mixing and entrainment in the red sea outflow plume part i plume structure when the salty and heavy water of the red sea exits from the strait of bab el mandeb it continues downslope into the gulf of aden mainly along two channels the 130 km long northern channel nc is topographically confined and is typically only 5 km wide in it the red sea plume shows unanticipated patterns of vertical structure turbulent mixing and entrainment above the seafloor a 25 120 m thick weakly stratified layer shows little dilution along the channel hence this bottom layer undergoes only weak entrainment in contrast a 35 285 m thick interfacial layer shows stronger entrainment and is shown in a companion paper to undergo vigorous turbulent mixing it is thus the interface that exhibits the bulk of entrainment of the red sea plume in the nc the interfacial layer also carries most of the overall plume transport increasingly so with downstream distance the southern channel sc is wider than the nc and is accessed from the latter by a sill about 33 m above the floor of the nc entrainment into the bottom layer of the sc is diagnosed to be strong near the entry into the sc such that the near bottom density and salinity are smaller in the sc than in the nc at the same distance from bab el mandeb in comparison with winter conditions the authors encountered weaker outflow with shallower equilibration depths during the summer cruise bulk froude numbers computed for the whole plume varied within the range 0 2 1 local maxima occurred in relatively steep channel sections and coincided with locations of significant entrainment

a regional modeling study of the entraining mediterranean outflow 1 we have evaluated a regional scale simulation of the mediterranean outflow by comparison with field data obtained in the 1988 gulf of cadiz expedition our ocean model is based upon the hybrid coordinate ocean model hycom and includes the richardson number dependent entrainment parameterization of xu et al 2006 given realistic topography and sufficient resolution the model reproduces naturally the major observed features of the mediterranean outflow in the gulf of cadiz the downstream evolution of temperature salinity and velocity profiles the mean path and the spreading of the outflow plume and most importantly the localized strong entrainment that has been observed to occur just west of the strait of gibraltar as in all numerical solutions there is some sensitivity to horizontal and vertical resolution when the resolution is made coarser the simulated currents are less vigorous and there is consequently less entrainment our richardson number dependent entrainment parameterization is therefore not recommended for direct application in coarse resolution climate models we have used the high resolution regional model to investigate the response of the mediterranean outflow to a change in the freshwater balance over the mediterranean basin the results are found in close agreement with the marginal sea boundary condition msbc a more saline and dense mediterranean deep water generates a significantly greater volume transport of the mediterranean product water having only very slightly greater salinity

descent and modification of the overflow plume in the denmark strait bulk properties of the denmark strait overflow dso plume observed in velocity and hydrography surveys undertaken in 1997 and 1998 are described despite the presence of considerable short term variability it is found that the pathway and evolution of the plume density anomaly are remarkably steady bottom stress measurements show that the pathway of the plume core matches well with a rate of descent controlled by friction the estimated entrainment rate diagnosed from the rate of plume dilution with distance shows a marked increase in entrainment at approximately 125 km from the sill leading to a net dilution consistent with previous reports of a doubling of overflow transport measured by current meter arrays the entrainment rate increase is likely related to the increased topographic slopes in the region compounded by a decrease in interface stratification as the plume is diluted and enters a denser background

eight glacial cycles from an antarctic ice core the antarctic vostok ice core provided compelling evidence of the nature of climate and of climate feedbacks over the past 420 000 years marine records suggest that the amplitude of climate variability was smaller before that time but such records are often poorly resolved moreover it is not possible to infer the abundance of greenhouse gases in the atmosphere from marine records here we report the recovery of a deep ice core from dome c antarctica that provides a climate record for the past 740 000 years for the four most recent glacial cycles the data agree well with the record from vostok the earlier period between 740 000 and 430 000 years ago was characterized by less pronounced warmth in interglacial periods in antarctica but a higher proportion of each cycle was spent in the warm mode the transition from glacial to interglacial conditions about 430 000 years ago termination v resembles the transition into the present interglacial period in terms of the magnitude of change in temperatures and greenhouse gases but there are significant differences in the patterns of change the interglacial stage following termination v was exceptionally long 28 000 years compared to for example the 12 000 years recorded so far in the present interglacial period given the similarities between this earlier warm period and today our results may imply that without human intervention a climate similar to the present one would extend well into the future

laboratory model studies of mediterranean outflow adjustment in the gulf of cadiz results are presented from a series of laboratory modelling studies carried out as part of the international multidisciplinary project canigo the focus of interest has been the phenomenon of flow adjustment within the gulf of cadiz of the mediterranean outflow plume modelled by a turbulent negatively buoyant jet discharging into a rotating stratified reservoir scaled to the idealised topography of the gulf the results show that the fully developed flow pattern that forms close to the entrance to the gulf consists of a relatively intense jet that divides when it encounters the southern iberian coastal boundary part of the jet is deflected westwards to form a boundary current at intermediate depth along the coastline and part is deflected eastwards and back towards the strait to form a closed weak circulation near the diverging right wall of the gulf within this closed circulation the typical horizontal dimensions of which scale with the source froude number when normalised with the inertial radius small scale eddy features are embedded the degree of entrainment into the descending turbulent jet is seen to be determined primarily by the receiving water froude number and is relatively insensitive to the burger and rossby numbers of the flow resulting in the formation of a boundary current of intermediate density that follows the iberian coastal boundary with a typical velocity determined primarily by the effective driving head at the source and weakly by the burger number of the flow

qualitative and quantitative approaches in palaeohydrography a case study from core top parameters in the gulf of cadiz palaeoceanography relies on the assumption that parameters measureable in sediment cores correlate reliably to hydrographic parameters areas of dynamic watermass mixing which generally have steep hydrographic gradients therefore provide both a tempting target large spatial and temporal differences in temperature and salinity and a significant challenge high flow velocities and high noise for palaeoceanographers here we investigate the ability of parameters measured on core tops to be used as the basis for simulating regional hydrography within one globally important area of watermass mixing the gulf of cadiz using grainsize sediment composition benthic foraminiferal assemblage and benthic foraminiferal stable isotope analysis it is possible to build an effective framework for qualitative constraint of the position of the modern mediterranean outflow water mw plume we propose that the gulf of cadiz slope can be sub divided into 8 hydrographic zones reflecting these parameters proximal mw core mw1 steep slopes core mw1 gentle slopes distal mw1 distal mw2 lower limit of mw plume north atlantic deep water and ambient atlantic water we anticipate that compilation of time slice data will reveal hydrographic sub divisions of the slope for the past in a similar manner improving our understanding of past changes in the size and position of the mw plume but more work needs to be done before a secure protocol for quantitative reconstruction can be created stable isotope analysis alone is insufficient for the task with 13c behaving in a non conservative manner and 18ocalcite acting in an ambiguous way due to the competing influences of 18owater and temperature it is likely that if a single parameter that unambiguously determines the difference between mediterranean water and atlantic water can be identified combination with 18ocalcite will allow extension of our qualitative analysis into a quantitative means of palaeohydrographic reconstruction for this region similar complexity to that found in this study would be expected in any mixing region and in particular on the majority of sediment drifts we recommend that core top surveys similar to this study are performed on these sediment drift systems before any form of quantitative palaeohydrology is attempted

a modified law of the wall applied to oceanic bottom boundary layers 1 near the bottom the velocity profile in the bottom boundary layer over the continental shelf exhibits a characteristic law of the wall that is consistent with local estimates of friction velocity from near bottom turbulence measurements farther from the bottom the velocity profile exhibits a deviation from the law of the wall here the velocity gradient continues to decrease with height but at a rate greater than that predicted by the law of thewall with the local friction velocity we argue that the shape of the velocity profile is made consistent with the local friction velocity by the introduction of a new length scale that near the boundary asymptotes to a value that varies linearly from the bottom farther from the boundary this length scale is consistent with the suppression of velocity fluctuations either by stratification in the upper part of the boundary layer or by proximity to the free surface the resultant modified law of the wall provides a good representation of velocity profiles observed over the continental shelf when a local estimate of the friction velocity from coincident turbulence observations is used the modified law ofthe wall is then tested on two very different sets of observations from a shallow tidal channel and from the bottom of the mediterranean outflow plume in both cases it is argued that the observed velocity profile is consistent with the modified law of the wall implicit in the modified law of the wall is a new scaling for turbulent kinetic energy dissipation rate this new scaling diverges from the law of the wall prediction above 0 2d where d is the thickness of the bottom boundary layer and agrees with observed profiles to 0 6d

current and density observations of packets of nonlinear internal waves on the outer new jersey shelf abstractclosely spaced observations of nonlinear internal waves nliws were made on the outer continental shelf off new jersey in june 2009 nearly full water column measurements of current velocity were made with four acoustic doppler current profilers adcps that were moored about 5 km apart on the bottom along a line approximately normal to the bathymetry between water depths of 67 and 92 m density profiles were obtained from two vertical strings of temperature and conductivity sensors that were deployed near each of the interior adcp moorings in addition a towed scanfish provided profiles and fixed level records of temperature and salinity through several nliw packets near the moorings several case studies were selected to describe the propagation of the nliws one to three solitary waves of depression were observed in five selected packets there were also occurrences of multiple phase dispersive wave packets the average propagation speed corrected for advection of the observed waves was 0 51

cause of the rupture and distribution of broken submarine carbonate chimneys in the gulf of cadiz southwestern spain abstract during the 2000 and 2001 r v cornide de saavedra cruises a vast field of hydrocarbon derived carbonate chimneys was discovered and sampled along the cadiz contourite channel and guadalquivir diapiric ridge photographs taken on board using an underwater camera revealed a spectacularly high density of pipe like chimneys lying on the sea floor though some of them were also found protruding from muddy sediment the major axes of the fallen chimneys which can be as long as 1 m exhibit a regular spatial distribution showing a conspicuous nw se trend furthermore their basal morphology shows a characteristic angular breakage associated to a flexo traction process both the uniform distribution of the major axis and the basal morphology of the chimneys imply that the cause of the rupture could be related either to strong bottom sea currents or to inertia forces associated with oscillatory ground motion due to seismic shaking the preferential orientation of the broken chimneys their mechanical properties the geological and topographical conditions of the area in relation to seismic ground motion amplification and the flow speed of the mediterranean outflow water mow along the cadiz contourite channel suggest that the most probable cause of the chimneys rupture was an earthquake the minimum magnitude of this seismic event is estimated to be more than 7 0 the causative fault being located directly under the chimney field or as far as 30 60 km away the chimneys rupture took place between ca 5 ka to present

ten years of marine current measurements in espartel sill strait of gibraltar more than 10 year of acoustic doppler current profiler observations collected at the westernmost sill espartel sill of the strait of gibraltar by a monitoring station have been carefully processed to provide the most updated estimation of the mediterranean outflow a comprehensive quality control of the factors affecting the uncertainty of the measurements has been carried out and great care has been paid to infer the current at the bottom layer where direct observations are lacking the mean outflow in the southern channel of the sill section has been estimated as 0 82 sv 1 sv 1 106 m3 s 1 with an average contribution of the eddy fluxes of 0 04 sv this figure is an overestimation as the mooring measurements assumed valid for the whole section ignore the lateral friction on the other hand it only gives the flow through the southern channel and disregards the fraction flowing through shallower northern part both drawbacks have been addressed by investigating the cross strait structure of the outflow from hindcasts produced by the mitgcm numerical model run in a high resolution domain covering the gulf of cadiz and alboran sea basins an overall rectifying factor of 1 039 was found satisfactory to correct the first estimate so that the final mean outflow computed from this data set is 0 85 sv complemented with an uncertainty of 0 03 sv based on the interannual variability of the series the temporal analysis of the series shows an outflow seasonality of around the 8 of the mean value with maximum outflow in early spring

a numerical study on the successive formation of meddy like lenses 1 we present a process study on the sustaining mechanism of lens formation using a series of numerical experiments of a density current over a sloping bottom with a cape along the coastline water parcels in the bottom density current are shed into the offshore region leading to periodic formation of anticyclonic lenses as part of baroclinic dipolar vortices the cyclonic partner is more prominent at the surface and the coupled vortices are carried by the mean current established in the offshore region parameter dependence of dipole generation is examined which suggests that the background current is necessary for the detached eddies to be coherent in the downstream direction and for shedding events to repeat nof and pichevin 1996 it is also shown that the density mixing of the bottom current provides criteria for cyclogenesis at the sea surface a detailed analysis is given by a five layer model forced by a water mass source sink which reproduces the baroclinic dipolar vortices similar to those observed in the preceding z coordinate model we find that the dipole generation is due to the finite amplitude divergence convergence of the baroclinic current passing the cape stern and chassignet 2000 the overall analyses suggest three necessary conditions for successive eddy formation 1 a localized variation in the coastline causing the finite amplitude disturbance 2 mixing of mediterranean water with the surrounding fluids leading to anticyclonic rotation of meddies as well as cyclogenesis at the surface and 3 background currents that advect the detached vortices out of the source region index terms 4255 oceanography general numerical modeling 4219 oceanography general continental shelf processes 4520 oceanography physical eddies and mesoscale processes 4528 oceanography physical fronts and jets keywords eddy shedding dense water plume numerical simulation

what can the data on late survival of australian megafauna tell us about the cause of their extinction abstract roberts et al 2001a new ages for the last australian megafauna continent wide extinction about 46 000 years ago science 292 1888 1892 concluded that the extinction of australia s late pleistocene megafauna was an abrupt event that took place about 46 ka ago throughout the continent by placing the extinctions soon after the arrival of people in australia but before climate changes associated with the last glacial maximum this study implicated human impact as cause however the study was controversial because it excluded evidence mainly from archaeological sites containing disarticulated megafauna remains for survival of some megafauna well past 46 ka ago here i ask how our interpretation of the extinctions would be changed if this evidence were accepted contrary to climate change models of extinction the young megafauna sites are not concentrated in mesic refuges around the coast these sites do suggest that relatively small bodied megafauna species were the last to disappear as predicted by a version of the overkill hypothesis more work is needed to test the evidence for late survival of megafauna species in australia but at present this evidence supports overkill not climate change as the cause of the extinctions

early human occupation at devil s lair southwestern australia 50 000 years ago abstract new dating confirms that people occupied the australian continent before the earliest time inferred from conventional radiocarbon analysis many of the new ages were obtained by accelerator mass spectrometry 14c dating after an acid base acid pretreatment with bulk combustion aba bc or after a newly developed acid base wet oxidation pretreatment with stepped combustion abox sc the samples charcoal came from the earliest occupation levels of the devil s lair site in southwestern western australia initial occupation of this site was previously dated 35 000 14c yr b p whereas the aba bc ages are indistinguishable from background beyond 42 000 14c yr b p the abox sc ages are in stratigraphic order to 55 000 14c yr b p the abox sc chronology suggests that people were in the area by 48 000 cal yr b p optically stimulated luminescence osl electron spin resonance esr ages u series dating of flowstones and 14c dating of emu eggshell carbonate are in agreement with the abox sc 14c chronology these results based on four independent techniques reinforce arguments for early colonization of the australian continent

continuous 150 k y monsoon record from lake eyre australia insolation forcing implications and unexpected holocene failure our reconstructed history of lake eyre provides the first continuous continental proxy record of australian monsoon intensity over the past 150 k y this continental record s broad correspondence to the marine isotope record demonstrates that this very large catchment with its hydrology dependent on a planetary scale climate element responds to milankovitch scale climate forcing abrupt transitions from dry phases to wet phases ca 125 and 12 ka coincide with northern hemisphere winter insolation minima rather than southern hemisphere summer insolation maxima indicating that northern hemi sphere insolation exerts a dominant control over the intensity of the australian monsoon stratigraphic and dating uncertainties of other wet phases preclude conclusive correlation to specific insolation signals but within the uncertainties are consistent with northern hemisphere forcing regardless of the hemispheric forcing the low intensity of the early holocene australian monsoon by comparison with the last interglacial and particularly the last high level lacustrine event at 65 60 ka when all forcing elements were modest is an enigma that can be explained by a change in boundary conditions within australia

internal hydraulic flows on the continental shelf high drag states over a small bank observations of currents hydrography and turbulence provide unambiguous evidence for hydraulic control of flow over an isolated three dimensional topographic feature on oregon s continental shelf the flow becomes critical at the crest of the bank forming a strong supercritical downslope flow in the lower layer farther downstream internal hydraulic jumps form as the bottom flow becomes subcritical as a consequence turbulence is greatly enhanced in the bottom boundary layer in the sheared interface above the swiftly flowing bottom current and in the internal hydraulic jump the dissipation rate of turbulent energy is consistent with the mean energy removal rate for a hydraulic jump in an idealized two layer flow this enhanced turbulence constitutes a high drag state of the flow in which the form drag introduced by the topography exerts significant influence on the flow around it and mixing is increased 102 103 times the background values

evidence of climatic control on hydrocarbon seepage in the miocene of the northern apennines the case study of the vicchio marls abstract the vicchio outcrop in the tuscan apennines contains excellent exposures of a miocene methane derived carbonate system made up of more than 80 carbonate bodies enclosed in marly sediments facies analysis composition and a detailed biostratigraphic study of the carbonates and enclosing vicchio marls allowed us to document the role of climatic changes and eustasy on seepage in these ancient deposits results of our study indicate that the stratigraphic horizon bearing seep carbonates is constrained by two planktonic foraminiferal events the acme end ae of turborotalita cf t quinqueloba 13 75 ma and the acme 1 beginning a 1 b of paragloborotalia siakensis 13 32 ma encompassing about 400 000 years the ae of t cf t quinqueloba approximates the mid miocene global cooling event mi3b as defined by the 18 o maxima 13 78 ma the glacio eustatic sea level drop associated with this cooling event is estimated to be approximately 60 m assuming that the highest rates of glacio eustatic sea level fall coincide with 18 o maxima the timing of the seepage onset at about 13 75 ma is concomitant with the phase of sea level lowering the ascent and emission of methane rich fluids may have been triggered by pressure drop due to the eustatic fall associated with the mi3b event the moderate intensity of fluid expulsion is suggested by the lack of brecciation by the scarcity of detrital particles in the carbonates and by the pervasive occurrence of carbonate in the enclosing marls this type of seepage differs from focused fault confined fluid flows typical of other tectonically controlled apenninic seep carbonates

momentum and energy balance of the mediterranean outflow field data taken in the gulf of cadiz have been analyzed to describe some aspects of the momentum and energy balance of the mediterranean outflow a crucial component of the momentum balance is the total stress entrainment stress and bottom drag which has been estimated from a form of the bernoulli function evaluated from density and current observations for the first 60 km west of the camarinal sill the outflow was confined within a narrow channel on the continental shelf at about 70 km downstream the outflow crossed over the shelf slope break and began to descend the continental slope the buoyancy force increased substantially and the outflow underwent a geostrophic adjustment albeit one heavily influenced by mixing and dissipation the current direction turned 90 degrees to the right at a near inertial rate in this region the estimated geostrophic velocity greatly underestimated the actual current and the estimated curvature rossby number was about 0 5 current speeds were in excess of 1ms 2 1 and the total stress was as large as 5 pa the entrainment stress estimated independently from property fluxes reached a maximum of about 1 pa or considerably smaller than the inferred bottom stress by about 130 km downstream the current was aligned approximately along the local topography the current amplitude and the estimated stress were then much less about 0 3 m s21 and 0 3 pa the entrainment stress was also very small in this region well downstream of the strait this slightly damped geostrophic flow continued on to cape st vincent where the outflow began to separate from the bottom bottom stress thus appears to be a crucial element in the dynamics of the mediterranean outflow allowing or causing the outflow to descend more than a kilometer into the north atlantic in the regions of strongest bottom stress the inferred drag coefficient was about 2 2 12 3 10 23 depending upon which outflow speed is used in the usual quadratic form entrainment stress was small by comparison to the bottom stress but the entrainment effect upon the density anomaly was crucial in eroding the density anomaly of the outflow the observed entrainment rate appears to follow roughly a critical internal froude number function

oceanographic processes and morphosedimentary products along the iberian margins a new multidisciplinary approach abstract our understanding of bottom currents and associated oceanographic processes e g overflows barotropic tidal currents including intermittent processes e g vertical eddies deep sea storms horizontal vortices internal waves and tsunamis is rapidly evolving many deep water processes remain poorly understood due to limited direct observations but can generate significant depositional and erosional features on both short and long term time scales this paper represents a review work which describes for the first time these oceanographic processes and examines their potential role in the sedimentary features along the iberian continental margins this review explores the implications of the studied processes given their secondary role relative to other factors such as mass transport and turbiditic processes and highlights three major results a contourite depositional and erosional features are ubiquitous along the margins indicating that bottom currents and associated oceanographic processes control the physiography and sedimentation b the position of interfaces between major water masses and their vertical and spatial variation in time specifically appears to exert primary control in determining major morphologic changes along the slope gradient including the contourite terraces development and c contourites deposits exhibit greater variation than the established facies model suggests therefore a consistent facies model however faces substantial challenges in terms of the wide range of oceanographic processes that can influence in their development an integrated interpretation of these oceanographic processes requires an understanding of contourites seafloor features their spatial and temporal evolution and the near bottom flows that form them this approach will synthesize oceanographic data seafloor morphology sediments and seismic images to improve our knowledge of permanent and intermittent processes around iberia and evaluate their conceptual and regional role in the margin s sedimentary evolution given their complexes three dimensional and temporally variable nature integration of these processes into sedimentary oceanographic and climatological frameworks will require a multidisciplinary approach that includes geology physical oceanography paleoceanography and benthic biology

late quaternary milankovitch scale climatic change and variability and its impact on monsoonal australasia four pollen and charcoal records derived from marine cores around the northern perimeter of australia are examined to provide a regional picture of patterns causes and impacts of climate change over the last 100 300 ka the availability of radiocarbon dates and oxygen isotope records for the cores provides primary chronological control spectral analysis of components of these records demonstrates an overall importance of milankovitch frequencies with clear glacial interglacial cyclicity dominated by variation in precipitation in addition a number of pollen taxa as well as charcoal particles exhibit a 30 ka frequency that is considered from its relationship with biomass burning and with results of past modelling to reflect changes in the intensity of el nino southern oscillation enso variability pollen components of all records show a decline frequently stepwise in more fire sensitive vegetation and its replacement with more fire tolerant vegetation there is some evidence that this trend is linked to an onset or general increase in enso activity and perhaps also to variation in monsoon activity dating from about 300 ka bp that was caused by changes to oceanic circulation within the indonesian region the trend may have accelerated within the last 45 ka due to burning by indigenous people c 2003 elsevier b v all rights reserved

gravity current dynamics and entrainment a process study based on observations in the arkona basin abstract a 19 h time series of dissipation stratification and horizontal velocities has been obtained for a dense gravity current flowing into the arkona basin in the western baltic sea the observations are compared with one dimensional quasi steady theory in which the gravity component in the flow direction is balanced by bottom friction while that in the cross flow direction is balanced by the coriolis force the observations deviate from the theory in that the bottom shear stress is more than 2 times as large as that required to balance the gravity several reasons for this discrepancy are discussed a 1d turbulence model is also compared with the observations profiles of velocity stratification and dissipation rates generally show similar variations with depth as the observations although the observed dissipation rates are somewhat larger than the modeled and the modeled transverse velocities are much larger than the observed subsequently the model is used to investigate the variation of t

stress on the mediterranean outflow plume part ii turbulent dissipation and shear measurements abstract bottom and interfacial stresses on the mediterranean outflow plume are estimated using vertical profiles of turbulent dissipation and velocity collected in the gulf of cadiz turbulent dissipation is high throughout the plume with a local minimum often present near the plume nose depth of maximum downstream velocity bottom stresses are estimated by applying a log layer model to the dissipation measurements the dissipation measurements are also divided by plume scale vertical shear from the horizontal velocity profiles to construct profiles of stress within the plume the mean stress estimates in the bottom layer agree well with those calculated in the log layer from the dissipation measurements alone the bottom layer means are slightly larger than those of the interfacial layer the maximum stresses in each layer are uncertain since they depend on the ill defined shape of the stress profiles within the plume dissipation derived stress estimates in the log layer and those from dissipation

mixing and entrainment in the red sea outflow plume part ii turbulence characteristics abstract south of the strait of bab el mandeb saline red sea water flows downslope into the gulf of aden mainly along the narrow 130 km long northern channel nc and the shorter and wider southern channel sc in the nc the red sea plume simultaneously exhibited weak entrainment into a 35 120 m thick weakly stratified bottom layer while a 35 285 m thick interfacial layer above showed signs of vigorous mixing overturns up to 30 m thick and extensive zones of gradient richardson numbers below 1 4 turbulent overturning scales or thorpe scales are extracted from regular ctd profiles and equated to ozmidov scales on this basis interfacial mixing is quantified in terms of estimated turbulent dissipation rates vertical turbulent salt flux and interfacial stress even though these estimates are subject to significant uncertainty they demonstrate the intensity of mixing during strong winter outflow in terms of eddy diffusivities k on the order of 10 2 m2 s 1 the large k occur in strong strati

dynamics of rotating shallow gravity currents passing through a channel part i observation of transverse structure abstract a detailed dataset describing a quasi stationary bottom gravity current approximately 10 m thick and 10 km wide passing through a channel like constriction in the western baltic sea is presented the data include full depth synoptic and highly resolved transects of stratification and turbulence parameters as well as detailed velocity transects across the gravity current at different down channel locations the velocity data reveal a persistent transverse circulation creating a characteristic wedge shaped density structure in the interface a strong asymmetry was also found in the interior of the gravity current where the evolution of a dynamically significant transverse density gradient to the right of the down channel flow was observed spectral analysis of the near bottom velocities showed a surprisingly strong contribution to the bottom stress from low frequency motions with periods up to 30 min that are possibly related to internal wave effects cross channel transects of shear microst

palaeontology extinction promoted fire the extinction of megafauna in australia roughly coincided with shifts in vegetation and fire regimes sediment geochemistry shows that the vegetation shift followed the extinction indicating that the loss of browsers promoted fire and altered plant composition

would the australian megafauna have become extinct if humans had never colonised the continent comments on a review of the evidence for a human role in the extinction of australian megafauna and an alternative explanation by s wroe and j field a critical comment on a review of the evidence for a human role in the extinction of australian megafauna and an alternative explanation by s wroe and j field is presented the authors have highlighted a range of ideas under consideration and provided a selective interpretation which does not come to terms with biology and ignores or misinterprets current evidence they rely heavily on the ages reported by roberts et al 2001 to argue for a gradual attenuation of the megafauna they propose a staggered series of extinctions throughout the middle and late pleistocene with many taxa lost during the penultimate glacial maximum pgm 140 130 ka and relatively few species persisting they ignore measurement uncertainties associated with the ages which when properly considered means that 20 rather than eight of the species they list have last appearance ages consistent with 45 ka

cryptic meteoric diagenesis in freshwater bivalves implications for radiocarbon dating shells of freshwater bivalves are commonly used for radiocarbon dating late pleistocene archaeological and vertebrate fossil sites thus providing important constraints on late pleistocene human dispersal and megafauna extinction hypotheses the reliability of bivalve shells for dating rests partly on the ease with which subsequent diagenetic alteration can be recognized typically wherein original shell aragonite is replaced by calcite in meteoric environments here we document late pleistocene freshwater bivalve shells wherein meteoric diagenesis involved syntaxial overgrowth of aragonite cement on original aragonite shell biocrystals aragonite cement was identified in situ using raman microspectroscopy and formed rather than calcite as a result of unusually high mg ca ratios in local groundwaters thus altered shells contain diagenetic 14c rendering their dates unreliable but they may slip past common vetting techniques because 1 epitaxial cements are not readily apparent petrographically because they are in optical continuity with adjacent biocrystals 2 x ray diffraction indicates that no calcite is present 3 alteration is not apparent in cathodoluminescence studies and 4 stable isotopes of c and o are difficult to interpret in shells that originate in terrestrial meteoric environments hence although freshwater with a high mg ca ratio is not common groundwater chemistry should be considered before accepting bivalve based radiocarbon dates uncritically more broadly meteoric diagenesis in carbonate rocks is generally characterized by the dissolution of aragonite or its conversion to calcite our data show that such is not invariably the case even in fully terrestrial freshwater systems

the contourite depositional system of the gulf of cadiz a sedimentary model related to the bottom current activity of the mediterranean outflow water and its interaction with the continental margin the present paper is mostly a synthesis of work conducted on the continental margin of the gulf of cadiz using a broad database collected by several cruises and projects supported by the spanish research council and the nrl and naval oceanographic office usa including bathymetry sidescan sonar imagery seismic profiles sediment cores submarine photographs and physical oceanographic data these data have enabled us to establish a detailed understanding of the morphologic development of the margin its pliocene and quaternary stratigraphy and a full characterization of the contourite depositional system cds generated by the mediterranean outflow water mow the northern margin of the gulf of cadiz shows the following distinct features a an active compressive framework where the cadiz allocthonous unit provides an unstable substratum for late miocene pliocene and quaternary sedimentation b a relative lack of submarine canyons except in the western area of the algarve margin c a very broad continental slope that lacks a marked continental rise d a middle slope dominated by along slope processes driven by the mow which has generated a complex cds during the pliocene and quaternary and e an irregular lower slope and abyssal plain region dominated by down slope processes that is partly detached from an upper slope source region the cds is composed of both depositional and erosive features the main depositional features are characterised by sedimentary wave fields sedimentary lobes mixed drifts plastered drifts elongated mounded and separated drifts and sheeted drifts the main erosive features are contourite channels furrows marginal valleys and moats these various depositional and erosive features have a specific location along the margin and their detailed distribution is essential to understand the present and past interaction of the mow with the middle slope based on this distribution five morphosedimentary sectors have been identified within the cds which from east to west are 1 proximal scour and sand ribbons 2 overflow sedimentary lobe 3 channels and ridges 4 active contourite drifts and 5 submarine canyons the development of the cds has been controlled in general by the pliocene and quaternary environmental and paleoceanographic changes and by the morphology of the margin but in detail the development of each of these sectors is related to systematic deceleration of the mow as it flows westwards to the interaction with the margin bathymetry and to the effects of coriolis force our comprehensive sedimentary model for the cds defines the gulf of cadiz margin as a mixed contourite turbidite system with a detached combined drift fan morphology this is different from many other contourite influenced margins where the contourite processes are dominant on the middle slope and separated from the down slope processes which are characteristic on the lower slope and abyssal plains

mixing in the stratified interface of the faroe bank channel overflow the role of transverse circulation and internal waves mixing in the stratified interface of the faroe bank channel overflow the role of transverse circulationand internal waves

observations of the faroe bank channel overflow using bottom following rafos floats abstract overflows do not easily lend themselves to study by lagrangian floats that remain on a constant isobaric pressure or isopycnal density surface since the mixing entrainment and descent of an overflow plume result in an increase of the pressure and typically a decrease in the density of the overflow waters a simple technique to maintain the float s altitude above the bottom was developed and 12 bottom following rafos floats were deployed at or downstream of the sill in the faroe bank channel in the summer of 2000 from the r s s discovery a technical problem resulted in the majority of the floats becoming stuck to the bottom nevertheless several floats were able to traverse the iceland basin and surface near the southeastern slope of iceland these floats made a descent from the mouth of the faroe bank channel only to shoal along the southern slope of the iceland faroe ridge before descending again when passing through the northwest corner of the iceland basin typical current speeds through the basin were 0 20 0 30 m s 1 with peaks of 0 40 0 50 m s 1 although the floats that were stuck on the bottom provided no trajectory information they were able to provide a time series of bottom or near bottom temperature in addition a crude estimate of the flow regime could be made by interpreting the pressure signals from these stuck floats as a response to strong or weak currents floats that were bottom stuck near the mouth of the channel experienced large fluctuations in temperature 0 5 c and height of the bottom and thus presumably speed on scales from 1 to 4 days another float stuck 100 km downstream of the sill underwent temperature and speed excursions on similar time scales albeit over a smaller range the behavior of the floats is assumed to be the result of the mesoscale variability of the overflow plume downstream of the faroe bank channel

paleoceanography of the atlantic mediterranean exchange overview and first quantitative assessment of climatic forcing 1 the mediterranean sea provides a major route for heat and freshwater loss from the north atlantic and thus is an important cause of the high density of atlantic waters in addition to the traditional view that loss of fresh water via the mediterranean enhances the general salinity of the north atlantic and the interior of the eastern north atlantic in particular it should be noted that mediterranean water outflowing at gibraltar is in fact cooler than compensating inflowing water the consequence is that the mediterranean is also a region of heat loss from the atlantic and contributes to its large scale cooling uniquely this system can be understood physically via the constraints placed on it by a single hydraulic structure the gibraltar exchange here we review the existing knowledge about the physical structure of the gibraltar exchange today and the evidential basis for arguments that it has been different in the past using a series of quantitative experiments we then test prevailing concepts regarding the potential causes of these past changes we find that 1 changes in the vertical position of the plume of mediterranean water in the atlantic are controlled by the vertical density structure of the atlantic 2 a prominent early holocene contourite gap within the gulf of cadiz is a response to reduced buoyancy loss in the eastern mediterranean during the time of sapropel 1 deposition 3 changes in buoyancy loss from the mediterranean during mis3 caused changes in the bottom velocity field in the gulf of cadiz but we note that the likely cause is reduced freshwater loss and not enhanced heat loss and 4 strong exchange at gibraltar during atlantic freshening phases implies that the gibraltar exchange provides a strong negative feedback to reduced atlantic meridional overturning given the very counterintuitive way in which the strait of gibraltar system behaves we recommend that without quantitative supporting work qualitative interpretations of how the system has responded to past external forcing are unlikely to be robust

bottom layer turbulence in the red sea outflow plume abstract turbulence in the red sea outflow plume in the western gulf of aden was observed with an upward looking five beam 600 khz acoustic doppler current profiler adcp the bottom lander adcp was deployed on the seafloor in two narrow topographically confined outflow channels south of bab el mandeb for periods of 18 40 h at three locations at 376 496 and 772 m depths two deployments were taken during the winter season of maximum outflow from the red sea and two in the summer season of minimum outflow these short term observations exhibit red velocity spectra with high frequency fluctuations of typically a few centimeters per second rms velocity during strong plume flow as well as strong subtidal variations in one winter season event the plume flow was reduced by a factor of 4 over an 18 h time span in variance preserving form velocity spectra show a separation at frequencies of 0 3 3 cycles per hour between low frequency and high frequency signals the latter show significant coherence

glacial to interglacial changes in the settling depth of the mediterranean outflow plume we present micropalaeontological and grain size records for a set of sediment cores from the gulf of cadiz southwest spain that reflect changes in the position and strength of the mediterranean outflow mo current the cores sample a sediment drift the gil eanes drift that is positioned lower on the slope in the gulf of cadiz than the position of the main current today the data indicate that the drift is of glacial age and that the glacial mo current was positioned lower on the slope than today but also that it was active over a considerably reduced area of the slope we argue that this observation is consistent with physical constraints on the gibraltar exchange and on the likely settling and spreading behavior of the mo plume along the iberian margin under glacial environmental and sea level conditions the deeper settling of the mo is likely to have influenced the formation of glacial north atlantic intermediate water and also may have exerted indirect influence on the formation of glacial north atlantic deep water

turbulence characteristics and dissipation estimates in the coastal ocean bottom boundary layer from piv data abstract turbulence characteristics in the coastal ocean bottom boundary layer are measured using a submersible particle image velocimetry piv system with a sample area of 20 20 cm2 measurements are performed in the new york bight at elevations ranging from 10 cm to about 1 4 m above the seafloor recorded data for each elevation consists of 130 s of image pairs recorded at 1 hz after processing the data at each elevation consist of 130 instantaneous spatial velocity distributions within the sample area the vertical distribution of mean velocity indicates the presence of large scale shear even at the highest measurement station the flow also undergoes variations at timescales longer than the present data series spatial spectra of the energy and dissipation are calculated from individual vector maps the data extend well beyond the peak in the dissipation spectrum and demonstrate that the turbulence is clearly anisotropic even in the dissipation range the vector maps are also patched together to

a theoretical two layer reduced gravity model for descending dense water flow on continental shelves slopes 1 a theoretical two layer reduced gravity model for descending dense water flow on continental shelves slopes has been developed to investigate the dynamics of bottom dense water plumes the model is nonsteady state and includes vertical viscosity the coriolis force and bottom friction an integral solution rather than a perfect analytical expression is derived and thus the simpson s 1 3 rule to approximate the integral is applied at the very bottom the dense water plume moves about 45 to the right left in the northern southern hemisphere looking downslope from the bottom the velocity vector rotates anticyclonically upward indicating a bottom ekman spiral that mimics the atmospheric ekman boundary layer the dense water within the bottom ekman layer obeys a three force balance while the dense water above the bottom ekman layer is governed by a two force balance which is a geostrophic flow with superimposed cycloidal inertial oscillations oriented from about 25 to 140 to the right left of the downslope direction in the northern southern hemisphere the transport within the bottom ekman layer is directed about 60 70 to the right left of the downslope direction in the northern southern hemisphere forming an offshore cross isobath transport in the absence of eddy flux and wind forcing the ratio of offshore transport to alongshore transport within the bottom ekman layer is about 0 19 19 while the ratio above the bottom ekman layer i e geostrophic layer of the dense water is only 3 negligible compared to its alongshore transport which however is equivalent in magnitude to its counterpart in the bottom ekman layer if o de h 0 1 where de is the bottom ekman layer thickness and h is the dense water layer thickness in other words the bottom ekman layer and the geostrophic dense layer contribute equivalent dense water offshore each contributes 50 the magnitude of the descending dense water velocity depends linearly on reduced gravity and sin slope angle

internal bores and breaking internal tides on the oregon continental slope abstractobservations of breaking internal tides on the oregon continental slope during a 40 day deployment of 5 moorings along 43 12 n are presented remotely generated internal tides shoal onto the slope steepen break and form turbulent bores that propagate upslope independently of the internal tide a high resolution snapshot of a single bore is captured from lowered acoustic doppler current profilers ladcp ctd profiles in a 25 h time series at 1200 m the bore is cold salty over 100 m tall and has a turbulent head where instantaneous dissipation rates are enhanced e 10 6 w kg 1 and sediment is resuspended at the two deepest slope moorings 1452 and 1780 m similar borelike phenomena are observed in near bottom high resolution temperature time series mean dissipation rates and diapycnal diffusivities increase by a factor of 2 when bores are present w kg 1 and m s 1 and observed internal tides are energetic enough to drive these enhanced dissipation rates globally the authors estimate

a review of the physical oceanography of the mediterranean outflow abstract the physical oceanography of the mediterranean sea is reviewed with particular emphasis on the mediterranean outflow in the gulf of cadiz in this region the dense mediterranean water forms a high velocity bottom current that interacts strongly with the sea floor the major energy source for the plume comes from the release of potential energy as the plume descends the continental slope and the major energy sink is work against bottom stress which is as large as 4 pa where the plume begins to descend the continental slope in this region the current makes a nearly inertial turn that would otherwise appear to be steered by the underlying topography the mediterranean plume entrains the overlying north atlantic central water and thereby loses much of its density anomaly the mixed mediterranean water becomes neutrally buoyant in the lower portion of the north atlantic thermocline near cape st vincent there are then two preferred transport modes having somewhat different temperature and salinity whose distinct characteristics can be found far into the open north atlantic the temperature salinity and volume of the mediterranean water in the strait of gibraltar and in the gulf of cadiz appear to be roughly constant since modern measurements have been made the estimated westward transport of mediterranean water has gone down considerably as direct measurement techniques have been applied a recent estimate is that the westward transport of pure mediterranean water is only about a half a sv 1 sv 106 m3 s 1 the transport of mixed mediterranean water in the western gulf of cadiz is larger by about a factor of three or four because of the entrainment of north atlantic water

mammalian responses to pleistocene climate change in southeastern australia resolving faunal responses to pleistocene climate change is vital for differentiating human impacts from other drivers of ecological change while 90 of australia s large mammals were extinct by ca 45 ka their responses to glacial interglacial cycling have remained unknown due to a lack of rigorous biostratigraphic studies and the rarity of terrestrial climatic records that can be related directly to faunal records we present an analysis of faunal data from the naracoorte caves in southeastern australia which are unique not only because of the species richness and time depth of the assemblages that they contain but also because this faunal record is directly comparable with a 500 k y speleothem based record of local effective moisture our data reveal that despite significant population fluctuations driven by glacial interglacial cycling the species composition of the mammal fauna was essentially stable for 500 k y before the late pleistocene extinctions larger species declined during a drier interval between 270 and 220 ka likely reflecting range contractions away from naracoorte but they then recovered locally persisting well into the late pleistocene because the speleothem record and prior faunal response imply that local conditions should have been favorable for megafauna until at least 30 ka climate change is unlikely to have been the principal cause of the extinctions