on the origin of the warm inflow to the nordic seas abstract warm and saline waters enter the nordic seas from the south as part of the warm to cold water transformation of the thermohaline circulation of the northern north atlantic one explanation for the origin of the nordic seas inflow is a shallow source hypothesis under which the inflow waters are a modification of upper ocean subtropical waters warm waters from the subtropical gyre are carried to the eastern north atlantic by the north atlantic current and branch northwards joined by poleward upper thermocline flow along the upper continental slope to provide the nordic seas inflow along this pathway the upper water column is progressively cooled and freshened by winter convection the subpolar mode water transformation process and this sets the inflow characteristics a deep source hypothesis provides an alternative explanation for the characteristics of the nordic seas inflow and the pathway delivering the waters to the inflow under this hypothesis inflow waters originate from the core of the mediterranean overflow waters in the gulf of cadiz carried northward at mid depth by the eastern boundary undercurrent in the subtropics continuing into the subpolar gyre along the eastern boundary and rising from depths near 1200m in the rockall trough to less than 600m to cross the wyville thomson ridge into the faroe shetland channel and thence to the nordic seas the deep source hypothesis focus is on lower thermocline source waters beneath the sill depth for the nordic seas inflow in contrast to the shallow source hypothesis focus of transformation of upper thermocline waters above the sill depth on the basis of regional water mass distributions geostrophic shear and direct current measurements we reject the deep source hypothesis in favor of the shallow source hypothesis rather than a flow of deep mediterranean overflow water along the eastern boundary rising from depth to feed the nordic seas the inflow is supplied directly by transformed north atlantic current waters from the same depth range as the inflow mediterranean overflow water is a constituent of the inflow but only through its contribution to defining the temperature salinity relationship of the interior of the subtropical gyre from which the north atlantic current draws its water rather than by direct northward advection of mediterranean outflow water via the eastern boundary undercurrent crossing from the subtropics into the subpolar domain instead the eastern boundary undercurrent wholly expels its transport of mediterranean overflow water into the eastern edge of the subtropical gyre creating the mid depth westward extending subtropical salinity plume for the lower thermocline isopycnal range of the core of the mediterranean overflow water measurements show that at the wyville thomson ridge the flow is southward and descending rather than northward and rising representing cold spillover from the faroese channels entraining warm upper ocean waters as the spillover plume descends and warms into the northern rockall trough no evidence is found in rockall trough northward of 54 n porcupine bank for direct advection of mediterranean overflow waters via an eastern boundary undercurrent extension and the very dilute mediterranean overflow waters in the trough reflect the interaction of the wyville thomson ridge plume descending into the trough from the north and lower thermocline waters delivered to the trough from the south

circulation mixing and production of antarctic bottom water abstract the antarctic source of bottom water to the abyssal layer of the world ocean is examined as well as its large scale flow pattern and ultimate entrainment rate into the deep water above we make use of the available high quality station data in the southern ocean to construct bottom maps of neutral density and mean property maps including chlorofluorocarbon cfc for the abyssal layer underneath a selected neutral density surface the maximum density at the sill depth of drake passage is used to distinguish between the voluminous deep water mass that is a continuous component of the antarctic circumpolar current from the relatively denser bottom water originated along the antarctic continental margins based on water density antarctic bottom water aabw is defined here generically to include all volumes of non circumpolar water of antarctic origin over the shelf regime multiple localized sources of specific aabw types contribute to the abyssal layer of the adjacent antarctic basins characteristics of these dense bottom waters reflect closely those observed on the parent shelf water mass spreading paths of newly formed deep and bottom waters over the slope regime and their subsequent oceanic circulation patterns are analyzed on the basis of global property maps for the aabw layer interior mixing and interbasin exchanges of aabw are deduced from mean characteristic curves following the southern streamline of the antarctic circumpolar current outflow and mixing of aabw from the weddell sea to the argentine basin is depicted using density and cfc distributions of two zonal hydrographic lines recirculation and mixing of deep and bottom waters within the weddell gyre are also detailed using a meridional section along the greenwich meridian the strength of all localized sources of aabw combined is estimated by two independent approaches an estimate of the total production rate of aabw is calculated based on the oceanic cfc budget for the aabw layer offshore of the 2500 m isobath the sum of all downslope inputs of well ventilated bottom water types underneath the top isopycnal must account for the measured cfc content in the bottom layer the resulting total aabw production rate is about 8 sv which is a conservative figure that neglects the loss of cfc bearing waters across the top isopycnal in recent years whereas about 9 5 sv is calculated assuming a well mixed bottom layer making use of their transient nature cfc distributions at the top of the aabw layer indicate that more direct and rapid entrainment of cfc rich bottom waters below occurs over localized areas with relatively strong upwelling rates and enhanced vertical mixing a second more ad hoc but independent oceanic mass budget of the bottom layer is also constructed a typical basin wide rate of deep upwelling of 3 10 7 m s 1 requires 10 sv 1 sv 106 m3 s 1 of newly formed aabw to sink down the slope around antarctica we have also formulated a spatial distribution of deep upwelling on the isopycnal at the top of the aabw it is expressed as a combination of wind topographic and turbulent components which in turn are functions of the isopycnal depth bottom depth and bottom layer thickness this non uniform upwelling field yields about 12 sv of aabw exported across the top isopycnal fortuitously the overall average of upward speed at the top isopycnal 3 7 10 7 m s 1 compares well with previous estimates of deep upwelling in the northern basins a series of likely sites for strong vertical entrainment of aabw are clearly identified in the modeled distribution of deep upwelling consonant with the observed cfc distributions on the top isopycnal altogether regions with relatively high upwelling rates 5 10 7 m s 1 occupy only a quarter of the total areal extent of the top isopycnal but they account for as much as 45 of the total vertical transport

the hafnium neodymium isotopic composition of atlantic seawater the first full water column hafnium isotopic compositions of atlantic seawater have been obtained at seven locations from the labrador sea to the drake passage despite subpicomolar concentrations in seawater a precision of the hf isotopic measurements of 03 epsilon hf units was achieved an overall epsilon hf range between 3 1 in the labrador sea and 4 4 in antarctic bottom water was determined the distribution of which broadly reflects continental weathering inputs within particular water column profiles significant differences of up to 4 epsilon hf units occur combined with nd isotope data of the same samples it is evident that the hf isotopic composition of seawater is too radiogenic for a given nd isotopic composition and that the largest difference between expected and measured hf isotopic compositions in seawater occurs near the oldest continental crust in the labrador sea this corroborates the previous proposition which was mainly based on ferromanganese crust data that the hf isotopic composition of seawater is controlled by incongruent weathering of continental crust and possibly to some extent by hydrothermal contributions hafnium concentrations in the ocean do not increase along the deep ocean conveyer indicating an oceanic residence time of only a few hundred years which is significantly shorter than previously assumed the hf isotopic composition of past seawater can therefore serve as a proxy for short distance basin scale mixing processes and the regime and intensity of nearby continental weathering processes c 2009 elsevier b v all rights reserved

accuracy of stable mg and ca isotope data obtained by mc icp ms using the standard addition method the standard addition method is evaluated to verify the accuracy and precision of mg and ca isotope data with complex matrices using the standard sample bracketing technique and analysis by mc icp ms the 44ca 42ca ratio of seawater expressed as 44 42ca relative to srm915a was determined as 0 93 0 03 95 confidence in agreement with estimates obtained by the double spike method using standard addition the seawater 26mg 24mg ratio expressed as 26mg relative to the dsm3 standard was determined as 0 80 0 06 95 confidence in agreement with previous estimates four terrestrial silicate rocks morb flood basalt glacial flour and granodiorite and olivine mineral separates from an island basalt are shown to exhibit no scatter within the error of the method averaging a 26mg of 0 20 0 05 95 confidence although a number of silicate rock data for mg isotope ratios have already been reported this is the first detailed effort to validate the accuracy of such data and test for residual analytical artifact after chemical purification of samples data regressions were evaluated statistically using the mean square weighted deviate mswd demonstrating that the uncertainty on individual data points are generally over estimated the external two standard deviation uncertainty on individual data points is estimated by monte carlo simulation as b0 075 about a factor of two improvement on early publications of mg isotope data the consistency of the standard addition estimates of 26mg in silicate rocks imply that if any residual matrix effects are present then they must be less than the spread of the data 0 11 given the diverse range of matrices in each of the samples the 26mg values of the silicate rocks suggest that mg isotope ratios in silicate material may only have a very restricted range the 26mg values of silicate material in the present study falls between the average values reported by teng et al teng f z wadhwa m helz r t 2007 investigation of magnesium isotope fractionation during basalt differentiation implications for a chondritic composition of the terrestrial mantle earth and planetary science letters 261 84 92 doi 10 1016 j epsl 2007 06 004 and wiechert and halliday wiechert u halliday a n 2006 non chondritic magnesium and the origins of the inner terrestrial planets earth and planetary science letters 256 360 371 doi 10 1016 j epsl 2007 01 007 and given the spread of published 26mg values for chondritic material a chondritic composition for terrestrial mg cannot be ruled out we suggest that some of the small discrepancies between our data and analysis of the same samples in earlier studies may have arisen because the chemical purification of mg prior to analysis can easily induce analytical artifact this method could be expanded to the isotope ratios of other elements which also rely on correcting for mass bias using the standard sample bracketing method where similar analytical discrepancies may also exist

gravitational symmetric and baroclinic instability of the ocean mixed layer abstract a hierarchy of hydrodynamical instabilities controlling the transfer of buoyancy through the oceanic mixed layer is reviewed if a resting ocean of horizontally uniform stratification is subject to spatially uniform buoyancy loss at the sea surface then gravitational instability ensues in which buoyancy is drawn from depth by upright convection but if spatial inhomogeneities in the ambient stratification or the forcing are present as always exist in nature then horizontal density gradients will be induced and within a rotation period horizontal currents in thermal wind balance with those gradients will be set up within the mixed layer there are two important consequences on the convective process upright convection will become modified by the presence of the thermal wind shear fluid parcels are exchanged not along vertical paths but rather along slanting paths in symmetric instability theoretical considerations suggest that this slantwise convection sets the potential vorticity of th

relevance of silicon isotopes to si nutrient utilization and si source assessment in antarctic waters 1 we analyzed 29si of dissolved silicate for eight water column profiles across the southern ocean south of australia in spring 2001 from the seasonal ice zone siz north to the subantarctic zone saz including the first isotopic compositions measured for si depleted seawaters all profiles display mixed layer enrichments in heavy si isotopes relative to deep water in accordance with preferential uptake of the light isotope by diatoms as silicate levels decrease from the siz northward across the polar front zone pfz to the saz surface and mesopelagic 29si signatures generally become progressively heavier but the most si depleted saz waters do not exhibit 29si values heavier than in the pfz this intricacy appears to derive from variations in the vertical and horizontal supply of silicate to surface waters and by applying a steady state open system model we estimate a fractionation factor 29 between diatoms and seawater of 0 45 0 17 independently of zones and phytoplankton community though encouraging these results are related to latitudinal changes in mesopelagic 29si values complexity in surface silicate 29si correlations and differences from previous studies which underline the need for caution in the use of silicon isotopes in paleoceanographic studies until systematic efforts have been undertaken to better understand modern variations

silicon isotope fractionation during nutrient utilization in the north pacific the distribution of silicon in the north pacific is controlled by the utilization of silicic acid by diatoms a process that fractionates silicon si isotopic compositions silicon isotope variations are presented for six water column profiles from the surface mixed layer down to the deep waters of the north pacific ocean although the observed si isotopic variations display an apparently simple inverse relationship with dissolved nutrient concentrations in fact they reflect mixing of surface waters undergoing active si isotope fractionation and deep waters with more uniform concentrations and isotope compositions samples from the surface of the subtropical gyre have the lowest dissolved si concentrations and heaviest si isotope compositions of marine waters measured thus far fractionation in the surface waters follows a typical rayleigh type distillation curve for a closed surface water reservoir resulting from stratification of the surface layer in the subarctic region in contrast an open system prevails within the subtropical gyre where there is significant recycling of silicic acid in the upper water column and lateral transport of silicon within surface currents for deep waters the si isotope composition distinguishes between the northern north pacific deep water npdw and the southerly derived bottom water the relatively light si isotope compositions measured from waters within the subarctic gyre provides evidence for isolation of the nutrient pool in the north pacific

southern ocean control of silicon stable isotope distribution in the deep atlantic ocean the fractionation of silicon si stable isotopes by biological activity in the surface ocean makes the stable isotope composition of silicon delta si 30 dissolved in seawater a sensitive tracer of the oceanic biogeochemical si cycle we present a high precision dataset that characterizes the delta si 30 distribution in the deep atlantic ocean from denmark strait to drake passage documenting strong meridional and smaller but resolvable vertical delta si 30 gradients we show that these gradients are related to the two sources of deep and bottom waters in the atlantic ocean waters of north atlantic and nordic origin carry a high delta 30 sisignature of 1 7 parts per thousand into the deep atlantic while antarctic bottom water transports si with a low delta si 30 value of around 1 2 parts per thousand the deep atlantic delta si 30 distribution is thus governed by the quasi conservative mixing of si from these two isotopically distinct sources this disparity in si isotope composition between the north atlantic and southern ocean is in marked contrast to the homogeneity of the stable nitrogen isotope composition of deep ocean nitrate delta n 15 no3 we infer that the meridional delta si 30 gradient derives from the transport of the high delta si 30 signature of southern ocean intermediate mode waters into the north atlantic by the upper return path of the meridional overturning circulation moc the basin scale deep atlantic delta si 30 gradient thus owes its existence to the interaction of the physical circulation with biological nutrient uptake at high southern latitudes which fractionates si isotopes between the abyssal and intermediate mode waters formed in the southern ocean

the δ15n of nitrate in the southern ocean nitrogen cycling and circulation in the ocean interior we report analyses of the nitrogen isotopic composition of nitrate in the eastern indian and pacific sectors of the southern ocean in this paper we focus on the subsurface data as well as data from the deep waters of other ocean basins nitrate 15n is relatively invariant in much of the abyssal ocean i e below 2 5 km with a value of 4 8 0 2 observed in lower circumpolar deep water north atlantic deep water and central pacific deep water the isotopic invariance of deep ocean nitrate stems fundamentally from the completeness of nitrate utilization in most of the global surface ocean the southern ocean surface being an important exception in the subantarctic zone north of the polar frontal zone the nitrate 15n of upper circumpolar deep water is 0 7 greater than that of lower circumpolar deep water this isotopic enrichment appears to result from denitrification in the low latitude water masses with which upper circumpolar deep water communicates the isotopic enrichment of upper circumpolar deep water is diminished in the antarctic probably because of the remineralization of sinking organic n which has a low 15n in the antarctic relative to the other water masses of the southern ocean the subantarctic thermocline has a very low nitrate 15n for its nitrate concentration because of exchange with the low latitude thermocline where this isotopic signature appears to originate this signature of the low latitude thermocline has two probable causes 1 mixing with low nitrate surface water and 2 the oxidation of newly fixed n

factors controlling the silicon isotope distribution in waters and surface sediments of the peruvian coastal upwelling we present the first systematic study of the silicon isotope composition in the water column 30sisi oh 4 and in diatoms 30sidiatom from the underlying surface sediments in a coastal upwelling region the surface waters upwelling on the shelf off peru are mainly fed by southward flowing subsurface waters along the coast which show a mean 30sisi oh 4 of 1 5 the concentration of dissolved silicic acid si oh 4 increases towards the south in these waters and with increasing water depth suggesting lateral mixing with water masses from the south and intense remineralisation of particulate biogenic silica bsio2 in the water column and in the surface sediments surface waters in the realm of the most intense upwelling between 5 s and 15 s have only marginally elevated 30sisi oh 4 values 30sisi oh 4 1 7 with respect to the source si isotope composition whereas further north and south where upwelling is less pronounced surface waters are more strongly fractionated 30sisi oh 4 up to 2 8 due to the stronger utilisation of the smaller amounts of available si oh 4 the degree of si oh 4 utilisation in the surface waters along the shelf estimated from the si oh 4 concentration data ranges from 51 to 93 the 30sidiatom values of hand picked diatoms in the underlying surface sediments vary from 0 6 to 2 0 which is within the range of the expected fractionation between surface waters and diatoms the fractionation signal in the surface waters produced during formation of the diatoms is reflected by the 30sidiatom values in the underlying sediments with the lowest 30sidiatom values in the main upwelling region the silicon isotope compositions of bsio2 30sibsio2 from the same surface sediment samples are generally much lower than the 30sidiatom signatures indicating a significant contamination of the bsio2 with biogenic siliceous material other than diatoms such as sponge spicules this shift towards lighter 30sibsio2 values by up to 1 3 compared to 30sidiatom signatures for the same surface sediment samples potentially biases the interpretation of 30si paleorecords from sediments with low bsio2 concentrations and thus the reconstruction of past si oh 4 utilisation in surface waters

global ocean meridional overturning abstract a decade mean global ocean circulation is estimated using inverse techniques incorporating air sea fluxes of heat and freshwater recent hydrographic sections and direct current measurements this information is used to determine mass heat freshwater and other chemical transports and to constrain boundary currents and dense overflows the 18 boxes defined by these sections are divided into 45 isopycnal neutral density layers diapycnal transfers within the boxes are allowed representing advective fluxes and mixing processes air sea fluxes at the surface produce transfers between outcropping layers the model obtains a global overturning circulation consistent with the various observations revealing two global scale meridional circulation cells an upper cell with sinking in the arctic and subarctic regions and upwelling in the southern ocean and a lower cell with sinking around the antarctic continent and abyssal upwelling mainly below the crests of the major bathymetric ridges

southern ocean thermocline ventilation abstract an approximate mass volume budget in the surface layer of the southern ocean is used to investigate the intensity and regional variability of the ventilation process discussed here in terms of subduction and upwelling ventilation resulting from ekman pumping is estimated from satellite winds the geostrophic mean component is assessed from a climatology strengthened with argo data and the eddy induced advection is included via the parameterization of gent and mcwilliams together with eddy mixing estimates all three components contribute significantly to ventilation finally the seasonal cycle of the upper ocean is resolved using argo data the circumpolar averaged circulation shows an upwelling in the antarctic intermediate water aaiw density classes which is carried north into a zone of dense subantarctic mode water samw subduction although no consistent net production is found in the light samw density classes a large subduction of subtropical mode water stmw is observed the s

the production of north atlantic deep water sources rates and pathways updating an earlier account by dickson et al 1990 this paper reviews the initial development phase of north atlantic deep water nadw production from the points where the dense inflows from nordic seas cross the greenland scotland ridge to the point off south greenland where the buildup of new production appears almost complete in particular three long term current meter arrays totaling 91 instruments and set at 160 km intervals south from the denmark strait sill are used to validate earlier short term arrays by others and in combination with these earlier arrays to describe the downstream evolution of mean speed depth and entrainment the variability of the overflow current in space and time and the likely contribution of the other three main constituents of nadw production at densities greater than 27 8 from the points of overflow 5 6 sv the transport within this range increases by entrainment and confluence with other contributory streams to around 13 3 sv at cape farewell while recirculating elements prevent us from determining the net southgoing transport a nadw transport of this order appears consistent with recent estimates of net abyssal flow passing south through the north and south atlantic

simulating the modern δ30si distribution in the oceans and in marine sediments the 30si of biogenic silica inline image in marine sediments is a promising proxy for the reconstruction of silicic acid utilization by diatoms in the geological past the application of this proxy however requires an understanding of the modern 30si distributions and their controlling mechanisms here we present results from a modern climate simulation with a coupled ocean sediment model that includes a prognostic formulation of biogenic silica production with concurrent silicon isotopic fractionation in agreement with previous studies biological fractionation combined with physical transport and mixing determines the oceanic distribution of simulated 30si a new finding is a distinct seasonal cycle of 30si in the surface ocean which is inversely related to that of silicic acid concentration and mixed layer depth we also provide the first simulation results of sedimentary 30si which reveal that 1 the inline image distribution in the surface sediment reflects the exported inline image signal from the euphotic zone and 2 the dissolution of biogenic silica in the sediment acts as a source of relatively light 30si into the bottom waters of the polar oceans while it is a source of heavier 30si to the subtropical south atlantic and south pacific

evidence of silicic acid leakage to the tropical atlantic via antarctic intermediate water during marine isotope stage 4 antarctic intermediate water aaiw and subantarctic mode water samw are the main conduits for the supply of dissolved silicon silicic acid from the deep southern ocean to the low latitude surface ocean and therefore have an important control on low latitude diatom productivity enhanced supply of silicic acid by aaiw and samw during glacial periods may have enabled tropical diatoms to outcompete carbonate producing phytoplankton decreasing the relative export of inorganic to organic carbon to the deep ocean and lowering atmospheric pco2 this mechanism is known as the silicic acid leakage hypothesis salh here we present records of neodymium and silicon isotopes from the western tropical atlantic that provide the first direct evidence of increased silicic acid leakage from the southern ocean to the tropical atlantic within aaiw during glacial marine isotope stage mis 4 60 70 ka this leakage was approximately coeval with enhanced diatom export in the nw atlantic and across the eastern equatorial atlantic and provides support for the salh as a contributor to co2 drawdown during full glacial development

north atlantic nordic seas exchanges abstract the northeastern part of the north atlantic is unique in the sense that it is much warmer in the surface than other ocean areas at similar latitudes the main reason for this is the large northward transport of heat that extends to high latitudes and crosses the greenland scotland ridge to enter the nordic seas and the arctic there the warm atlantic water is converted to colder water masses that return southwards over the ridge partly as surface outflows and partly as overflows through the deep passages across the ridge in this paper the state of knowledge on the exchanges especially across the eastern part of the greenland scotland ridge is reviewed based on results from the ices nansen north atlantic norwegian sea exchanges project from the nordic woce project and from other sources the accumulated evidence allows us to describe the exchanges in fair detail the origins of the waters the patterns of their flow towards and over the ridge and their ultimate fate there is also increasing information on temporal variations of the exchanges although dynamical changes are still not well understood quantitative estimates for the volume transport of most of the overflow branches seem reasonably well established and transport measurements of the atlantic inflows to the nordic seas are approaching acceptable levels of confidence which allows preliminary budgets to be presented the deep overflows are driven by pressure gradients set up by the formation of deep and intermediate water the dominance of deep overflows over surface outflows in the water budget argues that this thermohaline forcing also dominates over direct wind stress and estuarine forcing in driving the atlantic water inflow across the greenland scotland ridge while wind stress seems to influence the characteristics and distribution of the atlantic water north of the ridge

the continental si cycle and its impact on the ocean si isotope budget abstract the silicon isotope composition of biogenic silica 30sibsi in the ocean is a function of the 30si of the available dissolved si dsi h2sio4 the degree of utilisation of the available dsi and for some organisms the concentration of dsi this makes 30sibsi in sediment archives a promising proxy for past dsi concentrations and utilisation at steady state mean 30sibsi must equal a weighted average of the inputs the majority of which are of continental origin variation in the functioning of the continental si cycle on timescales similar to the residence time of dsi in the ocean 10 ka may therefore contribute to downcore variability in 30sibsi on millennial or longer timescales the direction and magnitude of change in published 30sibsi records over the last few glacial cycles is consistent among ocean basins and between groups of silicifiers they document glacial values that are typically 0 5 to 1 0 lower than interglacial values and together hint at coherent and predictable glacial interglacial variability in whole ocean 30si driven by a change in mean 30si of the inputs in this contribution we review the modern inputs of dsi to the ocean and the controls on their isotopic composition and assess the evidence for their variability on millennial plus timescales today 9 55 1012 mol yr 1 dsi enters the ocean of which roughly 64 and 25 are direct riverine inputs of dsi and dsi from dissolution of aeolian and riverborne sediment respectively the remainder derives from alteration or weathering of the ocean crust each input has a characteristic 30si with our current best estimate for a weighted mean being 0 74 although much work remains to be done to characterise the individual fluxes many aspects of the continental si cycle may have differed during glacial periods that together can cumulatively substantially lower the mean 30si of dsi entering the ocean these changes relate to i a cooler drier glacial climate ii lowered sea level and the exposure of continental shelves iii the presence of large continental ice sheets and iv altered vegetation zonation using a simple box model with a monte carlo approach to parameterisation we find that a transition from a hypothesised glacial continental si cycle to the modern si cycle can drive an increase in whole ocean 30si of comparable rate and magnitude to that recorded in 30sibsi this implies that we may need to revisit our understanding of aspects of the si cycle in the glacial ocean although we focus on the transition from the last glacial our synthesis suggests that the continental si cycle should be seen as a potential contributory factor to any variability observed in ocean 30sibsi on millennial or longer timescales

observed sources and variability of nordic seas overflow the overflows from the nordic seas maintain the deep branch of the north atlantic circulation that is an important part of the global climate system an analysis of observed ocean temperatures and salinities between 1950 and 2005 shows that the atlantic water circulating in the nordic seas is the main source for change in the overflow waters

a benthic si mass balance on the congo margin origin of the 4000 m dsi anomaly and implications for the transfer of si from land to ocean to elucidate the origin of the silicic acid dsi anomaly observed along the 4000 isobath on the congo margin we have established a benthic si mass balance and performed direct measurements of biogenic silica bsio2 dissolution in the deep waters and in the sediments results strongly suggest that the anomaly originates from the sediments the intensity of dsi recycling is consistent with the degradation of organic matter as observed from si o2 ratios in the benthic fluxes compared to that ratio observed in the anomalies strong imbalances observed in both the si and c mass balances suggest that the biogenic matter that degrades and dissolves in these sediments near 4000 m does not come from pelagic sedimentation it is probably not coming also from the deep channel because observations were similar in the deep channel vicinity site d and further south far from its influence site c the composition of the sediments with an si c ratio close to that observed on continental shelves suggests that this matter is coming from downslope transport a first estimate of the magnitude of this flux at global scale close to 12 t mol si yr 1 suggests that it may be an important path for transferring si from land to ocean

a first look at the distribution of the stable isotopes of silicon in natural waters the first data on the distribution of the stable isotopes of silicon in marine and freshwater systems are reported both marine and riverine 30si are more positive than 30si of igneous rocks suggesting isotopic fractionation during weathering and clay formation and or biomineralization the 30si value for dissolved silicic acid from several ocean basins is 1 1 0 3 n 69 more positive values are found in surface waters due to discrimination against 30si during biogenic opal formation by diatoms the deep pacific ocean is depleted in 30si relative to the deep atlantic by roughly 0 4 30si of riverine dissolved silicon averages 0 8 0 3 n 8 the biologic fractionation of silicon isotopes coupled with effects associated with thermohaline circulation appears to play a significant role in controlling the distribution of silicon isotopes in the sea the average 30si value of inputs of silicon to the ocean most likely falls between 0 3 and 0 9 and that of the outputs falls between 0 9 to 1 9 probably in the neighborhood of 1 0

large scale vertical and horizontal circulation in the north atlantic ocean abstract observations of large scale hydrography air sea forcing and regional circulation from numerous studies are combined by inverse methods to determine the basin scale circulation average diapycnal mixing and adjustments to air sea forcing of the north atlantic ocean dense overflows through the denmark strait and faroe bank channels are explicitly included and are associated with strong vertical and lateral circulation and mixing these processes in the far northern atlantic play a fundamental role in the meridional overturning circulation for the entire ocean accompanied by an upper cell of mode water and intermediate water circulation the two cells converge roughly at the mean depth of the midocean ridge crest the labrador sea water layer lies within this convergence south of the overflow region model derived mean diapycnal diffusivities are o 10 5 m2 s 1 or smaller at the base of the thermocline and diapycnal advection is driven primarily by air sea transformation on outcropping layers

silica cycling and isotopic composition in northern marguerite bay on the rapidly warming western antarctic peninsula the southern ocean is a key region for silica si cycling and the isotopic signatures established here influence the rest of the world s oceans the climate and ecosystem of the southern ocean are changing rapidly with the potential to impact si cycling and isotope dynamics this study examines high resolution time series dataset of dissolved si concentrations and isotopic signatures particulate si concentrations and diatom speciation at a coastal site on the western antarctic peninsula wap in order to characterise changes in si cycling with respect to changes occurring in productivity and diatom assemblages dissolved and particulate si phases reflect the dominant control of biological uptake and combined with isotopic fractionation were consistent with a season of low intermediate productivity biogenic si is tightly coupled to both chlorophyll and particulate organic carbon at the sampling site consistent with diatom dominated phytoplankton assemblages along the wap variability in diatom speciation has a negligible impact on the isotopic signature of dissolved si in surface waters although this is unlikely to hold for sediments due to differential dissolution of diatom species a continued decline in diatom productivity along the wap would likely result in an increasing unused si inventory which can potentially feed back into si limited areas promoting diatom growth and carbon drawdown further afield

silicic acid leakage from the southern ocean a possible explanation for glacial atmospheric pco2 1 using a simple box model we investigate the effects of a reduced si n uptake ratio by antarctic phytoplankton on the marine silica cycle and atmospheric pco2 recent incubation experiments demonstrate such a phenomenon in diatoms when iron is added hutchins and bruland 1998 takeda 1998 franck et al 2000 the southern ocean may have supported diatoms with reduced si n uptake ratios compared to today during the dustier glacial times petit et al 1999 a similar reduction in the uptake ratio may be realized with an increased production of nondiatom phytoplankton such as phaeocystis our model shows that reduced si n export ratios in the southern ocean create excess silicic acid which may then be leaked out to lower latitudes any significant consumption of the excess silicic acid by diatoms that leads to an enhancement in their growth at the expense of coccolithophorids diminishes caco3 production and therefore diminishes the carbonate pump in our box model the combination of a reduced carbonate pump and an open system carbonate compensation draw down steady state atmospheric co2 from the interglacial 277 to 230 242 ppm depending on where the excess silicic acid is consumed by comparison the atmospheric pco2 sensitivity of general circulation models to carbonate pump forcing is 3 5 fold greater which combined with carbonate compensation can account for peak glacial atmospheric pco2 we discuss the importance of the initial rain ratio of caco3 to organic carbon on atmospheric pco2 and relevant sedimentary records that support and constrain this silicic acid leakage scenario

on the intermediate and deep water flows in the south atlantic ocean a multiparameter analysis is applied on zonal and meridional hydrographic sections obtained for the south atlantic ventilation experiment save to determine the spreading and mixing of water masses in the south atlantic ocean focusing our interest on the large scale flow of intermediate and deep waters the method utilizes all information from the hydrographic data set including temperature salinity dissolved oxygen and nutrient fields mixing proportions are quantified and plotted along the eight sections considered results show no evidence of a primary route of antarctic intermediate water along the western boundary of the south atlantic in the eastern basin the eastward extension of the upper north atlantic deep water nadw in the guinea basin following the cyclonic subequatorial gyre is confirmed in the angola basin a weak but thick nadw core layer is observed in conjunction with very little presence of lower circumpolar deep water lcdw high lcdw concentrations in cape basin are indicative of the communication of this basin to cold water sources in the south the method is sensitive enough to detect for instance the presence of the congo river plume in the angola basin or the influence of the weddell sea deep water in the vicinity of the romanche and chain fracture zones in the equatorial region in conjunction with the multiparameter analyses along save sections an analysis of components of the residual vector r indicates a middepth minimum in the rn p utilization ratio both a suitable explanation for the minimum and the potential consequences for the multiparameter analyses of south atlantic water mass circulation are still to be found

using silicon isotopes to understand the role of the southern ocean in modern and ancient biogeochemistry and climate abstract the growth of siliceous phytoplankton mainly diatoms in the southern ocean influences the preformed nutrient inventory in the ocean on a global scale silicic acid use by diatoms and deep circulation combine to trap dissolved si in the southern ocean resulting in high levels of silica production and expansive diatom oozes in southern ocean sediments the analysis of the silicon isotope composition of biogenic silica or opal and dissolved silicic acid provide insight into the operation of the global marine silicon cycle and the role played by the southern ocean in nutrient supply and carbon drawdown both in the modern and in the past silicon isotope studies of diatoms have provided insight into the history of silica production in surface waters while the analysis of spicules from deep sea sponges has defined both the spatial and the temporal variability of silicic acid concentrations in the water column together these and other proxies reveal variations in the northward flow of southern ocean intermediate and mode waters and how changes in southern ocean productivity altered their preformed nutrient content we present a new hypothesis the silicic acid ventilation hypothesis savh to explain the geographical variation of opal based proxy records in particular the contrasting patterns of opal burial change found in the low and high latitudes by understanding the silicon isotope systematics of opal and silicic acid in the modern we will be able to use opal based proxies to reconstruct past changes in the southern ocean and so investigate its role in global carbon cycling and climate

barium isotopic fractionation in seawater mediated by barite cycling and oceanic circulation the marine biogeochemical cycle of ba is thought to be controlled by particulate baso4 barite precipitation associated with the microbial oxidation of organic carbon and its subsequent dissolution in the baso4 undersaturated water column despite many of these processes being largely unique to ba cycling concentrations of ba and si in seawater exhibit a strong linear correlation the reasons for this correlation are ambiguous as are the depth ranges corresponding to the most active baso4 cycling and the intermediate sources of ba to particulate baso4 stable isotopic analyses of dissolved ba in seawater should help address these issues as ba isotopic compositions are predicted to be sensitive to the physical and biogeochemical process that cycle ba we report a new methodology for the determination of dissolved ba isotopic compositions in seawater and results from a 4500 m depth profile in the south atlantic at 39 99 s 0 92 e that exhibit oceanographically consistent variation with depth these data reveal that water masses obtain their ba and ba isotopic signatures when at or near the surface which relates to the cycling of marine baso4 the shallow origin of these signatures requires that the substantial ba isotopic variations in the bathypelagic zone were inherited from when those deep waters were last ventilated indeed the water column below 600 m is well explained by conservative mixing of water masses with distinct ba and ba isotopic compositions this leads us to conclude that large scale oceanic circulation is important for sustaining the similar oceanographic distributions of ba and si in the south atlantic and possibly elsewhere these data demonstrate that the processes of organic carbon oxidation baso4 cycling and ba isotopic fractionation in seawater are closely coupled such that ba isotopic analyses harbor great potential as a tracer of the carbon cycle in the modern and paleo oceans

scale selection of baroclinic instability effects of stratification and nongeostrophy abstract a simple two dimensional linear stability analysis is presented to study the characteristics of the scale selection and structure of baroclinic cyclogenesis effects of a realistic vertical thermal structure of the environment and the related nongeostrophic wind are investigated in detail both short mesoscale waves trapped in the lower layer with a small static stability and long waves extending throughout the column are unstable which supports blumen s results the integral constraints imply that the vertical structure of the stratification can provide an internal lid to confine the wave into the mixed layer nongeostrophic effects become important as richardson number decreases as pointed out by stone the scale of the most unstable eady s mode becomes sensitive to the change of shear unlike the geostrophic case the measure of the horizontal scale of the disturbance is the modified rossby radius of deformationwhere is the vertical shear and provides o ri correction to the conventio

destruction of potential vorticity by winds the destruction of potential vorticity pv at ocean fronts by wind stress driven frictional forces is examined using pv flux formalism and numerical simulations when a front is forced by downfront winds that is winds blowing in the direction of the frontal jet a nonadvective frictional pv flux that is upward at the sea surface is induced the flux extracts pv out of the ocean leading to the formation of a boundary layer thicker than the ekman layer with nearly zero pv and nonzero stratification the pv reduction is not only active in the ekman layer but is transmitted through the boundary layer via secondary circulations that exchange low pv from the ekman layer with high pv from the pycnocline extraction of pv from the pycnocline by the secondary circulations results in an upward advective pv flux at the base of the boundary layer that scales with the surface nonadvective frictional pv flux and that leads to the deepening of the layer at fronts forced by both downfront winds and a destabilizing atmospheric buoyancy flux f b the critical parameter that determines whether the wind or the buoyancy flux is the dominant cause for pv

sediment efflux of silicon on the greenland margin and implications for the marine silicon cycle abstract the polar region is experiencing one of the most rapid environmental changes driven by atmospheric warming and feedbacks within the cryosphere under such a setting it is crucial to understand the biogeochemical cycling of the nutrient silicon si in the high latitudes which is regulating the nutrient supply to polar ecosystems and is linked to the global carbon cycle via diatom production however these efforts have been hindered by a lack of understanding of the benthic si cycle particularly the quantification of the sediment efflux of si and identification of the responsible mechanistic processes during early diagenesis here we address these issues using new pore water profiles and incubation experiments on sediment cores collected from the greenland margin and labrador sea combined with si isotope analysis and a mass balance model benthic si flux at our study sites is found to be greatly heightened from values sustained by pore water molecular diffusion the remainder of the flux is likely accountable with early dissolution of reactive biogenic silica phases at the upper sediments and advective transport of pore waters our results highlight an active benthic si cycle at a northern high latitude continental margin which could play a key role in recycling significant amounts of biologically available dissolved si to the overlying water and influencing the growth of benthic and planktonic communities in the polar region

evidence of deep and bottom water formation in the western weddell sea abstract during ice station polarstern ispol r v polarstern cruise ant xxii 2 november 2004 january 2005 hydrographic and tracer observations were obtained in the western weddell sea while drifting closely in front of the larsen ice shelf these observations indicate recently formed weddell sea bottom water which contains significant contributions of glacial melt water in its upper part and high salinity shelf water in its lower layer the formation of this bottom water cannot be related to the known sources in the south the filchner ronne ice shelf we show that this bottom water is formed in the western weddell sea most likely in interaction with the larsen c ice shelf by applying an optimum multiparameter analysis omp using temperature salinity and noble gas observations helium isotopes and neon we obtained mean glacial melt water fractions of about 0 1 in the bottom water on sections across the weddell gyre farther north melt water fractions are still on the order of 0 04 using chlorofluorocarbons cfcs as age tracers we deduced a mean transit time between the western source and the bottom water found on the slope toward the north 9 3 years this transit time is larger and the inferred transport rate is small in comparison to previous findings but accounting for a loss of the initially formed bottom water volume due to mixing and renewal of weddell sea deep water a formation rate of 1 1 0 5 sv in the western weddell sea is plausible this implies a basal melt rate of 35 19 gt year or 0 35 0 19 m year at the larsen ice shelf this bottom water is shallow enough that it could leave the weddell basin through the gaps in the south scotia ridge to supply antarctic bottom water these findings emphasize the role of the western weddell sea in deep and bottom water formation particularly in view of changing environmental conditions due to climate variability which might induce enhanced melting or even decay of ice shelves

geochemistry and mineralogy of a silica chimney from an inactive seafloor hydrothermal field east pacific rise 18 s an inactive vent field comprised of dead chimneys was discovered on the ultrafast east pacific rise epr at 18 degrees s during the research campaign naudur with the r v le nadir in december 1993 one of these chimneys was sampled studied and found to be largely composed of silica mineralized bacterial like filaments the filaments are inferred to be the result of microbial activity leading to silica fe oxyhydroxide precipitation the chimney grew from the most external layer precipitated 226 4 yr b p towards the central chimney conduit hydrothermal activity ceased 154 13 yr b p and the chimney conduit was completely sealed mixing between an end member hydrothermal fluid and seawater explains the sr nd isotopic composition of the chimney seawater was the major source of sr to the chimney whereas the dominant nd source was the local mid ocean ridge basalt morb leached by the hydrothermal fluids the mixing scenarios point to a dynamic hydrothermal system with fluctuating fluid compositions the proportion of seawater within the venting fluid responsible for the precipitation of the silica chimney layers varied between 94 and 85 pb isotope data indicates that all of the pb in the chimney was derived from the underlying morb the precipitation temperatures of the chimney layers varied between 55 and 71 degrees c and were a function of the seawater end member hydrothermal fluid mixing ratio delta si 30 correlates with the temperature of precipitation implying that temperature is one of the major controls of the si isotope composition of the chimney concentrations of elements across the chimney wall were a function of this mixing ratio and the composition of the end member hydrothermal fluid the inward growth of the chimney wall and accompanying decrease in wall permeability resulted in an inward decrease in the seawater hydrothermal fluid mixing ratio which in turn exerted a control on the concentrations of the elements supplied mainly by the hydrothermal fluids the silica chimney is significantly enriched in u likely a result of bacterial concentration of u from the seawater dominated vent fluid the chimney is poor in rare earth elements ree it inherited its ree distribution patterns from the parent end member hydrothermal fluids the dilution of the hydrothermal fluid with over 85 seawater could not obliterate the particular ree features positive eu anomaly of the hydrothermal fluids

barium stable isotopes in the global ocean tracer of ba inputs and utilization abstract barium has been used as a biogeochemical tracer for alkalinity productivity and riverine inputs in the ocean but its oceanic cycle remains poorly constrained barium stable isotope measurements may improve the use of ba as a tracer and better constrain the cycling of ba but data are only available in limited regions of the oceans in this study we present dissolved seawater ba isotopic compositions in a sample collection spanning the north atlantic south atlantic north pacific and southern oceans compiled global upper ocean ba data show a relatively constant ba 35 45 nm in the near surface waters throughout the global ocean with the exception of areas near river inputs or strong upwelling the relatively uniform distribution of ba in the upper ocean seawater indicates that ba removal is slow relative to supply and mixing and implies that near surface ba isotope values are controlled by basin scale balances rather than by regional or short term processes seawater ba isotopic compositions show a large variation of 138 134 ba values ranging from 0 24 to 0 65 and a tight relationship with ba this global relationship can be simply modelled assuming a primary deep southern ocean source for ba to yield a maximum isotope fractionation of 1 00058 0 00010 ba seawater 138 134 ba particle 138 134 this suggested isotope fractionation during ba removal from seawater is larger than implied by laboratory measurement during barite formation suggesting additional fractionating phases or a two stage fractionation process riverine input from the rio de la plata to the south atlantic has a signature of 138 134 ba 0 06 0 11 which is too light to explain the heavy values 0 58 observed in the surface open ocean globally the ba isotope composition of the upper ocean waters is correlated with the fraction of ba utilization at the basin scale which varies from 138 134 ba signals in the northern sourced 0 45 and the southern sourced water 0 25 trace mixing and allow identification of non conservative behaviour of ba reflecting additional inputs or sinks of ba during transport most likely addition from sediment or hydrothermal ba isotopes may be useful to trace such inputs in the present and past ocean

stress induced grain boundary migration in very soluble brittle salt abstract grain boundary migration gbm was studied in situ at room temperature atmospheric pressure and an applied diffmfwerential stress of 9 5 mpa under the optical microscope in a wet aggregate of an elastic brittle salt sodium chlorate the aggregate was previously deformed predominantly by a combination of grain boundary sliding pressure solution and cataclastic solution creep after deformation but when the sample was still under differential stress undeformed fracture free grains were observed to grow at the cost of deformed intensely fractured grains gmb rates typically fell in the range 2 10 m day gbm took place only as long as the sample was under stress boundaries stopped to migrate as soon as stress was taken off and started to migrate again as soon as the sample was stressed again our observations suggest that gbm in this material and under the prevailing experimental conditions is a stress induced process i e driven by differences in elastic strain between fractured and unfractured grains

subpolar mode water in the northeastern atlantic 2 origin and transformation 1 the processes that lead to the transformation and origin of the eastern north atlantic subpolar mode waters spmw are investigated from observational data using an extended walin framework air sea flux data from the national oceanography center southampton nocs and hydrographic data from the a24 cruise collected during the world ocean circulation experiment woce are used to estimate the contribution of diapycnal and isopycnal fluxes to the density classes that include spmw surface diapycnal volume flux is the dominant source of waters in the spmw density in the north atlantic subpolar gyre the diapycnal volume flux occurs along the main branches of the north atlantic current nac and it has an average transport of 14 6 5 sv with a maximum of 21 5 sv across the 27 35sq isopycnal the regional distribution of the diapycnal flux on isopycnal surfaces is computed to identify the areas with the largest diapycnal flux these regions coincide with the location of spmw based on potential vorticity the surface diapycnal flux is associated with obduction and subduction through the permanent pycnocline therefore the water involved in the transformation of spmws is continuously exchanged with the ocean interior in addition we suggest that subduction is not associated with smooth advection from the mixed layer to the ocean interior but is water mass loss entrainment into the deep overflows of the subpolar gyre the isopycnal component of the spmw throughput is estimated from the geostrophic transport across the a24 section from greenland to scotland and is 10 to 40 of the diapycnal flux

pervasive sources of isotopically light zinc in the north atlantic ocean abstract in this study we report seawater dissolved zinc zn concentration and isotope composition 66 zn from the geotraces ga01 geovide section in the north atlantic across the transect three subsets of samples stand out due to their isotopically light signature those close to the reykjanes ridge those close to the sediments and those pervasively in the upper ocean similar to observations at other locations the hydrothermal vent of the reykjanes ridge is responsible for the isotopically light zn composition of the surrounding waters with an estimated source 66 zn of 0 42 this isotopically light zn is then transported over a distance greater than 1000 km from the vent sedimentary inputs are also evident all across the trans atlantic section highlighting a much more pervasive process than previously thought these inputs of isotopically light zn ranging from 0 51 to 0 01 may be caused by diffusion out of zn rich pore waters or by dissolution of sedimentary particles the upper north atlantic is dominated by low 66 zn a feature that has been observed in all zn isotope datasets north of the southern ocean using macronutrient to zn ratios to better understand modifications of preformed signatures exported from the southern ocean we suggest that low upper ocean 66 zn results from addition of isotopically light zn to the upper ocean and not necessarily from removal of heavy zn through scavenging though the precise source of this isotopically light upper ocean zn is not fully resolved it seems possible that it is anthropogenic in origin this view of the controls on upper ocean zn is fundamentally different from those put forward previously

open ocean submesoscale motions a full seasonal cycle of mixed layer instabilities from gliders abstractthe importance of submesoscale instabilities particularly mixed layer baroclinic instability and symmetric instability on upper ocean mixing and energetics is well documented in regions of strong persistent fronts such as the kuroshio and the gulf stream less attention has been devoted to studying submesoscale flows in the open ocean far from long term mean geostrophic fronts characteristic of a large proportion of the global ocean this study presents a year long submesoscale resolving time series of near surface buoyancy gradients potential vorticity and instability characteristics collected by ocean gliders that provides insight into open ocean submesoscale dynamics over a full annual cycle the gliders continuously sampled a 225 km2 region in the subtropical northeast atlantic measuring temperature salinity and pressure along 292 short 20 km hydrographic sections glider observations show a seasonal cycle in near surface stratification throughout the fall september november

deconvolving the controls on the deep ocean s silicon stable isotope distribution abstract we trace the marine biogeochemical silicon si cycle using the stable isotope composition of si dissolved in seawater expressed as 30 si open ocean 30 si observations indicate a surprisingly strong influence of the physical circulation on the large scale marine si distribution here we present an ocean general circulation model simulation that deconvolves the physical and biogeochemical controls on the 30 si distribution in the deep oceanic interior by parsing dissolved si into its preformed and regenerated components we separate the influence of deep water formation and circulation from the effects of biogeochemical cycling related to opal dissolution at depth we show that the systematic meridional 30 si gradient observed in the deep atlantic ocean is primarily determined by the preformed component of si whose distribution in the interior is controlled solely by the circulation we also demonstrate that the 30 si value of the regenerated component of si in the global deep ocean is dominantly set by oceanic regions where opal export fluxes to the deep ocean are large i e primarily in the southern ocean s opal belt the global importance of this regionally dynamic si cycling helps explain the observed strong physical control on the oceanic 30 si distribution since most of the regenerated si present within the deep atlantic and indo pacific oceans is in fact transported into these basins by deep waters flowing northward from the southern ocean our results thus provide a mechanistic explanation for the observed 30 si distribution that emphasizes the dominant importance of the southern ocean in the marine si cycle

coupling of the distribution of silicon isotopes to the meridional overturning circulation of the north atlantic ocean abstract the distribution of silicon isotopes within silicic acid 30 si oh 4 was examined along a section in the north atlantic from the cape verde islands off africa to cape cod massachusetts in north america surface water displayed elevated 30 si oh 4 associated with biological fractionation of si during silica production below 300 m variations in 30 si oh 4 were closely tied to the distribution of water masses as diagnosed through optimum multiparameter analysis confirming a tight relationship between 30 si oh 4 and the meridional overturning circulation in the atlantic a linear relationship between 30 si oh 4 and the inverse of silicic acid concentration supported control of si isotope distribution by conservative mixing of end member water masses of different isotopic composition in the atlantic there was a suggestion of a weak local minimum in 30 si oh 4 in deep waters above the trans atlantic geotraverse hydrothermal zone on the mid atlantic ridge consistent with the light 30 si oh 4 of hydrothermal waters the lightest 30 si oh 4 values were observed in the deep western and deep eastern basins where antarctic bottom water aabw dominated the heaviest values in subsurface waters occurred in north atlantic deep water due to strong ventilation and the contribution of heavy northern source waters that are influenced by the arctic ocean the concept of a silicon isotope bipole is introduced to explain how the isotopic differences between the northern and southern end member water masses arise and how they influence si isotope distributions northern end member water masses are heavy due to the influence of the arctic ocean bottom topography prevents light deep waters from entering the arctic and the further removal of light isotopes through local biological productivity results in extremely heavy 30 si oh 4 within the arctic light aabw dominates the southern end member the southern ocean silicic acid trap distills heavier isotopes of si out of the southern ocean as preformed silicic acid within antarctic mode waters while retaining light isotopes that become incorporated into aabw the influence of the silicon isotope bipole is predicted to be strongest in the atlantic as the net flow of waters through the arctic ocean is from the pacific to the atlantic

mixed layer instabilities and restratification abstract the restratification of the oceanic surface mixed layer that results from lateral gradients in the surface density field is studied the lateral gradients are shown to be unstable to ageostrophic baroclinic instabilities and slump from the horizontal to the vertical these instabilities which are referred to as mixed layer instabilities mlis differ from instabilities in the ocean interior because of the weak surface stratification spatial scales are o 1 10 km and growth time scales are on the order of a day linear stability analysis and fully nonlinear simulations are used to study mlis and their impact on mixed layer restratification the main result is that mlis are a leading order process in the ml heat budget acting to constantly restratify the surface ocean climate and regional ocean models do not resolve the scales associated with mlis and are likely to underestimate the rate of ml restratification and consequently suffer from a bias in sea surface temperatures and ml depths in a

a switch from si oh 4 to no 3 depletion in the glacial southern ocean 1 phytoplankton in the antarctic deplete silicic acid si oh 4 to a far greater extent than they do nitrate no3 this pattern can be reversed by the addition of iron which dramatically lowers diatom si oh 4 no3 uptake ratios higher iron supply during glacial times would thus drive the antarctic towards no3 depletion with excess si oh 4 remaining in surface waters new 30si and 15n records from antarctic sediments confirm diminished si oh 4 use and enhanced no3 depletion during the last three glaciations the present low si oh 4 water is transported northward to at least the subtropics we postulate that the glacial high si oh 4 water similarly may have been transported to the subtropics and beyond this input of si oh 4 may have caused diatoms to displace coccolithophores at low latitudes weakening the carbonate pump and increasing the depth of organic matter remineralization these effects may have lowered glacial atmospheric pco2 by as much as 60 ppm

the silicon isotope composition of ethmodiscus rex laminated diatom mats from the tropical west pacific implications for silicate cycling during the last glacial maximum the cause of massive blooms of ethmodiscus rex laminated diatom mats ldms in the eastern philippine sea eps during the last glacial maximum lgm remains uncertain in order to better understand the mechanism of formation of e rex ldms from the perspective of dissolved silicon dsi utilization we determined the silicon isotopic composition of single e rex diatom frustules 30sie rex from two sediment cores in the parece vela basin of the eps in the study cores 30sie rex varies from 1 23 to 0 83 average 1 04 a range that is atypical of marine diatom 30si and that corresponds to the lower limit of reported diatom 30si values of any age a binary mixing model upwelled silicon versus eolian silicon accounting for silicon isotopic fractionation during dsi uptake by diatoms was constructed the binary mixing model demonstrates that e rex dominantly utilized dsi from eolian sources i e asian dust with only minor contributions from upwelled seawater sources i e advected from subantarctic mode water antarctic intermediate water or north pacific intermediate water e rex utilized only 24 of available dsi indicating that surface waters of the eps were eutrophic with respect to silicon during the lgm our results suggest that giant diatoms did not always use a buoyancy strategy to obtain nutrients from the deep nutrient pool thus revising previously proposed models for the formation of e rex ldms

silicic acid leakage from the southern ocean opposing effects of nutrient uptake and oceanic circulation 1 studies invoking the silicic acid leakage hypothesis to explain events of high biogenic opal deposition at low latitudes did not consider variations in formation rate of southern ocean intermediate waters comparison of 30si records from the southern ocean to records of biogenic opal accumulation from low latitude upwelling systems indicates a strong negative correlation between the silicic acid content of southern ocean surface waters and opal deposition at low latitudes during the last 80 kyr bp this implies that si leakage was somehow rendered ineffective when the silicic acid content of southern ocean waters was high comparison of opal accumulation records with records of intermediate water formation rates suggests that lower rates of intermediate water formation offset the higher silicic acid content of these water masses during glacial periods this indicates that physical processes that control intermediate water transport dominate over biological processes that set the silicic acid content of antarctic surface waters in determining the magnitude of silicic acid leakage from the southern ocean

mesoscale to submesoscale transition in the california current system part i flow structure eddy flux and observational tests in computational simulations of an idealized subtropical eastern boundary upwelling current system similar to the california current a submesoscale transition occurs in the eddy variability as the horizontal grid scale is reduced to o 1 km this first paper in a series of three describes the transition in terms of the emergent flow structure and the associated time averaged eddy fluxes in addition to the mesoscale eddies that arise from a primary instability of the alongshore wind driven currents significant energy is transferred into submesoscale fronts and vortices in the upper ocean the submesoscale arises through surface frontogenesis growing off upwelled cold filaments that are pulled offshore and strained in between the mesoscale eddy centers in turn some submesoscale fronts become unstable and develop submesoscale meanders and fragment into roll up vortices associated with this phenomenon are a large vertical vorticity and rossby number a large vertical velocity relatively flat horizontal spectra contrary to the prevailing view of mesoscale dynamics a large vertical buoyancy flux acting to restratify the upper ocean a submesoscale energy conversion from potential to kinetic a significant spatial and temporal intermittency in the upper ocean and material exchanges between the surface boundary layer and pycnocline comparison with available observations indicates that submesoscale fronts and instabilities occur widely in the upper ocean with characteristics similar to the simulations

symmetric instability in the gulf stream abstract analyses of wintertime surveys of the gulf stream gs conducted as part of the clivar mode water dynamic experiment climode reveal that water with negative potential vorticity pv is commonly found within the surface boundary layer sbl of the current the lowest values of pv are found within the north wall of the gs on the isopycnal layer occupied by eighteen degree water suggesting that processes within the gs may contribute to the formation of this low pv water mass in spite of large heat loss the generation of negative pv was primarily attributable to cross front advection of dense water over light by ekman flow driven by winds with a down front component beneath a critical depth the sbl was stably stratified yet the pv remained negative due to the strong baroclinicity of the current suggesting that the flow was symmetrically unstable a large eddy simulation configured with forcing and flow parameters based on the observations confirms that the observed structure of the sbl is consistent with the dynamics of symmetric instability si forced by wind and surface cooling the simulation shows that both strong turbulence and vertical gradients in density momentum and tracers coexist in the sbl of symmetrically unstable fronts si is a shear instability that draws its energy from geostrophic flows a parameterization for the rate of kinetic energy ke extraction by si applied to the observations suggests that si could result in a net dissipation of 33 mw m 2 and 1 mw m 2 for surveys with strong and weak fronts respectively the surveys also showed signs of baroclinic instability bci in the sbl namely thermally direct vertical circulations that advect biomass and pv the vertical circulation was inferred using the omega equation and used to estimate the rate of release of available potential energy ape by bci the rate of ape release was found to be comparable in magnitude to the net dissipation associated with si this result points to an energy pathway where the gs s reservoir of ape is drained by bci converted to ke and then dissipated by si and its secondary instabilities similar dynamics are likely to be found at other strong fronts forced by winds and or cooling and could play an important role in the energy balance of the ocean circulation

isopycnal mixing in ocean circulation models abstract a subgrid scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non eddy resolving ocean circulation models the mixing is applied in isopycnal coordinates to isopycnal layer thickness or inverse density gradient as well as to passive scalars temperature and salinity the transformation of these mixing forms to physical coordinates is also presented

stable silicon isotope signatures of marine pore waters biogenic opal dissolution versus authigenic clay mineral formation dissolved silicon isotope compositions have been analysed for the first time in pore waters 30sipw of three short sediment cores from the peruvian margin upwelling region with distinctly different biogenic opal content in order to investigate silicon isotope fractionation behaviour during early diagenetic turnover of biogenic opal in marine sediments the 30sipw varies between 1 1 and 1 9 with the highest values occurring in the uppermost part close to the sediment water interface these values are of the same order or higher than the 30si of the biogenic opal extracted from the same sediments 0 3 to 1 2 and of the overlying bottom waters 1 1 to 1 5 together with dissolved silicic acid concentrations well below biogenic opal saturation our collective observations are consistent with the formation of authigenic alumino silicates from the dissolving biogenic opal using a numerical transport reaction model we find that approximately 24 of the dissolving biogenic opal is re precipitated in the sediments in the form of these authigenic phases at a relatively low precipitation rate of 56 mol si cm 2 yr 1 the fractionation factor between the precipitates and the pore waters is estimated at 2 0 dissolved and solid cation concentrations further indicate that off peru where biogenic opal concentrations in the sediments are high the availability of reactive terrigenous material is the limiting factor for the formation of authigenic alumino silicate phases

dissolved silicon isotopic compositions in the east china sea water mass mixing vs biological fractionation we present the first set of dissolved silicon isotope data in seawater delta si 30 si oh 4 from the east china sea a large and productive marginal sea significantly influenced by the kuroshio current and freshwater inputs from the changjiang yangtze river in summer august 2009 the lowest surface delta si 30 si oh 4 signatures of 2 1 parts per thousand corresponding to the highest si oh 4 concentrations similar to 30 0 mu mol l 1 were observed nearshore in changjiang diluted water during advection on the east china sea inner shelf surface delta si 30 si oh 4 increased rapidly to 3 2 parts per thousand while si oh 4 became depleted indicating increasing biological utilization of the si oh 4 originating from the changjiang diluted water this is also reflected in the water column profiles characterized by a general decrease of delta si 30 si oh 4 and an increase of si oh 4 with depth on the east china sea mid shelf and slope in winter december 2009 january 2010 however the delta si 30 si oh 4 was nearly constant at 1 9 parts per thousand throughout the water column on the east china sea shelf beyond the nearshore which was a consequence of enhanced vertical mixing of the kuroshio subsurface water horizontal admixture of kuroshio surface water which is highly fractionated in si isotopes was observed only beyond the shelf break significant seasonal differences in delta si 30 si oh 4 were detected in the surface waters beyond the changjiang diluted water influenced region on the east china sea shelf where the winter values were similar to 1 0 parts per thousand lower than those in summer despite the same primary si oh 4 supply from the kuroshio subsurface water during both seasons this demonstrates significantly higher biological consumption and utilization of si oh 4 in summer than in winter

biogeochemical implications from dissolved rare earth element and nd isotope distributions in the gulf of alaska dissolved rare earth element ree concentrations and nd isotope compositions were measured for surface waters and full water column profiles of the gulf of alaska goa and compared to water mass properties and circulation in order to better understand the mechanisms controlling the input and transport of rees in the ocean the rees display a typical open ocean range of concentrations i e la 12 66 pm lu 0 2 2 5 pm and depth distributions i e surface ocean depletion and enrichment with water depth nd isotope signatures are highly radiogenic as expected for the north pacific margin ranging from 3 8 to 0 2 epsilon nd the most radiogenic values were found in the coastal waters but also in the cores of eddies indicating efficient export of rees from the margins and across the mixed layer this is the first time that distinct nd isotope distributions in near surface waters can be directly assigned to offshore eddy transport a distinct mid depth similar to 2200 m nd isotope signal was found that most likely reflects advection of a water mass that formed through past down welling in the northern pacific subsurface nd isotope compositions appear to behave conservatively and can be explained through a ree distribution model proposed here this model is based on multivariate analysis of the rees and invokes two distinct pools of dissolved rees a passive pool complexed by carbonate ions and a bio reactive pool that is microbially manipulated the latter pool is only significant in the upper water column and most likely reflects the indirect effects of microbial cycling of iron our model of the open ocean ree distribution contributes to explaining the conservative nature of nd isotopes and provides a mechanism linking surface ocean and pore water ree dynamics

radiocarbon based circulation age of the world oceans 1 the distributions of deep ocean 14c data are often used to illustrate the rate of deep ocean circulation the associated conventional 14c ages show a difference of 1000 years between the north atlantic and the southern ocean and a difference of another 1000 years between the southern ocean and the north pacific these differences may be interpreted directly and mistakenly as the timescale of circulation the characterization of the deep ocean circulation being millennial is common using objectively gridded natural 14c data of key et al 2004 i recast 14c in terms of circulation by accounting for 1 long surface ocean reservoir 14c ages and 2 two sources of deep water formed in the north atlantic and around antarctica the new distribution of circulation 14c ages is more consistent with the deep ocean being characterized by a centennial timescale than a millennial timescale also the role of the southern sourced deep water is now made more obvious by accounting for the two important controls on oceanic 14c that are not well known outside the field of chemical oceanography the new map will be useful as an illustration of global deep ocean circulation to the wider scientific community and as a pedagogical tool to new students in earth sciences and oceanography

radiocarbon decay and oxygen utilization in the deep atlantic ocean a parameter based on the sum of the concentrations of po4 and o2 divided by the redfield coefficient o2 po4 is used to separate the contributions of the northern and southern components to deep waters in the atlantic this separation allows the amount of radiocarbon lost by radiodecay and the amount of oxygen lost to respiration during residence in the deep atlantic to be calculated maps of these quantities reveal strong west to east gradients and weak north to south gradients consistent with ventilation along the western boundary from both ends of the ocean coupled with mixing outward from the boundary the o2 and 14c deficiences are highly correlated suggesting an o2 utilization rate of 12 m kg per century the apparent mean isolation time of water in the deep atlantic is about 200 years

differences between mono generic and mixed diatom silicon isotope compositions trace present and past nutrient utilisation off peru in this study we combine for the first time silicon si isotope compositions of small mixed diatom species 30sibsio2 and of large handpicked mono generic i e genus coscinodiscus diatom samples 30sicoscino with diatom assemblages extracted from marine sediments in the peruvian upwelling region in order to constrain present and past silicate utilisation the extension of a previous core top data set from the peruvian shelf demonstrates that 30sicoscino values record near complete si utilisation as these are similar to the isotopic composition of the subsurface source waters feeding the upwelling in contrast the 30sibsio2 of small mixed diatom species increase southward along the shelf as well as towards the shore we attribute highest 30sibsio2 values partly to transient iron limitation but primarily to the gradual increase of si isotope fractionation within the seasonal diatom succession which are mainly recorded by small diatom species during intense bloom events in contrast lower 30sibsio2 values are related to initial si isotope utilisation during periods of weak upwelling when low si oh 4 concentrations do not permit intense blooms and small diatom species record substantially lower 30si signatures as such we propose that the intensity of the upwelling can be deduced from the offset between 30sibsio2 and 30sicoscino 30sicoscino bsio2 which is low for strong upwelling conditions and high for prevailing weak upwelling we apply the information extracted from surface sediments to generate a record of the present day main upwelling region covering the past 17 700 years and find that this location has also been characterized by a persistent offset 30sicoscino bsio2 by comparison with the diatom assemblages we show that the coastal upwelling system changed markedly between weak and strong upwelling conditions in addition our model calculations to quantify species specific si isotope fractionation effects based on the diatom assemblages indicate an overall minor influence that cannot explain the high amplitude in the measured 30sibsio2 record

biological fractionation of silicon isotopes in southern ocean surface waters 1 the fractionation of silicon isotopes by diatoms during silicification i e opaline cell wall formation provides a new tool for paleoceanographic studies of the silicon cycle here we examine the natural variations of the 30si 28si ratio in silicic acid si oh 4 and biogenic silica bsio2 in surface waters of the antarctic circumpolar current acc along 170 w the results provide direct evidence of biologically mediated fractionation of silicon isotopes with an enrichment factor of between 1 1 and 1 9 depending on the model employed comparison of the mass flux of bsio2 captured in sediment traps deployed in the study area with the silicon isotopic composition of that material establishes a direct linkage between diatom dynamics in surface waters and the isotopic signature of exported particles we calculated si oh 4 no3 utilization ratios from variations in silicon and nitrogen isotopic ratios in surface waters and sediment traps that agree well with direct observations of 4 1 in this region this work supports the use of variations in silicon isotopes in sedimentary opal as a proxy for relative si oh 4 utilization in surface waters

sources and biological fractionation of silicon isotopes in the eastern equatorial pacific abstract silicon isotopes in dissolved silicic acid were measured in the upper four kilometers between 4 n and 3 s latitude at 110 w longitude in the eastern equatorial pacific silicon isotopes became progressively heavier with silicic acid depletion of surface water as expected from biological fractionation the value of e estimated by applying a steady state isotope fractionation model to data from all stations between 4 n and 3 s was 0 77 0 12 std err when the analysis was restricted to those stations whose temperature and salinity profiles indicated that they were directly influenced by upwelling of the equatorial undercurrent euc the resulting value of e was 1 08 0 27 std err similar to the value established in culture studies 1 1 when the non steady state rayleigh model was applied to the same restricted data set the resulting value of e was significantly more positive 0 61 0 16 std err to the extent that the equatorial system approximates a steady state these results support a value of 1 1 for the fractionation factor for isotopes of si in the sea without the assumption of steady state the value of e can only be constrained to be between 0 6 and 1 1 silicic acid in equatorial pacific deep water below 2000 m had a near constant 30 si of 1 32 0 05 that value is significantly more positive than obtained for north pacific deep water at similar depths at stations to the northwest of our study area 0 9 1 0 and it is slightly less positive than new measures of the 30 si of silicic acid from the silicic acid plume centered over the cascadia basin in the northeast pacific si oh 4 180 m 30 si 1 46 0 12 sd n 4 we show that the data from the equator and cascadia basin fit a general trend of increasing 30 si oh 4 with increasing silicic acid concentration in the deep sea but that the isotope values from the northeast pacific are anomalously light the observed level of variation in the silicon isotope composition of deep waters from this single ocean basin is considerably larger than that predicted by current models based on fractionation during opal formation with no isotope effect during dissolution confirmation of such high variability in deep water 30 si oh 4 within individual ocean basins will require reassessment of the mechanisms controlling the distribution of isotopes of silicon in the sea

the subpolar mode water of the north atlantic ocean abstract the warm waters of the subtropical and subpolar basins of the north atlantic have tight regional temperature salinity relationships and are conventionally called the regional central waters a volumetric census of the temperature salinity characteristics of the north atlantic by wright and worthington 1970 shows that waters characterized by certain segments of the t s relationships have large volumes compared to those of other segments volumetric mode waters such mode waters appear as layers with increased vertical separation between isopycnals pycnostads the present study reports on the existence of pycnostads in the central and eastern north atlantic these subpolar mode waters are formed by deep winter convection in the subpolar north atlantic and participate in the upper water circulation of the northern north atlantic the seasonal outcropping of the pycnostads occurs within and adjacent to the north atlantic current the irminger current the east and west greenland currents and

heavy silicon isotopic composition of silicic acid and biogenic silica in arctic waters over the beaufort shelf and the canada basin the silicon isotopic composition of silicic acid 30si oh 4 and biogenic silica 30si bsio2 were measured for the first time in marine arctic waters from the mackenzie river delta to the deep canada basin in the late summer of 2009 in the upper 100 m of the water column 30si oh 4 signals 1 82 to 3 08 were negatively correlated with the relative contribution of mackenzie river water the biogenic si isotope fractionation factor estimated using an open system model 30e 0 97 0 17 agrees well with laboratory and global ocean estimates nevertheless the 30si dynamics of this region may be better represented by closed system isotope models that yield lower values of 30e between 0 33 and 0 41 depending on how the contribution of sea ice diatoms is incorporated in the upper 400 m 30si bsio2 values were among the heaviest ever measured in marine suspended bsio2 2 03 to 3 51 a positive correlation between 30si bsio2 and sea ice cover implies that heavy signals can result from isotopically heavy sea ice diatoms introduced to pelagic assemblages below the surface bsio2 production zone the 30si oh 4 distribution followed that of major water masses vertical 30si oh 4 profiles showed a minimum average of 1 84 0 10 in the upper halocline 125 200 m composed of modified pacific water and heavier average values 2 04 0 11 in atlantic water 300 500 m deep in the canada basin deep water below 2000 m 30si oh 4 averaged 1 88 0 12 which represents the most positive value ever measured anywhere in the deep ocean since most si oh 4 enters the arctic from shallow depths in the atlantic ocean heavy deep arctic 30si oh 4 signals likely reflect the influx of relatively heavy intermediate atlantic waters a box model simulation of the global marine 30si oh 4 distribution successfully reproduced the observed patterns with the 30si oh 4 of the simulated deep arctic ocean being the heaviest of all deep ocean basins

the influence of water mass mixing on the dissolved si isotope composition in the eastern equatorial pacific silicon isotopes are a powerful tool to investigate the cycling of dissolved silicon si in this study the distribution of the si isotope composition of dissolved silicic acid 30si oh 4 was analyzed in the water column of the eastern equatorial pacific eep where one of the globally largest oxygen minimum zones omzs is located samples were collected at 7 stations along two meridional transects from the equator to 14 s at 85 50 w and 82 00 w off the ecuadorian and peruvian coast surface waters show a large range in isotope compositions 30si oh 4 2 2 to 4 4 with the highest values found at the southernmost station at 14 s this station also revealed the most depleted silicic acid concentrations 0 2 mol kg which is a function of the high degree of si utilization by diatoms and admixture with waters from highly productive areas samples within the upper water column and the omz at oxygen concentrations below 10 mol kg are characterized by a large range in 30si oh 4 which mainly reflects advection and mixing of different water masses even though the highly dynamic hydrographic system of the upwelling area off peru does not allow the identification of clear si isotope signals for distinct water masses therefore we cannot rule out that also dissolution processes have an influence on the 30si oh 4 signature in the subsurface water column deep water masses 2000 m in the study area show a mean 30si oh 4 of 1 2 0 2 which is in agreement with previous studies from the eastern and central pacific comparison of the new deep water data of this study and previously published data from the central pacific and southern ocean reveal substantially higher 30si oh 4 values than deep water signatures from the north pacific as there is no clear correlation between 30si oh 4 and silicic acid concentrations in the entire data set the distribution of 30si oh 4 signatures in deep waters of the pacific is considered to be mainly a consequence of the mixing of several end member water masses with distinct si isotope signatures including lower circumpolar deep water lcdw and north pacific deep water npdw

dissolved silicon and its isotopes in the water column of the bay of bengal internal cycling versus lateral transport abstract the concentration of dissolved si and its isotope composition are measured in the bay of bengal bob region of the northern indian ocean the isotope data are the first data set from the northern indian ocean the measurements are made in eight depth profiles closely along the 87 e transect gio1 section of the international geotraces program and in a few samples from the northern shelf of the bay dissolved si in the water column varies from 0 6 to 152 5 mol kg whereas the 30si data cover a range 1 2 to 3 6 the depth profiles of dissolved si show generally lower values in the surface increasing with depth whereas the pattern reverses in the case of 30si these vertical distribution patterns of si and 30si are similar to those reported in other oceanic regions and suggestive of the significant role of biological processes in governing si biogeochemistry in the upper layers top 1500 m in contrast dissolved si in near surface waters of the northern shelf and the southernmost station is exceptionally high these results indicate a continental supply of dissolved si from the ganga brahmaputra river system g b and submarine groundwater discharge sgd to the shelf region and an intrusion of high salinity waters from the arabian sea in the southern bay the 30si values of 1 34 0 10 for deep bottom waters of the bob depth 1500 m are similar to those reported for the deep southern ocean and indicate the dominant control of water mass mixing the dissolved si concentrations in the bottom waters of the bob are generally higher than those of the water mass endmembers which suggest the need for an additional source of si in situ particle dissolution and or benthic release in the central bay seem to be the potential candidate the annual si budget in the top 100 m of the bob seems to suggest that meso scale eddies frequently occurring during non monsoon periods can supply at the most 2 6 g si m2 year which is about 33 of the si requirement to support new production in the bay the supply of dissolved si 1 3 0 5 1011 mol year from the g b river system and sgd has been calculated based on the distributions of dissolved si concentration and 30si in the northern shelf waters a comparison of this supply with the reported si flux upstream of the estuarine zone indicates about 40 removal of dissolved si in the g b estuary the mass balance of si isotopes in the deep waters indicates that the dissolution of diatoms is the main cause of excess si in the bay

nutrient streams and their induction into the mixed layer 1 nutrients are transferred from the nutricline into the winter mixed layer through a combination of vertical and lateral advection referred to here as induction the reverse of the subduction process this advective supply of nutrients maintains the high productivity at high latitudes over timescales of several years and longer climatological diagnostics over the north atlantic reveal that the induction of nutrients into the winter mixed layer is dominated by lateral advection even in the subpolar gyre where there is significant ekman upwelling the induction flux of nutrients is itself sustained by nutrient streams strong nutrient transport associated with the western boundary currents in the north atlantic basin the integrated induction flux accounts for typically 40 of the nitrate flux in the nutrient stream at 36 n while the remaining fraction is recirculated in the thermocline of the subtropical gyre the relationship between the nutrient stream and induction flux is illustrated using a numerical model of the circulation and biogeochemistry of the north atlantic the model studies suggest that the nutrient stream preferentially supplies nutrients to isopycnals outcropping in the subpolar gyre the nutrient sources for the nutrient stream are either mode waters originating from the southern ocean or the tropics where the eastern upwelling circulation and particle fallout focuses nutrients onto the lighter isopycnal surfaces of the nutrient stream

a silicon depleted north atlantic since the palaeogene evidence from sponge and radiolarian silicon isotopes despite being one of earth s major geochemical cycles the evolution of the silicon cycle has received little attention and changes in oceanic dissolved silica dsi concentration through geologic time remain poorly constrained silicon isotope ratios expressed as 30si in marine microfossils are becoming increasingly recognised for their ability to provide insight into silicon cycling in particular the 30si of siliceous sponge spicules has been demonstrated to be a useful proxy for past dsi concentrations we analysed 30si in radiolarian tests and sponge spicules from the blake nose palaeoceanographic transect odp leg 171b spanning the palaeocene eocene ca 60 30 ma our 30si results range from 0 32 to 1 67 and 0 48 to 0 63 for the radiolarian and sponge records respectively using an established relationship between ambient dissolved si dsi concentrations and the magnitude of silicon isotope fractionation in siliceous sponges we demonstrate that the western north atlantic was dsi deplete during the palaeocene eocene throughout the water column a conclusion that is robust to a range of assumptions and uncertainties these data can constitute constraints on reconstructions of past ocean circulation previous work has suggested ocean dsi concentrations were higher than modern ocean concentrations prior to the cenozoic and has posited a drawdown during the early palaeogene due to the evolutionary expansion of diatoms our results challenge such an interpretation we suggest here that if such a global decrease in oceanic dsi concentrations occurred it must predate 60 ma

deep ocean biogeochemistry of silicic acid and nitrate 1 observations of silicic acid and nitrate along the lower branch of the global conveyor belt circulation show that silicic acid accumulation by diatom opal dissolution occurs at 6 4 times the rate of nitrate addition by organic matter remineralization the export of opal and organic matter from the surface ocean occurs at a si n mole ratio that is much smaller than this almost everywhere cf sarmiento et al 2004 the preferential increase of silicic acid over nitrate as the deep circulation progresses from the north atlantic to the north pacific is generally interpreted as requiring deep dissolution of opal together with shallow remineralization of organic matter broecker 1991 however sarmiento et al 2004 showed that the primary reason for the low silicic acid concentration of the upper ocean is that the waters feeding the main thermocline from the surface southern ocean are depleted in silicic acid relative to nitrate by implication the same southern ocean processes that deplete the silicic acid in the surface southern ocean must also be responsible for the enhanced silicic acid concentration of the deep ocean we use observations and results from an updated version of the adjoint model of schlitzer 2000 to confirm that this indeed the case

modeling the modern marine δ30si distribution 1 the silicon isotopic composition of opal sediments can be used as a proxy for past si utilization and hence paleoproductivity for a fixed ocean circulation interpretation of this si utilization proxy is limited by our understanding of the present marine si isotope distribution observations show significant isotopic variations is silicic acid between deep water masses that are not captured by a previous global climate model gcm here simple box models of the global oceans for the si cycle are presented which can account for the observed 0 3 o difference between atlantic and pacific deep water 30 si values using typical opal export fluxes the 30 si gradient results from both a high 30 si of dissolved si overturning in the north atlantic and the dissolution of low 30 si of opal in the deep waters around the southern ocean that feed into the pacific

baroclinic instability in an environment of small stability to slantwise moist convection part i two dimensional models abstract in the semigeostrophic system the growth rate of baroclinic waves varies with the inverse square root of the potential vorticity which acts as the effective static stability recent observations in the ascent regions of middle latitude cyclones show that the effective potential vorticity for saturated air is very near zero in this paper we examine the structure and rate of growth of baroclinic cyclones when the effective potential vorticity is small for upward saturated displacements but large in regions of descent analytic solutions for two dimensional disturbances in a two layer semigeostrophic model and numerical simulations using a multilevel semigeostrophic model show that when the effective potential vorticity is small in regions of upward motion growth rates are modestly increased and the region of ascent intensifies and collapses onto a thin ascending sheet in the limit of zero moist potential vorticity the fastest growing wave has a finite growth rate which is about 2 5 times the