the crustal structure of beira high central mozambique combined investigation of wide angle seismic and potential field data abstract the timing and geometry of the initial gondwana break up between africa and east antarctica is still poorly known due to missing information about the continent ocean boundaries along the rifted margins in this context the beira high off central mozambique forms a critical geological feature of uncertain crustal fabric based on new wide angle seismic and potential field data across beira high a p wave velocity model supported by amplitude and gravity modelling provides constraints on the crustal composition of this area in the mozambique basin mainly normal oceanic crust of 5 5 7 km thickness with velocities of 6 5 7 0 km s in the lower crust is present a sharp transition towards beira high marks the continent ocean boundary here the crust thickens to 23 km at maximum a small velocity depth gradient and a constant increase in velocity with basal velocities of maximum 7 0 km s are in good agreement with typical velocities of continental crust and continental fragments the density model indicates the existence of felsic material in greater depths and supports a fabric of stretched but highly intruded continental crust below beira high a gradual decrease in crustal thickness characterizes the transition towards the mozambican shelf area here in the zambezi delta depression 12 km of sediments cover the underlying 7 km thick crust the presence of a high velocity lower crustal body with velocities of 7 1 7 4 km s indicates underplated magmatic material in this part of the profile however the velocity structure in the shelf area allows no definite interpretation because of the experimental setup thus the crustal nature below the zambezi delta and consequently the landward position of the continent ocean boundary remains unknown the difference in stretching below the margins of beira high suggests the presence of different thinning directions and a rift jump during the early rifting stage

high calcification rates and inferred metabolic trade offs in the largest turritellid gastropod turritella abrupta neogene abstract turritella abrupta miocene pliocene is the largest species known from the diverse jurassic recent gastropod family turritellidae in addition to having achieved long length and substantial width the species produces shells of exceptional thickness even before secondary shell deposition we investigated the paleoecology of this species through analysis of drilling and peeling frequencies with comparisons to co occurring turritellid species we used oxygen isotopic sclerochronology to infer the growth rate and lifespan of t abrupta and to make comparisons with other modern and fossil tropical species including the living species turritella terebra with the first sclerochronologies for both of these species presented herein we find that t abrupta was fast growing long lived and comparatively effective at resisting predation t abrupta was notably widespread geographically and temporally but was a relatively rare component of faunas when compared with co occurring turritellids high rates of shell carbonate construction in excess of 35 g in the first year of life likely depended on habitats conducive to calcification late miocene cooling and eastern pacific carbonate limitation are implicated in the range contraction and eventual extinction of this species

lower pliocene gastropod assemblages from northwestern france palaeoceanographic and palaeobiogeographic implications abstract upper miocene to pleistocene assemblages of northwestern france are commonly grouped as redonian faunas although earlier authors recognized their diverse and heterochronic character redonian assemblages continued to be monographed as if they were homogeneous hindering understanding here we critically review the lower pliocene zanclean assemblage iii gastropod faunas of the nantes region 241 species within 145 genera and consider their palaeoenvironmental and palaeobiogeographic implications they occur in the wider context of a post miocene decrease in faunal diversity along the atlantic seaboard of europe this decline has been linked to the loss of thermophilic faunal elements due to a cooling in mean monthly sea surface temperature mmsst based on assemblage iii gastropods mmssts in the ligerian gulf of northwestern france were characterized by subtropical temperatures in zanclean times not cooler than approximately 16 c in february to 22 c in august and september compared to present day values of 9 5 c to 18 5 c for this latitude at generic level the assemblage iii fauna lies at the northern limit of the subtropical french iberian molluscan province which also included the pliocene faunas of the mondego basin of portugal however at species level it is unlike other redonian or french iberian assemblages of pliocene age and justifies the creation of a distinct ligerian molluscan subprovince the origin and evolution of the assemblage were strongly dependent on the geography of northwestern france during the pliocene once the ligerian gulf vanished so did the subprovince

resolving a late oligocene conundrum deep sea warming and antarctic glaciation abstract changes in ice volume and resulting changes in sea level were determined for the late oligocene 26 23 ma astronomical timescale ats by applying 18 o to sea level calibrations to deep sea 18 o records from odp sites 689 690 929 1090 and 1218 our results show that maximum global ice volume occurred during two late oligocene 18 o events oi2c 24 4 ma and mi1 23 0 ma inferred glacioeustatic lowering with volumes up to 25 greater than the present day east antarctic ice sheet eais ice volume during glacial minima was on the order of about 50 of the present day eais this is supported by late oligocene stratigraphic records from antarctica that contain evidence of cold climates and repeated episodes of glaciation at sea level and grounding lines of glacial ice on the antarctic continental shelf in the ross sea and prydz bay in contrast composite deep sea 18 o records show a significant decrease 1 between 26 7 and 23 5 ma which have long been interpreted as bottom water warming combined with deglaciation of antarctica however a close examination of individual 18 o records indicates a clear divergence after 26 8 ma between records from southern ocean locations i e ocean drilling program sites 689 690 744 and those of other ocean basins high 18 o values 2 9 3 3 from these southern ocean 18 o records are consistent with an ice sheet on the east antarctic continent equivalent to present day values and cold bottom water temperatures 2 0 c these differences suggest a reduction in deep water produced near the antarctic continent i e proto antarctic bottom water proto aabw which were quickly entrained and mixed with warmer and presumably more saline bottom water originating from lower latitudes expansion of a warmer deep water mass and the weakening of the proto aabw may explain the large intra basinal isotopic gradients that developed among late oligocene benthic 18 o records these conclusions are also supported by ocean modeling suggesting a reduction of deep water formed in the southern ocean strengthening of deep water from the northern hemisphere and decreasing temperatures in high southern latitudes occurred as the drake passage opened to deep water low 18 o values reported from deep sea locations other than the southern ocean are shown to bias estimates of antarctic ice volume calling for a re evaluation of the notion that antarctic ice volume was significantly reduced during the late oligocene

transition from the cretaceous ocean to cenozoic circulation in the western south atlantic a twofold reconstruction abstract the cretaceous oceanic circulation has been quite different from the modern with a different distribution of the continents on the globe this has resulted in a much lower temperature gradient between poles and equator we have studied seismic reflection data and used numerical simulations of atmosphere and ocean dynamics to identify important steps in modifications of the oceanic circulation in the south atlantic from the cretaceous to the cenozoic and the major factors controlling them starting in the albian we could not identify any traces of an overturning circulation for the south atlantic although a weak proto antarctic circumpolar current acc was simulated no change in circulation was observed for the paleocene early eocene south atlantic which indicated that this period has witnessed a circulation similar to the albian and cenomanian turonian circulation the most drastic modifications were observed for the eocene oligocene boundary and the oligocene early miocene with the onset of an acc and atlantic meridional overturning circulation amoc and hence southern sourced deep and bottom water masses in the western south atlantic a modern amoc which intensified in strength after closure of the central american seaway cas and a strong acc have resulted in current controlled sedimentary features and wide spread hiatusses in the south atlantic since the middle miocene the opening of drake passage in early oligocene times and the closure of the cas at 6 ma i e tectonic processes have been identified as the key triggers for the observed most severe changes in oceanic circulation in the south atlantic

modes of the southern extension of the east madagascar current data sets from satellite observations and a nested high resolution model are used to study a source region of the agulhas current altimeter derived geostrophic surface currents are averaged over varying periods providing evidence of the persistence of flow patterns in the extension of the southern branch of the east madagascar current semc south of madagascar the semc separates into one branch toward the agulhas current and into a second branch retroflecting and connecting to the subtropical indian ocean countercurrent sicc good agreement is found between long term mean patterns of observational and model dynamic heights two basic modes are identified in the semc extension with anticyclonic motion favoring retroflection in the northern mozambique basin when the extension is in a southwestward direction and cyclonic motion occurring in the case of the semc flowing westward along the southern madagascar slope a cross correlation sequence between model semc transports and the modal changes in the extension region displays a correlation at about 1 month lag which agrees with eddy propagation time from the semc to the outflow region mean model semc transports are determined using floats released at 21 degrees s and the contribution of the semc to the sicc is obtained using floats injected at 55 degrees e with the model running backward almost half of the semc volume transport contributes to the agulhas system and about 40 of sicc water originates from the semc

contourite erosive features caused by the mediterranean outflow water in the gulf of cadiz quaternary tectonic and oceanographic implications abstract contourite depositional systems have been the focus of much recent research but still relatively little is known about contourite erosive features and their associated processes based on multibeam bathymetry side scan imagery and different resolution seismic records a detailed description and classification of the major erosive submarine valley features of the contourite depositional system of the gulf of cadiz middle slope is presented for the first time four types of erosive features have been differentiated including contourite moats contourite channels marginal valleys and large isolated furrows and interpreted in terms of their tectonic and oceanographic implications during the quaternary the study of the distribution and characteristics of erosive features is essential to better understand the present and past interaction of the mediterranean outflow water mow with the middle slope seafloor and allows us to propose a new and more precise scheme for the mow circulation patterns this scheme includes a main along slope circulation responsible for the excavation of the contourite moat and channels and a secondary down slope circulation responsible for the erosion of marginal valleys and isolated furrows three evolutionary stages have been observed in the development of the erosive system during the quaternary that can be related to changes in the distribution and splitting of the mow as a consequence of the segmentation of the ne sw diapiric ridges by neotectonic effects 1 early pleistocene to mid pleistocene linear diapiric ridges 2 mid pleistocene to late pleistocene diapiric reactivation and 3 late pleistocene to holocene main recent phase of diapiric ridges segmentation and rotation this study provides important regional clues for establishing the evolution of the erosive features in relation with neotectonic effects and represents a good example of the potential of erosive features as evidences for the reconstruction of the paleoceanography and recent tectonic changes

interactions of synoptic and planetary waves scale dependent forcing of a gcm abstract in order to better understand planetary wave synoptic wave interactions in the atmosphere and to develop a tool for further studies the authors have applied a wavenumber dependent external forcing to a general circulation model gcm the forcing constrains various length scales of the gcm to be close to those in the evolving analyses of the european centre for medium range weather forecasts and the national centers for environmental prediction the forcing acts either on the planetary waves pw defined as zonal wavenumbers 0 5 the synoptic waves sw defined as zonal wavenumbers 6 20 the synoptic waves plus the zonal mean sw0 or the synoptic waves plus the zonal mean and wavenumber 1 sw01 the form of the forcing is a linear relaxation to the evolving analyses with a time constant of 8 h this forcing is applied only to the temperature and vorticity equations of the gcm which has a spectral truncation of t42 control integrations of length 30 days have been run starting on 15 dece

did the rifting of the atlantic ocean cause the cretaceous thermal maximum the cretaceous thermal maximum was a major turning point in the history of earth s climate this interval of peak warmth in the turonian has been attributed to very high atmospheric pco2 resulting from rapid outgassing rates although crustal cycling rates peaked in the aptian albian interval on the basis of coupled ocean atmosphere model simulations of the middle cretaceous we hypothesize that the formation of an atlantic gateway could have contributed to the cretaceous thermal maximum differences between prerifting and postrifting climate experiments demonstrate substantial regional oceanographic changes in the north and south atlantic basins that are consistent with oxygen isotopic evidence used to infer a cretaceous thermal maximum the model results help reconcile the paleoclimate record inferred from foraminiferal d 18 o with our understanding of climate dynamics and cretaceous tectonism

seismic stratigraphic framework and depositional history for cretaceous and cenozoic contourite depositional systems of the mozambique channel sw indian ocean this study describes previously unrecognized contourite depositional systems cdss in the mozambique channel which constrain palaeoceanographic models for this area the stratigraphic stacking patterns record nine seismic units su1 to su9 separated by eight major discontinuities a to h oldest to youngest key seismic markers in cds evolutionary history occur during aptian albian 122 ma late cenomanian 94 ma early 38 2 36 2 ma and late 25 23 ma oligocene and early middle miocene 17 15 ma epochs these record onset 122 to 94 ma growth 94 to 25 23 ma maintenance 25 23 to 17 15 ma and burial 17 15 ma to the actual time stages for cdss cdss first develop during the onset stage which coincides with the opening and deepening of the african southern ocean gateway at 122 and 100 ma respectively the growth stage beginning in the late cenomanian 94 ma correlates with the opening and deepening of the equatorial atlantic gateway during the growth stage two major shifts in sedimentary stacking pattern occur which coincide with palaeoceanographic changes during the early 38 2 36 2 ma and late 25 23 ma oligocene these in turn coincide with the onset and local enhancement of antarctic water masses cds growth continued until the early middle miocene during the maintenance stage 17 15 ma most cds growth ceased at the end of the maintenance stage circulation of the north atlantic water mass into the southern hemisphere led to a deepening of antarctic water masses in the area

spreading of red sea overflow waters in the indian ocean as a result of its remarkably high salinity and despite its small volume input remnants of red sea water rsw have been identified in the agulhas current over 6000 km distant of their source this provided the motivation to investigate the long term mean spreading of rsw throughout the indian ocean using a comprehensive set of observations taken from the national oceanographic data center archives and from the world ocean circulation experiment hydrographic program for the indian ocean after emerging from the gulf of aden into the arabian sea rsw spreads predominantly southwestward along the african continental slope as indicated by strongly inclined isohalines across the arabian sea there is some monsoon variability so that during the winter monsoon there is more rsw present in the gulf of aden and an intensification of southward spreading along the western boundary between 12 and 5 n elsewhere the intermediate depth salinity field of the indian ocean appears relatively stationary between 5 and 10 s in the region of the south equatorial current isohalines of the rsw layer become quasi zonal across the width of the indian ocean only dipping southward toward the western boundary west of 50 e south of here there is a strong tongue of rsw spreading southward through the mozambique channel and into the agulhas current these conclusions concur with previous localized investigations of intermediate water properties using a simple mixing model the percentage of rsw throughout the indian ocean was quantified it was found that the flux of salt into the gulf of aden from the red sea is similar to that estimated to cross 32 s in the agulhas current this result implies that all the rsw which is mixed into the interior of the indian ocean may eventually be exported at the western boundary furthermore it implies that rsw is the dominant component of the salt budget for the intermediate layer and that input from the indonesian seas and via diapycnic processes are small

cretaceous deep water formation in the indian sector of the southern ocean 1 the role that meridional overturning circulation moc patterns played in poleward heat transport during the extreme warmth of the early to late cretaceous is a fundamental and unresolved question in climate dynamics in order to address this question we must determine where deep waters formed and how they may have circulated during periods of extreme warmth here we present late albian through maastrichtian 105 to 65 ma nd isotope records from deep sea drilling project dsdp and ocean drilling program odp sites in the proto indian ocean and the tropical pacific comparison of these data with previously published records indicates deep water formation in the indian sector of the southern ocean began at least 105 ma extending the record of high latitude convection back into the early cretaceous prior to the peak warmth of the mid cretaceous the growing body of data supports a mode of moc in part characterized by high latitude downwelling during the peak of greenhouse warmth of the mesozoic and cenozoic however this mode of moc likely was characterized by numerous locations of deep convection that were regionally important but not significant in terms of a globally overturning circulation due to paleogeographic and bathymetric barriers

seismic stratigraphy along the amundsen sea to ross sea continental rise a cross regional record of pre glacial to glacial processes of the west antarctic margin abstract the seismic sediment record of the amundsen sea continental rise provides insight into the sedimentation processes from pre glacial to glacial times variations in ocean bottom circulation early ice sheet growth and intensification towards the present icehouse regime seismic reflection data acquired during the 2010 rv polarstern and the 2006 rv tangaroa expeditions created a 2000 km long continuous amundsen sea to ross sea seismic transect pre existing lines linked to this transect connect key seismic stratigraphy horizons from the ross sea shelf to the rise and farther along the west antarctic margin up to the amundsen sea embayment seismic units as 1 to as 3 constitute the cretaceous to eocene pre glacial pg sequence 79 34 ma units as 4 to as 6 the eocene to mid miocene transitional t sequence 34 15 5 ma and units as 7 to as 11 the mid miocene to quaternary full glacial fg climate sequence 15 5 0 ma the top pg sequence boundary horizon as u3 upg t links to unconformity rsu6 of the ross sea shelf and to the base of unit ii of the eastern amundsen sea and is interpreted as the first arrival of grounded ice on the shelf the top t sequence boundary as u6 ut fg links to rsu4 and the base of unit iii and is interpreted as the onset of the fg regime with intensified ice sheet advances onto the outer shelves the amundsen sea basin accumulated up to 3 9 km thick sediments in its centre seismic facies geometry analysis suggests paleocene eocene bottom current activity late eocene shelf grounding of the west antarctic ice sheet and no apparent difference in the deep sea sediment transport processes or temporal shift in deposition between the amundsen sea and ross sea implications for a marie byrd land uplift starting at 30 ma are observed by a progressive change in horizon dip in the central amundsen sea seismic sequences

contourite drifts as indicators of cenozoic bottom water intensity in the eastern agulhas ridge area south atlantic abstract high resolution multichannel seismic reflection profiles acquired in the agulhas ridge area eastern sub polar south atlantic were used in conjunction with multibeam bathymetry and ocean drilling program leg 177 borehole data to characterise deep water contourite formation in the area of the northeastern agulhas ridge and the cape rise seamounts the transverse ridge separates the cape basin from the agulhas basin and controls the exchange of water masses between these two basins small scale buried drifts moats and sheet like deposits indicate that sedimentation was controlled by bottom currents since the late eocene after a pronounced early oligocene erosional event resulting from the onset of lower circumpolar deep water lcdw flow drift formation intensified the type position and formation history of the interpreted drifts suggest that the pathways of lcdw flow have undergone little change during the last 33 ma and followed roughly today s 4900 m depth contour northwest of the cape rise seamount we found a mounded drift with an oval shape a height of 400 m and a width of 50 60 km indicating a clockwise circulating bottom water gyre in that area extensive drifts in the cape basin occur as features confined between the agulhas ridge and cape rise seamounts and as mounded and sheeted drifts further to the west the confined drifts show erosional features on both flanks suggesting a west setting bottom water flow along the northern flank of the agulhas ridge and an opposing eastward directed flow along the southern rim of the cape rise seamount group in contrast to the large drift deposits in the cape basin smaller confined drifts showing more erosional features are found south of the agulhas ridge together these findings suggest that the deepest lcdw flowed anticlockwise around the agulhas ridge before taking a major clockwise loop in the cape basin the returning bottom water then flowed around the cape rise seamounts before entering the indian ocean

slumps canyons and related features on the continental margin off east london se africa sw indian ocean abstract bathymetric and seismic reflection surveys in the natal valley east of east london show that the most important factors in the neogene development of the outer continental margin have been large scale slumping and penecontemporaneous canyon formation slope wasting and allochthonous mass movements have shifted the foot of the continental slope up to 25 km basinward and have constructed extensive continental rise lobes shelf break breaching by canyon heads is considered an important factor in transferring sediment entrained in the agulhas current from the continental shelf to the deep ocean basin a preliminary sediment budget suggests that the continental rise lobes could have been constructed during the last 2 69 m y following an episode of massive slumping

tectonically restricted deep ocean circulation at the end of the cretaceous greenhouse the evolution of global ocean circulation toward deep water production in the high southern latitudes is thought to have been closely linked to the transition from extreme mid cretaceous warmth to the cooler cenozoic climate the relative influences of climate cooling and the opening and closure of oceanic gateways on the mode of deep ocean circulation are however still unresolved here we reconstruct intermediate to deep water circulation for the latest cretaceous based on new high resolution radiogenic neodymium nd isotope data from several sites and for different water depths in the south atlantic southern ocean and proto indian ocean our data document the presence of markedly different intermediate water nd isotopic compositions in the south atlantic and southern ocean in particular a water mass with a highly radiogenic nd isotope signature most likely originating from intense hotspot related volcanic activity bathed the crest of walvis ridge between 71 and 69 ma which formed a barrier that prevented deep water exchange between the southern ocean and the north atlantic basins we suggest that the cenozoic mode of global deep ocean circulation was still suppressed by tectonic barriers in the latest cretaceous and that numerous mostly regionally formed and sourced intermediate to deep waters supplied the deep ocean prior to 68 million yr ago

neodymium isotopic reconstruction of late paleocene early eocene thermohaline circulation abstract high resolution fish tooth nd isotopic records for eight deep sea drilling project and ocean drilling program sites were used to reconstruct the nature of late paleocene early eocene deep water circulation the goal of this reconstruction was to test the hypothesis that a change in thermohaline circulation patterns caused the abrupt 4 5 c warming of deep and bottom waters at the paleocene eocene boundary the paleocene eocene thermal maximum petm event the combined set of records indicates a deep water mass common to the north and south atlantic southern and indian oceans characterized by mean nd values of 8 7 and different water masses found in the central pacific ocean nd 4 3 and caribbean sea nd 1 2 the geographic pattern of nd isotopic values before and during the petm suggests a southern ocean deep water formation site for deep and bottom waters in the atlantic and indian ocean basins the nd data do not contain evidence for a change in the composition of deep waters prior to the onset of the petm this finding is consistent with the pattern of warming established by recently published stable isotope records suggesting that deep and bottom water warming during the petm was gradual and the consequence of surface water warming in regions of downwelling

large scale short period sea ice atmosphere interaction changes in the microwave brightness temperature measured by the electrically scanning microwave radiometer esmr flown on board the nimbus v satellite reveal large scale sea ice fluctuations in the antarctic marginal ice zone these ice margin fluctuations arre predominantly wave numbers 1 4 with phase speeds of about 3 m s independent of wave number the spatial pattern and eastward advection of the sea ice anomalies match those of the atmospheric sea level pressure and are consistent with sea ice displacement due to surface wind stress examination of the outgoing longwave radiation indicates that suppression of high clouds in regions of increased sea ice inc reases the radiative cooling which contributes to maintaining the ice data from three winter seasons indicate about a one third probability of occurence of this large scale high frequency sea ice atmosphere interaction during any given 2 week period in winter

advanced insights into magmatism and volcanism of the mozambique ridge and mozambique basin in the view of new potential field data summary a new plate tectonic model is presented which describes the emplacement of the mozambique ridge off southeast africa as the result of long lasting volcanic activity 140 122 ma during the initial opening between africa and antarctica thus an oceanic origin for the mozambique ridge is proposed this model is based on a new and systematic high resolution magnetic anomaly data set acquired across the mozambique ridge and throughout the mozambique basin data from the mozambique basin allow the identification of mesozoic magnetic anomalies from m0r to m26 124 61 155 3 ma with previously unmatched accuracy small scale fracture zones are recognized by offsets of magnetic anomalies in the westernmost part of the basin additionally a bend in the major fracture zones f and e between m17r and m18n 142 84 144 04 ma and a recognized sinusoidal change in spreading direction with an amplitude of about 15 indicate that the basin experienced several small scale changes in spreading direction through time a maximum change in spreading direction to almost 0 at around m11n 135 69 ma is followed by a short lived increase in spreading half rate from 23 5 km ma 1 to about 27 5 km ma 1 in the time frame from m10r to m9n 134 30 132 83 ma we propose that this is related to the initial opening of the south atlantic ocean represented by the onset of seafloor spreading between the falkland plateau and africa in the conjugate georgia and natal basins across the mozambique ridge high amplitude magnetic anomalies at major structural boundaries suggest that the different plateaus of the ridge were formed at different times a simple 2 d gravity and magnetic model for the ridge supports the hypothesis of multiple volcanic episodes which formed the ridge though long lasting volcanic activity between about 140 and 122 ma together with new and verified rotation parameters from the mozambique basin and its conjugate the riiser larsen sea antarctica a series of plate tectonic reconstructions are presented which demonstrate when and how the different parts of the ridge evolved through time

sedimentary deposits on the southern south african continental margin slumping versus non deposition or erosion by oceanic currents seismic profiles extending from the southern south african shelf into the deep sea reveal a strong erosional activity which affects large parts of the continental margin quaternary to oligocene units and in places the complete sedimentary column appear to have been removed mass movements were previously considered as the origin of this erosion however structures indicating slumping can be identified in only a few places the erosional activity is confined to specific water depths which correlate well with the activity levels of water masses observed here we thus suggest that the agulhas current antarctic intermediate water north atlantic deep water and antarctic bottom water have intensively shaped the sedimentary units over a long period this indicates a stable circulation comparable to that at present day since the neogene

formation of southern component water in the late cretaceous evidence from nd isotopes constraining deep ocean circulation during past greenhouse climatic periods such as the cretaceous is important for understanding meridional heat transfer processes controls on ocean anoxia and the relative roles of climate and tectonics in determining paleocirculation patterns ocean circulation models for the late cretaceous and early paleogene suggest that significant deep water production occurred in the southern ocean but cannot constrain when this process commenced or what the temporal relationship was between opening tectonic gateways and late cretaceous climatic cooling nd isotope data obtained from biogenic apatite fish teeth and bones are presented from lower bathyal and abyssal sites in the south atlantic and indian oceans during the mid cretaceous relatively radiogenic nd isotope values suggest that deep water circulation in these basins was sluggish with inputs likely dominated by seawater particle exchange processes and possibly easily weathered volcanic terranes in the campanian maastrichtian the nd isotopic composition of proto indian and south atlantic deep waters became less radiogenic suggesting the onset of deep water formation in the southern ocean southern component water scw consistent with paleogene reconstructions and ocean circulation models a combination of southern hemisphere cooling and the opening of tectonic gateways during the campanian likely drove the onset of scw

comparison of three velocity sections of the agulhas current and agulhas undercurrent lowered acoustic doppler current profiles ladcp from an early march 1995 cruise across the agulhas current show a swift narrow undercurrent flowing northeast along the continental slope neither this agulhas undercurrent nor the adjacent deep extension of the agulhas current are evident from measurements of water properties alone and their absence from the conventional referencing of geostrophic current estimates biases net southward transport estimates high by several sverdrups here we refine the original transport calculation by removing barotropic tides and by estimating instrumental and sampling errors two additional ladcp sections from cruises in late march and june 1995 also show the undercurrent and the deep extension of the agulhas differences in the current structure are evident the agulhas current extends throughout the water column in march but extends only to 2300 m depth in june additionally the current extends further offshore in march of the three available ladcp sections only those from early march and june have sufficient sampling to calculate the net southward transport of the agulhas current and undercurrent the two estimates 78 3 and 76 2 sv are nearly identical consideration of water properties on density surfaces shows that although the undercurrent carries intermediate water with red sea water influence northward the bulk of this water mass is flowing southward away from its source in the agulhas current

a model for the growth of mudwaves in the presence of time varying currents abstract mudwaves are produced in a model when a periodic current produces lee waves that alter the deposition erosion rates of sediment at the bottom of an ocean a technique analogous to using a lagrangian representation for calculating the velocity perturbations associated with these lee waves is determined and then applied to a situation typical of one observed in the argentine basin the fact that the imposed current is variable results in the perturbation velocities also being time dependent with two major time scales one of these time scales is the period of the variable current which is taken to be on the order of weeks this is consistent with the dominant periods indicated in the data the other time scale is on the order of a day or two and is associated with the time required for the time dependent part of the imposed current total current minus its time averaged part to cross a ridge waves on this time scale are excited when the imposed current passes through a value that changes the character of the lee waves from vertically propagating to bottom trapped or vice versa the long term erosion deposition and mudwave growth rates are insensitive to the details of these shorter time scale variations

geomorphic tectonic control of sediment discharge to the ocean the importance of small mountainous rivers analysis of data from 280 rivers discharging to the ocean indicates that sediment loads yields are a log linear function of basin area and maximum elevation of the river basin other factors controlling sediment discharge e g climate runoff appear to have secondary importance a notable exception is the influence of human activity climate and geology on the rivers draining southern asia and oceania sediment fluxes from small mountainous rivers many of which discharge directly onto active margins e g western south and north america and most high standing oceanic islands have been greatly underestimated in previous global sediment budgets perhaps by as much as a factor of three in contrast sediment fluxes to the ocean from large rivers nearly all of which discharge onto passive margins or marginal seas have been overestimated as some of the sediment load is subaerially sequestered in subsiding deltas before the proliferation of dam construction in the latter half of this century rivers

origin signature and palaeoclimatic influence of the antarctic circumpolar current the antarctic circumpolar current acc is today the strongest current in the world s ocean with a significant influence on global climate its assumed history and influence on palaeoclimate while almost certainly equally profound are here called into question in this paper we review 30 years of accumulated data interpretation and speculation about the acc deriving mainly from dsdp and odp drilling in the southern ocean for most of this time a conventional view of acc development signature and influence has held sway among palaeoceanographers and marine geologists in this view the acc began at about 34 ma close to the eocene oligocene boundary the time of onset of significant antarctic glaciation and the time of creation of a deep water gap tasmanian seaway between australia and antarctica as the south tasman rise separated from north victoria land this is the smoking gun of synchroneity the southern ocean sediment record shows a latest eocene development and subsequent geographic expansion of a siliceous biofacies its northern limit taken to indicate the palaeoposition of the acc axis in addition the acc was considered to have caused antarctic glaciation by isolating the continent within a cold water annulus reducing north south heat transport a different and later date for antarctic south american opening drake passage was proposed but the timing of acc onset there was disputed and the simple story survived recent developments however call it into question modern physical oceanography shows that all or most of present day acc transport is confined to narrow jets within deep reaching circumpolar fronts and numerical modelling has suggested that a steady reduction in greenhouse gas concentration through the cenozoic could cause antarctic glaciation with or without a contribution from ocean circulation change the rapidity of antarctic glacial onset at the eocene oligocene boundary and coeval creation of a deep water gap south of tasmania both survive but in light of the new information the presence of a siliceous biofacies cannot be claimed as evidence of the existence of a continuous deep reaching oceanic front and therefore of an acc and the possibility arises that cool and cold sea surface temperatures were effects of antarctic glaciation rather than evidence of a major contributor to its cause in considering future work we emphasise the importance of additional information from ancillary fields better definition of the necessary and sufficient properties of oceanic fronts additional determinations of cenozoic atmospheric pco2 and further developments in models of antarctic glaciation but also suggest the way forward in marine geology our knowledge of the development and palaeoclimatic significance of the acc will be best served by grainsize studies of bottom current strength at selected locations and geochemical or mineralogical studies of clays and ird as a way of examining provenance and therefore surface and bottom current directions and the existence of interocean connections

cretaceous reactivation and intensified erosion in the archean proterozoic limpopo belt demonstrated by apatite fission track thermochronology abstract cratons are generally assumed to be regions of long lasting tectonic stability in particular the study of the phanerozoic exhumation history of cratons has been largely hampered by the scarcity of suitable stratigraphic controls onshore this fact is even more pronounced in terranes lacking mesozoic or younger penetrative structural fabrics and metamorphic overprinting our study in the limpopo belt shows that modern apatite fission track thermochronology provides a hitherto unavailable perspective in the study of these rocks and has profound implications for the crustal evolution of the zimbabwe craton apatite fission track data from 35 samples taken along two transects in the southern edge of the zimbabwe craton and in the central zone of the limpopo belt suggest that extensive regions experienced kilometer scale exhumation in two discrete events as recently as the cretaceous the first occurred at around 130 ma and the second at around 90 ma basin subsidence and sedimentation loads on the mozambique margin support the timing of these events and provide strong indications of the source and pathways for the eroded material further the results indicate that young and old surfaces in a geomorphological sense may be structurally juxtaposed in regions of high elevation in zimbabwe this is contrary to early ideas of surface chronologies based on summit accordances or invoking pediplanation

eustatic curve for the middle jurassic cretaceous based on russian platform and siberian stratigraphy zonal resolution we have used the stratigraphy of the central part of the russian platform and surrounding regions to construct a calibrated eustatic curve for the bajocian through the santonian the study area is centrally located in the large eurasian continental craton and was covered by shallow seas during much of the jurassic and cretaceous the geographic setting was a very low gradient ramp that was repeatedly flooded and exposed analysis of stratal geometry of the region suggests tectonic stability throughout most of mesozoic marine deposition the paleogeography of the region led to extremely low rates of sediment influx as a result accommodation potential was limited and is interpreted to have been determined primarily by eustatic variations the central part of the russian latform thus provides a useful frame of reference for the quantification of eustatic variations throughout the jurassic and cretaceous the biostratigraphy of the russian platform provides the basis for reliably determining age and eustatic events the mesozoic section of the central part of the russian platform is characterized by numerous hiatuses in this study we filled the sediment gaps left by unconformities in the central part of the russian platform with data from stratigraphic information from the more continuous stratigraphy of the neighboring subsiding regions such as northern siberia although these sections reflect subsidence the time scale of variations in subsidence rate is probably long relative to the duration of the stratigraphic gaps to be filled so the subsidence rate can be calculated and filtered from the stratigraphic data we thus have compiled a more complete eustatic curve than would be p ssible on the basis of russian platform stratigraphy alone relative sea level curves were generated by backstripping stratigraphic data from well and outcrop sections distributed throughout the central part of the russian platform for determining paleowater depth we developed a model specifically designed for this region based on paleoecology sedimentology geochemistry and paleogeography the curve describes a series of high frequency eustatic events superimposed on longer term trends many of the events identified from our study can be correlated to those found by haq et al 1988 and other sea level studies from other parts of the world but there are significant differences in the relative magnitudes of events because the eustatic curve resulting from this study is based on a stable reference frame the curve can be used in sedimentary basin modeling and as a tool for quantifying subsidence history from the stratigraphy of passive margins basins and other active regions

global miocene tectonics and the modern world abstract an amazing congruence of seemingly unrelated diverse global events began in the middle and upper miocene and established our modern world two global orogenic belts were active mostly in the middle and upper miocene while backarc basins formed along the eastern margin of asia coincident with these events global temperatures cooled in both the ocean and atmosphere desertification occurred from central asia into and across most of northern africa and also in australia and in southern south america coincident with the expansion of the antarctic ice cap at 14 ma there was initial widespread deep sea erosion and changes in patterns of deep sea sedimentation muddy pelagic sedimentation increased six fold in the north and central atlantic and pacific oceans and global changes in circulation lead to more diatomites in the pacific and fewer in the atlantic by the end of the miocene most of the mediterranean sea had evaporated broadly coincident with these events many old large river systems were destroyed and new ones formed as much of the world s landscape changed collectively these global on shore tectonic and ocean atmospheric events provide the foundation for our modern world a mixture of new and rejuvenated orogenic belts and their far field effects distant epiorogenic uplift rain shadow deserts large alluvial aprons and distant deltas as inherited gondwanan landscapes persisted remote from plate boundaries thus at the end of the miocene much of the world s landscape except for that changed by pleistocene continental glaciation would be recognizable to us today we argue that all of these events had the same ultimate common cause an internal earth engine that drove plate motions in two broad ways first the opening and closing of seven key gateways to deep water oceanic currents radically altered global heat transfer and changed a lingering greenhouse to an icehouse world secondly these events were in part coincident with renewed heat flow in the african and pacific superplumes that energized global plate motions in the middle and upper miocene we hope this global synthesis will stimulate more research on the many global events of the miocene to understand better both our modern world and earlier global orogenies

giant mounded drifts in the argentine continental margin origins and global implications for the history of thermohaline circulation abstract partially buried giant drifts are located in the southern most sector of the argentine continental margin generating a bathymetric jump at the base of the slope they are characterised as giant asymmetrical elongated mounded contourite drifts and are described in detail here for the first time this description is mainly based on the bathymetric and multichannel seismic reflection profiles data their origin and evolution in the argentine basin are decoded and their implications in an area crucial for geologic and palaeoceanographic reconstruction between the atlantic and antarctica are discussed the buried giant drifts are divided into two zones by a large seamount the major giant drift 50 km wide 300 km long and having a sedimentary thickness of nearly 1 km is located to the south of this seamount and oriented to the north where its summit outcrops at present seafloor here its asymmetric external shape is characterised by a steep western side and a gently dipping smooth eastern side with internal reflections prograding eastward based on its position morphology and internal characteristics it has been deduced that this giant drift was generated in the argentine basin by the southward branch of a confined large loop of the antarctic bottom deep water from the eocene oligocene boundary coevally with the drake passage opening until the middle miocene however after a major palaeoceanographic change in the mid to late miocene a new oceanographic scenario was established that fossilised these giant drifts these changes produced a new depositional style in the margin resulting in initiation of the present contourite depositional system and morphologic features over the slope extension of north atlantic deep water circulation into the southern hemisphere and deepening of antarctic bottom water circulation in the argentine basin are herein postulated as the factors that controlled said changes these giant drifts show strong seismic evidence of hydrates and free gas that are ripe for energy resources exploration thus demonstrating the economic potential of contourite deposits in deep marine environments

reconstructing sea level pressure variability via a feature tracking approach abstractthe aim of this paper is to quantify the contribution of synoptic transients to the full spectrum of space time variability of sea level pressure slp in middle latitudes in previous work by the authors it was shown that tracking cyclones and anticyclones in an idealized atmospheric model allows one to reconstruct a surprisingly large fraction of the model s variability including not only synoptic components but also its large scale low frequency component motivated by this result the authors performed tracking of cyclones and anticyclones and estimated cyclone and anticyclone size and geometry characteristics in the observed slp field using the 1948 2008 ncep ncar reanalysis dataset the reconstructed synoptic field was then produced via superimposing radially symmetrized eddies moving along their actual observed trajectories it was found that similar to earlier results for an idealized model the synoptic reconstruction so obtained accounts for a major fraction of the full observed slp v

the offshore east african rift system structural framework at the toe of a juvenile rift the cenozoic east african rift system ears extends from the red sea to mozambique here we use seismic reflection and bathymetric data to investigate the tectonic evolution of the offshore branch of the ears the data indicate multiple and time transgressive neotectonic deformations along 800km of the continental margin of northern mozambique we observe a transition from a mature rift basin in the north to a juvenile fault zone in the south the respective timing of deformation is derived from detailed seismic stratigraphy in the north a 30km wide and more than 150km long n s striking symmetric graben initiated as half graben in the late miocene extension accelerated in the pliocene causing a continuous conjugate border fault and symmetric rift graben coevally the rift started to propagate southward which resulted in a present day 30km wide half graben approximately 200km farther south since the pleistocene the rift has continued to propagate another 300km where the incipient rift is reflected by subrecent small scale normal faulting estimates of the overall brittle extension of the matured rift range between 5 and 12km with an along strike southward decrease of the extension rate the offshore portion of the ears evolves magma poor similar to the onshore western branch the structural evolution of the offshore ears is suggested to be related to and controlled by differing inherited lithospheric fabrics preexisting fabrics may not only guide and focus extension but also control rift architecture

sedimentary basin in fill in the northernmost natal valley hiatus development and agulhas current palaeo oceanography sedimentation over large areas of a marginal plateau the northernmost natal valley nnv is influenced by the tropical sub tropical agulhas current through mobilization of detritus and concentration of sedimentation into localized depo centres current action provides a mechanism for hiatus development both in on shelf and off shelf areas post gondwana break up sedimentary basin in fill is interrupted by several regional hiatuses these correlate with coastal regressions global sea level low stands proposed phases of invigorated oceanic circulation and with global phases of slow deep sea sedimentation an assessment of these mechanisms indicates detritus supply related to sea level changes cannot explain correlation of on shelf and off shelf hiatuses current action may have been a contributory factor in development of a cenomanian turonian hiatus and facies change and a cretaceous tertiary boundary hiatus an early oligocene hiatus clearly marks a change in the agulhas current regime current moulded features asymmetric sediment distributions and re working attest to enhanced current action establishment of a cold bottom layer in the ocean at this time stimulated indian ocean surface near surface circulation new circulation patterns were established after a miocene pliocene boundary hiatus since then the main flow paths have been fairly stable although local depo centre shifts late in the pliocene recent sequence suggest eastward movement of a counter current and cyclonic eddy off maputo

observed characteristics of mozambique channel eddies 1 the flow in the mozambique channel is dominated by large southward propagating anti cyclonic eddies as opposed to a steady western boundary current these mozambique channel eddies feed their waters into the agulhas current system where they are thought to have a significant influence on the formation of the natal pulse and agulhas ring shedding here we use in situ hydrographic and nutrient data together with satellite altimetry and surface velocity profilers to provide a detailed characterization of the mozambique channel eddies two warm eddies in the channel at 20 s and 24 s had diameters of over 200 km they rotated anti cyclonically with a tangential velocity of over 0 5 m s 1 vertical sections show that the eddies reached to the bottom of the water column relative to the surrounding waters the features were warm and saline the total heat and salt anomalies for the southernmost eddy were computed relative to a reference station close by at 24 s the total anomalies were 1 3 1020 j and 6 9 1012 kg respectively being on par with agulhas rings mozambique channel eddies thus have the potential to form a major contribution to the southward eddy heat flux in the agulhas current system the feature also had positive nutrient and negative oxygen anomalies the large magnitude of the water mass anomalies within the eddy suggests that interannual variability in mozambique channel eddy numbers would have a significant impact on downstream water mass characteristics

ocean overturning since the late cretaceous inferences from a new benthic foraminiferal isotope compilation 1 benthic foraminiferal oxygen isotopic d 18 o and carbon isotopic d 13 c trends constructed from compilations of data series from multiple ocean sites provide one of the primary means of reconstructing changes in the ocean interior these records are also widely used as a general climate indicator for comparison with local and more specific marine and terrestrial climate proxy records we present new benthic foraminiferal d 18 o and d 13 c compilations for individual ocean basins that provide a robust estimate of benthic foraminiferal stable isotopic variations to 80ma andtentatively to 110ma first order variations ininterbasinal isotopicgradients delineate transitions from interior ocean heterogeneity during the late cretaceous 65 ma to early paleogene 35 65 ma homogeneity and a return to heterogeneity in the late paleogene early neogene 35 0 ma we propose that these transitions reflect alterations in a first order characteristic of ocean circulation the ability of winds to make water in the deep ocean circulate we document the initiation of large interbasinal d 18 o gradients in the early oligocene and link the variations in interbasinal d 18 o gradients from the middle eocene to oligocene with the increasing influence of wind driven mixing due to the gradual tectonic opening of southern ocean passages and initiation and strengthening of the antarctic circumpolar current the role of wind driven upwelling possibly associated with a tethyan circumequatorial current in controlling late cretaceous interior ocean heterogeneity should be the subject of further research

the crustal structure of the central mozambique continental margin wide angle seismic gravity and magnetic study in the mozambique channel eastern africa abstract the continental margin of mozambique formed during the initial dispersal of gondwana about 180 ma due to the lack of deep seismic and dense potential field data many details of the timing and geometry of the early breakup in this region remained unknown to date to close this gap a research project mobamasis mozambique basin marine seismic survey with the french research vessel r v marion dufresne ii was conducted in 2007 this paper presents the results of p wave magnetic and 2d gravity modelling along two parallel seismic refraction profiles between 37 and 41 e crossing the mozambique rifted margin the crust shows the characteristics of normal to slightly thickened oceanic crust a lower crustal high velocity body with p wave velocities of 7 0 7 5 km s is observed along both profiles its origin is discussed in the context of upper mantle convection and thermal properties the existing magnetic anomaly identifications have been extended to older ages we postulate that the oldest oceanic crust near the central mozambique continental margin has been formed around m41n 166 ma closer to the coast a pronounced negative magnetic anomaly exists that we interpret to coincide with the continent ocean transition this implies that the position of the continent ocean transition is located significantly closer to the shoreline than proposed before

seaward extension of the east african rift the continental margin of northern mozambique is edged by a north south trending alignment of submarine peaks forming the davie ridge fig la from earlier studies1 5 the ridge has been interpreted as a remnant transform fault that controlled the late jurassic early cretaceous drift of madagascar and the opening of the somali basin6 8 during md40 macamo cruise june 1984 7 500 km of water gun seismic reflexion lines have been implemented throughout the region where two flat bottomed troughs the north south trending kerimbas and lacerda deep grabens interrupt the continental slope alongside the ridge these grabens cut into tertiary sediments correspond westward to a miocene tilting or a recent normal fault and eastward to an old discontinuity several times reactivated and located above an abrupt uplift of the moho the two grabens are separated from each other by a rigid region of thickened crust affected by intense seismicity and supporting volcanoes we report here that the eastern branch of the east african rift joins the submerged grabens across the tanzanian continental shelf fig 1b

southern caribbean neogene palaeobiogeography revisited new data from the pliocene of cubagua venezuela abstract based on new data from the pliocene gastropod assemblage of cubagua island venezuela the palaeobiogeography of the southern caribbean is reviewed the data obtained from the southern caribbean assemblages demonstrate a highly endemic fauna along the northern coasts of south america distinct from that found in the neighbouring isthmian region to the west which has probably been in place since at least the early neogene these assemblages are characterised by a relative stability at generic level but an extremely high turnover rate at specific level the name colombian venezuelan trinidad subprovince is chosen over puntagavilanian subprovince and chorotypes and chronotypes are selected for the gatunian province and the colombian venezuelan trinidad subprovince

reconstruction of madagascar and africa evidence from the davie fracture zone and western somali basin new seismic reflection gravity and magnetic data from offshore east africa allow the davie fracture zone to be traced from 11 s to its intersection with the kenyan coast at 2 s constraining the relative motion of madagascar and africa seasat derived free air gravity anomalies and slope rise positive magnetic anomalies observed in shipboard data help to locate the continent ocean boundaries cob off the shore of east africa and madagascar seismic reflection data further document a diapirprovince off madagascar presumably conjugate to that observed off kenya and somalia the dhow and very large crude carrier vlcc basement ridges are complex features and do not appear to be simple fracture zones owing their existence entirely to the separation of madagascar and africa from these data we determine a predrift fit of madagascar and africa involving a 14 2 rotation of madagascar to africa about a pole at 10 n 150 e the geometry of the reconstruction adheres to seismic and potential field data indicating the oceanic nature and extent of the comoros basin and of the somali basin between kenya and the seychelles and it does not conflict with onshore or offshore stratigraphy timing of the opening of the western somali basin is constrained by mesozoic marine magnetic anomalies and extrapolation to the interpreted cob and occurred between approximately 165 and 130 ma

contourite depositional system on the argentine slope an exceptional record of the influence of antarctic water masses a significant contourite depositional system cds on the continental slope of the southern argentine margin is described here for the first time this system contains both erosive and depositional features that have resulted from several factors including topographic intensification of the antarctic sourced water masses the systematic northward decrease in speed of these water masses a northward increase of downslope sedimentary processes and local tectonic influences this system is an exceptional example of a cds that started to develop at the time of the eocene oligocene boundary potentially coeval with the opening of the drake passage however a new margin morphology characterized by a complex terraced slope lacking any continental rise developed after a major paleoceanographic change in the middle to late miocene we infer that this change resulted from the extension of north atlantic deep water circulation into the southern hemisphere and the deepening of antarctic bottom water circulation in the argentine basin

geographical heterochrony comtemporaneous coexistence of neogene and recent molluscan faunas in the americas abstract geographical heterochrony is a biogeographical pattern that within a single faunal province represents the contemporaneous coexistence of newly evolved faunas and those which have persisted unchanged from earlier geological ages this array of discrete adjacent and asynchronous faunules produces a mosaic pattern in both time and space based on the faunas of three anachronistic subregions within the recent caribbean molluscan province one off the yucatan peninsula one off roatan island and one near the gulf of venezuela this geographically heterochronous province is shown to contain two types of relict pockets the term primary relict pocket is proposed for a faunule that contains the same genera and species as the older more extensive parent fauna and differs only in being reduced spatially the term secondary relict pocket is proposed for a faunule that contains relict genera that were once widespread but have persisted and speciated within the boundaries of the pocket based on the faunas of the caribbean relict pockets on patterns seen in the fossil record and on the unusual ecological conditions of the gulf of venezuela area a new reconstruction of neogene caribbean molluscan provinciality is proposed and two new neogene molluscan provinces the caloosahatchian and the gatunian are described to explain the existence of the caribbean relict pockets and their affinities to the recent panamic and neogene floridian faunas a new interpretation of upper pliocene biological catastrophism and extinction is proposed incorporating aspects of the effects of the closing of the isthmus of panama and glacially induced sea level fluctuations and temperature changes a new scheme of recent tropical american marine molluscan provinciality is presented taking into account the two paleoprovinces and the distribution of the post pleistocene caribbean fauna

deep inflow into the mozambique basin more than 200 conductivity temperature depth ctd stations were worked around the southwest indian ridge and del cano rise as part of the world ocean circulation experiment a selection of these data provides information about the inflow of bottom water into the mozambique basin the basin is closed below 3000 m yet the inflow is significantly large of order 1 sv 1 sv 106 m3 s 1 estimates of the basin scale upwelling at 4000 m suggest that the vertical velocity is also large 10 10 5 cm s 1 or more an order of magnitude greater than global ocean estimates examination of the characteristics of the bottom water in the mozambique and agulhas basins and the prince edward fracture zone shows that bottom water enters the mozambique basin from the agulhas basin and also directly from the enderby basin most of the transport enters the mozambique basin via the agulhas basin where two regions of northward flow below 4000 m are found the major flow on the eastern flank of the mozambique ridge is through and above the deep extending 5900 m trench that connects the agulhas and mozambique basins the second weaker flow enters the transkei basin along the deep eastern flank of the agulhas plateau then turning east into the mozambique basin the only source of bottom water to the agulhas basin is the enderby basin but a more direct route between the enderby and mozambique basins exists via the prince edward fracture which extends deeper than 4000 m throughout its length and links the two basins directly across the southwest indian ridge full depth ctd stations trace the changing characteristics of the deep and bottom water in the fracture and moored current meter data show the strength and persistence of the throughflow strong mixing with the overlying deep water elevates the salt content of the bottom water by comparison with the other water in the mozambique basin thus two distinct bottom waters of the mozambique basin originate in the same place the enderby basin and their different characteristics are solely a function of the routes they have taken and the processes encountered along the different pathways

geophysical evidence for widespread cenozoic bottom current activity from the continental margin of nova scotia canada abstract two dimensional and three dimensional multichannel seismic reflection data are utilized to examine previously unrecognized contourite depositional systems along the continental margin of nova scotia canada prior to this study these features were thought to be of limited extent in the study area the new data show that contourite drifts are widespread with greatest drift development during the late miocene to pliocene giant sediment waves form stacked aggrading sequences of bedforms and their development is linked to pre existing seafloor morphology small sediment drifts developed locally throughout the late paleogene and neogene either southwest and down current of seafloor obstacles or form channel fills major erosional pulses form regional seismic markers first along the continental rise in the early oligocene then along the continental slope during the late miocene and pliocene three dimensional seismic data show that erosion surfaces preserve along slope amplitude anomalies two dimensional low relief sediment waves and barchanoid bedforms the geophysical evidence implies that a northeast to southwest along slope bottom current influenced cenozoic depositional patterns throughout the study area

the crustal structure of the madagascar ridge summary the madagascar ridge is an elongated aseismic plateau in the southwest indian ocean which separates two ocean basins of mid to late cretaceous age from interpretations of a reversed refraction line on the ridge and a continuous gravity profile across it we conclude that it formed during the cretaceous as thickened oceanic crust at a mid ocean ridge which possibly overlay a mantle hot spot the seismic data consist of record sections from five recording sonobuoys forty one shots were fired along a 210 km long nne ssw line across the central part of the ridge simple horizontal layer analysis of first arrivals reveals a lower crustal refractor with vp 7 kms 1 at a depth of 8 8 5 km and the moho at an apparent depth of 18 km but fails to characterize the crustal structure as either oceanic or continental we obtain a definitively oceanic structure by using synthetic seismogram techniques together with tau p analysis and second arrival correlations the model which best fits the data consists of 2 2 km of water and 1 km of sediments overlying a velocity gradient from 4 to 7 kms 1 beneath this at a depth of 7 5 km is a 9 5 km thick layer with vp 6 9 7 1 kms 1 a 7 5 kms 1 layer not observed in the first arrival analysis is found at the base of the crust the moho is a transition zone 1 km thick and occurs at 25 km depth the gravity data consist of a continuous ne sw profile across the ridge and on to the flanking basins two dimensional gravity modelling allows a structure consistent with the synthetic seismogram results and supports the existence of a high density basal layer in the crust we use in addition a time series analysis approach to estimate the depth of compensation assuming an airy model and lithospheric thickness assuming an elastic plate model convolution of bathymetry along the profile with a range of model filters gives minimum rms gravity residuals for a crustal compensation depth of 23 5 km and a plate thickness of less than 2 5 km

development of the mozambique and ruvuma sedimentary basins offshore mozambique two major sedimentary basins have been identified on the continental margin of mozambique mozambique basin and ruvuma basin the formation of the basins is related to the break up of gondwana and opening of the western part of the indian ocean the basins are relatively young having developed discordantly to the structural plan of gondwana sedimentary basins the history of the formation of the east african continental margin sedimentary basins within mozambique has been studied on the basis of the present day concept of gondwana break up and madagascar s drift with respect to africa two stages in the history of the east african basins can be recognized late gondwana and post gondwana the late gondwana stage 303 157 ma is typified by sedimentary and volcanogenic rocks of the karoo group the post gondwana stage 157 0 ma corresponds to the period of active gondwana break up and the formation of indian ocean marginal basins the mozambique basin occupies both central and southern parts of the coastal plain of mozambique extending onto the continental shelf and slope the sedimentary fill is composed of upper jurassic cretaceous and cenozoic rocks which discordantly overly the karoo basalts the upper jurassic occurs as continental red beds mostly distributed within buried grabens cretaceous rocks occur as terrigenous sediments of continental and marine genesis cenozoic deposits are of predominantly marine and deltaic origin the ruvuma basin situated in the north of mozambique is part of an extended east african marginal basin which includes parts of the coastal plains and continental margins of tanzania and kenya the basement of the basin is composed of crystalline and metamorphic rocks of pre cambrian age the sedimentary fill is represented by continental terrigenous karoo sediments marine and lagoonal jurassic deposits and marine and deltaic rocks of cretaceous and cenozoic age the main difference between the sedimentary fill of the mozambique and ruvuma basins is related to the changing sedimentary environment in the period of gondwana break up jurassic and early cretaceous when the drift of madagascar and sea floor spreading took place and the marine basin gradually extended southwards between the east african margin and madagascar from the late cretaceous on the sedimentary environment within both basins was similar the mozambique and ruvuma basins forming a single system on the east african continental margin the main stages of the basins development are illustrated by a series of palaeogeographic reconstruction maps

solid sediment load history of the zambezi delta abstract we calculate the solid sediment load history of the zambezi river for the last 120 ma i e middle cretaceous recent times our starting point is a detailed grid of two dimensional seismic reflection profiles calibrated by well log information after conversion from two way travel time to depth we made simple assumptions about the compaction history in order to construct isochore maps of solid sediment load and yield as a function of geological time our results show that solid sediment load has varied by 1 order of magnitude over the last 120 ma there have been three periods of elevated flux the first period occurred in late cretaceous times 90 65 ma and was synchronous with the rapid denudation of southern africa recognised by for example apatite fission track modelling the second period occurred in oligocene times 34 24 ma during proposed rapid regional uplift of southern africa the youngest phase of elevated flux started in late miocene times 10 ma and has continued to increase rapidly through to the present day a large proportion of this increase can be attributed to a doubling of the size of the zambezi catchment during the pliocene at other times we suggest that load variations can be used to bound the uplift history of the region which encompasses the catchment finally changes in the shape of the zambezi delta through time suggest that the mozambique current which sweeps through the mozambique channel from northeast to southwest initiated at the start of the miocene

shackleton fracture zone no barrier to early circumpolar ocean circulation the opening of southern ocean gateways was critical to the formation of the antarctic circumpolar current and may have led to cenozoic global cooling and antarctic glaciation drake passage was probably the final barrier to deep circumpolar ocean currents but the timing of opening is unclear because the shackleton fracture zone could have blocked the gateway until the early miocene geophysical and geochemical evidence presented here suggests that the shackleton fracture zone is an oceanic transverse ridge formed by uplift related to compression across the fracture zone since ca 8 ma hence there was formerly i e in the miocene no barrier to deep circulation through drake passage and a deep water connection between the pacific and atlantic oceans was probably established soon after spreading began in drake passage during the early oligocene

a model of plate kinematics in gondwana breakup summary an accurate model of relative plate motions in gondwana breakup is based on visual fitting of seafloor isochrons and fracture zones fz from the riiser larsen sea and mozambique basin used predictively the model precisely locates kinematic markers in the west somali basin which allows the conclusion that the spreading centres in the west somali and mozambique basins and the riiser larsen sea formed parts of the boundary between the same two plates the locations of fz and less well defined isochrons from neighbouring regions are also consistent with their formation on other lengths of this same boundary and with its relocation from the west somali basin and northern natal valley to the west enderby basin and lazarev sea during chron m10n small independently moving plates thus played no role in the breakup of this core part of gondwana in an inversion procedure the data from these areas yield more precise finite rotations that describe the history of the two plates separation breakup is most simply interpreted to have occurred in coincidence with karoo volcanism and a reconstruction based on the rotations shows the lebombo and mateke sabi monoclines and the mozambique and astrid ridges as two sets of conjugate volcanic margins madagascar s pre drift position can be used as a constraint to reassess the positions of india and sri lanka in the supercontinent

evolution of middle to late cretaceous oceans a 55 m y record of earth s temperature and carbon cycle a new 55 m y global compilation of benthic foraminifera 13 c and 18 o for the middle to late cretaceous shows that there was widespread formation of bottom waters with temperatures 20 c during the cretaceous greenhouse world these bottom waters filled the silled north atlantic and probably originated as thermocline or intermediate waters in the tropical oceans carbon burial during the cretaceous oceanic anoxic events produced a positive 13 c shift in global carbon reservoirs but this is not particularly large especially by comparison with the remarkable late paleocene carbon maximum the interbasin 13 c gradient was unusually large during the cretaceous hot greenhouse probably because the north atlantic sills prevented the free exchange of waters in the deep basin the hot greenhouse ended when the equatorial atlantic gateway opened sufficiently to flood the deep north atlantic with relatively cool polar waters formed in the southern ocean

deep sea paleotemperature record of extreme warmth during the cretaceous oxygen isotope analyses of well preserved foraminifera from blake nose 30 n paleolatitude north atlantic and globally distributed deep sea sites provide a long term paleotemperature record for the late albian maastrichtian interval that is difficult to reconcile with the existence of significant cretaceous ice sheets given reasonable assumptions about the isotopic composition of cretaceous seawater our results suggest that middle bathyal water temperatures at blake nose increased from 12 c in the late albian through middle cenomanian to a maximum of 20 c during the latest cenomanian and earliest turonian bottom waters were again 12 c during the middle campanian and cooled to a minimum of 9 c during the maastrichtian correlative middle bathyal foraminifera from other ocean basins yield paleotemperature estimates that are very similar to those from blake nose comparison of global bottom water temperatures and latitudinal thermal gradients suggests that global climate changed from a warm greenhouse state during the late albian through late cenomanian to a hot greenhouse phase during the latest cenomanian through early campanian then to cool greenhouse conditions during the mid campanian through maastrichtian

geology basin analysis and hydrocarbon potential of mozambique and the mozambique channel abstract the phanerozoic structural and stratigraphic history of mozambique can be separated into a karoo and a post karoo phase outcrops of karoo rocks limited to the zambezi valley and small outliers in the mozambique belt to the north show an eastward facies change marked by an increased argillaceous content and the appearance of carbonates the wide coastal plain and immediate offshore of southern mozambique is marked by extensional horsts and grabens with a mesozoic fill covered by cenozoic sediments little is known of lower stratigraphic horizons in this coastal province there is a facies transition from continental clastics in the northwest and west to more lacustrine and marine deposits to the southeast and east upwards the succession passes into the cenozoic without a noticeable break in the offshore in the north seismic lines help define two basins the zambezi delta basin and the mozambique channel basin separated by the beira high in the south several offshore grabens have been defined one of which the south mozambique graben has been examined here by an integrated basin analysis technique the technique which uses available depositional structural and thermal data provides information on maturation and the timing of hydrocarbon maturation the analysis showed variation in the extension rate with time and two phases of rifting the greatest rate of extension accumulation and subsidence between 96 and 76 ma depends upon interpretational assumptions during this time interval fault blocks were systematically tilted domino fashion this event broadly coincides with the rifting between madagascar and antarctica a period of plate reorganization the earlier rifting phase was probably related to the strike slip separation of madagascar from africa modelling heat flow suggests that the later event is the more important from the maturation hydrocarbon standpoint the offshore and potentially the deeper onshore post karoo basins have traps reservoirs and seals with the presumed source rock within lower cretaceous or jurassic strata there is at the present time no means to evaluate the potential of the karoo rocks

evolution of cenozoic seaways in the circum antarctic region abstract a complete circum antarctic seaway did not open until both the south tasman rise cleared the oates land coast of east antarctica and drake passage opened between the southern tip of south america and the northern end of the antarctic peninsula major plate motions based on dated seafloor spreading anomalies and distinct fracture zone lineations constrain the age of the opening of a seaway between the south tasman rise and antarctica as very close to the eocene oligocene boundary with an unrestricted opening deeper than 2000 m dating from 32 ma timing of the opening of drake passage is more circumstantial because the exact motions of certain micro continental fragments are not known the motion of africa with respect to south america as well as the motion of east antarctica with respect to africa are well constrained for the cenozoic these major plate motions are used with the reasonable assumption of no cenozoic motion of the antarctic peninsula with respect to east antarctica to constrain the location of the antarctic peninsula with respect to the southern tip of south america for the critical period of late eocene to late oligocene uncertainty of motion of the south georgia and south orkney microcontinents and other possible continental fragments make an exact time for opening of drake passage difficult to ascertain even so the early oligocene position of the antarctic peninsula with respect to south america requires a through going deep water seaway to have been open at drake passage prior to 28 ma even given the unconstrained motion of various high standing crustal fragments in the scotia sea with reasonable assumptions concerning motion of the crustal fragments in the western and central scotia sea it is likely that drake passage or passage through powell basin was open to deep water circulation by 31 2 ma

cenozoic glaciation in the southern hemisphere with the end of glaciation in gondwana in early permian time a prolonged interval began when the earth was without ice sheets though not necessarily free of mountain glaciers not until sometime during the tertiary era did ice sheets build up once more first in antarctica and later in the northern hemisphere the fundamental cause of the cooling that led to the formation of the antarctic ice sheet and eventually to major glaciation in lower latitudes is generally held to have been the increasing thermal isolation of antarctica as australia and later south america separated from it allowing zonal oceanic circulation to develop rifting between australia and antarctica began between 1 10 and 90 m y ago cande mutter 1982 but not until much later probably sometime between 30 and 25 m y ago had it progressed far enough for deep circulation to begin as the south tasman rise cleared north victoria land kennett et a1 1974 kennett 1977 weissel et ai 1977 however antarctica did not reach its present high level of thermal isolation until south america had separated from it enabling the antarctic circumpolar current to become established savin et ai 1975 according to barker burrell 1977 1982 coherent spreading between the antarctic peninsula and tierra del fuego started about 29 m y ago and deep circulation began 23 5 2 5 m y ago thereafter increasing in volume the initial ice fields and ice caps that later expanded to become the

mozambique upper fan origin of depositional units the upper mozambique fan includes a stable down stream region with a north south channel flanked by thick 1 5 sec two way traveltime asymmetric levees and a migrating upstream region where at least two main feeding paths have been successively dominant from the oligocene to early miocene the north south serpa pinto valley acted as the main conduit for the north mozambique terrigenous sediments from the middle miocene the west east zambezi valley became the dominant path and supplied the fan with sediments transported by the zambezi river from the central part of mozambique the transfer from one sediment feeding system to the other is related to the abandonment of the serpa pinto valley because of graben formation along the davie ridge which trapped the sediments and the increase of the zambezi river sediment supply because of the creation and erosion of the east african rift

deep flow in the madagascar mascarene basin over the last 150 000 years the sw indian ocean contains at least four layers of water masses with different sources deep antarctic lower circumpolar deep water flow to the north midwater north indian deep water flow to the south and upper circumpolar deep water to the north meridional convergence of intermediate waters at 500 1500 m and the shallow south equatorial current flowing west sedimentation rates in the area are rather low being less than 1 cm ka 1 on madagascar ridge but up to 4 cm ka 1 at amirante passage bottom flow through the madagascar mascarene basin into amirante passage varies slightly on glacial interglacial time scales with faster flow in the warm periods of the last interglacial and minima in cold periods far more important are the particularly high flow rates inferred from silt grain size which occur at warm to cold transitions rather than extrema this suggests the cause is changing density gradient driving a transiently fast flow corroboration is found in the glacial interglacial range of benthic d18o which is ca 2 suggesting water close to freezing and at least 1 2 more saline and thus more dense glacial bottom waters than present significant density steps are inferred in isotope stage 6 the 5e 5d and 5a 4 transitions oxygen isotope data suggest little change by mixing in glacial bottom water on their northward path benthic carbon isotope ratios at amirante passage differ from glacial southern ocean values due possibly to absence of a local productivity effect present in the southern ocean

a geotectonic history of the northern mozambique basin including the beira high a contribution for the understanding of its development abstract the interpretation of a comprehensive two dimensional 2d seismic reflection data set discloses several rifting events for a typical passive rifted margin two major phases have been identified rift i phase is in agreement with the initial rifting of gondwana postulated for the early jurassic this rift resulted in break up stage 1 governed by a north eastern motion of antarctica along the reactivated pebane shear zone indications for lava flows were identified in the seismic profiles along the offshore zambezi depression they form a link between early rifting the initial break up and early sea floor spreading the nature of these lava flows may be associated with the emplacement of thick volcanic dykes during post rift magmatism that occurred when the antarctica plate with the beira high drifted dextrally from the west to east until the mid jurassic times it is more likely that the break up in stage 1 resulted from high tension due to strain relief of rift i phase above all the extensional deformation occurred in a narrow rift mode therefore the v shape of the offshore zambezi depression suggests a possible rift failure structure this is best explained by the reeves and de wit model 2000 this model postulates that the motion of antarctica changed to southward direction at around 170 ma consequently a rift jump from the offshore zambezi depression in the northwest to the south eastern edge of the beira high occurred and rift ii phase may have started leading to the break up in stage 2 during this stage rift ii phase the extension migrated towards the east thus thinning the crust and exhuming the sub continental mantle in the continental ocean transition zone the rift ii phase shows a sequence of half graben morphologies confining the syn rift infill that is subdivided in three units syn rift i syn rift ii and rift sag all three units appear to have developed under minor extensional regimes in the crust evidenced by gently dipping low angle detachment faults with respect to the presence of the two rift phases it is deduced that break up and sea floor spreading are diachronous within rift i and rift ii segments

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

the jurassic history of the africa antarctica corridor new constraints from magnetic data on the conjugate continental margins abstract finding the best fit for east and west gondwana requires a detailed knowledge of the initial jurassic movements between africa and antarctica this study presents results of systematic and densely spaced aeromagnetic measurements which have been conducted in 2009 2010 across the astrid ridge antarctica and in the western riiser larsen sea to provide constraints for the early seafloor spreading history between both continents the data reveal different magnetic signatures of the northern and southern parts of the astrid ridge which are separated by the astrid fracture zone the southern part is weakly magnetised corresponding to the low amplitude anomaly field of the southwestern riiser larsen sea in contrast the northern astrid ridge bears strong anomalies of positive value furthermore several sets of trends are visible in the data in the mozambique channel we extended the existing magnetic spreading anomaly identifications combined with the spreading anomalies in the conjugate riiser larsen sea they were used to establish a new model of the relative movements of africa and antarctica after the breakup of gondwana in jurassic times a detailed model for the emplacement of the mozambique ridge is now incorporated the model postulates a tight fit between africa and antarctica and two stages of breakup the first of which lasting until 159 ma m33n during this stage antarctica rotated anticlockwise with respect to africa the grunehogna craton cleared the coastal plains of mozambique to a position east of the mozambique fracture zone the southern astrid ridge is interpreted to consist of oceanic crust also formed during this first stage prior to the riiser larsen sea during the second stage antarctica moved southward with respect to africa forming the mozambique basin and the conjugate riiser larsen sea the mozambique ridge and the northern natal valley were formed at different spreading centres being active subsequently

space time spectral structure of a glas general circulation model and a comparison with observations abstract the wavenumber frequency spectra of geopotential height have been computed from a winter simulation of a glas general circulation model and are compared to the spectra obtained from 15 winters of observed analyses the variances in several broadly defined space time categories are presented and their distribution with latitude and height discussed the low frequency planetary waves wavenumbers 1 4 in the model have substantially less variance than is observed and their latitudinal structure also differs from observations the same is true for the medium frequency planetary waves the synoptic scale waves wavenumbers 5 10 with both low and medium frequencies are quite well reproduced by the model with the total variances and their latitude height structure comparing well to the observations the net propagation tendency is examined for low frequency planetary waves and the medium frequency synoptic scale waves the model results show too much tendency for eastward propagation the discrepan

late neogene chronology and sequence stratigraphy of mixed carbonate siliciclastic deposits of the cibao basin dominican republic a stratigraphic study of late miocene pliocene mixed carbonate and siliciclastic sediment upper part of the yaque group was conducted in northern hispaniola to refine the chronostratigraphy of uplifted nearshore marine sequences in the cibao basin classic late neogene sections in these sequences include a spectacularly preserved faunal record for the northern caribbean refined ages of the cercado formation 6 6 6 0 ma the gurabo formation ca 5 8 4 0 ma and the mao formation 3 9 3 5 ma allow correlation to regional and global sea level events each of the three formations corresponds to one depositional sequence composed of mixtures at different proportions of skeletal debris mostly coral and mud size silt siliciclastic sediment distinct stratal geometries and lithofacies individualize the three formations each separated by pebble conglomerates the cercado formation depositional sequence consists of prograding inner shelf siliciclastic sands and patch reefs the basal transgressive unit of the gurabo formation is made of seaward prograding low angle clinothems of bedded and burrowed siliciclastic sand with molluscs and free living corals the upper gurabo unit consists of massive middle shelf clinothems of silt with bedded reef debris and in situ branching coral thickets these clinothems grade basinward into massive deeper water silt deposits at the shelf edge and basin slopes the mao formation sequence records the transition from lowstand fluvial deltaic facies to transgressive and highstand marine clinothems made mostly of coral debris in a silt matrix in the mao formation outcrops the coral silt clinothems can be divided into at least 10 high frequency cycles 40 k y based on lithofacies channelized and slumped coarse siliciclastic sand gravel and nonmarine alluvium cap the sequence likely associated with lowered sea level starting at ca 3 3 ma and perhaps the onset of regional uplift the major lithologic and sequence geometry changes in the cibao basin correlate with trends in the oxygen isotope ice volume proxy and likely reflect deposition responding to major periods of marine transgression and highstand an especially prominent feature is the major deepening and margin back step near the miocene pliocene boundary continuing in the early pliocene which resulted in the deposition of the gurabo formation transgression and sea level highstand in the late early pliocene 3 9 3 5 ma produced distinctive margin progradation primarily by coral debris this progradation event was synchronous with progradation in the nearby western great bahama bank the different stratal configurations of the three sequences can be attributed to a combination of factors including 1 shelf morphology and location on the shelf 2 paleoceanographic conditions such as warm sea surface temperatures and reduced upwelling which controlled coral productivity and reef geometry and 3 the influx of siliciclastic mud from the adjacent mountains

bedform velocity matrix the estimation of bottom current velocity from bedform observations a wide variety of bedforms both depositional and erosional in origin has been recognized on the deep seafloor and attributed to the influence of bottom currents these range in scale from those visible in bottom photographs centimeter to decimeter to those recorded with seafloor bathymetric imaging meter to kilometer in many cases it has been possible to provide some quantification of substrate grain size and flow velocity responsible for each bedform type we have synthesized both our own and published data in order to present a bedform velocity matrix which facilitates the estimation of bottom current velocity based on bedform type despite imperfections we believe this to be a valuable model for assessing strength and variability of bottom currents that can have a significant influence on the siting of submarine cables pipelines and other seafloor installations

the sources and mixing characteristics of the agulhas current recent observations taken at four principal latitudes in the agulhas current show that the watermass properties on either side of its dynamical core are significantly different inshore of its velocity core are found waters of predominantly arabian sea red sea and equatorial indian ocean origin while offshore waters are generally from the atlantic ocean the southern ocean and the southeast indian ocean for the most part the inshore waters approach the agulhas current through the mozambique channel while those offshore are circulated within the southern indian ocean subtropical gyre before joining the current these disparate water masses remain distinct during their 1000 km journeys along the south african continental slope despite the convergence extreme velocity shears and high eddy kinetic energies found within the agulhas current both potential vorticity conservation and kinematic arguments are discussed as potential inhibitors of along isopycnal mixing it is concluded that a high cross stream gradient of potential vorticity is the dominant mechanism for watermass separation near the surface while the kinematic steering of water particles by the current is dominant at intermediate depths where cross stream potential vorticity is homogeneous hence three lateral mixing regimes for the agulhas current are suggested the surface and thermocline waters are always inhibited from mixing by the presence of both a strong cross frontal potential vorticity gradient and kinematic steering at intermediate depths mixing is inhibited by steering alone and thus in this regime periodic mixing is expected during meander events such as natal pulses when the steering level will rise and allow cross frontal exchange below the steering level in the deep waters there is a regime of free lateral mixing the deep waters of the agulhas current are homogeneous in the cross stream sense being from the same north atlantic source and their salinity steadily and rather rapidly decreases to the north here it is suggested that mixing must be dominated by vertical processes and a large vertical mixing coefficient of order 10 cm 2 s 1 is estimated

on the origins of deep and bottom waters of the indian ocean the characteristics of the deep and bottom waters of the indian ocean when illustrated on potential density anomaly surfaces indicate that the waters enter from both the atlantic and pacific oceans the paths of spreading are constrained by the complex topography and characteristics are seen to be altered by exchange with the overlying and underlying water and with the sediments especially in the northern indian ocean the weddell sea contributes to the densest waters found in the western basins and the ross sea and adelie coast to the densest waters found in the eastern basins both dense water varieties are altered by and incorporated in the less dense water above initially water carried by the circumpolar current then water from the north atlantic and finally by deep water whose characteristics are derived in the northern indian ocean contact with the sediments increases the silica content of the bottom water in the southern ocean in the northern indian ocean the sediments alter the silica of the water at the bottom and together with enhanced salinity from diffusion of saline overflows from the marginal seas above imprint unique markers to the deep water that flows back to the south at middepths the series of ridges between madagascar and australia confine the flow to a series of gyres that carry characteristics from the circumpolar current equatorward and the northern indian ocean characteristics southward within the circumpolar current low oxygen deep water from the pacific is carried across the atlantic and into the indian ocean south of africa part flows around the cyclonic weddell sea gyre and part extends across the southern ocean water from another pacific source can be seen near 2000 m extending westward from the tasman sea south of aus tralia and across the indian ocean and perhaps to the aghulas current region southeast of africa in this study we show some maps of deep and bottom water characteristics of the indian ocean which may be used in a qualitative sense to infer pathways of spreading of water below a depth of about 2000 m the indian ocean has numerous ridges and basins figure 1 that severely restrict possible pathways of deep and bottom wa ter flow in the following we will begin by looking at char acteristics at the bottom of the ocean to reveal the path ways of spreading of the densest bottom waters from basin to basin then a series of six potential density anomaly surfaces at progressively shallower depths from about 4000 m up to about 2000 m will be shown to see what character istics influence the deep waters and how pathways of flow are constrained by the bathymetry of the indian ocean the choice of the specific density levels table 1 was based primarily on various property extrema shown by the 18os atlantis ii cruise 93 colored sections that accompany warren s 1981 detailed discussion of the major features of the central indian ocean between madagascar and aus tralia the sections approximately midway between antarctica and the northern indian ocean boundaries are well positioned to show features originating from both the north and the south as well as some brought into the indi an ocean by the antarctic circumpolar current the full ocean area isopycnal maps provide essential evidence on the origins and fates of features seen in the sections and

saucer shaped intrusions occurrences emplacement and implications abstract magmatic intrusions and fluidized injected sands represent the two main types of sheet intrusions in the earth s crust in this paper we show that both intrusion types often display a saucer like geometry as revealed by 3d seismic imaging and field observations saucer shaped intrusions are fairly common in sedimentary basins as for example offshore in the norwegian and north sea basins and onshore in the karoo basin of south africa the formation of the saucer geometry is controlled by the low viscosity of the injected fluid and by the interaction between a growing shallow hydraulic fracture and overburden deformations statistics gathered from observations and modelling show a linear relationship between the depth of emplacement and the size of the saucer shaped intrusions we anticipate future cross disciplinary studies aiming to discover other occurrences of saucer shaped intrusions and to identify the physical processes controlling the development of this fundamental geometry

bottom current sediment waves sediment drifts and contourites in the northern rockall trough four types of large scale sediment bedform occur in the northeast rockall trough broad sheeted drifts elongate drifts sediment waves and thin contourite sheets a large sheeted drift occupies the northern basin its crest can be traced for 40 km perpendicular to the bottom current flow direction elongate drifts occur adjacent to steep areas of continental slope separated from the break of slope by a moat drift crests are oriented alongslope parallel to bottom current flow elongate drifts are asymmetric with the steeper flank facing upslope sedimentation rates are higher on the downslope flank sediment waves with a wavelength of 1 2 km and wave height up to 20 m occur locally on the flanks of sheeted and elongate drifts larger waves wavelength of 3 4 km and wave height of 50 100 m occur on the flank of the wyville thomson ridge wave crests are oblique to bathymetric contours to drift crests and to bottom current flow the association of sediment waves and drifts their orientation oblique to regional slopes and the presence of contourite sediments in cores indicate that these are bottom current rather than turbidity current waves a contourite sheet typically 10 25 cm in thickness and composed of well sorted sand covers the present day seafloor along the lower slope bedform evolution through time has involved a complex interaction between bottom currents of variable intensity sea level change and sediment input the present sea level highstand is characterised by minimal sediment input and redistribution of sediments by strong bottom currents giving active bedform growth and contourite development variability of sediment input is the main factor affecting sedimentation during sea level lowstands where input was high as when ice sheets reached the shelf edge downslope sediment transport dominated overwhelming any bottom currents that were active when input was low such as during the last lowstand some evidence for active currents can be observed although these were less energetic than those during highstands