instability waves in the equatorial atlantic ocean abstract evidence is presented for the generation of planetary waves by barotropic instability within the cyclonic shear region of the atlantic ocean s south equatorial current sec immediately following the springtime intensification of the southeast trade wind which accelerates the sec westward a packet of waves with central periodicity of around 25 days is observed lasting for about three cycles independent wavenumber analyses on 1983 and 1984 data give newly identical zonal wavelengths and phase speed estimates of around 1100 km and 50 cm s 1 the waves are anisotropic and spatially inhomogeneous with generation confined primarily to the mixed layer an energetics analysis using 1983 data centered upon the equator at 28 w shows a rapid increase in total perturbation energy tpe reaching values of 2000 erg cm 3 within two weeks the subsequent decrease in tpe at this location is due primarily to meridional pressure work divergence baroclinic instability is negligible because both the meridional

pleistocene to holocene offshore tephrostratigraphy of highly explosive eruptions from the southwestern cape verde archipelago thirteen sediment gravity cores from the seafloor around the southern cape verdean islands fogo and brava as well as the cadamosto seamount recovered 43 mafic and 5 phonolitic pleistocene to holocene primary ash layers twelve of these layers could be identified in several cores they evidently cover areas of at least 6200 17 650km2 corresponding to minimum tephra volumes of 1km3 volcanic explosivity index vei 5 and thus are attributed to eruptions of sub plinian to plinian dimensions provenance analyses based on geochemical compositions and geological evidence link the mafic tephras to eruptions on fogo island the detailed foraminiferal 18o stratigraphy of one core yields a high resolution age scale that can be applied to other cores via correlated ash beds and background sediment intervals sedimentation rates of the background sediment then constrain ages of other ash beds in the cores the resulting temporal distribution of the marine tephras indicates that relatively large magnitude highly explosive eruptions occurred about every 3000years on fogo during the last 150kyrs during the past 30kyrs the rate has increased to one in 2000years in addition we identified a 117kyr old thick turbidite sequence containing both mafic and phonolitic ash we interpret this as the deposit of density currents generated by the monte amarelo flank collapse on fogo and its associated tsunami on the other hand only a single widespread phonolitic tephra layer 145 kyrs old could be correlated to brava island indicating that highly explosive activity producing deposits spread widely beyond the island s shores did not occur anymore over the last 145 kyrs no equivalents of the caldera filling ignimbrites on brava could be found in the cores and therefore we infer that the caldera formed earlier than the 155 ka reached by coring two widespread phonolitic tephra layers 17 ka and 40 ka old are correlated to the cadamosto seamount and thus derived from eruptions that occurred at 1380 m water depths where thermal granulation producing blocky glass shards was a major fragmentation process we interpret that these eruptions produced huge submarine volcanic ash water plumes that spread widely across the seafloor in conclusion our marine tephrostratigraphy provides the first evidence of frequent highly explosive volcanic eruptions at the southwestern part of the cape verdes during the past 155 kyrs a somewhat surprising result is that such large eruptions occurred much more frequently from mafic alkalic magmas at fogo than from highly evolved phonolitic magmas on brava

on the deep western boundary current south of cape cod abstract using ctd oxygen data from eight cruises in the decade from 1994 2003 we have constructed a mean section of properties across the deep western boundary current dwbc south of cape cod near 70 w since all sections included direct velocity measurements our composite section enables us to portray the flow field as well as the mean water mass structure inshore of the gulf stream between the 2500 and 4000 m isobaths the flow is to the southwest along the bathymetry and is remarkably barotropic the equatorward flowing labrador sea water is shown to have high dissolved oxygen low salinity and low potential vorticity while the underlying overflow water is high in oxygen transport estimates for the cold limb of the thermohaline circulation give a range of 14 to 19 sv for the n atlantic deep water found on the section the greatest uncertainty is due to the presence of a warm core ring on one of the sections which apparently completely reversed the flow in the dwbc offshore of the dwbc some of the deep source waters are returned to the north in the deep gulf stream the section is compared to two other widely separated locations abaco and 55 w that have markedly different dwbc characteristics

from swallow floats to argo the development of neutrally buoyant floats neutrally buoyant floats have been a vital tool in the exploration of the global ocean circulation and now provide a central element of the in situ ocean observing system through the argo project the paper traces the development of neutrally buoyant floats from their origins in the mid 1950s to the present day and highlights areas of ocean science to which floats made significant contributions

on the average absolute transport of the deep western boundary currents east of abaco island the bahamas abstract data obtained from a line of absolute current profiler pegasus and ctd stations at 26 5 n east of abaco island in the northern bahamas are used to estimate the average transport and overall level of variability of the deep below 800 m southward flow associated with the so called deep western boundary current dwbc at this latitude from april 1985 to september 1987 a total of 11 sections were made along a line extending from near the island boundary to 85 km offshore as part of the subtropical atlantic climate studies stacs program in all but two cases when pegasus data were obtained over the total depth absolute velocities were determined from 0 to about 3000 db while ctd derived relative velocities were determined over the total depth using the combined datasets it is possible to reference the relative geostrophic velocity with pegasus data thereby obtaining an estimate of the absolute southward transport of the deep flow the average value of transport determined in this man

dynamic impact of oscillating sources on the north atlantic deep circulation a process study in this process study the dynamic effects of fluctuating deep water sources on the deep circulation of the north atlantic are examined by means of a 1 layer model the model is used with a realistic topography of the atlantic ocean between 65 n and 25 s and has an open southern boundary according to observations the northern inflow of deep water is subject to variability on timescales between days and decades the same holds for the deep western boundary current dwbc which acts as a link between the deep water inflow and the interior circulation the timescales of the observed variability motivate the forcing functions used in the model they determine the nature of the models response the flow response relative to the local mean flow can be considerably larger than the original perturbation at the source normalizing the perturbed flow with the local steady state flow leads to the identification of sensitive areas their location and shape are determined by the difference between the characteristic travel paths of the perturbation and the streamlines of the steady state circulation in the eastern equatorial atlantic a forcing with periods between 1 and 3 months gives rise to larger flow perturbations than a forcing with periods between half a year and 1 year forcing with the latter periods leads to a stronger response in the western equatorial atlantic and in the interior ocean changes in the direction of the deep equatorial flow are driven by low frequency oscillations of the dwbc in the model this is consistent with observations in the area perturbations radiate into the interior away from the eastern boundary as long rossby waves as this propagation is along geostrophic contours consequently large parts of the abyssal interior are inaccessible the results support the notion that the area where the dwbc encounters the equator is not only a key region for the slow interhemispheric exchange of heat and tracers but also for the transport of information by fast internal waves along the equator

comparison of lake and land tephra records from the 2015 eruption of calbuco volcano chile tephra layers in lake sediment cores are regularly used for tephrostratigraphy as isochronous features for dating and recording eruption frequencies however their value for determining volcanic eruption size and style may be complicated by processes occurring in the lake that modify the thickness and grain size distributions of the deposit to assess the reliability of data from lake cores we compare tephra deposited on land during the 2015 eruption of calbuco volcano in chile to records in sediment cores from three lakes of different sizes that are known to have received primary fall deposits in general the thickness and granulometry of the deposit in lake cores and nearby terrestrial sections are very similar as anticipated however cores sampled close to here within 300 m of fluvial inflows were affected by sediment deposition from the lake s catchment they differed from primary deposits not only in their greater thickness and organic content but also in poor sorting and lack of grading cores 850 m away from the inlet were not affected we consider our results in the context of the particle settling regime as well as each lake s location bathymetry and catchment area we find that the particle settling regime is important in more distal settings where the ash particles are small and particle settling occurs in density plumes rather than as individual particles we conclude that lake cores can be useful for physical volcanology providing consideration is given to eruption parameters such as particle size and mass flux as well as lake features such as bathymetry and catchment area

hemispheric asymmetry of deep water transport modes in the western atlantic subtropical studies of the atlantic meridional cold water flow show a hemispheric contrast in the dominant southward transport mode below 2000 m in the north atlantic lower deep water ldw 1 8 2 4 c dominates with small transport of middle deep water mdw 2 4 3 2 c while in the south atlantic the opposite is observed we use numerous observations in the western basins of the tropics to show that the transition occurs rapidly near the equator in the western atlantic a meridional section in the central brazil basin suggests zonal flows are responsible for the transition ldw transport from the guiana basin north of the equator flows eastward in the northern brazil basin and is inferred to continue on through the romanche fracture zone into the eastern atlantic an opposing flow of mdw from the eastern tropical atlantic flows toward the western boundary where it bifurcates to supply mdw to the deep western boundary current dwbc of the brazil basin as well as to feed the northward flow of mdw in the guiana basin offshore of the dwbc the magnitude of each of these oppositely directed flows is roughly 7 sv we furthermore speculate that they are connected predominantly by upwelling from ldw to mdw within the low latitude eastern basin the overall deep water transport system below 2000 m in the western basins of the mid and low latitude atlantic is thus found to comprise the following three distinct components 1 a strong dwbc transport of ldw with associated recirculation dominates the guiana basin north of the equator 2 in the northern brazil basin just south of the equator a narrow eastward flow absorbs the ldw and carries it eastward while a somewhat broader westward flow imports mdw into the western basin 3 this mdw flow then bifurcates with the southward branch causing the mdw dominance in the brazil basin where the mdw dominated dwbc and associated recirculations are the third component of the deepwater transport system

volcanic tephra bed formation and condensation processes a review and examination from devonian stratigraphic sequences abstractthe formation and preservation of airfall volcanogenic tephras is the result of various interacting factors explanations of the depositional history of individual tephra beds lie along a spectrum between two end member models of a primary single event and a complex depositional history nonrecognition of the history of a tephra bed may lead to errors in geochronology correlation and interpretation of the record of explosive volcanism in sedimentary successions increased understanding of sequence stratigraphic principles and condensation processes favors the model of tephras with a complex depositional history this is supported by studies that indicate many tephras in the rock record result from amalgamation and or reworking in the depositional environment a prediction of increased tephra bed formation preservation within transgressive systems tracts due to condensation processes is tested through 11 lochkovian to eifelian lower to lower middle devonian depositional sequences the character

k bentonites volcanic ash preservation and implications for early to middle devonian volcanism in the acadian orogen eastern north america lower to middle devonian marine strata in the appalachian foreland basin feature up to 80 or more thin k bentonites that represent ancient volcanic ashes the time vs space distribution of k bentonites through the lochkovian to eifelian stages representing 30 m y shows a distinct pattern of clustered multiple beds several scattered beds and thick intervals with no k bentonites four clusters of 7 to 15 individual closely spaced layers occur in the middle lochkovian bald hill k bentonites kalkberg new scotland formations late pragian or early emsian sprout brook k bentonites esopus formation and early eifelian two clusters the tioga middle coarse zone and tioga a g k bentonites onondaga formation detailed examination of these devonian k bentonites shows that in many cases they do not represent a single eruption multilayered beds fossil layers within beds authigenic minerals e g glauconite and phosphate nodules subjacent hardgrounds and an irregular distribution of beds through space and time raise questions about the depositional history and preservation potential of volcanic ash in marine environments and the degree to which the beds represent a primary record of volcanism these and other lines of evidence indicate that postdepositional physical biological and geochemical processes e g sedimentation rate event and background physical processes burrowing have modified the primary record of these water laid ash fall events these factors may lead to preservation of primary ash deposits or to their resedimentation and or partial to complete mixing with background sediments the preservation potential and resulting distribution of the devonian k bentonites can be analyzed across a spectrum of preservational magnafacies in this paper i present a model of ash preservation the model incorporates environmentally related physical biological and chemical processes active in epicontinental seas and marine foreland basins conclusions based on the model indicate that the middle lochkovian early emsian and early eifelian were times of peak volcanic activity in eastern north america related to times of increased tectonism in the acadian orogen

deep flow along the western boundary south of the blake bahama outer ridge abstract in june july 1990 hydrographic chlorofluorocarbon cfc and velocity observations were taken along the western boundary of the north atlantic south of the blake bahama outer ridge from 30 to 24 n between the northern bahamas and 71 w the deep flow in the region associated with the deep western boundary current forms a pattern of strong narrow currents and cyclonic gyres close to the continental slope with broad slower southward flow offshore the cfcs reveal that the most recently ventilated water i e having the highest cfc concentrations due to more recent contact with the atmosphere in the northern north atlantic is found along the western boundary in two distinct cores between potential temperatures 4 6 c and 1 9 2 4 c geostrophic transport streamlines were constructed for the deep flow referenced using direct velocity observations at 26 5 n and assuming mass conservation between closed areas bounded by the hydrographic sections the tracers and transports are used together

subaqueous eruption fed density currents and their deposits density currents fed directly from subaqueous eruptions can be divided conceptually into three groups based on modes of fragmentation and transport i explosive fragmentation with deposition from a gas supported current ii explosive fragmentation with deposition from a water supported current iii fragmentation of flowing lava with deposition from a water supported current fig 1 group i products include subaqueously emplaced welded ignimbrite and other high temperature emplaced subaqueous pyroclastic flow deposits group ii products are the most varied and include representatives of both high and low concentration turbidity currents grainflows and debris flows and are termed eruption fed aqueous density currents some clasts in such currents are transported and even deposited at high temperature but the transporting currents ranging from grain flows to dilute turbidity currents are water dominated even though steam may be developed along large clasts margins group iii products formed from lava flow fed density currents tend to be weakly dispersed down gradient along the seafloor and generally consist largely of fragments formed by dynamo thermal quenching and spalling bursting of bubbles formed by vapor expansion probably contributes to some group iii beds distinctive column margin fall deposits may form in water excluded zones that developed very locally around vents in association with group i and ii deposits and are distinguished by heat retention structures indicators of gas phase transport and absence of current formed depositional features 2000 elsevier science b v all rights reserved

deposition and preservation of tephra in marine sediments at the active hikurangi subduction margin abstract tephra volcanic ash deposits are important isochronous markers for correlating marine sediments or events recorded in marine sediment cores however the active tectonics that are commonly associated with volcanic activity at plate tectonicboundaries also drive large scale deformation leading to steep and variable local and regional bathymetry e g ridges basins and canyons systems this complex bathymetry influences gravity flow behaviour and paths which can rework and redeposit tephras resulting in stratigraphic complexities such as the mis identification of primary versus reworked tephra deposits and in turn lead to the development of inaccurate chronostratigraphies here we present 36 tephra deposits from 21 shallow marine sediment cores that traverse the length of the southern and central margin of eastern north island new zealand using major and trace element geochemical compositions for glass shards from the tephras we correlate these deposits to three major rhyolitic eruptions from the taup volcanic zone tvz approximately 200 km west including taup 1718 cal yrs bp kaharoa 636 cal yrs bp and kawakawa oruanui kot 25 4 ka based on their morphology depositional character and inferred emplacement mechanisms the tephra deposits are grouped into four lithofacies types 1 primary deposits 2 volcaniclastic rich turbidites 3 blebs pods of volcaniclastic rich material and 4 complex deposits primary deposits form syn eruptively through airfall onto the ocean surface settling over hours to days through the water column under diffuse vertical gravity currents volcaniclastic rich turbidites are formed through secondary redeposition and entrainment by post eruptive turbidity currents while blebs pods of material are interpreted to have formed by erosion and or bioturbation complex deposits form through the interaction of all these mechanisms producing an overthickened array of primary and redeposited units within a single facies herein we argue that redeposited units of volcaniclastic rich turbidites or small blebs pods can be used as tentative chronological markers if the geochemical composition of the glass shards have a homogeneous signature i e a single eruptive source where the glass shards in redeposited units have mixed geochemical compositions and are not stratigraphically associated with a primary deposit source they cannot be used as chronological marker horizons this emphasises the need for accurate and rigorous data reduction without overlooking the importance of data points that are statistical outliers we also show that the highest preservation of tephra deposits is found in semi confined isolated basin settings including a wide range of deposit types due to erosive sediment flows that bypass through submarine distributary systems these major sediment dispersal pathways preserve few volcaniclastic deposits our findings have important implications not only for identifying primary or redeposited characteristics in marine tephras for building accurate chronostratigraphies but also as a guide geomorphic sub environments with the best preservation of tephras in marine sedimentary systems

direct observations of the current structure east of the bahamas quasisynoptic absolute velocity sections were obtained in the western north atlantic to the east of the bahamas on five cruises from 1992 to 1994 showing complex spatial and temporal variability of the currents throughout the water column lowered acoustic doppler current profilers were used on all cruises for comparison an acoustically tracked free fall profiler was used on the august 1992 and june 1993 cruises where simultaneous profiles from both methods are available depth averaged differences are less than 0 01 m s 1 in the ensemble mean and standard deviation though small the mean difference appears to be statistically non zero for one of the two doppler profilers used no cause has been found the individual velocity sections show a banded structure of currents in the offshore direction with strong baroclinic and depth averaged components extending over the full sampling domain two sections show a picture of the southward flowing deep western boundary current dwbc differing from previous observations during august 1992 and june 1993 the mean dwbc core was located more than 100 km offshore in observations prior to 1990 it was usually near 50 km offshore net southward transport below 800 m integrated from the coast to 400 km offshore varied from 15 to 50 sv with a mean of about 40 sv 1 sv 1 106 m s 1 our mean is consistent with previous estimates our sections add new evidence that the transport is highly variable to at least 400 km offshore

temperature and salinity variability in the deep western boundary current abstract a ten year time series 1984 1993 of repeat hydrographic sections from offshore abaco island the bahamas 26 5 n is used to define the mean and time dependent characteristics of the deep western boundary current dwbc the dwbc flow is divided into four vertical layers based on chlorofluorocarbon cfc concentration and formation regions upper layer cfc core 3 9 5 0 c second layer classical labrador sea water 3 2 3 9 c third layer cfc minimum 2 4 3 2 c deepest layer cfc core 1 85 2 4 c time series analysis of mean layer properties and their anomalies showed that the temperature and salinity of each layer did not increase or decrease monotonically with time variations in temperature and salinity were characterized by 2 3 yr period oscillations variability between years is illustrated by subtracting repeat sections of temperature and salinity along levels of both constant pressure and constant potential density to determine an original water mass modificati

a statistical description of the mean circulation and eddy variability in the northwestern atlantic using sofar floats abstract velocity data obtained from trajectories of sofar floats launched as part of several experiments carried out in the northwest atlantic during the last two decades are used to give a statistical description of the mean gulf stream and its recirculation the distribution of eddy variability in the region is also presented a mean gulf stream which bifurcates between 55 and 45 w to feed the north atlantic current and a southern recirculation can be clearly identified a weak interior sverdrup flow is also suggested the spatial distribution of eddy variability is consistent with previous descriptions showing a maximum near the gulf stream

historical variability in atlantic meridional baroclinic transport at 26 5 n from boundary dynamic height observations the strength of the atlantic meridional overturning circulation moc at 26 5 n may be inferred from the combination of florida straits transport derived from cable measurements ekman transport estimated from wind stress climatologies and mid ocean geostrophic shear traditionally obtained from hydrographic sections with application of mass balance to the section to yield the mid ocean barotropic flow the recent rapid monitoring project has provided time series information for the mid ocean geostrophic shear since 2004 this work presents methods to assemble a comparable dataset from ctd end stations and boundary mooring temperature and pressure time series to estimate the past variability from 1980 to 2005 variability in the end station derived transport anomalies suggests that the moc has fluctuated by more than 10 sv encompassing all moc estimates reported in the literature interannual changes in moc transport are masked by this variability and calculated trends in layer transports are not statistically significant more extensive datasets of ctd casts and moored temperature records at the western boundary do show significant changes with warming in the thermocline and long term freshening of the deep waters between the 1980s and 2005 these changes are associated with stronger southward flow in the upper waters and weaker southward flow in the deep waters and suggest a decrease in the moc strength of 2 4 sv any such decrease however is masked by the scale of variability in layer transports derived from the historical database of ctd end stations

fallout of volcanic ash to the deep south china sea induced by the 1991 eruption of mount pinatubo philippines an ongoing sediment trap experiment in the central south china sea provided the opportunity to investigate directly the mode and rates of a medial fallout of volcanic ash following the june 15 1991 eruption of mount pinatubo philippines within less than three days after the release of the major eruption plume ash accumulation 9 kg m 2 was simultaneously recorded by traps positioned in 1190 and 3730 m water depth at a distance of 586 km to the west of the volcano a numerical simulation of the fallout reveals that the vertical trajectories of the pyroclasts in both the atmosphere and ocean were controlled by particle aggregation the aggregation process caused a premature subaerial fallout of fine grained ash after crossing the air sea interface vertical settling of the ash clusters was enhanced by absorption of water leading to settling rates of more than 1670 m d aggregates were observed at all depths and their rapid settling is reflected in identical pyroclast spectra in intermediate and deep water this implies that particle sorting must have been complete in the upper water column and that the fallout was not perturbed by oceanic currents the tephra blankets an area of 37 10 4 km 2 in a fan shaped westward distribution which reflects the prevailing direction of the upper level winds our data show that atmospheric tephra dispersal patterns are recorded almost instantaneously on the deep sea floor and corroborate the key role of deep sea ash layers as highly reliable chronostratigraphic markers and as a geological tool to infer ancient wind fields

comparison of surface seismic sources at the co2sink site ketzin germany 3d seismic time lapse surveys 4d seismics are an essential tool for large scale reservoir characterization the target reservoir of the ketzin pilot project for co2 storage is a saline aquifer of the heterogeneous stuttgart formation upper triassic in the northeast german basin the focus of this project is on testing and further developing monitoring co2 storage technologies for time lapse seismic monitoring three seismic surface sources were characterized with respect to s n signal to noise ratios signal penetration and frequency content by analysis of raw shot gathers and stacked sections along two lines at the ketzin site differences in reflectivity between these 2d lines reflect the differences in the nature of the sources tested and how they influence the signal bandwidth resolution and signal energy all three sources image the target horizon the weight drop source was recommended as the primary source for 3d surveys based mainly on logistics and cost results of processing including equalization of a 4d 3d time lapse data set from the ketzin site and cross correlation indicate that the injected co2 can be monitored the highly irregular amplitude response on the time lapse data can be attributed to the reservoir heterogeneity time lapse seismic processing petrophysical measurements on core samples and geophysical logging of co2 saturation levels allow for an estimate of the total amount of co2 visible in the seismic data to be made in spite of some uncertainty the close agreement between the injected and observed amount is encouraging for quantitative monitoring of a co2 storage site using seismic methods by integrating seismic modeling and multiphase fluid flow simulations the impact of the reservoir temperature on the 4d seismic data from ketzin was estimated the modeled time lapse seismic differences for two temperature scenarios present in the reservoir are minor regarding the qualitative analysis however the influence of temperature on the volumetric estimation of the co2 using the 4d seismic data is significant future issues to be considered include expanding the temperature range 34 38 c in this study to be investigated and the resulting effects on the seismic response as well as the role of the reservoir heterogeneity in addition it would be important to investigate the impact of temperature on the co2 storage at other sites with favorable p t conditions in the reservoir further seismic modeling using results of petrophysical experiments for estimating the effect of the co2 injection at the ketzin site on the ava avo response on time lapse seismic data was performed two effects were considered the co2 saturation and the pore pressure related effects the results indicate that it is worth investigating if it is possible to discriminate between these effects on future 3d repeat surveys at the ketzin site

variability of the deep western boundary current at 26 5 n during 2004 2009 five years of data from a line of dynamic height moorings dhm bottom pressure recorders bpr and pressure equipped inverted echo sounders pies near the atlantic ocean western boundary at 26 51n are used to evaluate the structure and variability of the deep western boundary current dwbc during 2004 2009 comparisons made between transports estimated from the dhm bpr and those made by the pies demonstrate that the two systems are collecting equivalent volume transport information correlation coefficient r 0 96 root mean square difference 6 sv 1 sv 10 6 m 3 s 1 integrated to 450 km off from the continental shelf and between 800 and 4800 dbar the dwbc has a mean transport of approximately 32 sv and a standard deviation during these five years of 16 sv both the barotropic full depth vertical mean and baroclinic flows have significant variability changes exceeding 10 sv on time scales ranging from a few days to months with the barotropic variations being larger and more energetic at all time scales the annual cycle of the deep transport is highly dependent on the horizontal integration distance integrating 100 km offshore yields an annual cycle of roughly similar magnitude but shifted in phase relative to that found from current meter arrays in the 1980 1990s while the annual cycle becomes quite weak when integrating 450 km offshore variations in the dwbc transport far exceed those of the total basin wide meridional overturning circulation standard deviations of 16 sv vs 5 sv transport integrated in the deep layer out to the west side of the mid atlantic ridge still demonstrates a surprisingly high variance indicating that some compensation of the western basin deep variability must occur in the eastern basin published by elsevier ltd

fine ash content of explosive eruptions abstract in explosive eruptions the mass proportion of ash that is aerodynamically fine enough to cause problems with jet aircraft or human lungs 50

crystal capture sorting and retention in convecting magma the mystery of producing strong compositional diversity among suites of comagmatic igneous rocks is investigated by considering the dynamic evolution of basaltic magma in a sheet like chamber a central conclusion is that inward progressing crystallization produces strong viscosity and temperature gradients that promote convection only near the leading edge of the upper thermal boundary layer convection is apparently confined to an essentially isoviscous isothermal region that hugs the downward growing roof zone strong changes in viscosity with crystallization divide the upper and lower thermal boundary layers into regions of decreasing viscosity and crystallinity n called rigid crust n 0 5 mush 0 5 n 0 25 and suspension n 0 25 the strong increase in viscosity near the mush suspension interface acts as a capture front that overtakes and traps slowly settling crystals initial phenocrysts mostly escape capture but crystals nucleated and grown in the suspension zone can escape only if the capture front slows to a critical rate attainable only in bodies thicker than about 100 m escaping crystals are redistributed and sorted by convection driven by the advance of the capture front itself crystal laden plumes traverse the central hot core of the body and deposit partially resorbed and sorted crystals within the lower suspension zone convection is never vigorous but is part of an overall intimate balance between roofward heat loss rigid crust growth crystallization kinetics and transport and sorting of sinking escaped crystals there is a strong similarity between these processes and those producing both varves and saline pan deposits it is clear that lavas lava lakes and sills are indeed examples of true magma chambers strongly exhibiting certain aspects of this over all process these aspects commonly also characterize the large mafic magmatic bodies because strong compositional changes in the residual melt occur largely outward that is at lower temperatures and higher crystallinities of the capture front which is immobile and mostly within rigid crust the possible range in comagmatic compositions available for eruption anywhere within the active magma is very limited this is in broad agreement with the compositional range observed in basaltic lava lakes sills and plutons like skaergaard the tuning of convection crystallization kinetics and phase equilibria in chambers of this type can produce a variety of textures and layering but not a diversity of compositions

observations of the boundary current system at 26 5 n in the subtropical north atlantic ocean abstract five current meter moorings and four horizontal electric field records spanning june 1990 february 1992 are used to describe the mean structure and variability of the vertically averaged velocity field and volume transport extending 425 km east of abaco the bahamas at 26 5 n examination of zonal and meridional velocity sections shows that while meandering may explain part of the variability there is substantial evidence for pulsation of the core dwbc velocity in the record intermittently strong northward flow is observed 225 km east of abaco that is significantly coherent and out of phase with the currents closer to the boundary at periods of 50 100 d and longer suggesting recirculation this is never observed 315 km offshore at 380 km offshore and extending east at least 60 km a strong dominantly southward flow is observed that is coherent with both that near abaco in phase and in the intervening recirculation zone out of phase at long periods the net mean transport over 17 month

on deep western boundary currents the stommel and arons 1960a b theory of the deep circulation of the world ocean does not directly address the physics of deep boundary currents in the present paper a maximally simple steady state physics for deep western boundary currents is developed and integrated into the stommel arons model this is accomplished by modifying the assumptions of the latter to include effective horizontal eddy viscous terms everywhere in the deep water including in the interior the resulting solution for the meridional velocity in a rectangular ocean on a beta plane is mathematically analogous to that of munk 1950 for the wind driven circulation in the interior the meridional velocity reduces identically to that of stommel and arons among other benefits the solution eliminates the theoretical difficulties associated with the crossing of the equator by the deep western boundary current as an initial test of the theory s consistency with observation theoretical plots of pressure and meridional velocity as a function of longitude are qualitatively compared with hydrographic sections and inferred meridional velocity fields in the eastern and western basins of the abyssal north atlantic also the theoretical profile for the meridional velocity as a function of longitude is qualitatively compared with observations of longshore velocity as a function of offshore distance in the deep western boundary current of the north atlantic deep water

submarine deposits from pumiceous pyroclastic density currents traveling over water an outstanding example from offshore montserrat iodp 340 pyroclastic density currents have been observed to both enter the sea and to travel over water for tens of kilometers here we identified a 1 2 m thick stratified pumice lapilli ash cored at site u1396 offshore montserrat integrated ocean drilling program iodp expedition 340 as being the first deposit to provide evidence that it was formed by submarine deposition from pumice rich pyroclastic density currents that traveled above the water surface the age of the submarine deposit is ca 4 ma and its magma source is similar to those for much younger soufriere hills deposits indicating that the island experienced large magnitude subaerial caldera forming explosive eruptions much earlier than recorded in land deposits the deposit s combined sedimentological characteristics are incompatible with deposition from a submarine eruption pyroclastic fall over water or a submarine seafloor hugging turbidity current derived from a subaerial pyroclastic density current that entered water at the shoreline the stratified pumice lapilli ash unit can be subdivided into at least three depositional units with the lowermost one being clast supported the unit contains grains in five separate size modes and has a 12 phi range particles are chiefly subrounded pumice clasts lithic clasts crystal fragments and glass shards pumice clasts are very poorly segregated from other particle types and lithic clasts occur throughout the deposit fine particles are weakly density graded we interpret the unit to record multiple closely spaced 2 d hot pyroclastic density currents that flowed over the ocean releasing pyroclasts onto the water surface and settling of the various pyroclasts into the water column our settling and hot and cold flotation experiments show that waterlogging of pumice clasts at the water surface would have been immediate the overall poor hydraulic sorting of the deposit resulted from mixing of particles from multiple pulses of vertical settling in the water column attesting to complex sedimentation slow settling particles were deposited on the seafloor together with faster descending particles that were delivered at the water surface by subsequent pyroclastic flows the final sediment pulses were eventually deflected upon their arrival on the seafloor and were deposited in laterally continuous facies this study emphasizes the interaction between products of explosive volcanism and the ocean and discusses sedimentological complexities and hydrodynamics associated with particle delivery to water

application of the continuous wavelet transform on seismic data for mapping of channel deposits and gas detection at the co2sink site ketzin germany conventional seismic data are band limited and therefore provide limited geological information every method that can push the limits is desirable for seismic data analysis recently time freque

encounter of a deep western boundary current with the equator a numerical spin up experiment the encounter of a deep western boundary current with the equator is studied using an ultra high resolution numerical model of the equatorial ocean in a study of the origin of the deep equatorial current system the integration was carried out from a state of rest to 3500 days the spin up experiment shows that the deep western boundary current upon encountering the equator first turns along the equator the time scale of spin up is fast along the equator and the boundaries about 250 days but is considerably slower off the equator after this initial response the western boundary current slowly begins to penetrate into the opposite hemisphere as expected from the linear theory the eastward equatorial current gradually slows down the total water mass transport in the equatorial branch decreases steadily until by day 3000 it has dwindled to zero and all the mass transport in the western boundary current crosses into the other hemisphere however a vestige of the equatorial branch remains until the end of the integration with the speed of less than 3 cm s and with intermittent meandering an analysis of the momentum balance indicates that this meander may be a free equatorial inertial oscillation we postulate that a permanent steady bifurcation of the mid depth western boundary current that carries substantial mass does not occur and eventually all of the mass transport will continue into the opposite hemisphere the asymptotic flow field seems incompatible with the tracer field observed in the atlantic which clearly shows a splitting at the equator of the tracer signal associated with the upper north atlantic deep water we speculate that an inherently time dependent explanation of the tracer tongue is called for

enhancing seismic data resolution using the prestack blueing technique an example from the ketzin co2 injection site germany restricted resolution imposes limits on the detection of subsurface geologic features using surface seismic data despite great improvements in acquisition and processing techniques in recent years seismic data still suffer from limited resolution detailed subsurface information on the geologic conditions at depth cannot always be attained from conventional surface seismic data for the area of this study the carbon dioxide c o2 storage site at ketzin germany high seismic resolution is required for mapping the internal structure of the main target reservoir processing of the 3d surface seismic data using conventional methods did not provide optimum resolution the spectral blueing technique attempts to enhance the surface seismic data resolution well log data generally show a blue spectrum with higher amplitudes at the higher frequencies than the commonly assumed white reflection series by designing and applying one or several operators to poststack data it has been possible to better match th

the climatology of the north atlantic abstract data from approximately 144 000 hydrographic stations in the north atlantic have been retrieved from the national oceanic data centre and analysed to produce maps of mean pressure temperature salinity and oxygen on selected potential density surfaces for the domain bounded by 0 85 w and 0 65 n the data span the period from 1904 to 1990 with the majority of the data from the last four decades the data set for this region is 60 larger than that used in the production of levitus climatological world atlas this increase in stations coupled with smoothing scales specific to the north atlantic rather than the global ocean considerably improves the resolution of the basin s features the mean property fields and their associated standard deviations are resolved on a one degree grid with little smoothing contrasting with the levitus atlas where properties although presented on a one degree grid have been smoothed on the order of 1000km another important feature of this database is the process by which irregularly spaced data are averaged onto a regular grid in a significant departure from the levitus analysis which averaged on depth surfaces these data were averaged on potential density surfaces thus eliminating an artificial mixing of water mass properties the database is used to describe the baroclinicity of well known features such as the gulf stream the north atlantic current and the deep western boundary current and to elucidate the recirculations associated with these currents it additionally resolves several new features in the intermediate and deep north atlantic these features include the signature of a large scale deep recirculation that extends southwestward from the eastward extension of the north atlantic current to the separation point of the gulf stream near cape hatteras this recirculation which spans approximately 2000m of the water column encompasses more local recirculations and potentially mixes subpolar and subtropical waters furthermore in the upper thermocline this database reveals a coherent azores current that stretches from the gulf stream system south of the tail of the grand banks to madeira this flow is marked by divergences to the south and convergences from the north such that its downstream transport is not much changed

combination of seismic reflection and constrained resistivity inversion with an application to 4d imaging of the co2 storage site ketzin germany abstracta combination of seismic and geoelectric processing was studied by means of a structurally constrained inversion approach structural constraints were interpreted from the seismic data and integrated into the geoelectric inversion through a local regularization which allowed inverted resistivities to behave discontinuously across defined boundaries this arranged seismic processing and constrained resistivity inversion in a sequential workflow making the generic assumption that the petrophysical parameters of both methods change across common lithostructural boundaries we evaluated the approach using a numerical example and a real data example from the ketzin co2 pilot storage site germany the latter demonstrated the efficiency of this approach for combining 4d seismic and surface downhole geoelectric data in consistence with the synthetic example the constrained resistivity inversions produced clearer delineated images along the boundary between caprock and reservoir formation near the co

overflow into the deep caribbean effects of plume variability the deepest connection between the eastern caribbean and the atlantic is over the jungfern grappler sill complex at 1815 m depth through these gaps flows the sole source of water for the deep caribbean presumably balanced by diffusively driven upwelling elsewhere in the basin fourteen month long moored observations at the sills in 1991 1992 reveal a mean transport of 0 11 0 05 sv 1 sv 106 m3 s l of atlantic water colder than 0 3 965oc flowing into the caribbean one quarter of which comes over the previously unmeasured grappler sill this is about twice the transport seen in previous shorter experiments the overflow is highly episodic with 10 events per year the range of overflow density is comparable to the total vertical strati fication of the deep caribbean a numerical stream tube model of the overflow plume is run with 13 different initial conditions representing the observed range of overflow strength and density results indicate that much of the overflow only penetrates to 2200 3000 m depth and was dense enough to descend to the bottom of the caribbean only 1 of the time combined plume model results are used to drive a model of the basin stratification a steady balance is found to be possible with small diapycnal diffusivity in the basin between 0 1 and 0 6 cm2 s 1 the reason for the small diffusivity is that the stratification is essentially built in place by the different detrainment depths and so less diffusive transformation is needed below a depth of 1815 m the eastern caribbean is topog raphically isolated from the rest of the worlds ocean figure l a middepth water flows from the atlantic through anegada pass 1915 m into the virgin islands basin vib figure lb a small intermediate basin between the atlantic and caribbean the source water for the inflow is upper north atlantic deep water unadw which lies between depths of 1200 2000 m along the western boundary of the north atlantic leaman and harris 1990 show the freon maximum at 26 5on associated with labrador sea water nominally the source of unadw to be at 1200 m considerably shallower than the anegada sill depth however labrador sea water still appears to form the bulk of what flows into the deep caribbean atlantic water at that depth is also influenced to a lesser degree by the high salinity mediterranean ouflow water as is apparent from the contour map of salinity on the 3 8oc potential temperature surface by worthington and wright 1970 from the vib water flows through jungfern pass 1815 m and nearby grappler pass 1710 m as reported by fratantoni et al 1997 forming a dense overflow plume that cascades into the deep caribbean throughout this paper

the atlantic deep western boundary current water masses and transports near the equator the distributions and transports of deepwater masses at the western boundary in the tropical atlantic off brazil have been studied on three surveys along 35 degrees w and 5 degrees s and one at 10 degrees s transports are obtained from direct measurements of the velocity fields pegasus profiling system and lowered acoustic doppler current profiler and from geostrophic computations using chlorofluoromethane cfm and hydrographic distributions four water masses could be identified forming the north atlantic deep water nadw system two of these have a high cfm content the shallow upper nadw sunadw and the overflow lower nadw olnadw these exhibit the highest velocity signals at 35 degrees w where distinct flow cores seem to exist most of the southeastward flow of the sunadw centered around 1600 m occurs 320 km offshore between 3 degrees 09 s and 1 degrees 50 s 9 7 3 3 sv farther north in that section a highly variable reversing flow is found in a second velocity maximum the transport of olnadw centered around 3800 m of 4 6 2 6 sv is guided by the parnaiba ridge at 1 degrees 45 s 35 degrees w the water masses located between the two cfm maxima the labrador sea water lsw and the lnadw old water mass lnadw old did not show any persistent flow features however a rather constant transport of 11 1 2 6 sv was observed for these two layers the total southeastward flow of the nadw at 35 degrees w showed a transport of 26 8 7 0 sv if one neglects the reversing sunadw north of 1 degrees 50 s at 5 degrees s the flow of all deepwater masses shows vertically aligned cores the main southward transport occurred near the coast 19 5 5 3 sv the boundary current is limited offshore by a flow reversal present in all three surveys but located at different longitudes at 10 degrees s a southward transport of 4 7 sv was observed in november 1992 however the section extended only to 32 degrees 30 w so that probably a significant part of the flow has been missed an important result is the large transport variability between single cruises as well as variability of the spatial distribution of the flow at 35 degrees w which could lead to large uncertainties in the interpretation of single cruise observations despite these uncertainties we suggest a circulation pattern of the various deepwater masses near the equator by combining our mean transport estimates with other observations

crossing of the equator by the deep western boundary current in the western atlantic ocean abstract property distributions and geostrophic shear from a hydrographic section near 37 w in the atlantic ocean show the deep western boundary current dwbc in the north atlantic deep water nadw established against the western boundary of the brazil basin immediately south of the equator between 2 and 5 s the dwbc thus has directly crossed the equator to the south atlantic following the east southeast trend of the continental slope isobaths the estimated dwbc transport of nadw is 35 106 m3 s 1 similar to other estimates from the tropics discussed here these large dwbc transports are opposed by flow of deep water to the north atlantic immediately offshore of the dwbc with as much as two thirds of the dwbc transport being returned as these recirculations one recirculation center is the guiana basin north of the equator but extends at least a few hundred kilometers south of the equator another is visible at 11 s in the brazil basin the degree of connection of these two observed recirculatio

low frequency variability in the equatorial atlantic abstract low frequency v and u component oscillations in the eastern equatorial atlantic are addressed using lime series of up to 1 5 years duration from the lower portion of the main thermocline a distinct frequency separation is observed with characteristic time scales of 1 month for v and 4 8 months for u the principal v component oscillations correspond to seasonally modulated linear equatorially trapped rossby gravity waves apparently generated at the surface in the central equatorial atlantic by shear instability between the south equatorial current and the north equatorial countercurrent their length and time scales are well defined and they agree with theoretical predictions the principal u component oscillations correspond to the deep zonal jets observed by recent profiling experiments in the indian and pacific oceans the vertical scales found here agree with the results from the other oceans however insufficient data are available anywhere to ascertain zonal scales linear theory would

antarctic intermediate water circulation in the tropical and subtropical south atlantic recent hydrographic data from the south atlantic ventilation experiment cruises and others are combined with historical data and used to map the isopycnal properties corresponding to the antarctic intermediate water aaiw in the atlantic ocean the low salinity of the aaiw extends eastward across the south atlantic just south of the equator 3 4 s evidence of a weak eastward flow just north of the equator 1 2 n is also shown lateral and vertical homogenization of properties in the aaiw is found at the equator between 2 s and 2 n there is no clear zonal gradient in salinity just along the equator these observations suggest enhanced mixing within the equatorial baroclinic deformation radius the south atlantic tropical gyre is shown to consist of the following three cells one cyclonic cell centered at about 7 s another centered at about 19 s in the west and 23 s in the east and one anticyclonic cell centered at about 13 s these cells are associated with a westward extension at 10 s of high salinity and low oxygen which originates in the eastern tropical south atlantic and a front in these properties at about 15 s in the west and about 20 s in the east

compositional heterogeneity of tephras from the 1980 eruptions of mount st helens samples collected at hourly intervals on may 18 19 1980 at three sites 200 km downwind from mount st helens have made possible a detailed reconstruction of the conditions that contribute to the compositional heterogeneity of mineral and glass components observed in distal tephra layers the air fall tephra deposited at the sites during the first 7 hours of the may 18 eruption is mostly coarse grained microlite rich nonjuvenile glass and feldspar grain size maxima in this initial tephra can be related to the cataclysmic blast at 0832 and a subsequent pulse of the eruption at 1200 juvenile microlite free glass increases in relative abundance at the sampling sites beginning at about 1900 such a change between nonjuvenile and juvenile tephra can be related to a 5 km increase in column height associated with the last major pulse of the eruption which occurred at 1700 at the volcano electron microprobe study of both microlite rich and microlite free pumice in the time series samples reveals significant compositional differences interstitial glass in nonjuvenile pumice deposited during the first few hours at the sampling sites is enriched in sio2 and k2o and depleted in tio2 feo and mgo relative to juvenile glass by comparison major element composition of the least evolved juvenile glass sampled during the last several hours of the eruption displays a slight trend toward less evolved composition least squares calculations suggest that the more evolved character of the nonjuvenile glass can be explained by greater fractional crystallization brought about by enhanced cooling in a cryptodome prior to eruption whereas the temporal changes observed in juvenile glass composition during the last several hours of the eruption suggest the presence of a small slightly zoned magma chamber at depth electron microprobe study of glass coated ilmenites magnetites and plagioclases provides the following estimates of the physical conditions in this reservoir 865 50 c ph2o 2 2 kbar and log o2 11 7 analyses of bulk pumice glass and selected mineral phases from may 25 june 12 july 22 and october 16 18 pumices erupted from mount st helens indicate that the bulk pumice magma compositions have become slightly more andesitic with time while mineral and co existing glass compositions have changed significantly in post may 18 eruptions with both being more highly evolved than those associated with the may 18 eruption an application of the magnetite ilmenite geothermometer to june 12 and july 22 samples indicates temperatures of 919 30 c and 930 50 c respectively least squares calculations suggest that such evolved post may 18 glass and mineral phases can be derived by fractional crystallization of a magma composition like bulk may 18 pumice into approximately 50 crystals and 50 residual liquid such partitioning between crystals and residual liquid appears to have occurred on the scale of centimeters and is interpreted as a consequence of accelerated crystallization under reduced water pressure

influence of convective sedimentation on the formation of widespread tephra fall layers in the deep sea the formation of widespread volcanic ash fall layers in deep sea sediments was investigated experimentally to examine the settling behavior of tephra 20 180 m diameter as it travels from the atmosphere into water using a fallout mass flux rate that was constrained by measurements of distal fallout from the 1980 eruption of mount st helens 0 2 g cm 2 hr the experiments show that particle settling in the water column is dramatically accelerated by the formation of diffuse vertical gravity currents the currents form as a result of convective instabilities that develop in a surface boundary layer when the local particle concentration becomes large at the air water interface the settling velocity of particles drops abruptly and the concentration of particles increases because of the differential in mass flux above and below the fluid surface the implication of the experiments is that deposition of distal tephra fall layers in deep sea sediments may be dominantly controlled by diffuse vertical gravity currents as opposed to passive settling of individual particles through the water column this process greatly reduces the residence time of fine ash in the ocean and diminishes the role of ocean currents in influencing the distribution patterns of individual tephra layers support for this mechanism comes from observations of greatly accelerated tephra settling rates in the south china sea following the 1991 eruption of mount pinatubo in the philippines

3d baseline seismics at ketzin germany the co2sink project a 3d 25 fold seismic survey with a bin size of 12 by 12 m and about 12 km 2 of subsurface coverage was acquired in 2005 near a former natural gas storage site west of berlin as part of the fiv

an investigation of the spectral properties of primary reflection coefficients this paper investigates the form of the nonwhiteness found in the reflection coefficients from a wide variety of rock sequences around the world in all but one case densities are taken as constant due to the paucity of suitable density data the reflection sequences are pseudo white only above a corner frequency below which their power spectrum falls away according to a power law f where is between 0 5 and 1 5 this spectrum can be adequately modelled in practice very simply with an arma 1 1 process which acts on a white innovation sequence the corollary of this is that before wavelet estimation methods are applied all of which except those based on synthetic seismograms presuppose white reflection sequences or deconvolution filters estimated seismic traces should be filtered with the inverse of this process interestingly the estimated arma processes group themselves into two clearly differentiated categories having very different indices of predictability or strictly indices of linear determinism the two categories apparently correlate precisely with two kinds of sedimentation one which consists largely of sequences of rocks with repeating properties called repetitive in this paper but perhaps loosely describable as cyclic and the other which is randomly bedded with no apparent pattern of components the former has indices of predictability which are two to four times as great as those of the latter another probably related property is that for the repetitive sequences tends to be greater than that for non repetitive rock columns the observed power spectra are shown to be consistent with a simple model for the logarithm of acoustic impedance consisting of a mixture of processes where the distribution of time scale parameters is reciprocal detailed effects of block averaging and sampling the logs are shown to depend on the type of sequence under examination

a new view of the dynamics stability and longevity of volcanic clouds abstract powerful explosive volcanic eruptions inject ash high into the atmosphere which spreads to form umbrella clouds identifying key physical processes governing the dynamics stability and longevity of umbrella clouds is central to assessing volcanic hazards as well as the nature of volcanic forcings on climate here we present a series of laboratory experiments producing turbulent particle laden jets and subsequent axisymmetric intrusive gravity currents into a stratified environment our experiments reproduce many of the main dynamical regimes observed during the formation of an explosive volcanic column and highlight new dynamics for the umbrella cloud theoretical predictions of column collapse from a simple model of a turbulent jet are in good agreement with experimental observations as well as previous studies depending on the flow intensity the strength of the initial environmental density stratification and the particle concentration at the source resulting umbrella clouds can however evolve through a series of new regimes as a result of the dynamics of particle sedimentation within these flows as well as from their bases using scaling theory we show that during cloud spreading internal sedimentation drives the growth and intermittent overturn of thin gravitationally unstable particle boundary layers pbls as particle rich plumes this pbl driven convection can have remarkable effects ranging from progressive dilution of clouds to their catastrophic overturn and collapse in natural eruptions whether the dynamics of pbls play a major role in particle sedimentation depends on the grain size distribution inside the cloud and on eruption column height in general particles larger than 60 m 1 mm are expected to settle individually whereas finer particles will accumulate pbls resulting in the formation of armless mammatus clouds or dangerous gravity currents at much larger distances from the volcanic vent than ever considered before such dynamics is apparent in observations of numerous modern eruptions and is inferred from the deposits of historic and prehistoric eruptions for where there exist appropriate data consideration of the consequences of these phenomena for problems such as volcanic hazards to humans and climate change may thus be very important in the assessment of future eruptions

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

variability of the horizontal velocity structure in the upper 1600 m of the water column on the equator at 10 w this paper presents initial results from new velocity observations in the eastern part of the equatorial atlantic ocean from a moored current meter array during the equalant program 1999 2000 a mooring array was deployed around the equator near 10 w that recorded one year of measurements at various depths horizontal velocities were obtained in the upper 60 m from an upward looking acoustic doppler current profiler adcp and at 13 deeper levels from current meters between 745 and 1525 m to analyze the quasiperiodic variability observed in these records a wavelet based technique was used quasiperiodic oscillations having periods between 5 and 100 days were separated into four bands 5 10 10 20 20 40 and 40 100 days the variability shows i a strong seasonality the first half of the series is dominated by larger periods than the second one and ii a strong dependence with depth some oscillations are present in the entire water column while others are only present at certain depths for the oscillations that are present in the entire water column the origin of the forcing can be traced to the surface while for the others the question of their origin remains open phase shifts at different depths generate vertical shears in the horizontal velocity component with relatively short vertical scales this is especially visible in long duration events 100 days of the zonal velocity component comparison with a simultaneous lowered acoustic doppler current profiler ladcp section suggests that some of these flows may be identified with equatorial deep jets a striking feature is a strong vertical shear lasting about 7 months between 745 and 1000 m these deep current meter observations would then imply a few months of duration for the jets in this region

kinematic elements of antarctic intermediate water in the western south atlantic the northward flowing antarctic intermediate water aaiw is a major contributor to the large scale meridional circulation of water masses in the atlantic together with bottom and thermocline water aaiw replaces north atlantic deep water that penetrates into the south atlantic from the north on the northbound propagation of aaiw from its formation area in the south western region of the argentine basin the aaiw progresses through a complex spreading pattern at the base of the main thermocline this paper presents trajectories of 75 subsurface floats seeded at aaiw depth the floats were acoustically tracked covering a period from december 1992 to october 1996 discussions of selected trajectories focus on mesoscale kinematic elements that contribute to the spreading of aaiw in the equatorial region intermittent westward and eastward currents were observed suggesting a seasonal cycle of the aaiw flow direction at tropical latitudes just offshore the intermediate western boundary current the southward advection of an anticyclonic eddy was observed between 5 degrees s and 11 degrees s farther offshore the flow lacks an advective pattern and is governed by eddy diffusion the westward subtropical gyre return current at about 28 degrees s shows considerable stability with the mean kinetic energy to eddy kinetic energy ratio being around one farther south the eastward deeper south atlantic current is dominated by large scale meanders with particle velocities in excess of 60 cm s 1 at the brazil falkland current confluence zone a cyclonic eddy near 40 degrees s 50 degrees w seems to act as injector of freshly mixed aaiw into the subtropical gyre in general much of the mixing of the various blends of aaiw is due to the activity of mesoscale eddies which frequently reoccupy similar positions c 1999 elsevier science ltd all rights reserved

eddies of newly formed upper labrador sea water a new type of submesoscale eddy has been observed south of the labrador sea during late winter embedded in equatorward flow along the western boundary the eddy radius is 20 km with a weak dynamic signature swirl speed of 0 5 cm s the center of the eddy is characterized by weak stratification elevated concentrations of oxygen and anthropogenic tracers and low tracer ages all indicative of newly ventilated water strong lateral intrusions distort the shape of the feature the water mass contained in the eddy is not classical labrador sea water from the central labrador sea but is significantly fresher and hence lighter it is of the correct density to be the source of the high chlorofluorocarbon layer of the shallow deep western boundary current observed further south and hence is termed upper labrador sea water using a combination of hydrographic data sets along the western boundary to implement a simple lateral diffusion model it is shown that such eddies decay of the order of several months and are difficult to observe equatorward of the grand banks of newfoundland this is in contrast to deeper lenses of classical labrador sea water which persist further equatorward tracer derived ages of the upper labrador sea water eddy range from 3 to 5 years much older than the lifetime deduced from their lateral diffusion a simple convection model of tracer age shows that this age discrepancy is caused by gas exchange being unable to maintain equilibrium between the deep convecting mixed layer and the atmosphere during formation

vertical density currents a review of their potential role in the deposition and interpretation of deep sea ash layers marine tephra layers form important chronostratigraphic markers and may also provide information on eruption dynamics and chronology recent experiments and field data from the 1991 pinatubo eruption challenge tacit assumptions that following atmospheric fallout on the ocean surface marine sedimentation of ash grade material occurs by stokes law settling of individual grains laboratory experiments have demonstrated that vertical density currents can be initiated at representative mass flux rates further experiments described herein involving sustained particle fluxes entering stratified environments demonstrate that ash particles can decouple from the transporting fluid whose vertical motions are limited by the stable density gradient in the ocean vertical density currents generated by ash loading and gravitational destabilization of the water column can overcome the strong stable density gradients in the ocean and transport ash particles vertically 1 3 orders of magnitude faster than possible by stokesian settling greatly reducing their residence time sea bed sedimentation is inferred to involve settling of individual coarse or dense grains through the entire water column plus suspension fallout from the polydisperse turbid layer delivered by convective plumes yielding normal distribution grading the turbid layer may continue laterally as a nepheloid current under the influence of bottom slopes and or geostrophic forces either mechanism possibly modifying the grain size distribution of the suspension and hence the resulting ash layer by transporting selected fractions into different depositional locations consequently and regardless of any subsequent reworking deep sea ash isopachs may not directly reflect an eruptive signal and the ash layers may not directly record the order rate or location of arrival of ash particles at the ocean surface

float trajectories in the deep western boundary current and deep equatorial jets of the tropical atlantic abstract fourteen neutrally buoyant sofar floats at a nominal depth of 1800 m were tracked acoustically for 3 7 yr in the vicinity of the western boundary and the equator of the atlantic ocean the trajectories revealed a swift narrow southward flowing deep western boundary current dwbc extending from 7n across the equator two floats crossed the equator in the dwbc and went to 10s two other floats left the dwbc and drifted eastward in the equatorial band 3s 3n three floats entered the dwbc from the equatorial current system and drifted southward these results suggest that at times the dwbc flows directly southward across the equator with a mean velocity of 8 9 cm s averaged over long distances 2800 km at other times dwbc water is diverted eastward near the equator for long periods 2 3 yr which can reduce the mean along boundary velocity to 1 2 cm s this is much less than the instantaneous along boundary velocities in the dwbc which are often above 25 cm s and occasionally exceed 50 cm s mean eastward flowing jets were observed near 2n and 2s bounding a mean westward jet centered on the equator 1s 1n the southern jet at 2s coincides with a cfc rich plume centered south of the equator the cfc plume is inferred to have been advected by the southern jet across the atlantic and into the gulf of guinea

an improved quantitative measure of the tendency for volcanic ash plumes to form in water implications for the deposition of marine ash beds laboratory experiments and numerical simulations have shown that volcanic ash particles immersed in water can either settle slowly and individually or rapidly and collectively as particle laden plumes the ratio of timescales for individual and collective settling in the form of analytical expressions provides a dimensionless quantitative measure of the tendency for such plumes to grow and persist which has important implications for determining particle residence times and deposition rates however existing measures in the literature assume that collective settling obeys stokes law and is therefore controlled by the balance between gravitational forces and viscous drag despite plume development actually being controlled by the balance between gravitational forces and inertial drag even in the absence of turbulence during early times this paper presents a new measure for plume onset which takes into account the inertial drag controlled rather than viscous drag controlled nature of plume growth and descent a parameter study comprising a set of numerical simulations of small scale volcanic ash particle settling experiments highlights the effectiveness of the new measure and by comparison with an existing measure in the literature also demonstrates that the timescale of collective settling is grossly under estimated when assuming that plume development is slowed by viscous drag furthermore the formulation of the new measure means that the tendency for plumes to form can be estimated from the thickness and concentration of the final deposit the magnitude and duration of particle flux across the water s surface do not need to be known the measure therefore permits the residence times of particles in a large body of water to be more accurately and practically determined and allows the improved interpretation of layers of volcaniclastic material deposited at the seabed

the intermediate depth circulation of the western south atlantic the subsurface oceanic circulation is an important part of the earth climate system subsurface currents traditionally are inferred indirectly from distributions of temperature and dissolved substances occasionally supplemented by current meter measurements neutrally buoyant floats however now enable us to obtain for the first time directly measured intermediate depth velocity fields over large areas such as the western south atlantic here our combined data set provides unprecedented observations and quantification of key flow patterns such as the subtropical gyre return flow 12 sv 1 sverdrup 10 6 m 3 s 1 its bifurcation near the santos plateau and the resulting continuous narrow and swift northward intermediate western boundary current 4 sv this northward flowing water passes through complex equatorial flows and finally enters into the north atlantic

sofar float observations of an intermediate depth eastern boundary current and mesoscale variability in the eastern tropical atlantic ocean two neutrally buoyant sofar floats vigorously looped and meandered at depths of 950 1150 m in the eastern tropical atlantic ocean the float trajectories illustrate a poleward flow along the tropical eastern boundary and significant intermediate depth mesoscale variability in the low latitude eastern basin one float caught within an energetic cyclonic eddy near the eastern boundary looped cyclonically 14 times while translating 600 km northward parallel to the african coastline a second float launched near the mid atlantic ridge meandered eastward with a lagrangian zonal wavelength of 400 km and meridional amplitude exceeding 200 km satellite infrared imagery indicates that horizontal shear associated with the system of near surface zonal equatorial currents may contribute to the observed intermediate depth variability

western boundary current structure and variability east of abaco bahamas at 26 5 n abstract during april 1987 to june 1988 a four mooring transport resolving current meter array was deployed in a section extending 70 km eastward from abaco bahamas at 26 5 n mean currents in the upper layer 800 m showed a clockwise rotation that appears to be associated with a small scale quasi permanent anticyclonic gyre centered just northeast or the bahamas deep mean currents were persistenly southward and indicated a strong deep jet the deep western boundary current dwbc reaching core speeds of 20 cm s 1at 2500 m depth about 25 km seaward of the western boundary vertically integrated meridional volume transport over the section showed a surprisingly large variability ranging from approximately 20 sv northward to 70 sv southward 1 sv 106m3s 1 the mean meridional volume transport was 30 sv to the south with about 3 sv flowing northward above 800 m which could be produced by a weak antilles current and 33 sv flowing southward below 800 m the deep southward transport of 33 sv i

deep circulation in the equatorial atlantic ocean in the atlantic ocean the northward export of warm surface water is compensated by a southward flow of cold north atlantic deep water nadw the nadvv is transported southward along the american continental margin within the deep western boundary current dwbc some tracer and float observations have shown that part of the dwbc water flows eastward along the equator here we present three meridional velocity sections which give an instantaneous image of the lop to bottom zonal circulation along the equatorial atlantic they reveal the presence of equatorial deep jets edjs between 1 30 n and 1 30 s alternating eastward westward currents with short vertical scale surrounded by columns of eastward currents the extra equatorial jets or eejs at 2 n and 2 s in addition to direct velocity measurements tracer distributions give indications of water mass feeding of the edjs and eejs by the dwbc

fluidity a fully unstructured anisotropic adaptive mesh computational modeling framework for geodynamics we present a new computational modeling framework fluidity for application to a range of two and three dimensional geodynamic problems with the focus here on mantle convection the approach centers upon a finite element discretization on unstructured simplex meshes which represent complex geometries in a straightforward manner throughout a simulation the mesh is dynamically adapted to optimize the representation of evolving solution structures the adaptive algorithm makes use of anisotropic measures of solution complexity to vary resolution and allow long thin elements to align with features such as boundary layers the modeling framework presented differs from the majority of current mantle convection codes which are typically based upon fixed structured grids this necessitates a thorough and detailed validation which is a focus of this paper benchmark comparisons are undertaken with a range of two and three dimensional isoviscous and variable viscosity cases in addition model predictions are compared to experimental results such comparisons highlight not only the robustness and accuracy of fluidity but also the advantages of anisotropic adaptive unstructured meshes significantly reducing computational requirements when compared to a fixed mesh simulation

the mechanics of eddy transport from one hemisphere to the other summary the trajectory of a dense eddy propagating along the bottom of a meridional channel of parabolic crosssection from the southern to the northern hemisphere is described by a hamiltonian system with two degrees of freedom two simplie ed types of motion exist in which the meridional acceleration vanishes in midlatitudes the motion is geostrophic poleward equatorward directed along the western eastern e ank of the channel while on the equator the motion consists of zonal oscillations along the potential well generated by the bottom parabolic cross section of the channel the eddy s propagation along the equator is much faster than that in midlatitudes which enhances its dissipation via mixing with the overlying ocean water for motions that occur slightly off the equator the eastward segment isstable while the westward segment isunstable so an expulsion from the equatorial regime takes place during the latter a dense eddy that arrives near the equator along the west eank of the channel has to cross the channel to its east eank where it can either oscillate back westward to the other side or move poleward from the equator along the channel s east e ank the eddy s dissipation during the equatorial part of its trajectory is very large and the probability of the dissipated eddy leaving the equator to either hemisphere is identical the non integrability of the system is manifested in the sensitive combination of the equatorial and the midlatitude regimes that renders the dynamics of the transport of dense eddies across the equator chaotic this description explains both the sharp decrease in the amount of antarctic bottom water mass in the immediate vicinity of the equator in the western atlantic ocean and the splitter effect of the equator this effect encountered in earlier e uid dynamical numerical simulations causes a current and a cloud of particles to chaotically split into two parts eowing in different hemispheres

tracing lower north atlantic deep water across the equator water whose ultimate source is the overflow at the denmark strait can be traced using maps of salinity dissolved oxygen and silica concentration on deep density surfaces south along the american continent to at least 5 s several property extrema whose resolution is at the limit of accuracy of the measurements occur on these density surfaces which nevertheless may indicate areas of stronger mixing along the crest of the mid atlantic ridge transport estimates for that part of the southward flow within 600 km of the western boundary near 10 n suggest that 10 3 106 m3 s 1 of lower north atlantic deep water moves south there the total southward deep water transport near 10 n is estimated to be closed to 25 106 m3 s 1