nonuniform upwelling in a shallow water model of the antarctic bottom water in the brazil basin a numerical model based on the shallow water equations is developed to represent the flow of antarctic bottom water aabw in the brazil basin southwest atlantic ocean the aim is twofold first an attempt is made to identify in a model that includes both simplified dynamics and realistic bathymetry at 1 68 resolution the impacts of the elevated diapycnal mixing rates near the mid atlantic ridge mar documented by dissipation data of the deep basin experiment dbe to this end different assumptions regarding the distribution of the velocity across the aabw layer interface w are considered second the extent to which the shallow water model can replicate observations relative to aabw circulation in the basin in particular the trajectory and velocity of neutrally buoyant floats released in the aabw during the dbe is examined the model flows are characterized by small rossby numbers except in the northward flowing western boundary current where kinetic energy is largely concentrated to interpret the flows model streamlines are compared with isopleths of linear potential vorticity f h0 of the shallow water theory f is the planetary vorticity and h0 is the layer thickness in the absence of motion the f h0 contours are oriented northwest southeast in the western part of the basin and southwest northeast in the eastern part reflecting the bowl shaped topography of the southern hemisphere basin with a spatially uniform positive w the ubiquitous vortex stretching produces a flow to the southeast consistent with the stommel arons theory this flow occurs in most of the basin interior even in the east where f h0 contours converge to the northeastern end of the basin with strongly positive w near the ridge and zero or slightly negative w elsewhere the flow follows more closely f h0 contours in the western interior and intersects them near the ridge the confinement of the diapycnal mass flux near the mar drastically reduces the southward flow in the interior or even reverses its direction leading to a circulation quite distinct from that of the stommel arons theory the model results compare favorably to some but not all hydrographic estimates of aabw circulation patterns and rates on the other hand the model streamlines and velocities show important differences with respectively the trajectory and the velocity of the floats launched in the aabw layer the prescription of vanishing w in the interior does not systematically improve the fit of the model streamlines to the float trajectories and the model velocities simulated with spatially uniform w or spatially variable w are on average smaller by one order of magnitude than the float velocities a variety of mechanisms which are not included in the numerical experiments may explain the differences between the model results and the float data

application of an inverse method to interpret 231pa 230th observations from marine sediments constraints then sediment 231 pa 230 th data for each time interval are combined with an advection scavenging model in order to determine their in consistency with the modern circulation estimate we find that the majority of sediment 231 pa 230 th data for the holocene lgm or h1 can be brought into consistency with the modern circulation if plausible assumptions are made about the large scale distribution of 231 pa and about model uncertainties moreover the adjustments in the data needed to reach compatibility with a hypothetical state of no flow no advection are positively biased for each time interval suggesting that the 231 pa 230 th data including that for h1 are more consistent with a persistence of some circulation than with no circulation our study does not imply that earlier claims of a circulation change during the lgm or h1 are inaccurate but that these claims cannot be given a rigorous basis given the current uncertainties involved in the analysis of the 231 pa 230 th data

ocean margin densities and paleoestimates of the atlantic meridional overturning circulation a model study an eddy permitting numerical ocean model is used to test to what extent the atlantic meridional overturning circulation moc can be reconstructed from sea water densities at the ocean margins by gradually reducing the number of locations where the densities are known and by adding systematic and random errors comparable to those expected in paleoreconstructions of sea water densities we test if a reliable picture of the moc can still be obtained our results show that even with only 0 07 of the boundary densities the mean state of the moc as well as its variability can be reconstructed the addition of a systematic error results in an overestimation or underestimation of the meridional transport with a random error similar to the one expected for paleoestimates of the density the mean state of the moc can still be reproduced especially north of 40 n however the random noise largely masks the relatively modest moc variability observed in the model run for both systematic and random noise in the density field there is a larger sensitivity for moc reconstructions the closer a location is from the equator for a measuring campaign that aims to infer past states of the atlantic moc on the basis of boundary densities the present study suggests that the best strategy is to collect data that allow estimation of the basin wide vertical density structure between 40 n and 50 n information from only a few latitudes is sufficient for a reasonable representation of the mean moc state

the salinity temperature and δ18o of the glacial deep ocean we use pore fluid measurements of the chloride concentration and the oxygen isotopic composition from ocean drilling program cores to reconstruct salinity and temperature of the deep ocean during the last glacial maximum lgm our data show that the temperatures of the deep pacific southern and atlantic oceans during the lgm were relatively homogeneous and within error of the freezing point of seawater at the ocean s surface our chloride data show that the glacial stratification was dominated by salinity variations in contrast with the modern ocean for which temperature plays a primary role during the lgm the southern ocean contained the saltiest water in the deep ocean this reversal of the modern salinity contrast between the north and south atlantic implies that the freshwater budget at the poles must have been quite different a strict conversion of mean salinity at the lgm to equivalent sea level change yields a value in excess of 140 meters however the storage of fresh water in ice shelves and or groundwater reserves implies that glacial salinity is a poor predictor of mean sea level

meridional circulation during the last glacial maximum explored through a combination of south atlantic δ18o observations and a geostrophic inverse model 1 the vertical profile of meridional transport in the south atlantic is examined by combining paleoceanographic observations with a geostrophic circulation model using an inverse method 18ocalcite observations along the margins of the south atlantic show that upper ocean cross basin differences were weaker during the last glacial maximum lgm than the holocene the 18ocalcite observations can be explained by a shift of water mass properties without any change in the overturning circulation alternatively they may indicate a reduced lgm cross basin density difference and via the thermal wind relation a reduced vertical shear model inversions of 18ocalcite are found to require meridional transports different from the modern only after three assumptions are made temperature and salinity distributions are spatially smooth the relationship between salinity and 18owater is linear and spatially invariant and lgm temperatures are known to within 1 c along the margins the last assumption is necessary because an independent constraint on temperature or salinity is required to determine density from 18ocalcite observations 18ocalcite observations are clearly useful but before any firm constraints can be placed on lgm meridional transport it appears necessary to better determine the relationship between 18ocalcite and density

atlantic meridional overturning circulation during the last glacial maximum the circulation of the deep atlantic ocean during the height of the last ice age appears to have been quite different from today we review observations implying that atlantic meridional overturning circulation during the last glacial maximum was neither extremely sluggish nor an enhanced version of present day circulation the distribution of the decay products of uranium in sediments is consistent with a residence time for deep waters in the atlantic only slightly greater than today however evidence from multiple water mass tracers supports a different distribution of deep water properties including density which is dynamically linked to circulation

on the abyssal circulation in the glacial atlantic an inverse method is used to evaluate the information contained in sediment data for the atlantic basin during the last glacial maximum defined here as the time interval 18 21 kyr before present the data being considered are an updated compilation of the isotopic ratios 18 o 16 o 18 o and 13 c 12 c 13 c of fossil shells of benthic foraminifera bottom dwelling organisms first an estimate of the abyssal circulation in the modern atlantic is obtained which is consistent with i climatologies of temperature and salinity of the world ocean circulation experiment ii observational estimates of volume transport at specific locations and iii the statements of a finite difference geostrophic model second estimates of water properties 18 o of equilibrium calcite or 18 oc and 13 c of dissolved inorganic carbon or 13 cdic derived from sediment data are combined with this circulation estimate to test their consistency with the modern flow it is found that more than approximately 80 of water property estimates 18 oc or 13 cdic are compatible with the modern flow given their uncertainties the consistency of glacial 13 cdic estimates with the modern flow could be rejected after two assumptions are made i the uncertainty in these estimates is 0 1 this uncertainty includes errors in sediment core chronology and oceanic representativity of benthic 13 c which alone appears better than this value on average and ii 13 cdic in the glacial deep atlantic was dominated by a balance between water advection and organic c remineralization measurements of 13 c on benthic foraminifera are clearly useful but the current uncertainties in the distribution and budget of 13 cdic in the glacial atlantic must be reduced to increase the power of the test

numerical modeling of the global semidiurnal tide in the present day and in the last glacial maximum 1 a hydrodynamic model incorporating a self consistent treatment of ocean self attraction and loading sal and a physically based parameterization of internal tide it drag is used to assess how accurately barotropic tides can be modeled without benefit of data and to explore tidal energetics in the last glacial maximum lgm m2 solutions computed at high resolution with present day bathymetry agree with estimates of elevations from satellite altimetry within 5 cm rms in the open ocean this accuracy and agreement with atlimetric estimates of energy dissipation are achieved only when sal and it drag are included in the model solutions are sensitive to perturbations to bathymetry and inaccuracies in available global databases probably account for much of the remaining error in modeled elevations the 100 m drop in sea level during the lgm results in significant changes in modeled m2 tides with some amplitudes in the north atlantic increasing by factors of 2 or more dissipation is also significantly changed by the drop in sea level if it drag estimated for the modern ocean is assumed dissipation increases by about 50 globally and almost triples in the deep ocean however it drag depends on ocean stratification which is poorly known for the lgm tests with modified it drag suggest that the tendency to a global increase in dissipation is a robust result but details are sensitive to stratification significant uncertainties about paleotides thus remain even in this comparatively simple case where bathymetry is well constrained

benthic foraminiferal δ18o in the ocean s temperature salinity density field constraints on ice age thermohaline circulation benthic 18o data from 95 core sites are used to infer possible temperature salinity t s fields of the atlantic and pacific oceans at the last glacial maximum lgm a constraint of stable density stratification yields logically consistent scenarios for both t and s the solutions are not unique but are useful as a thinking tool using geosecs data we solve for the modem relationship between 18owater w and salinity in the deep sea w smow 1 529 s 53 18 as a starting point we assume that the slope of this equation applies to lgm conditions and predict 18ocalcite c gradients in equilibrium with probable t s fields of lgm deep and bottom waters benthic foraminiferal 18o data from the deep pacific 2 4 km depth and the bottom atlantic 4 km depth are 0 1 0 2 lower than from the deep atlantic 2 4 km depth at the lgm if the modern w s slope applies atlantic deep and bottom waters were more dense than pacific deep waters this assumption would imply bottom waters both fresher s 0 5 and colder t 3 c than overlying deep waters in conflict with other data suggesting ice age deep water much colder than at present it is also possible that the observed c gradients are an artifact of laboratory intercalibration if atlantic deep and bottom water c values were similar to deep pacific values this would be consistent with the hypothesis of a stronger southern ocean versus north atlantic source for deep ocean ventilation at the lgm taking the observed gradients at face value however a solution could be that the lgm w s slope in deep and bottom waters was higher than at present conceivably because of a stronger contribution of salt to the deep ocean via more intense sea ice freezing this would allow pacific deep waters and atlantic bottom waters to have a common source again in the antarctic both would be more dense than atlantic deep waters even though the deep waters were much colder than at present to better constrain these inferences drawn from the spatial distribution of benthic 18o we must reduce scatter in the 18o data with more high quality measurements in high sedimentation rate cores this is especially true at bottom water sites also we must intercalibrate mass spectrometers at different isotope laboratories more accurately to insure our isotope data are compatible

13c record of benthic foraminifera in the last interglacial ocean implications for the carbon cycle and the global deep water circulation the 13c 12c ratios of upper holocene benthic foraminiferal tests genera cibicides and uvigerina of deep sea cores from the various world ocean basins have been compared with those of the modern total carbon dioxide tco2 measured during the geosecs program the 13c difference between benthic foraminifera and tco2 is 0 07 0 04 for cibicides and 0 83 0 07 for uvigerina at the 95 confidence level 13c analyses of the benthic foraminifera that lived during the last interglaciation isotopic substage 5e about 120 000 yr ago show that the bulk of the tco2 in the world ocean had a 13c value 0 15 0 12 lower than the modern one at the 95 confidence level reflecting a depletion compared to the present value of the global organic carbon reservoir regional differences in 13c between the various oceanic basins are explained by a pattern of deep water circulation different from the modern one the antarctic bottom water production was higher than today during the last interglaciation but the eastward transport in the circumpolar deep water was lower

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

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

till as a glacial tectomict its internal architecture and the development of a typing method for till differentiation abstract till has been studied for over a century yet still a clear understanding of the mechanics of deposition and or emplacement of till eludes us tills in the past have been generally subdivided into sediments formed in various sub environments of the glacial system as such it has been generally agreed that tills can be separated in terms of these types based on various attributes such as clast fabric increasingly such separation into various sub types of till has become however highly debatable and tenuous based upon the use of micromorphology and a greater in depth appreciation of the internal architecture of till it has become apparent that previously established classifications of till are erroneous micromorphology reveals that deposition and or emplacement of tills is a complex tectonic process such that lodgement melt out and flow tills as previously defined do not appear to exist rather a new term better describes tills as glacial tectomicts by using micromorphology in establishing the presence of sets of microstructures a statistical method is introduced that permits differentiation of tills from other diamictons the implications for this new understanding of tills allied to statistical typing is considerable in terms of till formation glacial stratigraphy and understanding glacial environments in terms of stress histories porewater transmission subglacial deformation mechanics till rheological phase changes and the timeline of till formation

transport of bottom water in the romanche fracture zone and the chain fracture zone abstract two moored arrays deployed in the romanche fracture zone and chain fracture zone in the equatorial atlantic ocean provide two year long time series of current and temperature in the lower north atlantic deep water and the antarctic bottom water total time averaged transport of antarctic bottom water potential temperature 1 9 c across the mid atlantic ridge amounts to 1 22 106 m3 s 1 eastward with a standard deviation of 0 25 106 m3 s 1 a time averaged transport of 0 36 0 23 106 m3 s 1 eastward is found for the lower north atlantic deep water in the 1 9 2 1 c temperature range but this may represent only a fraction of the total flow of this water mass across the ridge contributions of the romanche fracture zone and chain fracture zone to the antarctic bottom water transport are similar while the chain fracture zone has the greater share of lower north atlantic deep water transport semiannual and annual periods are detected in the transport time series and together expl

ice sheets and sea level of the last glacial maximum this paper outlines the general issues regarding ice sheets and sea level of the last glacial maximum lgm which formed the basis of an epilog project workshop papers in this special issue of quaternary science reviews provide a comprehensive assessment of these issues from the perspective of geological reconstructions of ice sheet extent records of sea level change ice sheet modelling geophysical models of glacial isostatic adjustment and geochemical proxies of ice volume this new assessment has substantially narrowed the uncertainties in the total changes in ice sheets and sea level and their proxies suggesting a net decrease in eustatic sea level at the lgm ranging from 120 to 135 m r 2001 elsevier science ltd all rights reserved

a geostrophic transport estimate for the florida current from the oxygen isotope composition of benthic foraminifera we present a new method for the quantitative reconstruction of upper ocean flows for during times in the past for the warm t 5 c surface ocean density can be accurately reconstructed from calcite precipitated in equilibrium with seawater as both of these properties increase with decreasing temperature and increasing salinity vertical density profiles can be reconstructed from the oxygen isotopic composition of benthic foraminifera the net volume transport between two vertical density profiles can be calculated using the geostrophic method using benthic foraminifera from surface sediment samples from either side of the florida straits florida keys and little bahama bank we reconstruct two vertical density profiles and calculate a volume transport of 32 sv using this method this agrees well with estimates from physical oceanographic methods of 30 32 sv for the mean annual volume transport we explore the sensitivity of this technique to various changes in the relationship between temperature and salinity as well as salinity and the oxygen isotopic composition of seawater

transequatorial flow of antarctic bottom water in the western atlantic ocean abyssal geostrophy at the equator abstract in its general northward flow along the western trough of the atlantic antarctic bottom water aabw must pass over several sills separating the various abyssal basins at the equator the western trough is deformed by major east west offsets of the mid atlantic ridge and the continental margin of brazil forming a newly zonal abyssal channel about 250 km wide centered at the equator and extending approximately 1000 km along its axis in which the aabw is confined thus the general northward flow of aabw is topographically constrained to be westward as it crosses the equator a hydrographic section across this channel at 37 w shows the aabw isopycnals to be bowl shaped within and beneath the level of the channel walls the equatorial geostrophic relation permits us to compute a zonal velocity from the well defined parabolic distribution of dynamic height relative to a reference level at the transition between aarw and the overlying deep water here 4 3 106 m3 s 1 is estimated for the wes

can paleoceanographic tracers constrain meridional circulation rates abstract the ability of paleoceanographic tracers to constrain rates of transport is examined using an inverse method to combine idealized observations with a geostrophic model considered are the spatial distribution accuracy and types of tracers required to constrain changes in meridional transport within an idealized single hemisphere basin measurements of density and radioactive tracers each act to constrain rates of transport conservative tracers while not of themselves able to inform regarding rates of transport improve constraints when coupled with density or radioactive observations it is found that the tracer data would require an accuracy one order of magnitude better than is presently available for paleo observations to conclusively rule out factor of 2 changes in meridional transport even when assumed available over the entire model domain when data are available only at the margins and bottom of the model radiocarbon is unable to constrain transport while density remains effective o

glacial water mass geometry and the distribution of δ13c of σco2 in the western atlantic ocean 1 oxygen and carbon isotopic data were produced on the benthic foraminiferal taxa cibicidoides and planulina from 25 new piston cores gravity cores and multicores from the brazil margin the cores span water depths from about 400 to 3000 m and intersect the major water masses in this region these new data fill a critical gap in the south atlantic ocean and provide the motivation for updating the classic glacial western atlantic 13c transect of duplessy et al 1988 the distribution of 13c of co2 requires the presence of three distinct water masses in the glacial atlantic ocean a shallow 1000 m southern source water mass with an end member 13c value of about 0 3 0 5 vpdb a middepth 1500 m northern source water mass with an end member value of about 1 5 and a deep 2000 m southern source water with an end member value of less than 0 2 and perhaps as low as the 0 9 values observed in the south atlantic sector of the southern ocean ninnemann and charles 2002 the origins of the water masses are supported by the meridional gradients in benthic foraminiferal 18o a revised glacial section of deep water 13c documents the positions and gradients among these end member intermediate and deep water masses the large property gradients in the presence of strong vertical mixing can only be maintained by a vigorous overturning circulation

antarctic bottom water flow through the hunter channel the rio grande rise acts as a natural barrier for the equatorward flow of antarctic bottom water in the subtropical south atlantic in addition to the vema channel the hunter channel cuts through this obstacle and offers a separate route for bottom water import into the southern brazil basin on the occasion of the deep basin experiment a component of the world ocean circulation experiment woce the expected deep flow through the hunter channel was directly observed for the first time by an array of moored current meters and thermistor chains from december 1992 to may 1994 main results are i the hunter channel is in fact a conduit for bottom water flow into the brazil basin our new mean transport from moored current meters 2 92 1 24 106 m3 s 1 is significantly higher than earlier estimates that were based on geostrophic calculations ii during the woce observational period a tendency toward increased bottom water temperatures was observed this observation from the hunter channel is consistent with findings from the vema channel iii the overflow through the hunter channel is highly variable and puts in perspective earlier synoptic geostrophic transport estimates

large scale circulations forced by localized mixing over a sloping bottom abstract a simple nonlinear two layer planetary geostrophic model of the large scale circulation forced by localized mixing over a sloping bottom is discussed the model is forced by parameterized diapycnal mixing at the density interface and or by a mass flux downward into unresolved deep topographic canyons two nondimensional parameters are identified the ratio of the change in coriolis parameter over the horizontal mixing length scale to the nominal coriolis parameter and the ratio of the advective speed to the rossby wave phase speed the former controls the strength of horizontal recirculation gyres that are forced by spatially variable diapycnal mixing while the latter is a measure of the importance of nonlinearity in the density equation when bottom topography is introduced bottom pressure torque becomes important and the traditional strong horizontal recirculation gyre found for mixing over a flat bottom beta plume is gradually replaced by a zonal flow into or out of the mixing region

thermohaline instability in the north atlantic during meltwater events stable isotope and ice rafted detritus records from core so75 26kl portuguese margin a benthic isotope record has been measured for core so75 26kl from the upper portuguese margin 1099 m water depth to monitor the response of thermohaline overturn in the north atlantic during heinrich events evaluating benthic 18o in ts diagrams in conjunction with equilibrium c fractionation implies that advection of mediterranean outflow water mow to the upper portuguese margin was significantly reduced during the last glacial 15 compared to 30 today the benthic isotope record along core so75 26kl therefore primarily monitors variability of glacial north atlantic conveyor circulation the 14c accelerator mass spectrometry ages of 13 54 07 and 20 46 12 ka for two ice rafted detritus ird layers in the upper core section and an interpolated age of 36 1 ka for a third ird layer deeper in the core are in the range of published 14c ages for heinrich events h1 h2 and h4 marked depletion of benthic 13c by 0 7 1 1 during the heinrich events suggests reduced thermohaline overturn in the north atlantic during these events close similarity between meltwater patterns inferred from planktonic 18o at site 609 and ventilation patterns inferred from benthic 13c in core so75 26kl implies coupling between thermohaline overturn and surface forcing as is also suggested by ocean circulation models benthic 13c starts to decrease 1 5 2 5 kyr before heinrich events hl and h4 fully increased values are reached 1 5 3 kyr after the events indicating a successive slowdown of thermohaline circulation well before the events and resumption of the conveyor s full strength well after the events benthic 13c changes in the course of the heinrich events show subtle maxima and minima suggesting oscillatory behavior of thermohaline circulation a distinct feature of thermohaline instability in numerical models inferrred gradual spin up of thermohaline circulation after hl and h4 is in contrast to abrupt wanning in the north atlantic region that is indicated by sudden increases in greenland ice core 18o and in marine faunal records from the northern north atlantic from this we infer that thermohaline circulation can explain only in part the rapid climatic oscillations seen in glacial sections of the greenland ice core record

constraints from paleotracer data on the north atlantic circulation during the last glacial maximum we investigate the extent to which the paleotracers 13c and 18o constrain the north atlantic ocean general circulation during the last glacial maximum lgm the distinction is made in this work between the depiction of water mass distributions and the inference of the oceanic circulation the 13c data indicate that water masses formed in the north atlantic and in the southern ocean were distributed differently during the lgm but the extent of the differences cannot be quantified because of uncertainties in the end member composition reasonable choices of values for the end members produce volumes of lower north atlantic deep water nadw significantly smaller than in the modern atlantic in agreement with previous studies to explore the range of circulations consistent with this water mass distribution a coarse resolution inverse model of the north atlantic is constrained with 13c and 18o paleodata the model shows that the glacial period tracer distribution is fully consistent with the circulation rates estimated for the modern ocean however the paleodata are also consistent with a circulation rate of one half the modern flux of nadw and indeed there are an infinite number of rates of deep water movement that are possible to remove this ambiguity the central need is for a data type capable of setting the overall rates of water movement i e a clock such as that provided by geostrophy or a well sampled radioactive tracer

an inverse model of the north atlantic general circulation using lagrangian float data abstract a nonlinear finite difference inverse model is used for estimating the north atlantic general circulation between 20 and 50 n the inverse model with grid spacing 2 latitude and 2 5 longitude is based on hydrography and is in geostrophic and hydrostatic balances the constraints of the inverse model are surface and subsurface float mean velocities ekman pumping derived from wind data conservations of mass heat and salt and the planetary vorticity equation at the reference level the mass heat and salt conservations are applied in a vertically integrated form the model does not have explicit mixing or air sea flux terms vertical velocities result from the nondivergence of the 3d velocity field after inversion float velocities hydrographic data and dynamical constraints are generally compatible within error bounds a few float velocities are however rejected by the model mainly due to inadequate time or space sampling of the 2 latitude by 5 longitude boxes for which mean float v

the information content of pore fluid δ18o and cl abstractpaleoceanographic proxies indicate that the ocean state during the last glacial maximum lgm differed from the modern ocean state depth profiles of ocean sediment pore fluid 18o and cl have been used to reconstruct the 18o and salinity at the lgm here it is investigated whether pore fluid profiles can constrain ocean 18o and salinity at other times and simultaneously their ability to constrain the lgm 18o and salinity an inverse framework is developed that relies on bayesian parameter estimation thus allowing formal separation of prior assumptions from the information in observations synthetic problems are used to explore the information about past ocean tracers that can be recovered from pore fluid profiles it is concluded that prior knowledge of deep ocean mixing time scales is essential to an accurate inverse estimate of lgm ocean salinity and 18o from modern pore fluid profiles the most recent 10 000 years of ocean salinity and 18o and the error in their estimates are bette

circulation and variability at the southern boundary of the brazil basin as a contribution to the woce deep basin experiment an array of current meters with individual record lengths exceeding ii years was set across the southern boundary of the brazil basin between early 1991 and early 1996 the array spanned the santos plateau the vema channel and the hunter channel all areas believed to be important for transport of antarctic bottom water between the argentine and brazil basins from the combination of geostrophic velocities computed from hydrographic stations and those directly measured the total transport of bottom water potential temperature below 2 degrees c is estimated to be about 6 9 sv sv 10 6 m 3 s 1 northward with about 4 sv coming through the vema channel and the remainder through the hunter channel properties of the eddy field are also discussed eddy energy levels and their spatial distribution are similar to comparable regimes in the north atlantic integral timescales vary from a few days to several weeks with distance from the brazil current and the western boundary the eddy heat bur is in the same direction as the heat advection by the mean flow but considerably smaller

calibration of stable isotopic data an enriched δ18o standard used for source gas mixing detection and correction we present empirically based calibrations of our measurements made on a finnigan mat252 equipped with a kiel device to vienna pee dee belemnite using an enriched 18o standard calibrations include corrections for biases caused by the differences in isotopic composition of carbonate standards measured on the two parallel extraction lines of the kiel device and for decreases in the isotopic difference between the reference and sample gas caused by mixing in the source after correcting for these biases the precision of 2200 nbs19 analyses 10 300 g is 0 07 for 18o and 0 03 for 13c we have shared our standard enriched in 18o with 18 laboratories engaged in paleoceanographic research producing the first large scale interlaboratory calibrations for this community using correction procedures reported here water mass reconstructions using data produced on multiple mass spectrometers may now be possible with a precision approaching the level necessary to reconstruct temperature salinity and density variability in the deep ocean

pre permian sedimentary basins in the north sea images from reprocessed and pre stack depth migrated mona lisa data pre permian sedimentary basins in the southeastern north sea have been previously interpreted from potential field data but only poorly imaged on seismic sections due to the presence of salt layers and a thick mesozoic and cenozoic cover furthermore potentially low prospectivity for hydrocarbon resources has resulted in sparse penetration of pre permian sediments in the southeastern north sea we present images of large palaeozoic basins obtained through reprocessing and pre stack depth migration of seismic data from the mona lisa project the depth images show in detail the structure of pre permian basins beneath the danish basin and the ringkobing fyn high providing new insights into upper palaeozoic basin formation in the north sea

the oxygen isotopic composition of seawater during the last glacial maximum high resolution oxygen and hydrogen isotope measurements were made on pore fluids from deep sea sediments from sites in the north and south atlantic the data provide direct measurements of changes in the isotopic composition of bottom waters during the last glacial maximum lgm results from ocean drilling program odp site 981 in the north atlantic currently bathed in north atlantic deep water nadw reproduces previous results from the ceara and bermuda rises constraining the glacial interglacial change in 18 o of the deep atlantic to be 0 7 0 8 results from site 984 which is located north of site 981 and at a shallower water depth yield a similar value 0 8 providing insight into the properties of glacial north atlantic intermediate water gnaiw sites from odp leg 177 in the south atlantic span the modern boundary between northern and southern sources of deep water data from the northern site 1088 yield a similar result to sites in the tropical and north atlantic 0 7 at the southern site 1093 located south of the polar front the change is substantially larger 1 1 representing the change in 18 o of southern source waters since the lgm these results confirm previous estimates that the global average change in 18 o of seawater is 1 0 0 1 hydrogen isotopes measured on pore fluids from three sites are consistent with the oxygen isotopes from these locations giving further support to these results at all sites studied the temperature of the deep ocean during the lgm calculated by combining the pore fluid results with oxygen isotope data from benthic foraminifera was within 1 c of the freezing point of seawater

slowing of the atlantic meridional overturning circulation at 25 n the atlantic meridional overturning circulation carries warm upper waters into far northern latitudes and returns cold deep waters southward across the equator its heat transport makes a substantial contribution to the moderate climate of maritime and continental europe and any slowdown in the overturning circulation would have profound implications for climate change a transatlantic section along latitude 25 n has been used as a baseline for estimating the overturning circulation and associated heat transport here we analyse a new 25 n transatlantic section and compare it with four previous sections taken over the past five decades the comparison suggests that the atlantic meridional overturning circulation has slowed by about 30 per cent between 1957 and 2004 whereas the northward transport in the gulf stream across 25 n has remained nearly constant the slowing is evident both in a 50 per cent larger southward moving mid ocean recirculation of thermocline waters and also in a 50 per cent decrease in the southward transport of lower north atlantic deep water between 3 000 and 5 000 m in depth in 2004 more of the northward gulf stream flow was recirculating back southward in the thermocline within the subtropical gyre and less was returning southward at depth

antarctic bottom water flux in the equatorial western atlantic a moored array at the equator in the western basin of the atlantic provides a 604 day time series of abyssal currents and temperatures spanning the full breadth of the antarctic bottom water aabw flowing from the brazil basin to the guiana basin mean aabw transport is estimated to be 2 0 sv sv 106 m3 s21 comprising organized westward flow of 2 24 sv and return flow of 0 24 sv the low frequency variability is dominated by a quasi annual transport cycle of amplitude 0 9 sv and a 120 day period of amplitude 0 6 sv maximum transports occur in september october while minimum transports occur in february march allowing for this quasiannual cycle and extrapolating the 604 day record to a full two years adds about 7 to the estimated mean aabw transport the array also provides limited sampling in the overlying lower north atlantic deep water lnadw where a southern boundary intensified flow of lnadw gives the strongest recorded mean speed through the array 9 9 cm s21 into the brazil basin the lnadw records also have a quasi annual cycle with strong lnadw flow episodes occurring in april may time series of temperature indicate that the lnadw aabw transition layer rises and falls in synchrony with the quasi annual aabw transport cycle uplifted transition layer during strong aabw transport periods an observed overall warming trend appears to be accompanied by a decline in aabw transport

glacial deep water geometry south atlantic benthic foraminiferal cd ca and δ13c evidence a depth transect of cores from 1268 to 3909 m water depth in the western south atlantic are ideally situated to monitor the interocean exchange of deep water and variations in the relative strength of northern and southern sources of deep water production benthic foraminiferal cd ca and 13c data suggest that glacial north atlantic intermediate water gnaiw extended at least as far south as 28 s in the western south atlantic the core of nutrient depleted water was situated at 1500 m above and below water masses with higher nutrient concentrations when examined in conjunction with published paired cd ca and 13c from intermediate depth cores from other basins it appears that the extent of gnaiw influence on the intermediate waters of the world s oceans was less than suggested previously differentiating among possible pathways for the glacial deep ocean 3 km requires a better understanding of the controls on cd ca and 13c values of benthic foraminifera

determining paleoceanographic circulations with emphasis on the last glacial maximum the inference from radiocarbon and other data that the north atlantic ocean circulation during the last glacial maximum was reduced is examined because glacial periods are thought to be windier than non glacial ones a reduced circulation is difficult to rationalize a first step is to note that the circulation of mass is unlikely to correspond simply to the circulation of heat or freshwater salt by merely permitting an enhanced input of southern hemisphere water into the north atlantic one can accommodate significantly increased radiocarbon ages such an input is equally interpretable as owing to an increased mass circulation rather than a decreased one inferred estimates from paleo shear estimates in the florida current are re interpreted as producing an indeterminate net flow there determination of the paleocirculation rates is not simpler than the problem in the modern ocean it is fully three dimensional and requires an adequate data base to determine the mass flux rates

cadmium and delta13c paleochemical ocean distributions during the stage 2 glacial maximum changes in heat and salt transport by ocean currents have been implicated in ice age climate cycles weyl 1 968 newell 1 974 broecker et al 1 990b in addition to the direct physical effects of the ocean on climate the ocean carbon system controls atmospheric co2 and the co2 fluctuations documented in ice core bubbles barnola et a1 1 987 neftel et a1 1 988 are thought to be a significant factor in climate oscillations hansen et al 1 984 dramatic changes in oceanic circulation patterns and carbon system chemistry have occurred during the late quaternary glacial interglacial global climate cycles boyle keigwin 1 982 curry lohmann 1 983 shackleton pisias 1 984 boyle keigwin 1 987 duplessy et al 1 988 curry et al 1 988 broecker et al i 990a these changes in the ocean system may be significant driving forces in the progression of ice age co2 cycles toggweiler sarmiento 1 985 ennever mcelroy 1 985 wenk siegenthaler 1 985 boyle 1 988a b broecker peng 1 990 the oceanic distributions of c513c cd and other carbon system linked properties are controlled by the interaction between biological uptake and decomposition with the general circulation of the ocean because records of past oceanic cd and c5 1 3c are preserved in shells of planktonic and benthic foraminifera

mixing and spreading of the mediterranean outflow abstract hydrographic and current profiler data taken during the 1988 gulf of cadiz expedition have been analyzed to diagnose the mixing spreading and descent of the mediterranean outflow the s properties and the thickness and width of the outflow were similar to that seen in earlier surveys the transport of pure mediterranean water i e water with s 38 4 psu was estimated to be about 0 4 106 m3 s 1 which is lower than historical estimates most of which were indirect but comparable to other recent estimates made from direct velocity observations the outflow transport estimated at the west end of the strait of gibraltar was about 0 7 106 m3 s 1 of mixed water and the transport increased to about 1 9 106 m3 s 1 within the eastern gulf of cadiz this increase in transport occurred by entrainment of fresher north atlantic central water and the salinity anomaly of the outflow was consequently reduced the velocity weighted salinity decreased to 36 7 psu within 60 km of the strait and decre

the δ13c in benthic foraminiferal tests of fontbotia wuellerstorfi schwager relative to the δ13c of dissolved inorganic carbon in southern ocean deep water implications for glacial ocean circulation models on a transect between 20 and 70 s in the eastern atlantic ocean and weddell sea water samples from 19 hydrographic stations and bottom water from 55 surface sediment samples taken with a multiple corer were investigated for the stable carbon isotopic composition of the total dissolved inorganic carbon 13c co2 these measurements were compared to 13c values determined on live specimens of the benthic foraminifer fontbotia wuellerstorfi and closely related genera from the same stations in addition at 16 stations the stable carbon isotope composition of sedimentary organic carbon was measured general deepwater and bottom water mass circulation patterns as inferred from the 13c co2 are in close agreement with those known from other nonconservative tracers very low 13c values of upper circumpolar deep water 0 3 pee dee belemnite pdb in the polar front region and the eastern limb of the weddell gyre coincide with nutrient maxima however a significant decoupling of the dissolved phosphate signal from the 13c co2 signal is indicated in the abyssal weddell sea we attribute this to temperature dependent fractionation processes during gas exchange of surface waters with the atmosphere at sites of bottom water formation multiple corer water from the sediment water interface is slightly 13c depleted relative to deepwater and bottom water 13 co2 the surface sediment organic carbon 13c is 3 to 4 lower south of the polar front than north of it and the 13corg in freshly accumulated phytodetritus is 3 to 4 lower than surface sediment organic carbon 13c comparison of live f wuellerstorfi 13c and related genera with bottom water 13c co2 exhibits at most stations between the subtropical front 41 s and the southern boundary of the antarctic circumpolar current 55 s a significant lowering of foraminiferal 13c values compilation of a mean last glacial interglacial 13c amplitude 13c from six published southern ocean cores results in a shift of 0 99 0 13 pdb this shift is greater than that in all other regions however all of these cores are from positions close to recent oceanic fronts thus for these peripheral areas of the southern ocean we suggest about half of the glacial interglacial shift can be explained by varying frontal zone positions and widths accompanied by a change in mode and height of export production

abyssal atlantic circulation during the last glacial maximum constraining the ratio between transport and vertical mixing transport to vertical diffusivity in the deep atlantic our calculations suggest that the ratio was at least twice as large at the lgm we speculate that the primary cause was reduced mixing between northern and southern component waters associated with movement of this water mass boundary away from the zone of intense mixing near the seafloor the shallower water mass boundary yields an order of magnitude increase in the volume of southern component water suggesting its residence time may have increased substantially our analysis supports the idea that an expanded volume of antarctic bottom water and limited vertical mixing enhanced the abyssal ocean s ability to trap carbon during glacial times

changes in east atlantic deepwater circulation over the last 30 000 years eight time slice reconstructions using 95 epibenthic 13c records eight time slices were reconstructed to trace the distribution of east atlantic deepwater and intermediate water masses over the last 30 000 years our results show that there have been three distinct modes of deepwater circulation near the stage 3 2 boundary the origin of north atlantic deep water nadw was similar to today mode 1 however after late stage 3 the source region of the nadw end member shifted from the norwegian greenland sea to areas south of iceland mode 2 a reduced nadw flow persisted during the last glacial maximum with constant preformed 13c values the nutrient content of nadw increased markedly near the azores fracture zone from north to south probably because of the mixing of upwelled antarctic bottom water aabw from below which then advected with much higher flux rates into the northeast atlantic later the spread of glacial meltwater over the north atlantic led to a marked short term ventilation minimum below 1800 m about 13 500 14c years ago mode 3 the formation of nadw recommenced abruptly north of iceland 12 800 12 500 years ago and reached a volume approaching that of the holocene in the younger dryas 10 800 10 350 years b p another short term shutdown of deepwater formation followed between 10 200 and 9 600 years b p linked to a further major meltwater pulse into the atlantic each renewal of deepwater formation led to a marked release of fossil co2 from the ocean the likely cause of the contemporaneous 14c plateaus over the last 9000 years deepwater circulation varied little from today apart from a slight increase in aabw about 7000 14c years ago it is also shown that the oxygenated mediterranean outflow varied largely independent of the variations in deepwater circulation over the last 30 000 years

benthic isotope evidence for changes of the mediterranean outflow during the late quaternary a suit of sediment cores close to and south of the strait of gibraltar 12 36 n 500 2800 m water depth were analyzed for stable isotopes in epibenthic foraminifers cibicidoides wuellerstorfi and planulina ariminensis during peak glacial times the data exhibit higher 13c values of up to 1 6 at intermediate depths near the strait of gibraltar 36 n the values decrease to the south as evidenced by our data but also to the north as revealed by data of intermediate depth cores north of 38 n in duplessy et al 1987 thus the distribution pattern of 13c provides crucial evidence for an increased influence of nutrient depleted mediterranean outflow water mow on the glacial northeast atlantic hydrography during oxygen isotope terminations i and ii the meridional carbon isotope gradient indicates a significantly decreased but still active mow as deduced from the 18o fluctuations the temperatures of the mow in the atlantic were lower during glacial times by as much as 5 c during glacial times and during termination i the maximum 13c values of the mow correlate with minimum values of the north atlantic deep water nadw and vice versa this inverse response to climatic change of the carbon isotope signals of both water masses indicates that the supply of saline mow to the north atlantic may be less important for the formation of nadw than previously assumed

the inference of north atlantic circulation patterns from climatological hydrographic data we present a review of spiral techniques which have recently been developed for the determination of absolute velocity profiles from hydrographic observations a specific technique is then designed and applied to the north atlantic part of levitus 1982 climatological hydrographic atlas with the aim of estimating reference velocities and diffusivities for heat salt and potential vorticity these quantities are determined on a 1 grid from the local gradients of temperature and salinity under the constraints of the thermal wind relations and the conservation of the respective tracers including diapycnic and isopycnic mixing terms the estimation procedure includes the statistical framework of inverse modeling in the weighting of the constraints by the data noise variances and the determination of the covariances of the model parameters the resulting circulation pattern bears strong resemblance to the classical view of the north atlantic circulation as put foreward by wust 1935 and defant 1941 the upper layers are dominated by the gulf stream north atlantic current system with a broad subtropical gyre recirculation in the lower layer a western boundary current is fed from norwegian sea overflow penetrating the gibbs fracture zone and partly circulating around the labrador sea as a consequence of the climatological averaging the currents appear in broad shape with much reduced velocities in particular in the upper layer the vertical structure reveals an almost horizontal level of no motion pattern much along the concepts of defant 1941 diffusion coefficients were determined for an upper layer depth of mixed layer to 800 m depth and a lower layer 800 m to 2000 m the spatial pattern of these coefficients correlates with maps of eddy activity showing higher values in the strong current regimes and low values within the subtropical and subpolar gyre average values in the lower layer of the quiet regions are 10 5 m s and 10 m s for the diapycnal and isopycnal diffusivity respectively and 10 1 m s for the vertical diffusivity of vorticity which yields 10 m s for the lateral diffusivity of potential vorticity toward the regions of strong currents and in the upper layer these values roughly increase by an order of magnitude