new constraints on the active tectonic deformation of the aegean 1 site velocities from six separate global positioning system gps networks comprising 374 stations have been referred to a single common eurasia fixed reference frame to map the velocity distribution over the entire aegean we use the gps velocity field to identify deforming regions rigid elements and potential microplate boundaries and build upon previous work by others to initially specify rigid elements in central greece the south aegean anatolia and the sea of marmara we apply an iterative approach tentatively defining microplate boundaries determining best fit rigid rotations examining misfit patterns and revising the boundaries to achieve a better match between model and data short term seismic cycle effects are minor contaminants of the data that we remove when necessary to isolate the long term kinematics we find that present day aegean deformation is due to the relative motions of four microplates and straining in several isolated zones internal to them the rms misfit of model to data is about 2 sigma very good when compared to the typical match between coseismic fault models and gps data the simplicity of the microplate description of the deformation and its good fit to the gps data are surprising and were not anticipated by previous work which had suggested either many rigid elements or broad deforming zones that comprise much of the aegean region the isolated deforming zones are also unexpected and cannot be explained by the kinematics of the microplate motions strain rates within internally deforming zones are extensional and range from 30 to 50 nanostrain year nstrain year 10 9 year 1 to 2 orders of magnitude lower than rates observed across the major microplate boundaries lower strain rates may exist elsewhere within the microplates but are only resolved in anatolia where extension of 13 4 nstrain year is required by the data our results suggest that despite the detailed complexity of active continental deformation revealed by seismicity active faulting fault geomorphology and earthquake fault plane solutions continental tectonics at least in the aegean is to first order very similar to global plate tectonics and obeys the same simple kinematic rules although the widespread distribution of aegean seismicity and active faulting might suggest a rather spatially homogeneous seismic hazard the focusing of deformation near microplate boundaries implies the highest hazard is comparably localized

biases in thorpe scale estimates of turbulence dissipation part i assessments from large scale overturns in oceanographic data abstractoceanic density overturns are commonly used to parameterize the dissipation rate of turbulent kinetic energy this method assumes a linear scaling between the thorpe length scale lt and the ozmidov length scale lo historic evidence supporting lt lo has been shown for relatively weak shear driven turbulence of the thermocline however little support for the method exists in regions of turbulence driven by the convective collapse of topographically influenced overturns that are large by open ocean standards this study presents a direct comparison of lt and lo using vertical profiles of temperature and microstructure shear collected in the luzon strait a site characterized by topographically influenced overturns up to o 100 m in scale the comparison is also done for open ocean sites in the brazil basin and north atlantic where overturns are generally smaller and due to different processes a key result is that lt lo increases with overturn size in a fashion similar to that observed in numeric

mixing at camarinal sill in the strait of gibraltar during the gibraltar experiment in 1985 1986 observations of shear stratification microstructure and acoustic backscatter revealed a complex mixing regime at camarinal sill in the strait of gibraltar the mixing is forced by strong semidiurnal tidal currents and weaker but more steady baroclinic flows flows are characterized as outflow or inflow nearly equivalent to westward or eastward rising water at gibraltar coincides with outflow at all depths we observed two modes of outflow at the sill in may 1986 the transition layer separating atlantic and mediterranean water was between 0 75 mpa and 1 mpa when we sampled east of the sill near neap tide the transition turned down just before reaching the sill crest where the flow became critical increasing shear soon destabilized the transition producing billows which grew to 30 75 m on the west flank of the sill the overturning generated intense turbulence with peak dissipation rates e exceeding 10 2 w kg 1 this turbulence rapidly thickened the transition from z 30 50 m east of the sill to z 130 150 m within 1 2 km west of the sill from the rate of thickening we estimate the rate of formation of new transitional water during neap tide as 0 8 1 2 105 m3 s 1 this outflow mode did not release eastward propagating internal bores when outflow turned to inflow but the inflow did appear to arrest the deep outflow in may the largest average dissipation rate over and just west of the sill had the same magnitude as estimates of the net energy lost by adjustment of velocity and mass across the sill during outflow other averages are smaller than the energy loss presumably because the limited sampling severely underestimates the average dissipation rate during outflow strong shears were observed to rise to 100 times those in the background internal wave field however probably owing to spatial offsets between the ship s acoustic doppler current profiler and our tethered free fall profiler the only statistically significant correlations were found between overturning scales measured directly and those estimated from e in the same profile we observed the other outflow mode in october 1985 when we profiled over the sill near spring tide and the transition was east of the sill centered near 1 5 mpa after encountering the sill the transition rose sharply and remained within 10 20 m of the surface across the sill plunging steeply downward just past the west edge of the sill this was accompanied by intense turbulence and at least one rebound of the transition we did not observe release of eastward propagating bores when inflow turned to outflow but observations by armi and farmer 1988 in april 1986 found bores released at spring tide in regimes similar to the one we observed in october 1985

turbulence and entrainment in a buoyant surface plume in winter tidal outflow from the leschenault estuary forms a buoyant sheared stratified plume in koombana bay on each outgoing tide the turbulence in the plume and the local entrainment into the plume are examined based on measurements collected in an intensive field program coherency measurements between horizontally separated sensors are used to examine the anisotropy of the turbulence the effects of viscosity and buoyancy on the turbulence are studied using an activity diagram which is also an indicator of the mixing efficiency and total mixing the osborne cox model is tested and found to be valid in the plume

the contribution of salt fingers to vertical mixing in the north atlantic tracer release experiment abstract the north atlantic tracer release experiment natre was performed in an area moderately favorable to salt fingers however the classic finger signature of a distinct thermohaline staircase caused by upgradient density flux was absent this is likely because mixing by turbulence was sufficiently strong to disrupt the formation of permanent step and layer systems despite the lack of a staircase optical shadowgraph profiles revealed that small scale tilted laminae previously observed in a salt finger staircase were abundant at the natre site using microstructure observations the strength of salt finger mixing has been diagnosed using a nondimensional parameter related to the ratio of the diffusivities for heat and buoyancy the dissipation ratio by examining the dissipation ratio in a parameter space of density ratio r and richardson number ri the signal of salt fingers was discerned even under conditions where turbulent mixing also occurred while the model for turbulence descr

role of overturns in optimal mixing in stratified mixing layers turbulent mixing plays a major role in enabling the large scale ocean circulation the accuracy of mixing rates estimated from observations depends on our understanding of basic fluid mechanical processes underlying the nature of turbulence in a stratified fluid several of the key assumptions made in conventional mixing parameterizations have been increasingly scrutinized in recent years primarily on the basis of adequately high resolution numerical simulations we add to this evidence by compiling results from a suite of numerical simulations of the turbulence generated through stratified shear instability processes we study the inherently intermittent and time dependent nature of wave induced turbulent life cycles and more specifically the tight coupling between inherently anisotropic scales upon which small scale isotropic turbulence grows the anisotropic scales stir and stretch fluid filaments enhancing irreversible diffusive mixing at smaller scales we show that the characteristics of turbulent mixing depend on the relative time evolution of the ozmidov length scale compared to the so called thorpe overturning scale which represents the scale containing available potential energy upon which turbulence feeds and grows we find that when the mixing is most active and efficient since stirring by the largest overturns becomes optimal in the sense that it is not suppressed by ambient stratification we argue that the high mixing efficiency associated with this phase along with observations of in oceanic turbulent patches together point to the potential for systematically underestimating mixing in the ocean if the role of overturns is neglected this neglect arising through the assumption of a clear separation of scales between the background mean flow and small scale quasi isotropic turbulence leads to the exclusion of an highly efficient mixing phase from conventional parameterizations of the vertical transport of density such an exclusion may well be significant if the mechanism of shear induced turbulence is assumed to be representative of at least some turbulent events in the ocean while our results are based upon simulations of shear instability we show that they are potentially more generic by making direct comparisons with data from ocean and lake observations which represent a much wider range of turbulence inducing physical processes

three dimensional double ridge internal tide resonance in luzon strait abstractthe three dimensional 3d double ridge internal tide interference in the luzon strait in the south china sea is examined by comparing 3d and two dimensional 2d realistic simulations both the 3d simulations and observations indicate the presence of 3d first mode semi diurnal standing waves in the 3 6 km deep trench in the strait as in an earlier 2d study barotropic to baroclinic energy conversion flux divergence and dissipation are greatly enhanced when semidiurnal tides dominate relative to periods dominated by diurnal tides the resonance in the 3d simulation is several times stronger than in the 2d simulations for the central strait idealized experiments indicate that in addition to ridge height the resonance is only a function of separation distance and not of the along ridge length that is the enhanced resonance in 3d is not caused by 3d standing waves or basin modes instead the difference in resonance between the 2d and 3d simulations is attributed to the topographic blocking

large eddy simulation of flow and turbulence at the steep topography of luzon strait steep generation sites for topographic internal gravity waves can also be sites of turbulence the present work uses turbulence resolving large eddy simulation les for tidal flow over multiscale steep bathymetry patterned after a portion of the west ridge of luzon strait at 20 6 n oceanic values of the slope criticality froude number excursion number and aspect ratio are matched the amplitude and phasing of velocity overturns and dissipation are similar to observations made at a deep mooring however a tidal phase is noted in which there is a discrepancy that could arise from the resonant interaction with waves generated at the east ridge that is not included in the model turbulent dissipation originates from several mechanisms including breaking lee waves during flow reversal downslope jets critical slope boundary layer high mode internal wave beams off slope lee wave breaking and wave breaking in a valley between subridges the model baroclinic energy budget is closed with negligible residual and the local turbulence loss of 23 5 is large

unusual manganese enrichment in the mesoarchean mozaan group pongola supergroup south africa abstract an unusual sediment hosted manganese deposit is described from the mesoarchean mozaan group pongola supergroup south africa mno contents up to 15 wt were observed in marine clastic and chemical sedimentary rocks mn enrichment is interpreted to have resulted from the hydrothermal alteration of manganiferous shale and bif parent rocks the primary mno contents of which are as high as 8 5 wt a detailed mineralogical and petrographic study shows that these parent rocks are characterized by manganoan siderite ferroan rhodochrosite and other mn fe rich mineral phases such as kutnohorite and fe mn chlorite their hypogene alteration gave rise to a diversification of mineral assemblages where ferroan tephroite calcian rhodochrosite rhodochrosite pyrochroite pyrophanite cronstedtite manganoan fe rich chlorite and manganoan phlogopite partially or totally replaced the previous mineral assemblage thermodynamic modeling performed on chlorite phases associated with the described mineral assemblages illustrates a decrease of average crystallization temperatures from ca 310 c during early metamorphic stages to ca 250 c during a hydrothermal stage mineral transformation processes were thus related to retrograde metamorphism and or hydrothermal alteration post dating metamorphism and gave rise to progressive mn enrichment from unaltered parent to altered rocks the timing of hypogene alteration was constrained by 40 ar 39 ar dating to between about 1500 and 1100 ma ago reflecting tectonic processes associated with the namaqua natal orogeny along the southern kaapvaal craton margin manganiferous shale and bif of the mozaan group may represent the oldest known examples of primary sedimentary mn deposition related to oxidation of dissolved mn ii by free oxygen in a shallow marine environment oxygenic photosynthesis would have acted as a first order control during mn precipitation this hypothesis opens a new perspective for better constraining secular evolution of sediment hosted mineral deposits linked to oxygen levels in the atmosphere hydrosphere system during the archean eon

p and s deep velocity structure of the hellenic area obtained by robust nonlinear inversion of travel times we derived a new model for the p and s velocity structure of the lithosphere in the subduction area of the hellenic arc and the aegean sea from the inversion of travel times of local events the inversion technique applied is nonlinear since three dimensional ray tracing is incorporated at the same time an appropriate preconditioning of the final linearized system is used in order to reduce ray density effects on the results the p s coherency is controlled by an additional damping of the vp vs ratio the study focuses mainly on the structure of the hellenic subduction in the southern aegean region interesting features and details of the subducted slab can be recognized in the final tomographic images on the western part the shallower section of the slab is dipping at a very small angle 10 this changes to approximately 25 for the deeper part of the subduction resulting in a prominent kink at a depth of about 70 km which is in accordance with the general characteristics of the associated benioff zone the smaller eastern part of the slab is not resolved with the same detail but a clearly steeper subduction zone can be recognized moreover detailed information about the crustal thickness variations are inferred from the velocity structure and correlate very well with the existing results from refraction experiments finally the results confirm the recent suggestion concerning the existence of a low velocity crustal layer at shallow depths 10 15 km in the accretionary prism of the alpine belt

spatial variability of turbulent mixing in the abyssal ocean ocean microstructure data show that turbulent mixing in the deep brazil basin of the south atlantic ocean is weak at all depths above smooth abyssal plains and the south american continental rise the diapycnal diffusivity there was estimated to be less than or approximately equal to 0 1 x 10 4 meters squared per second in contrast mixing rates are large throughout the water column above the rough mid atlantic ridge and the diffusivity deduced for the bottom most 150 meters exceeds 5 x 10 4 meters squared per second such patterns in vertical mixing imply that abyssal circulations have complex spatial structures that are linked to the underlying bathymetry

estimates of the temperature flux temperature gradient relation above a sea floor the relation between the ux of temperature or buoyancy the verti cal temperature gradient and the height above the bottom is investigatedin an oceanographic context using high resolution temperature measure ments the model for the evolution of a strati ed layer by balmforthet al 1998 is reviewed and adapted to the case of a turbulent owabove a wall model predictions are compared to the average observa tional estimates of the ux exploiting a ux estimation method proposedby winters d asaro 1996 this estimation method enables the dis entanglement of the dependence of the average ux on the height abovethe bottom and on the background temperature gradient the classicaln shaped ux gradient relation is found in the observations model andobservations show similar qualitative behaviour despite the strong sim pli cations used in the model the results shed light on the modulationof the temperature ux by the presence of the boundary and support theidea of a turbulent ux following a mixing length argument in a strati edow furthermore the results support the use of thorpe scales close to aboundary if su cient averaging is performed suggesting that the thorpescales are a ected by the boundary in a similar way as the mixing length

turbulent mixing in a deep fracture zone on the mid atlantic ridge abstractmidocean ridge fracture zones channel bottom waters in the eastern brazil basin in regions of intensified deep mixing the mechanisms responsible for the deep turbulent mixing inside the numerous midocean fracture zones whether affected by the local or the nonlocal canyon topography are still subject to debate to discriminate those mechanisms and to discern the canyon mean flow two moorings sampled a deep canyon over and away from a sill contraction a 2 layer exchange flow accelerated at the sill transports 0 04 0 10 sv 1 sv 106 m3 s 1 up canyon in the deep layer at the sill the dissipation rate of turbulent kinetic energy e increases as measured from microstructure profilers and as inferred from a parameterization of vertical kinetic energy cross sill density and microstructure transects reveal an overflow potentially hydraulically controlled and modulated by fortnightly tides during spring to neap tides e varies from o 10 9 to o 10 10 w kg 1 below 3500 m around the 2 layer inte

prefrontal bore mixing rainbow ridge a 1 950 m deep upthrusted ultramafic block along the axis of the mid atlantic ridge has an active hydrothermal vent system at 2 400 m on its western slope however within one kilometer from the vent excessive temperatures are barely measurable probably due to strong turbulent mixing this mixing is studied here using a 400 m long high resolution temperature sensor array moored with a 600 m ranging 75 khz acoustic doppler current profiler rich internal wave turbulence was recorded characterized by 100 200 m upshoots and 200 m large overturning in particular near the end of the warming phase of the up and down moving tide these highly nonlinear internal waves of tides interacting with buoyancy frequency waves extend up to 400 m above the sloping bottom of the ridge whilst a turbulent bottom boundary layer could barely be defined the more intense turbulence higher up in the water column is suggested to lead to the strong dispersion of the hydrothermal plume

detailed observations of a naturally occurring shear instability simultaneous profiles of microstructure horizontal velocity and acoustic backscatter allow one of the most complete descriptions of a naturally occurring shear instability to date shear increased rapidly after passing through a lateral constriction which formed a hydraulic control a kilometer long set of 20 m tall billows grew on a middepth density interface where the richardson number fell below 0 25 the velocity interface thickened steadily after the billows formed consistent with rapid momentum mixing across a shear layer with a reynolds number of 3 106 the billows generated large density overturns and dissipation rates greater than 10 5 w kg 1 even within the first large overturn indicating that these structures were fully turbulent early in their development as the billows grew a well mixed layer developed at the interface and survived as an actively turbulent layer for up to 6 buoyancy periods 3 times longer than in laboratory studies at low reynolds number variations in the mean density of the billows lead us to infer that the vertical offset of the velocity and density interfaces varied with time where the billows first formed with data from the large overturns within the shear layer we find e n2 3 104 an average root mean square overturn scale lrms of 2 6 m and a buoyancy scale lb of 2 7 m despite having sampled the billows at varying stages of their evolution we find no indication that the ratio lrms lb is ever significantly different than 1 for this shear instability

late archean initiation of manganese metallogenesis its significance and environmental controls abstract manganese metallogenesis was initiated in a modest scale during the late archean time selectively in india and brazil sedimentary deposits occur in both greenstone belts and high grade granulite terranes in shallow water shelf regimes only and are often intimately associated with stromatolites the changes in the rates of continental crustal growth mantle heat flux and outgassing along with the evolution of tectonic regimes sedimentary environments the state of the biosphere and finally the compositions of interacting atmosphere hydrosphere system from the early to the late archean time have been traced as interdependent parameters and interpreted to explain the first appearance of sedimentary manganese deposits promoted by possibly localized oxygenated environments compared to the spectacular manganese event represented by the giant paleoproterozoic kalahari deposit south africa which is interpreted to have been formed as a consequence of the rapid increase in atmospheric oxygen level ca 2 5 2 4 ga ago the modest initiation of the late archean manganese depositions was triggered by localized photosynthetic oxygen supply forming basin margin oxygen oases as a prologue for the succeeding stratified ocean system the dissolved manganese in the basin water that was oxidized to form shelf type shallow water deposits is suspected by analogy with contemporary banded iron formations bifs but not proved to be of hydrothermal ancestry the initiation of concentrated manganese sedimentation during the late archean by itself provides an evidence for an increase in the oxygen content in the contemporary atmosphere hydrosphere system compared to that in the earlier period supporting the geochemical indications from paleoweathering profiles

some geochemical indicators for discrimination between diagenetic and hydrothermal metalliferous sediments abstract two types of metalliferous sediments are known hydrothermal associated with tectonic faults and diagenetic enriched in precipitates from seawater they are similar in their appearance and chemical composition only the first type is an indicator for hydrothermal discharge therefore in exploration criteria for differentiating between the two are needed four possible indicators have been investigated and are discussed a differences in p y correlation b differences in cr zr correlation c different distribution patterns of ree and d different as contents the as content is easy to determine and seems to be at the present time the most suitable criterion for determining the genesis of the metalliferous sediments

mixing associated with sills in a canyon on the midocean ridge flank to close the global overturning circulation the production and sinking of dense water at high latitudes must be balanced elsewhere by buoyancy gain and upward vertical motion hydrographic and microstructure observations from the brazil basin in the south atlantic ocean indicate that most of the abyssal mixing there takes place on the topographically rough flank of the midocean ridge in previous studies it has been suggested that the high level of abyssal mixing observed on the ridge flank is primarily caused by breaking internal waves forced by tidal currents here the results from a detailed analysis of velocity hydrographic and microstructure data from a ridge flank canyon are presented two year long current meter records indicate that within the canyon there is a significant along axial mean flow down the density gradient toward the ridge crest five hundred meters above the canyon floor the kinetic energy in the subinertial band exceeds that associated with the semidiurnal tides by approximately a factor of 2 the mean dissipation of kinetic energy inside the canyon exceeds that above the ridge flank topography by approximately a factor of 5 the largest dissipation values were observed downstream of a narrow 1000 m high sill that extends across the full width of the canyon along the entire canyon there is a strong association between the presence of sills and along axial density gradients while there is no similar association between the presence of depressions and density gradients together these observations suggest that sill related mixing contributes at least as much to the diapycnal buoyancy flux in the canyon as tidally forced internal wave breaking which is not expected to be associated preferentially with sills while only 15 of the interfacial area between antarctic bottom water and north atlantic deep water in the brazil basin lie inside canyons the available data suggest that approximately one half of the diapycnal buoyancy fluxes take place there in comparison the region above the ridge flank topography accounts for about one third of the buoyancy fluxes the apparent importance of sill related processes for mixing in ridge flank canyons is therefore of global significance especially considering that such canyons occur on average every 50 km along 2 3 of the global midocean ridge system and that sills partially block the canyon axes every few tens of kilometers

mixing in the romanche fracture zone the romanche fracture zone is a major gap in the mid atlantic ridge at the equator which is deep enough to allow significant eastward flows of antarctic bottom water from the brazil basin to the sierra leone and guinea abyssal plains while flowing through the romanche fracture zone bottom water properties are strongly modified due to intense vertical mixing the diapycnal mixing coefficient in the bottom water of the romanche fracture zone is estimated by using the finestructure of ctd profiles the microstructure of high resolution profiler data and by constructing a heat budget from current meter data the finestructure of density profiles is described using the thorpe scaleslt it is shown from microstructure data taken in the bottom water that the ozmidov scale lo is related to lt by the linear relationship lo 5 0 95lt similar to other studies which allows an estimate of the diapycnal mixing coefficient using the osborn relation the thorpe scale and the diapycnal mixing coefficient estimates show enhanced mixing downstream eastward of the main sill of the romanche fracture zone in this region a mean diapycnal mixing coefficient of about 1000 3 1024 m2 s21 is found for the bottom water estimates of cross isothermal mixing coefficient derived from the heat budgets constructed downstream of the current meter arrays deployed in the romanche fracture zone and the nearby chain fracture zone are in agreement with the finestructure estimates of the diapycnal mixing coefficient within the romanche fracture zone although the two fracture zones occupy only 0 4 of the area covered by the sierra leone and guinea abyssal plains the diffusive heat fluxes across the 1 4 8c isotherm in the romanche and chain fracture zones are half that found over the abyssal plains across the 1 88c isotherm emphasizing the role of these passages for bottom water property modifications

source process and tectonic implications of the 8 january 2006 mw 6 7 kythira earthquake southern greece abstract on 8 january 2006 at 11 34 gmt a strong m w 6 7 subcrustal earthquake occurred near kythira island in southern greece causing damage to the surrounding area while the shock was also felt in most of the eastern mediterranean the event and its aftershocks were recorded by a regional network of 31 stations equipped with three component broad band seismometers that cover most parts of the greek region initial locations of the earthquake sequence showed a diffuse pattern therefore catalog and cross correlation differential travel times were input to a shrinking grid search relocation algorithm in order to obtain accurate relative locations after this relocation most aftershocks form a tight cluster around the mainshock at depths between 44 and 53 km subsequently waveforms from 12 stations giving the best possible azimuthal coverage around the kythira event were inverted in order to investigate its source process a parallel non negative least squares inversion technique utilizing multiple time windows was used to derive the spatiotemporal slip distribution the fault surface was parameterized by subfaults of 3 km 3 km size covering a length and down dip width of 91 and 100 km respectively inversion results indicate that the fault plane is probably the one dipping towards se the slip distribution model showed that there are two areas of large slip 60 cm the largest extending between 38 and 80 km depth and a smaller one at depths 65 82 km the moment rate function consists of a large initial peak with a duration of about 11 s followed by a number of smaller peaks probably due to coseismic slip on some of the subfaults furthermore the hypocenters of the relocated aftershocks coincide with the areas of minimum slip

energy flux and dissipation in luzon strait two tales of two ridges abstractinternal tide generation propagation and dissipation are investigated in luzon strait a system of two quasi parallel ridges situated between taiwan and the philippines two profiling moorings deployed for about 20 days and a set of nineteen 36 h lowered adcp ctd time series stations allowed separate measurement of diurnal and semidiurnal internal tide signals measurements were concentrated on a northern line where the ridge spacing was approximately equal to the mode 1 wavelength for semidiurnal motions and a southern line where the spacing was approximately two thirds that the authors contrast the two sites to emphasize the potential importance of resonance between generation sites throughout luzon strait baroclinic energy energy fluxes and turbulent dissipation were some of the strongest ever measured peak to peak baroclinic velocity and vertical displacements often exceeded 2 m s 1 and 300 m respectively energy fluxes exceeding 60 kw m 1 were measured at spring tide at the wester

evidence for enhanced mixing over rough topography in the abyssal ocean the overturning circulation of the ocean plays an important role in modulating the earth s climate but whereas the mechanisms for the vertical transport of water into the deep ocean deep water formation at high latitudes and horizontal transport in ocean currents have been largely identified it is not clear how the compensating vertical transport of water from the depths to the surface is accomplished turbulent mixing across surfaces of constant density is the only viable mechanism for reducing the density of the water and enabling it to rise however measurements of the internal wave field the main source of energy for mixing and of turbulent dissipation rates have typically implied diffusivities across surfaces of equal density of only approximately 0 1 cm2 s 1 too small to account for the return flow here we report measurements of tracer dispersion and turbulent energy dissipation in the brazil basin that reveal diffusivities of 2 4 cm2 s 1 at a depth of 500 m above abyssal hills on the flank of the mid atlantic ridge and approximately 10 cm2 s 1 nearer the bottom this amount of mixing probably driven by breaking internal waves that are generated by tidal currents flowing over the rough bathymetry may be large enough to close the buoyancy budget for the brazil basin and suggests a mechanism for closing the global overturning circulation

buoyancy forcing by turbulence above rough topography in the abyssal brazil basin abstract observations of turbulent dissipation above rough bathymetry in the abyssal brazil basin are presented relative to regions with smooth bathymetry dissipation is markedly enhanced above rough topography of the mid atlantic ridge with levels above bathymetric slopes exceeding levels observed over crests and canyon floors furthermore mixing levels in rough areas are modulated by the spring neap tidal cycle internal waves generated by barotropic tidal flow over topography are the likely mechanism for supplying the energy needed to support the observed turbulent dissipation a model of the spatial and temporal patterns in the turbulent dissipation rate is used to constrain the diapycnal advection in an inverse calculation for the circulation in an area of rough bathymetry this inverse model uses both beta spiral and integrated forms of the advective budgets for heat mass and vorticity and contains sufficient information to resolve the full three dimensional flow the inverse model solution re

geochemistry and sedimentology of a facies transition from microbanded to granular iron formation in the early proterozoic transvaal supergroup south africa abstract a transition from microbanded kuruman to granular griquatown iron formation is described in terms of sedimentological petrographic and geochemical characteristics major minor trace and rare earth elements as well as whole rock carbon and oxygen isotopic compositions transitions between microbanded and granular iron formation are uncommon in the global stratigraphic record with microbanding absent from most known granular iron formations the kuruman griquatown transition zone differs from other granular iron formation sequences in being non oolitic i e peloidal and magnetite rich instead of hematite rich it is concluded that the coarsening upward microbanded granular iron formation units in the kuruman griquatown sequence represent upward shallowing storm dominated deposits that is primary microbanded relatively deep water fair weather deposits having been intermittently reworked by storm wave action form wavy sharp based graded and poorly sorted irregular interbeds of granular iron formation this is in contrast to typical hematitic oolitic iron formation units which represent true shoal deposits geochemical results strongly support the storm reworking hypothesis microbanded and granular iron formation beds have similar ree patterns with negative ce anomalies and depletion in light ree reflecting deposits in a marine setting below the zone of shallow surface water positive eu anomalies in the ree patterns suggest admixture of a hydrothermal component of the ocean water carbonates in the iron formation are depleted in 13c indicating involvement of organic matter during diagenetic alteration of the iron formations there are also indications that total dissolved carbon dioxide in deeper water was more depleted in 13c than in shallower water deposition of the iron formations was sporadically interrupted by influx of tuffaceous material now represented by centimeter thick shard and lapilli bearing stilpnomelane lutite beds these beds differ chemically from the iron formations in being enriched in elements such as al sc y zr nb hf ta th u and the lanthanides and displaying ree patterns with strong negative eu and slight positive ce anomalies the composition of the parent magma of the volcanics remains uncertain because of probable post depositional alteration of the volcanic glass however it was most probably of intermediate to acidic composition as concluded from the widespread distribution of the ash beds typical of plinian type deposits chemical variations in the minor trace and ree compositions of the iron formations are closely correlated with the amounts of admixed volcanic ash material most of the iron formations appear chemically very uncontaminated being highly depleted in all minor trace and ree and organic carbon contents the iron formations were therefore most probably deposited as a chemical precipitate on a submerged shallow cratonic platform with volcanic ash as the only exogenic material sporadically reaching the depository

mixing dissipation rate and their overturn based estimates in a near bottom turbulent flow driven by internal tides abstractdirect numerical simulation dns and large eddy simulation les are employed to study the mixing brought about by convective overturns in a stratified oscillatory bottom layer underneath internal tides the phasing of turbulence the onset and breakdown of convective overturns and the pathway to irreversible mixing are quantified mixing efficiency shows a systematic dependence on tidal phase and during the breakdown of large convective overturns it is approximately 0 6 a value that is substantially larger than the commonly assumed value of 0 2 used for calculating scalar mixing from the turbulent dissipation rate diapycnal diffusivity is calculated using the irreversible diapycnal flux and for tall overturns of o 50 m the diffusivity is found to be almost 1000 times higher than the molecular diffusivity the thorpe overturn length scale is often used as a proxy for the ozmidov length scale and thus infers the turbulent dissipation rate from overturns the accuracy of overturn based es

sedimentary manganese metallogenesis in response to the evolution of the earth system abstract the concentration of manganese in solution and its precipitation in inorganic systems are primarily redox controlled guided by several earth processes most of which were tectonically induced the early archean atmosphere hydrosphere system was extremely o2 deficient thus the very high mantle heat flux producing superplumes severe outgassing and high temperature hydrothermal activity introduced substantial mn2 in anoxic oceans but prevented its precipitation during the late archean centered at ca 2 75 ga the introduction of photosystem ii and decrease of the oxygen sinks led to a limited buildup of surface o2 content locally initiating modest deposition of manganese in shallow basin margin oxygenated niches e g deposits in india and brazil rapid burial of organic matter decline of reduced gases from a progressively oxygenated mantle and a net increase in photosynthetic oxygen marked the archean proterozoic transition concurrently a massive drawdown of atmospheric co2 owing to increased weathering rates on the tectonically expanded freeboard of the assembled supercontinents caused paleoproterozoic glaciations 2 45 2 22 ga the spectacular sedimentary manganese deposits at ca 2 4 ga of transvaal supergroup south africa were formed by oxidation of hydrothermally derived mn2 transferred from a stratified ocean to the continental shelf by transgression episodes of increased burial rate of organic matter during ca 2 4 and 2 06 ga are correlatable to ocean stratification and further rise of oxygen in the atmosphere black shale hosted mn carbonate deposits in the birimian sequence ca 2 3 2 0 ga west africa its equivalents in south america and those in the francevillian sequence ca 2 2 2 1 ga gabon are correlatable to this period tectonically forced doming up attenuation and substantial increase in freeboard areas prompted increased silicate weathering and atmospheric co2 drawdown causing glaciation on the neoproterozoic rodinia supercontinent tectonic rifting and mantle outgassing led to deglaciation dissolved mn2 and fe2 concentrated earlier in highly saline stagnant seawater below the ice cover were exported to shallow shelves by transgression during deglaciation during the sturtian glacial interglacial event ca 750 700 ma interstratified mn oxide and bif deposits of damara sequence namibia was formed the varangian marinoan ca 600 ma cryogenic event produced mn oxide and bif deposits at urucum jacadigo group brazil the datangpo interglacial sequence south china liantuo nantuo varangian event contains black shale hosted mn carbonate deposits the early paleozoic witnessed several glacioeustatic sea level changes producing small mn carbonate deposits of tiantaishan early cambrian and taojiang mid ordovician in black shale sequences china and the major mn oxide carbonate deposits of karadzhal type central kazakhstan late devonian the mesozoic period of intense plate movements and volcanism produced greenhouse climate and stratified oceans during the early jurassic oae organic rich sediments host many mn carbonate deposits in europe e g urkut hungary in black shale sequences the late jurassic giant mn carbonate deposit at molango mexico was also genetically related to sea level change mn carbonates were always derived from mn oxyhydroxides during early diagenesis large mn oxide deposits of cretaceous age at groote eylandt australia and imini tasdremt morocco were also formed during transgression regression in greenhouse climate the early oligocene giant mn oxide carbonate deposit of chiatura georgia and nikopol ukraine were developed in a similar situation thereafter manganese sedimentation was entirely shifted to the deep seafloor and since ca 15 ma b p was climatically controlled glaciation deglaciation assisted by oxygenated polar bottom currents aabw nadw the changes in climate and the sea level were mainly tectonically forced

observations of small scale secondary instabilities during the shoaling of internal bores on a deep ocean slope abstractthe rockall bank area located in the northeast atlantic ocean is a region dominated by topographically trapped diurnal tides these tides generate up and downslope displacements that can be locally described as swashing motions on the bank using high spatial and time resolution of moored temperature sensors the transition toward the upslope flow cooling phase is described as a rapid upslope propagating bore likely generated by breaking trapped internal waves buoyant anomalies are found during the bore propagation likely resulting from small scale instabilities the imbalance between the rate of disappearance of available potential energy and the dissipation rate of turbulent kinetic energy suggests that these instabilities are growing i e young and have high mixing potential

a volcanic exhalative origin for the world s largest kalahari manganese field a discussion of the paper by d h cornell and s s schutte

how variable is mixing efficiency in the abyss author posting american geophysical union 2020 this article is posted here by permission of american geophysical union for personal use not for redistribution the definitive version was published in geophysical research letters 47 2020 e2019gl086813 doi 10 1029 2019gl086813

physicochemical environments for the formation of quartz free manganese oxide ores from the early proterozoic hotazel formation kalahari manganese field south africa physicochemical environments for the formation of quartz free manganese oxide ores from the hotazel formation of the kalahari manganese field northern cape province south africa are discussed quantitatively on the basis of the phase relations in the system mn fe si o and qualitatively in the system ca mn fe si o the phase relations involve major oxide minerals of the ores namely braunite braunite ii hausmannite bixbyite jacobsite and hematite and were constructed under quartz deficient conditions using existing and estimated thermody namic data and mineral assemblages from the ores standard enthalpy delta hm 0 sub f 298 and gibbs free energy delta g 0 sub f 298 values of braunite are estimated to be 1 011 9 or 5 and 937 3 or 5 kcal mole respectively on the basis of published experimental data and predicted entropy and heat capacity the low grade diagenetic to very low grade metamorphic mamatwan type ores consisting mainly of braunite hausmannite jacobsite hematite kutnohorite and calcite are altered by hydrothermal solutions into the high grade high temperature wessels type ores consisting of braunite ii bixbyite hausmannite hematite calcite and andradite with trace amounts of braunite the peak temperature of formation of the high grade ores appears not to have exceeded about 300 degrees to 400 degrees c the ores are oxidized from oxygen fugacity levels defined by the assemblage hausmannite jacobsite hematite to that defined by the assemblage bixbyite hausmannite with increasing temperature under hydrothermal quartz deficient conditions the stability of braunite is enlarged to higher temperatures with increasing silica activity a sub sio sub 2 aq because braunite is much less abundant in the high grade wessels type ore than in the low grade mamatwan type ore the maximum a sub sio sub 2 aq in the solutions could be estimated at 10 super 4 to 10 super 3 at 300 degrees to 400 degrees c from the stable assemblage bixbyite hausmannite braunite kutnohorite and to some extent calcite was a major source of ca reacting with silica released from braunite to form braunite ii and andradite

spatial patterns of mixing in the solomon sea the solomon sea is a marginal sea in the southwest pacific that connects subtropical and equatorial circulation constricting transport of south pacific subtropical mode water and antarctic intermediate water through its deep narrow channels marginal sea topography inhibits internal waves from propagating out and into the open ocean making these regions hot spots for energy dissipation and mixing data from two hydrographic cruises and from argo profiles are employed to indirectly infer mixing from observations for the first time in the solomon sea thorpe and finescale methods indirectly estimate the rate of dissipation of kinetic energy and indicate that it is maximum in the surface and thermocline layers and decreases by 2 3 orders of magnitude by 2000 m depth estimates of diapycnal diffusivity from the observations and a simple diffusive model agree in magnitude but have different depth structures likely reflecting the combined influence of both diapycnal mixing and isopycnal stirring spatial variability of is large spanning at least 2 orders of magnitude within isopycnal layers seasonal variability of reflects regional monsoonal changes in large scale oceanic and atmospheric conditions with increased in july and decreased in march finally tide power input and topographic roughness are well correlated with mean spatial patterns of mixing within intermediate and deep isopycnals but are not clearly correlated with thermocline mixing patterns