of the delaware bay and river system part 1 description of the model and tidal analysis abstract a three dimensional time dependent numerical model is used to simulate the dynamics and thermodynamics of delaware bay river and adjacent continental shelf this study describes the first attempt to model an estuary and the contiguous shelf as a coupled hydrodynamic and thermodynamic system here in part 1 a description of the model boundary conditions and forcing information is provided numerical results are compared with surface elevation data at several locations throughout the bay and the river as well as with the observations collected by the national ocean service during their 1984 1985 circulatory study it is shown that a vertically integrated two dimensional version of the model predicts realistic amplitudes but with some phase error the full three dimensional model reduces the phase error but underpredicts the tidal range this is due to the higher values of horizontal viscosity required by the three dimensional model the model accounts for non linear shallow water effects and reproduces the observed amplification of the high frequency tidal components from the mouth of the bay to the head of the river at trenton

comparison of δ18o analyses on individual planktic foraminifer orbulina universa shells by sims and gas source mass spectrometry author s wycech jb kelly dc kozdon r orland ij spero hj valley jw abstract 2018 elsevier b v the oxygen isotope 18o compositions of final chamber fragments of individual shells of the planktic foraminifer orbulina universa were measured in situ via secondary ion mass spectrometry sims and by traditional gas source mass spectrometry gsms entailing acid digestion of sampled calcite the paired sims gsms analyses were performed on final chamber fragments of fossil shells taken from the top of a sediment core holocene as well as shells grown in laboratory culture multiple iterations of sims gsms analyses were conducted on final chamber fragments treated with a variety of cleaning protocols the series of paired analyses yielded an average sims gsms 18o offset 18osims gsms of 0 9 0 1 2 se the volume of material analyzed in 10 m sims spots is 105 times smaller than that analyzed by gsms hence the extent to which these 18osims gsms values represent real differences in analyte vs instrumental factors remains unclear possible contributing factors to the sims gsms 18o difference include sample standard mismatch by sims differences in standardization of sims and gsms and non calcite contaminants in samples although the two datasets are consistently offset sims values reproduce inter shell 18o variability delineated by shell fragment gsms values this strong positive covariance proved useful for bringing the two datasets into agreement i e 18osims gsms 0 and confirms that sims based foraminifer 18o values record changes in calcification temperature and or 18o of seawater whether shells of foraminifer taxa with differing microcrystalline structures chemical composition and or preservation histories register a similar 18osims gsms value is a subject of ongoing testing

spatial variability of diapycnal mixing and turbulent dissipation rates in a stagnant fjord basin two microstructure profilers two ships and four moorings with acoustic doppler current profilers and conductivity temperature loggers were used in an intensive effort to map the spatial and temporal variations of vertical mixing in the stagnant deep basin of gullmar fjord sweden during three days in the beginning of august 2001 a continuous time series of turbulent kinetic energy dissipation profiles was obtained with one microstructure profiler at a fixed position near the deepest part of the fjord during the same period the other microstructure profiler was used to obtain six sections of dissipation through the length of the basin two moorings were deployed in the fjord basin for one month from the end of july to the end of august the mapping of dissipation rates reveals that the dissipation in the deep basin is confined to areas just inside the sill more than 77 of the dissipation in the fjord basin happens above the sloping bottoms closest to the sill

resistance to barotropic tidal flow in straits by baroclinic wave drag energy transfer from barotropic tides to baroclinic motions may take place at the ends of straits connecting stratified basins implying generation of internal waves propagating into the basins different aspects of this have been described in the literature including quantification of the energy transfer studies of the resulting internal tides and the relationship to diapycnal mixing in the basins however the accompanying resistance to oscillatory barotropic strait flows by so called baroclinic wave drag has not been implemented earlier in models for strait flow in the present paper a formulation for the instantaneous baroclinic wave drag force is suggested this leads to a linear relationship between the strait flow and the synoptic sea level difference between the adjacent basins the model is used to study tidal response in land locked basins the response is computed for three fjords where the resistance to the barotropic flow comes essentially from baroclinic wave drag it is concluded that the strait flow model handles baroclinic wave drag in a satisfactory way since the observed responses are well predicted by the model baroclinic wave drag should provide the major resistance to surface tides in the deep ocean which indicates its importance in a wider context several mechanisms may cause resistance to barotropic flow in straits and channels friction against the sea bed is certainly the most commonly considered type of flow resistance it is often modeled with a quadratic relationship between the bottom stress and the barotropic velocity using a drag coefficient to characterize the frictional properties of the sea bed another type of flow resistance is due to large scale longitudinal variations of the vertical cross sectional area of the strait causing contraction followed by expansion of the flow this class of flow resistance appropriately termed barotropic form drag includes the resistance caused by contraction expansion induced by the ends of straits having smaller vertical cross sectional areas than the adjacent basins in the cases of frictional drag and largescale form drag barotropic energy is transferred to turbulence in the bottom boundary layer in the strait and to large scale eddies in the expanding current downstream of the strait respectively the present paper however draws attention to a third class of resistance operating on oscillating barotropic strait flow it is due to generation of baroclinic internal waves in the adjacent stratified basins by the ends of the strait and is

cospatial eocene and miocene granitoids from the jiru cu deposit in tibet petrogenesis and implications for the formation of collisional and postcollisional porphyry cu systems in continental collision zones abstract jiru is a poorly studied cu deposit located in the west segment of the gangdese porphyry cu belt gpcb 200 km west of lhasa the deposit consists of both collisional and postcollisional stage porphyry type cu systems which are genetically associated with the early eocene granitoid batholith and the miocene jiru porphyry stock respectively in this study we present zircon u pb la icp ms dates and hf isotopes whole rock geochemical and pb isotope geochemical data for the main intrusions in the jiru deposit the early eocene granitoid samples 49 ma are characterized by magmatic arc geochemical features slightly concave ree patterns and well developed negative eu anomalies these geochemical characteristics suggest that the granitoid melts were generated by partial melting of a metasomatized mantle and that the melt had undergone fractional crystallization of amphibole and plagioclase in contrast the miocene porphyry intrusions 16 4 15 5 ma at jiru are characterized by high k contents adakitic affinities e g high sr y and la yb ratios subduction signatures e g enriched cs rb ba and depleted nb ta ti positive zircon ehf t values 1 6 and variable 208pb 204pb ratios 38 5 39 0 similar to other post collisional porphyry intrusions in the gangdese belt based on the above features we propose that the miocene porphyry intrusions at jiru were generated by partial melting of subduction modified lower crust well developed negative eu anomalies and low sr y ratios generally

influence of trench width on subduction hinge retreat rates in 3 d models of slab rollback subduction of tectonic plates limited in lateral extent and with a free trailing tail i e free subduction is modeled in a three dimensional 3 d geometry the models use a nonlinear viscoplastic rheology for the subducting plate and exhibit a wide range of behaviors depending on such plate characteristics as strength width and thickness we investigate the time evolution of this progressive rollback subduction measure the accompanying return flow in the upper mantle and quantify the plate kinematics due to the 3 d geometry flow is allowed to accompany slab rollback around the lateral edges of the slab the toroidal component as opposed to 2 d geometry where material is forced to flow underneath the slab tip the poloidal component a simple force balance is provided which relates the speed of backward trench migration to the resistive forces of generating flow and weakening the plate our results indicate most of the gravitational energy of the system i e the negative buoyancy of the subducting slab is converted into a toroidal flow 69 a much smaller amount goes into weakening the plate 18 and the remaining amount goes into driving flow parallel to displacement of the slab 13 for the trench widths w we investigate 1500 km a maximum trench retreat rate occurs for trenches 600 km wide which is attributed to the interaction between a plate of finite width and the induced flow which has a lengthscale in the horizontal direction these numerical results quantitatively agree with comparable 3 d laboratory experiments using analogue models with a purely viscous plate material schellart 2004a 2004b including correlations between increasing retreat rate with increasing plate thickness trench width for maximum retreat rate 500 km and estimated amount of slab buoyancy used to drive rollback induced flow 70 several implications for plate tectonics on earth result from these models such as rollback subduction providing a physical mechanism for ephemeral slab graveyards situated above the more viscous lower mantle and endothermic phase transition prior to a flushing event into the lower mantle mantle avalanche

effects of seafloor diagenesis on planktic foraminiferal radiocarbon ages radiocarbon 14c ages obtained from planktic foraminiferal calcite are a mainstay for reconstructing ocean climate change and carbon cycle dynamics of the past 30 k y yet the effects of diagenesis on this vital chronometer are poorly constrained here we address this shortcoming by comparing 14c ages and trace element ratios mg ca mn ca of planktic foraminifera with white opaque shells deemed well preserved by traditional standards to those with exquisitely preserved translucent shells results support a diagenetic mechanism as opaque shells yield 14c ages invariably older and trace element ratios consistently higher than those of translucent shells radiocarbon age offsets are particularly pronounced in mono specific samples taken from stratigraphic horizons proximal to the 18o maximum marking the last glacial maximum lgm and the subsequent deglacial radiocarbon based calendar ages of translucent shells from the two intervals are congruent with the established age ranges for these climate events whereas those of co occurring opaque shells overestimate the lgm and deglacial by 8 15 k y and 14 22 k y respectively these results demonstrate that the use of translucent foraminifera enhances reproducibility and accuracy of 14c ages by minimizing the deleterious effects of diagenesis this study serves as a cautionary tale since white opaque foraminifera are common in pelagic sediments and 14c ages derived from their ostensibly well preserved shells can lead to discrepancies in the timing of quaternary climate events and ocean circulation reconstructions

deep water renewal in the upper basin of loch sunart a scottish fjord abstract recording current meters were deployed near the surface and bottom in the upper basin of loch sunart during the summers of 1987 1989 and 1990 the measurements revealed frequent though irregular deep water renewal events when the basin water was replaced by density driven intrusions the data suggested that these events were induced by wind enhanced estuarine circulation during periods of low fresh water runoff a semi implicit laterally integrated estuarine model was modified to allow variable vertical grid spacing and to permit the sea surface to move vertically through the grid both modifications being necessary in order to apply the model to the sea loch environment a turbulence closure scheme was also incorporated the model can simulate the evolution of the salinity field over periods of several weeks and was used to examine the mechanisms that induce deep water renewal model results suggested that very weak stratification in the basin is required for renewal to occur and indicated

reconstructing past sea surface temperatures correcting for diagenesis of bulk marine carbonate abstract a numerical model which describes oxygen isotope exchange during burial and recrystallization of deep sea carbonate is used to obtain information on how sea surface temperatures have varied in the past by correcting measured 18o values of bulk carbonate for diagenetic overprinting comparison of bulk carbonate and planktonic foraminiferal 18o records from odp site 677a indicates that the oxygen isotopic composition of bulk carbonate does reflect changes in sea surface temperature and 18o at odp site 690 we calculate that diagenetic effects are small and that both bulk carbonate and planktonic foraminiferal 18o records accurately reflect paleogene warming of high latitude surface oceans biased from diagenesis by no more than 1 c the same is likely to be true for other high latitude sites where sedimentation rates are low at dsdp sites 516 and 525 the effects of diagenesis are more significant measured 18o values of eocene bulk carbonates are more than 2 lower at deeply buried site 516 than at site 525 consistent with the model prediction that the effects of diagenesis should be proportional to sedimentation rate model corrections reconcile the differences in the data between the two sites the resulting paleotemperature reconstruction indicates a 4 c cooling of mid latitude surface oceans since the eocene at low latitudes the contrast in temperature between the ocean surface and bottom makes the carbonate 180 values particularly sensitive to diagenetic effects most of the observed variations in measured 18o values are accounted for by diagenetic effects rather than by sea surface temperature variations we show that the data are consistent with constant equatorial sea surface temperatures through most of the cenozoic with the possible exception of the early eocene when slightly higher temperatures are indicated we suggest that the lower equatorial sea surface temperatures for the eocene and oligocene reported in other oxygen isotope studies are artifacts of diagenetic recrystallization and that it is impossible to reconstruct accurately equatorial sea surface temperatures without explicitly accounting for diagenetic overprinting

radiocarbon dating of planktonic foraminifer shells a cautionary tale 1 sedimentation rate bioturbation winnowing and calcite dissolution produce significant radiocarbon age offsets among multiple species of coexisting planktonic foraminifers and pteropod fragments we compare the radiocarbon age of foraminifer species and pteropod fragments with estimates of percent calcite dissolved made with a sedimentary proxy globorotalia menardii fragmentation index mfi to delineate the effect of dissolution on radiocarbon age of foraminifers data from two core top transects on the rio grande rise rio and ontong java plateau ojp and from down core sediments of varying sedimentation rates in the tropical pacific me 27 md98 2177 and mw91 9 56ggc reveal that sediments with the greatest accumulation rates produce the least age offsets among coexisting species age offsets among coexisting foraminifers are about 3500 years on rio and 1000 years on ojp two core tops from rio yield an age of the last glacial maximum possibly due to mass displacement of younger sediments downslope foraminifer age increases with increasing dissolution and there is a consistent pattern of older foraminifer fragments coexisting with younger whole shells of the same species the only exception is sediments which have experienced high dissolution where fragments are younger than whole shells the age offset between fragments of g menardii and its coexisting whole shells does not exceed the age offset among other coexisting foraminifer species in the same core tops

the simulation of deep water exchange in a fjord indian arm british columbia canada abstract a numerical model is used to simulate deep water renewals that were observed to occur in burrard inlet indian arm over a time period of about 100 days during winter 1984 85 the renewals bottom water flowing from burrard inlet into indian arm occurred or intensified during neap tide and caused a steady increase in the density near the bottom in indian arm the model successfully reproduces the renewal events for a simulation time of at least 85 days it uses a mellor yamada level two turbulence closure scheme for which the local turbulent energy production is balanced by local dissipation and for which the turbulent length scale is prescribed the length scale varies linearly with depth near the surface and bottom and is constrained to never be larger than 1 20 of the water depth and 2 the ozmidov length scale a lower bound is imposed on the vertical diffusion coefficients that depends on the brunt vaisala frequency n its magnitude for a given n is different in burrard inlet and in

late mesozoic tectonic evolution and growth of the tibetan plateau prior to the indo asian collision abstract the elevation of the tibetan plateau is a revolutionary event in the earth history which resulted in change not only of cenozoic regional and global climate but also of monsoon intensity a critical aspect for the development of the tibetan plateau that remains open to intense debate is whether the collision among the tibetan continental blocks or the subduction of the tethys prior to the indo asian collision in the cenozoic contributed to the plateau growth we propose here that an andean type orogen could have been present in southern tibet from the jurassic to early cretaceous but collapsed during the middle cretaceous based on a synthesis of existing structural sedimentary geochronological and geochemical data the orogeny is marked by north verging jurassic early cretaceous thrusting in the lhasa block a result of flat subduction of the yarlung zangpo neo tethys that is recorded by a narrow belt of 205 174 ma adakitic rocks along the southernmost margin of the lhasa block denudation of the orogen produced a thick up to 15 km sequence of jurassic turbidites and molasse type sediments on the northern lhasa block widespread middle cretaceous 135 100 ma postorogenic strongly peraluminous to calc alkaline magmatism and extensional deformation in the lhasa block and an extensive marine transgression in tibet indicate that the orogen broke down due to extension resulting from lithosphere delamination and asthenosphere upwelling meanwhile the north dipping subduction of the bangong meso tethys produced a wide jurassic middle cretaceous magmatic arc along the southern margin of the qiangtang block findings of 132 108 ma aged ophiolites rich with middle cretaceous radiolarians in central tibet indicate that the bangong meso tethys did not close until the late cretaceous repeated flat slab subduction of the yarlung zangpo neo tethys occurred during the late cretaceous 90 78 ma as indicated by the adakites in the southernmost lhasa block which together with the ongoing qiangtang lhasa collision could have contributed to the growth of the tibetan plateau

copper enrichment in arc magmas controlled by overriding plate thickness the characteristics of magmas typically associated with porphyry copper deposits are thought to be imparted in the mantle statistical assessment of over 40 000 geochemical analyses of magmatic rocks formed in subduction zones worldwide however shows that the characteristics of these magmatic rocks are systematically controlled by the thickness of the arc crust

a numerical model of the circulation in knight inlet british columbia canada abstract during spring 1988 a period of low freshwater runoff and summer 1989 a period of high freshwater runoff month long observations of velocity temperature and salinity were made throughout the water column in knight inlet both up inlet and down inlet of the sill measurements were made at depths of 2 4 6 9 and 12 m using s4 current meters at depths down to about 200 m using profiling current meters and at depths deeper than 200 m using aanderaa current meters anemometers were deployed at two locations along the inlet a laterally integrated two dimensional numerical model of the inlet that uses the mellor and yamada level 2 5 turbulence closure scheme and that accounts for the combined influence of the winds tides and freshwater runoff has been used to produce 30 day simulations of the velocity and density field in the inlet the vertical coordinate is transformed in the model so that very fine vertical resolution can be attained near the surface even though the tidal range is larg

implications of shortening in the himalayan fold thrust belt for uplift of the tibetan plateau 1 recent research in the himalayan fold thrust belt provides two new sets of observations that are crucial to understanding the evolution of the himalayan tibetan orogenic system first u pb zircon ages and sm nd isotopic studies demonstrate that 1 greater himalayan medium to high grade metasedimentary rocks are much younger than true indian cratonic basement and 2 these rocks were tectonically mobilized and consolidated with the northern margin of gondwana during early paleozoic orogenic activity these observations require that greater himalayan rocks be treated as supracrustal material in restorations of the himalayan fold thrust belt rather than as indian cratonic basement in turn this implies the existence of greater himalayan lower crust that is not exposed anywhere in the fold thrust belt second a regional compilation of shortening estimates along the himalayan arc from pakistan to sikkim reveals that 1 total minimum shortening in the fold thrust belt is up to 670 km 2 total shortening is greatest in western nepal and northern india near the apex of the himalayan salient and 3 the amount of himalayan shortening is equal to the present width of the tibetan plateau measured in an arc normal direction north of the indus yalu suture zone this information suggests that a slab of greater indian lower crust composed of both indian cratonic lower crust and greater himalayan lower crust with a north south length of 700 km may have been inserted beneath the tibetan crust during the cenozoic orogeny we present a modified version of the crustal underthrusting model for himalayan tibetan orogenesis that integrates surface geological data recent results of mantle tomographic studies and broadband seismic studies of the crust and upper mantle beneath the tibetan plateau previous studies have shown that incremental mesozoic and early cenozoic shortening had probably thickened tibetan crust to 45 55 km before the onset of the main cenozoic orogenic event thus the insertion of a slab of greater indian lower crust with maximum thickness of 20 km tapering northward could explain the cenozoic uplift of the tibetan plateau the need for tibetan crust to stretch laterally as the greater indian lower crust was inserted may explain the widespread east west extension in the southern half of the plateau our reconstruction of the himalayan fold thrust belt suggests that indian cratonic lower crust of presumed mafic composition and high strength should extend approximately halfway across the tibetan plateau to the banggong suture from there northward we predict that the tibetan plateau is underlain by more felsic and therefore weaker lower crust of greater himalayan affinity two slab break off events are predicted by the model the first involved neotethyan oceanic lithosphere that foundered 45 35 ma and the second consisted of greater indian lithosphere most likely composed of greater himalayan material that delaminated and foundered 20 10 ma asthenospheric upwelling associated with the break off events may explain patterns of cenozoic volcanism on the tibetan plateau although the model predicts a northward migrating topographic front due solely to insertion of greater indian lower crust the actual uplift history of the plateau was complicated by early middle tertiary shortening of tibetan crust

the accuracy of mid pliocene δ 18 o based ice volume and sea level reconstructions abstract understanding the sensitivity of the polar ice caps to a modest global warming 2 3 c above preindustrial is of paramount importance if we are to accurately predict future sea level change knowledge that will inform both social and economic policy in the coming years however decades of study of the pliocene 2 6 5 3 ma an epoch in recent earth history characterized by atmospheric co2 levels similar to today have so far failed to provide definitive sea level and ice volume estimates for that time here we review the sources of uncertainty in the paired mg ca 18o methodology used to estimate past sea level ice volume and ocean temperature as well as discuss common assumptions that may bias our interpretation of ocean geochemical records including the lr04 benthic 18o stack a global compilation of 57 oxygen isotope records that forms the standard template of climate change history for the last 5 3 million years

increased magmatic water content the key to oligo miocene porphyry cu mo au formation in the eastern gangdese belt tibet the eastern gangdese magmatic belt between 88 e and 94 e in southern tibet preserves a series of cenozoic collision related igneous rocks that have formed since the start of the india asia collision at 55 to 50 ma the paleocene eocene intrusive rocks have intermediate la yb n ratios average 12 0 8 6 n 150 range 1 15 64 0 intermediate to low sr y ratios mostly 40 and weak or absent eu anomalies oligocene average eun eu 0 99 0 27 n 35 range 0 67 1 58 miocene average eun eu 0 95 0 13 n 93 range 0 44 1 46 and their mineralogy consists mainly of plagioclase quartz and amphibole as phenocrysts these geochemical and mineralogical characteristics suggest that the paleocene eocene magmas were relatively dry and evolved primarily by fractionation of pyroxene and plagioclase whereas the oligo miocene magmas were more hydrous and fractionated significant amounts of hornblende and lesser plagioclase prior to upper crustal emplacement oligocene and miocene magmatic rocks crop out as small volume intrusions and are associated with several large to giant sized porphyry cu mo au deposits as well as numerous smaller porphyry and skarn deposits in contrast earlier more voluminous paleocene eocene magmatism is only known to be associated with three small porphyry deposits erosional loss of subvolcanic porphyry systems from the older paleocene eocene sequence is not thought to be a controlling factor in this temporal difference of porphyry deposit distribution because exhumation rates after 55 ma were relatively low and coeval linzizong volcanic rocks are extensively preserved in the gangdese belt instead we suggest that differences in magmatic history and petrogenesis led to this restricted temporal distribution of porphyry deposits the paleocene eocene magmas were generated during the onset of collision between india and asia and were triggered by rollback of the neo tethyan oceanic slab this magmatism records the final stage of subduction of oceanic lithosphere beneath southern tibet and the relatively low water contents of these magmas may reflect final dehydration of the remnant neo tethyan slab after oceanic slab breakoff at 40 to 38 ma the greater india slab began to subduct beneath the gangdese belt until hard collision with the indian continent at 35 ma and breakoff of the greater india slab at 25 to 10 ma this last event is suggested to have caused asthenospheric mantle upwelling and partial melting of subduction modified hydrated tibetan lithosphere the resultant partial melts may have remobilized metals residual in the deep lithosphere from previous arc magmatism giving rise to a suite of postsubduction porphyry type ore deposits in the oligo miocene

on the influence of a surface coastal front on near inertial wind induced internal wave generation 1 a numerical model is used to examine the wind induced near inertial internal waves in the coastal region in the presence of a surface front initial calculations with the front separating a well mixed coastal region from a stratified offshore area in an infinite sea domain showed that away from the front surface inertial oscillations are dominant near the front on the positive vorticity side superinertial internal waves are generated and propagate offshore in an analogous manner to that found when stratification intersected a coastal boundary however in the case of a coastal boundary the no flow condition at the coast produced inertial oscillations at depth phase shifted by 180 from those at the surface on the negative vorticity side of the front internal waves at the subinertial frequency are trapped between the well mixed region and the front some inertial energy is found at depth in the frontal region although the large scale features of the distribution of inertial energy are independent of vertical eddy viscosity parameterization its magnitude depends upon it in the case of a stratified nearshore region and an offshore front near inertial waves generated at the coast were trapped between the coastal boundary and the front which results in a surface region of enhanced inertial energy this gradually decreased with time as inertial energy diffused to depth this timescale depended upon vertical eddy viscosity magnitude

radiocarbon age offsets of foraminifera resulting from differential dissolution and fragmentation within the sedimentary bioturbated zone 1 shells of coexisting species of planktonic foraminifera from the ontong java plateau reveal radiocarbon age offsets of up to 2200 years similar offsets are found between fragments and whole shells of single species steady state modelling of dissolution and bioturbation within the sedimentary mixed layer predicts age differences of up to several kiloyears due to the interplay between differential dissolution and fragmentation of foraminifer shells and bioturbation the observation that fragile foraminiferal shells are systematically older than those of more robust species is more difficult to explain mechanisms of chemical erosion interface dissolution and sediment redistribution are all apparently unable to explain this phenomenon a possible solution is presented in which a particular species may be represented by two distinct classes of shells which are more or less robust in this case differential dissolution and fragmentation causes an increase in the mean age as the fragile class contributes less to the remaining intact shells this study highlights the vulnerability of low sedimentation rate cores to the effects of dissolution and bioturbation

a three dimensional simulation of the hudson raritan estuary part ii comparison with observation abstract results from a time dependent three dimensional numerical simulation of the hudson raritan estuary are compared with observations the comparison includes 1 instantaneous salinity contours across a transect in the estuary 2 amplitudes and phase of tidal constituents at four tide gauge and five current meter stations 3 mean currents at nine meter locations and mean salinity in the hudson river 4 kinetic energy spectra and 5 response to wind forcing of subtidal current at an observational station near the mouth of the estuary observations confirm the model s prediction of existence of density advection instabilities induced by differential advection of the three dimensional density field these instabilities produce intense vertical mixing and should significantly modify dispersion processes in the estuary effects of neap spring tides on vertical stratifications are also simulated by the model simulated m2 phase at three tide gauge stations show improvement over the m2 phases obtaine

wind stress and surface waves observed on lake washington abstract new results from turbulent flux measurements made over lake washington include the following 1 the only direct measure of the vertical transport of the horizontal momentum heat and matter in the surface boundary layer is the so called eddy correlation method however even if the measurement errors are negligible the results obtained from point observations may show large scatter due to lack of stationarity and horizontal homogeneity in the turbulent field and to the sampling variability scatter may be greatly reduced by spatial averaging in this study such an effect is achieved by determining the surface roughness length hence the neutral drag coefficient from the measured wave height spectrum which reflects the atmospheric input integrated over the fetch applicability and usefulness of the approach for general field measurements and remote sensing is discussed 2 the evolution of the wave field observed on lake washington agrees in peak frequency and the slope of the equilibrium ra

three dimensional nature of flow near a sill velocity and density sections across knight inlet british columbia demonstrate that lateral recirculations are a first order feature of the flow in the lee of the sill the flow over this sill has been used as a typical example of two dimensional hydraulics with a lower layer that thins and accelerates as it moves downstream below an almost stagnant layer that widens downstream creating a distinctive wedge shape however we find that rather than being stagnant the velocity in this wedge shaped layer is actually quite large consisting of a swiftly recirculating dipole vortex during flood tide and a monopole vortex during ebb tide though we may have missed the matching half of a dipole with our sampling during ebb the recirculations during flood tide carry an amount of water equal to 25 of the tidal flux while the monopole during ebb tide carries 20 of the tidal flux these recirculations bias along channel estimates of volume flux especially in the middle wedge shaped layer and demonstrate that accurate volume fluxes in the lee wave are only possible if three dimensional surveys are made our three dimensional survey shows that there is a net isopycnal convergence of water in the middle layer at a rate adequate to close the volume budget of the flow without recourse to diapycnal fluxes we also calculate the strength of the vorticity in the recirculations observed during flood tide and attribute their formation to boundary layer separation

spatial and temporal offsets between proxy records in a sediment drift chronologies for late quaternary marine sediment records are usually based on radiocarbon ages of planktonic foraminifera signals carried by other sedimentary components measured in parallel can provide complementary paleoclimate information a key premise is that microfossils and other indicators within a given sediment horizon are of equal age we show here that haptophyte derived alkenones isolated from bermuda rise drift sediments are up to 7000 years older than coexisting planktonic foraminifera this temporal offset which is apparently due to lateral transport of alkenones on fine grained particles from the nova scotian margin markedly influences molecular estimates of sea surface temperatures more broadly the observation raises questions about both the temporal and the geographic fidelity of paleoenvironmental records encoded by readily transported components of sediments

energy fluxes due to the surface and internal tides in knight inlet british columbia abstract a laterally integrated two dimensional nonlinear numerical model is used to examine the flux of m2 tidal energy in knight inlet the simulated flux of tidal energy into the inlet is somewhat smaller than that estimated using the change in phase of the m2 tidal height along the inlet a method that does not account for the effect of the internal tide on the surface elevation the simulated energy flux into the inlet is close to the energy flux of the internal tide away from the sill determined from observations using an acoustic doppler current profiler adcp the net flux due to the internal tide is significantly less than 1 2 of the rate at which energy is removed from the surface tide earlier linear models of the internal tide produced energy fluxes that agreed with those estimated from the phase change of the tidal height but were larger than the fluxes that could be found in the observations the reason for this discrepancy is not that these simple models neglected nonlinear effects b

a three dimensional simulation of the hudson raritan estuary part i description of the model and model simulations abstract a time dependent three dimensional finite difference simulation of the hudson raritan estuary is presented the calculation covers july september 1980 the model estuary is forced by time dependent observed winds tidal elevation at open boundaries and river and sewage discharges turbulence mixing coefficients in the estuary are calculated according to a second moment turbulence closure submodel horizontal diffusivities are zero in the simulation and small scale eddies produced by the interaction of unsteady three dimensional velocity and salinity fields with coastline and bottom bathymetry were resolved by the model these eddies are important physical elements in shear dispersion processes in an estuary model results show unstably stratified water columns produced by advection of waters of different densities these instabilities produce intense mixing with vertical eddy diffusivities reaching 2 3 times their neutral values they occur most frequently at slack currents during initial s

the role of upstream waves and a downstream density pool in the growth of lee waves stratified flow over the knight inlet sill abstract observations and modeling simulations are presented that illustrate the importance of a density contrast and the upstream response to the time dependence of stratified flow over the knight inlet sill repeated sections of velocity and density show that the flow during ebb and flood tides is quite different a large lee wave develops early in flood tide whereas lee wave growth is suppressed until the second half of ebb tide there is a large upstream response that displaces as much water as accumulates in the lee wave one that is large enough to also block the flow at a depth roughly consistent with simple dynamics there is a large density contrast between the seaward and landward sides of the sill and a salty pool of water is found in the seaward basin that is not found landward the interface with this salty pool demarks the point of flow separation during ebb initially suppressing the lee wave and then acting as its lower boundary a simple two dimensional numerical model of the inlet wa

postsubduction porphyry cu au and epithermal au deposits products of remelting of subduction modified lithosphere porphyry cu mo au and some epithermal au deposits are formed from hydrothermal fluids exsolved from cooling water rich calc alkaline magmas emplaced in volcanoplutonic arcs above subduction zones these magmas originate by partial melting of the metasomatized asthenospheric mantle wedge however there is increasing evidence for the existence of a suite of porphyry cu au and epithermal au deposits related to magmas generated after subduction beneath the arc has ceased associated magmas tend to be mildly alkaline relatively sulfur poor and emplaced as isolated complexes rather than in voluminous volcanoplutonic arcs they are likely formed by remelting of previously subduction modified arc lithosphere triggered by postsubduction lithospheric thickening lithospheric extension or mantle lithosphere delamination metasomatized mantle lithosphere or hydrous lower crustal cumulates residual from first stage arc magmatism contain small amounts of chalcophile and siderophile element rich sulfides and constitute a fertile source for hydrous au rich but relatively sulfur poor magmas during later remelting the recognition that porphyry cu au and related epithermal au systems can also form in postsubduction and collisional tectonic settings expands the range of geological environments and geographical terranes that are prospective for such deposits

a numerical study of the tidal circulation and buoyancy effects in a scottish fjord loch torridon 1 the tidal and buoyancy driven circulation in loch torridon a scottish fjord is investigated using a three dimensional hydrostatic primitive equation model the predictions from the model have been quantitatively compared to available data gathered between 1998 and 2001 the model performed well in reproducing the tidal characteristics and density evolution of the fjord over the 3 month simulation period results show that tidal currents are dominated by the constraint imposed on the flow by the shallow sill separating the upper and middle basins the flow is strongly accelerated during flood and ebb tides and significant residual currents are generated that may influence exchange and residence times of the middle basin a strong baroclinic response to the flow over the sill is predicted throughout the fjord with a propagating linear m2 tide displacing isopycnals vertically by up to 32 m at the sill the formation of stationary lee waves on both flood and ebb tides is predicted with the wave generated during flood tide being notably larger as the growth of the ebb wave is inhibited by deep dense water this allows more energy to transfer into the seaward propagating internal tide as the tidal flow weakens the lee waves propagate over the sill and upstream estimates of the total vertical turbulent buoyancy flux in the fjord are made and the distribution of mixing over a tidal cycle is shown vertical mixing is seen to be highly spatially and temporally variable and strongly linked to the presence and dissipation of lee waves near the sill

a model study of density intrusions into and circulation within a deep silled estuary puget sound a laterally averaged hydrodynamical model is used to investigate episodic intrusions of denser more oceanic water across the shallow double silled entrance of puget sound and the resulting circulation within the interior deep basin tidal freshwater discharge and wind effects are also encompassed in model results model simulations were made of the 4 month period from december 1983 through march 1984 during that period hourly measurements of currents and salinity were made at the estuary entrance and on four moorings in the interior basin the first 60 days of salinity data were used with the model to choose rates of vertical turbulent mixing and the lag times between fresh water appearance at river gauges and its arrival in the main basin of the sound comparison of model results with the full 120 days of salinity and velocity data shows that the model incorporates the important circulation features of this estuary the model was then used to study intrusions and mixing in the sill region the model identifies these factors as controls on the occurrence and strength of intrusions the trans sill salinity gradient the intensity of tidal stirring and mixing over the sills the supply of fresher surface water resulting from river runoff and the winds time series of salinities in the interior basin show local maxima following local maxima in the trans sill salinity difference vertical transport in the sill region at both tidal and subtidal time scales is dominated by advection rather than diffusion though much of the vertical exchange occurs over the shallower sill a depth depression between the two entrance sills appears to be the site of significant overturning downwelling of surface water occurs on the landward side of both sills substantial freshening of the interior basin occurs during the winter months because river discharges are large and are stirred downward in the sill region to be entrained into landward bottom flows extraordinarily large river discharges can suppress the effect of large trans sill salinity differences with the effect of weakening potential intrusions winds can indirectly influence intrusions by longitudinally positioning fresher river influenced surface water in the sill region winds can directly influence intrusions via bottom compensation flows in admiralty inlet with northward winds encouraging intrusions and by changing salinities at the entrance to the sound and thus the trans sill salinity gradient

petrology of the chilliwack batholith north cascades washington generation of calc alkaline granitoids by melting of mafic lower crust with variable water fugacity calc alkaline granitoid rocks of the oligocene pliocene chilliwack batholith north cascades range from quartz diorites to granites 57 78 sio2 and are coeval with small gabbroic stocks modeling of major element trace element and isotopic data for granitoid and mafic rocks suggests that 1 the granitoids were derived from amphibolitic lower crust having ree rare earth element and sr nd isotopic characteristics of the exposed gabbros 2 lithologic diversity among the granitoids is primarily the result of variable water fugacity during melting the main effect of fh 2 o variation is to change the relative proportions of plagioclase and amphibole in the residuum the ree data for intermediate granitoids quartz diorite granodiorite eu eu 0 84 0 50 are modeled by melting with fh 2 o 1 kbar leaving a plagioclase pyroxene residuum in contrast data for leucocratic granitoids leuco granodiorites and granites eu eu 1 0 0 54 require residual amphibole in the source and are modeled by melting with fh 2 o 2 3 kbar consistent with this model isotopic data for the granitoids show no systematic variation with rock type 87sr 86sri 0 7033 0 7043 end 0 3 3 to 5 5 and overlap significantly with data for the gabbroic rocks 87sr 86sri 0 7034 0 7040 end 0 3 3 to 6 9 the fh 2 o variations during melting may reflect additions of h2o to the lower crust from crystallizing basaltic magmas having a range of h2o contents chillwack gabbros document the existence of such basalts one dimensional conductive heat transfer calculations indicate that underplating of basaltic magmas can provide the heat required for large scale melting of amphibolitic lower crust provided that ambient wallrock temperatures exceed 800 c based on lithologic and geochemical similarities this model may be applicable to other cordilleran batholiths