periplatform drift the combined result of contour current and off bank transport along carbonate platforms hydroacoustic and sedimentological data from the western leeward flank of the great bahama bank document the interplay of off bank sediment export along slope transport and erosion which together shape facies and thickness distribution of slope carbonates the integrated data set depicts the combined product of these processes and allows formulation of a comprehensive model of a periplatform drift that significantly amends established models of carbonate platform slope facies distribution and geometry the basinward thinning wedge of the periplatform drift at the foot of the bank escarpment displays along slope and downslope variations in sedimentary architecture sediments are muddy carbonate sands that coarsen basinward the drift wedge has a pervasive cover of cyclic steps in zones of lower contour current speed depth related facies belts develop whereas strike discontinuous sediment lobes scarps and gullies characterize areas with higher current speed this understanding of the impact of currents on carbonate slope sedimentation has wider implications for seismic and sequence stratigraphic interpretation of carbonate platforms and for applied aspects such as hydrocarbon exploration

a scenario of mesozoic cenozoic ocean circulation over the blake plateau and its environs after the continental breakup of north america africa and europe a sluggish wind driven circulation gyre probably formed in the juvenile north atlantic basin surface flow in the western atlantic ocean during the early cretaceous likely was northward along the seaward edge of an extensive reef that flourished offshore of north america sometime during the late cretaceous the surface circulation in the gulf of mexico had evolved into a clockwise rotary pattern as the exchange of water with the pacific ocean diminished by latest cretaceous time a vigorous easterly current flowed through the suwannee strait of southern georgia and northern florida injecting copious amounts of water and sediment into the western blake plateau basin this flow pattern apparently persisted into paleocene time during late paleocene and early eocene time strong northerly flow through the straits of florida and across the blake plateau was established this represented the onset of gulf stream circulation over the blake plateau the position of the gulf stream shifted repeatedly across the northern and central plateau during the tertiary and quaternary periods the westernmost track was located along the inner blake plateau parallel to the florida hatteras slope the easternmost track was 500 m lower bathymetrically and crossed the central plateau and continental slope just north of the blake spur the gulf stream apparently shifted in response to changes in water depth on the blake plateau that accompanied global fluctuations of sea level during rises of sea level the gulf stream drifted westward until it impinged on the florida hatteras slope and outer shelf during lowerings of sea level the gulf stream was deflected progressively eastward across the central plateau by the charleston bump a prominent bathymetric bulge of the northern blake plateau

quantifying rates of syndepositional marine cementation in deeper platform environments new insight into a fundamental process abstract syndepositional marine cementation on carbonate platforms is a fundamental process that results in physical stabilization and lithification of sediments as well as reduction in primary porosity and permeability which may affect the migration of later diagenetic fluids rapid submarine cementation is commonly invoked to explain various aspects of carbonate platform geometry especially related to the preservation of steep slopes but the rates typically reported for cementation in deeper parts of the platform are mostly conjectural in an attempt to quantify the rates of syndepositional cementation in marginal platform environments substrates containing samples of carbonate sand ooids suspended above the sea floor were emplaced at various depths across a modern platform in the bahamas to evaluate the rates of cement growth through direct observation initial results indicate that cementation may lead to partial lithification of carbonate sands within a minimum of 8 months in water depths of up to 60 m these results provide some of the first direct evidence for rates at which cementation can occur in deeper marginal environments and confirms earlier hypotheses that very rapid syndepositional marine cementation may not be limited to very shallow subtidal and intertidal environments

transport potential vorticity and current temperature structure across northwest providence and santaren channels and the florida current off cay sal bank currents and temperatures were measured using pegasus current profilers across northwest providence and santaren channels and across the florida current off cay sal bank during four cruises from november 1990 to september 1991 on average northwest providence 1 2 sv and santaren 1 8 sv contribute about 3 sv to the total florida current transport farther north e g 27 n partitioning of transport into temperature layers shows that about one half of this transport is of 18 c water 17 c 19 5 c this can account for all of the excess 18 c water observed in previous experiments this excess is thought to be injected into the 18 c layer in its region of formation in the northwestern north atlantic ocean due to its large thickness potential vorticities in this layer in its area of formation are very low in our data lowest potential vorticities in this layer are found on the northern end of northwest providence channel and are comparable to those observed on the eastern side of the florida current at 27 n on average a low potential vorticity 18 c layer was not found in the florida current off cay sal bank

closure of the isthmus of panama the near shore marine record of costa rica and western panama the final closure of the isthmus of panama at 3 5 ma divided the american tropical ocean into two separate and different oceanographic regions consequences for the marine biota were profound but hitherto correlation of the pacific and caribbean coastal sections has not been precise enough to track biologic patterns we present here a correlation of 31 sections from the pacific and caribbean coasts of costa rica and western panama using calcareous nannofossils and planktonic foraminifera at both the tops and bottoms of each formation we estimate that the caribbean section ranges from 8 2 ma to 1 7 ma and the pacific sequence from 3 6 ma to 1 7 ma these intervals bracket postulated dates for final closure of the isthmus and provide the first well dated record of middle and late pliocene faunas from the region the caribbean and pacific sections include very different environments of deposition yet there is sufficient overlap and diversity of habitats to permit meaningful biological comparisons on the caribbean side formations tied together by the overlap of the upper pliocene markers sphenolithus abies and pseudoemiliana lacunosa 3 5 ma to 3 6 ma range from very shallow to shallow inner shelf 200 m and upper slope 200 800 m the pacific coast sections were mostly deposited in a trench slope environment which is absent on the caribbean side these sections fortuitously include abundant thick intra formational slumps containing shallow water fauna more appropriate for biological comparison with the caribbean biota similarly the 1 9 ma to 1 5 ma interval well constrained by various taxa includes middle to outer shelf and inner shelf to upper slope deposits on the caribbean side and marginal marine to inner shelf deposits on the pacific coast using our new biostratigraphic framework to correlate previously poorly constrained mollusc collections we show that evolutionary divergence of the pacific and caribbean near shore marine faunas had occurred by 3 5 ma this strongly suggests that the isthmus was effectively closed by this time

active sand transport along a fjord bottom channel bute inlet british columbia an underwater channel system is incised in the holocene fjord basin sediments of bute inlet british columbia high resolution side scan sonar swaths and seismic profiles reveal two channels within a zone of extensive rotational sliding on the slopes of a fjord head delta the channels coalesce into a single sinuous channel that extends 14 km downfjord on a 0 9 slope where it splits into two distributaries which continue another 18 km ending within stacked depositional lobes piston cores show sands in the channels and lobes in contrast to deep water silty clays in the basin floor the sea floor features and sediment distributions result from active highly mobile slide generated sediment flows which transport sands via the channels from the delta to the fjord basin

bottom current controlled sand deposits a review of modern shallow to deep water environments abstract different examples of modern marine sand accumulations generated or strongly influenced by the action of bottom currents are here presented they are drawn from a variety of tectonic and morphological settings and grouped into three water depth zones deep water 2000 m mid water 300 2000 m and outer shelf upper slope 50 300 m deposits in the first two of these depth zones are normal contourites according to their original definition heezen and hollister 1971 being those sediments that have been transported and deposited by contour currents in deep water environments those deposited at shallower depths under the influence of surficial geostrophic currents combined with other hydrodynamic factors shelf currents induced by wind tide and waves gyres internal waves etc are more properly referred to as outer shelf upper slope bottom current sands or shallow water bottom current sands we have elaborated a facies model for each bathymetric zone deep water sandy contourites are relatively rare thin and very thin bedded highly bioturbated and mainly of bioclastic composition they are interbedded with muddy contourites and pelagites or in some areas with turbidites in the latter case thin bottom current reworked sandy tops of turbidites provide a different and distinct facies mid water sandy contourites are more common ranging up to a metre in thickness and may form extensive sandy sheets in a variety of slope bank and channel settings they are mainly of mixed siliciclastic bioclastic composition typically bioturbated and associated with muddy silty contourites in coarsening up fining up complete or truncated sequences shallow water bottom current sands occur in particular outer shelf upper slope settings where they may develop relatively thick 1 20 m laterally extensive sheets covered by fields of sandwaves megaripples and ribbons internal structures may be preserved along with much bioturbation their composition varies from mainly siliciclastic to bioclastic and they may be interbedded with both inner shelf facies and slope hemipelagites the principal factors that control the deposition of sandy contourites and shallow water bottom current sands are the hydrodynamic regime of the basin the availability of coarse grained sandy sediments and the physiographic context of the area swept by the currents the greater the depth the finer and rarer the bottom current or sandy contourite deposits global sea level and climatic changes and the time involved in the depositional history play an ultimate role in the development of important sand accumulations of this sort by controlling the ocean circulation pattern and its long term persistence from the present analysis we conclude that mid depth sandy contourites are the most commonly found in modern environments and that shallow water bottom current sands constitute the most significant potential oil reservoirs to be found in the geological record

new insights into the morphology and sedimentary processes along the western slope of great bahama bank new high quality multibeam and seismic data image the western slope of the great bahama bank and the adjacent floor of the straits of florida the extensive survey reveals several unexpected large and small scale morphologies these include bypass areas channel leveelobe systems gullied slopes and products of slope instabilities at various scales including long slump scars at the lower slope and mass transport complexes that extend 30 km into the adjacent basin floor the toe of the slope is irregularly covered with deep water carbonate mounds the abundance of the individual morphological features varies from north to south from 26 00 n to 25 20 n the slope is dissected by numerous deep canyons that abruptly end southward where the slope is characterized by a smooth lower portion and small regularly spaced furrows in its upper part further south two long 25 50 km scars document instability at the lower slope one of these scars is the source area of a large mass transport complex in addition to this large scale feature several types of gravity induced sedimentary processes are revealed most of the morphologies and inferred processes of this carbonate system are similar to those observed in siliciclastic systems including mass transport complexes gravity currents initiated by density cascading and overspilling channeled turbidity currents for the first time a clear asymmetric channel levee system has been identified along the slope suggesting similitude in sorting processes between carbonate and siliciclastic systems and enhancing the reservoir bearing potential of carbonate slopes notable differences with siliciclastic systems include the lack of connection with the shallow and emerged part of the system i e bank top and the small size of the sedimentary system 2012 geological society of america

itrax description and evaluation of a new multi function x ray core scanner a new automated multi function core scanning instrument named itrax has been developed that records optical radiographic and elemental variations from sediment half cores up to 1 8 m long at a resolution as fine as 200 m an intense micro x ray beam focused through a flat capillary waveguide is used to irradiate samples to enable both x radiography and x ray fluorescence xrf analysis data are acquired incrementally by advancing a split core via a programmable stepped motor drive through the flat rectangular section x ray beam traditional xrf determination of element composition in sediments provides high quality data but it takes a considerable time and normally consumes gram quantities of material that is often only available in limited quantities the itrax core scanner non destructively collects optical and x radiographic images and provides high resolution elemental profiles that are invaluable for guiding sample selection for further destructive detailed sampling this paper presents a description of the construction characteristics and capabilities of the itrax system high resolution itrax data obtained from sediment cores are also presented and compared with results from traditional wavelength dispersive xrf analysis at lower resolution finally some recent technical developments linked to the second generation itrax are presented

variability of cold water coral mounds in a high sediment input and tidal current regime straits of florida cold water coral mound morphology and development are thought to be controlled primarily by current regime this study however reveals a general lack of correlation between prevailing bottom current direction and mound morphology i e footprint shape and orientation as well as current strength and mound size i e footprint area and height these findings are based on quantitative analyses of a high resolution geophysical dataset collected with an autonomous underwater vehicle from three cold water coral mound sites at the toe of slope of great bahama bank the three sites 80 km2 total have an average of 14 mounds km 2 indicating that the great bahama bank slope is a major coral mound region at all three sites living coral colonies are observed on the surface of the mounds documenting active mound growth morphometric analysis shows that mounds at these sites vary significantly in height 1 to 83 m area 81 to 6 00 000 m2 shape mound aspect ratio 0 1 to 1 and orientation mound longest axis 0 to 180 the autonomous underwater vehicle measured bottom current data depict a north south flowing current that reverses approximately every six hours the tidal nature of this current and its intermittent deviations during reversals are interpreted to contribute to the observed mound complexity an additional factor contributing to the variability in mound morphometrics is the sediment deposition rate that varies among and within sites at most locations sedimentation rate lags slightly behind mound growth rate causing mounds to develop into large structures where sedimentation rates are higher than mound growth rates sediment partially or completely buries mounds the spatial distribution and alignment of mounds can also be related to gravity mass deposits as indicated by geomorphological features for example slope failure and linear topographic highs in the three dimensional bathymetry in summary variability in sedimentation rates current regime and underlying topography produce extraordinarily high variability in the distribution development and morphology of coral mounds on the great bahama bank slope

canyon morphology on a modern carbonate slope of the bahamas evidence of regional tectonic tilting new high quality multibeam data presented here depict the northern slope of the little bahama bank bahamas the survey reveals the details of large and small scale morphologies that look like siliciclastic systems at a smaller scale including large scale slope failure scars and canyon morphologies previously interpreted as gullies and creep lobes the slope exhibits mature turbidite systems built by mass fl ow events and turbidity currents the sediment transport processes are probably more complex than expected slope failures show sinuous head scarps with various sizes and most of the scars are fi lled with recent sediment canyons have amphitheater shaped heads resulting from coalescing slump scars and are fl oored by terraces that are interpreted as slump deposits canyons rapidly open on a short channel and a depositional fan shaped lobe the entire system extends for 40 km the development of these small turbidite systems similar to siliciclastic systems is due to the lack of cementation related to alongshore current energy forcing the transport of fi ne particles and fl ow differentiation detailed analyses of bathymetric data show that the canyon and failurescar morphology and geometry vary following a west east trend along the bank slope the changing parameters are canyon length and width depth of incision and canyon and channel sinuosity accordingly failure scars are larger and deeper eastward these observations are consistent with a westward tectonic tilt of the bank during the cenozoic

early cretaceous platform re entrant and escarpment erosion in the bahamas a multichannel 24 fold seismic profile crossing the bahama escarpment south of san salvador is tied stratigraphically to deep sea drilling project site 99 and dredge hauls on the eroded platform slopes in the bahamas we conclude from the study that 1 a deep water sequence crops out in exuma canyon west of the bahama escarpment between 2 600 and 3 800 m it includes valanginian through cenomanian chalk and limestone presumably deposited in a re entrant between the platforms 2 beneath these carbonates the seismic data indicate a possible reef structure at 5 700 m and prograding clinoforms suggestive of rapidly deposited rift sediments below 6 500 m 3 the bahama escarpment like the blake escarpment is an erosional slope exposing jurassic to early cretaceous back reef limestones geometry indicates an erosional retreat of ca 5 km of the base of the escarpment since early cretaceous time 4 major faults may separate the re entrant sequence from the ridge forming the escarpment and may also define exuma canyon and the escarpment itself

contourite drift molded by deep mediterranean outflow since its inception in early pliocene time the deep mediterranean outflow has eroded and sculptured the southern iberian margin the 50 km long 300 m thick faro drift constructed by this powerful bottom current has been the subject of detailed study sandy silty and muddy contourite facies can be clearly characterized and a distinctive vertical arrangement or sequence of these facies can be related to hydrodynamic fluctuation these results are important for the identification of ancient contourites the reconstruction of paleocirculation patterns and documentation of the effects of quaternary climates on the mediterranean outflow

contour current imprints and contourite drifts in the bahamian archipelago new data collected along the slopes of little and great bahama bank and the abyssal plain of the bahama escarpment provides new insights about contour current related erosive structures and associated deposits the bahamian slope shows abundant evidence of bottom current activity such as furrows comet like structures sediment waves and drifts at a seismic scale large erosion surfaces and main periods of drift growth resulted from current acceleration related to plate tectonic processes and progressive opening and closure of gateways and long term palaeoclimate evolution at present day erosion features and contourite drifts are either related to relatively shallow currents 2500 m water depth it appears that the carbonate nature of the drifts does not impact the drift morphology at the resolution addressed in the present study classical drift morphologies defined in siliciclastic environments are found such as mounded plastered and separated drifts in core contourite sequences show a bi gradational trend that resembles classical contourite sequences in siliciclastic deposits showing a direct relationship with a change in current velocity at the sea floor however in a carbonate system the peak in grain size is associated with increased winnowing rather than increased sediment supply as in siliciclastic environments in addition the carbonate contourite sequence is usually thinner than in siliciclastics because of lower sediment supply rates little bahama bank and great bahama bank contourites contain open ocean input and slope derived debris from glacial episodes inner platform platform edge and open ocean pelagic input characterize the classical periplatform ooze during interglacials in all studied examples the drift composition depends on the sea floor topography surrounding the drift location and the type of sediment supply carbonate particles are derived from either the slope or the platform in slope and toe of slope drifts very deep contourites have distant siliciclastic sources of sediment supply the recent discovery of the importance of a large downslope gravitary system along bahamian slopes suggests frequent interactions between downslope and along slope contour currents processes the interlayering of mass flow deposits and contourites at a seismic scale or the presence of surface structures associated with both contour currents and mass flow processes shows that both processes act at the same location finally contour currents have an important impact on the repartition of deep water coral mounds currents can actively interact with mounds as a nutrient and oxygen supplier or have a passive interaction with mounds solely being obstacles orienting erosion and deposition

controls on grain size patterns in periplatform carbonates marginal setting versus glacio eustacy the importance of grain size as a fundamental property controlling other physically derived properties is well known recent grain size studies on periplatform sediments of the caribbean and western north atlantic have shown that grain size variability is coupled with changes in carbonate platform productivity and export in response to pleistocene holocene glacio eustacy these findings showed that periplatform sediments are dominated by fine grained material during sea level highstands and coarser material during sea level lowstands this paper addresses the question of grain size patterns in periplatform settings and their relationship to glacio eustacy furthermore it considers whether these patterns are applicable to different types of margin settings accretionary erosional here existing data pleistocene holocene from the accretionary western leeward margin of great bahama bank odp leg 166 and new data from the erosional eastern windward margin odp leg 101 are used the results show that the grain size patterns of both margins are controlled by late neogene sea level fluctuations the accretionary leeward margin sediments indicate the same grain size pattern as previously described in the literature however the erosional windward margin shows a reverse pattern with coarser sediments in interglacials and finer deposits in glacials this proposes that the leeward margin grain size pattern is controlled by the productivity export mode of the platform this is indicated by the platform top derived fine muds transported off bank to form a thick sediment wedge on the low angled slope during interglacials in contrast on the windward margin the sediment flux pattern is controlled by the impact of mass transport processes this is indicated by the increased occurrence of turbidites during interglacials that might be more frequent at this time because the steep angled erosional slope is less apt for the deposition of excess fine material this material is eroded and or bypasses the slopes and is redeposited in the periplatform basin setting

bahamian carbonate platform development in response to sea level changes and the closure of the isthmus of panama in this paper we show that the development of the sediment architecture at the leeward toe of slope of great bahama bank ocean drilling project leg 166 bahama transect during the last 6 ma is not only a response to sea level fluctuations but also to major paleo oceanographic and climatic changes a major sequence boundary close to the miocene pliocene boundary dated at 5 6 5 4 ma is interpreted to reflect a major sea level drop that was followed by a sea level rise which led to the re flooding of the mediterranean sea at the end of the messinian and increasing sea surface temperatures at great bahama bank distinct erosional horizons occurred during the pliocene dated at 4 6 and 3 3 3 6 ma related to sea level change and the intensification of the gulf stream when the emergence of the isthmus of panama reached a critical threshold the gulf stream brings warm saline and nutrient poor waters to the bahamas starting at the early late pliocene boundary at 3 6 ma this paleo oceanographic reorganization in combination with enhanced sea level fluctuations associated with the late pliocene main intensification in northern hemisphere glaciation since 3 2 ma led to 1 a gradual change from a ramp type to a flat topped type morphology and 2 a change from a skeletal to a non skeletal dominated sedimentary system mainly peloidal increased sea level fluctuations during the second half of the pleistocene led to an intensified high stand shedding depositional pattern within the surrounding basins

middle miocene oceanographic event in the eastern gulf of mexico implications for seismic stratigraphic succession and loop current gulf stream circulation analysis of 1 500 km of high resolution single channel air gun seismic reflection profiles correlated with two drill cores demonstrates that the central west florida carbonate ramp slope experienced an oceanographic event in the middle miocene that resulted in a major stratigraphic break or succession of depositional sequences during the long term eustatic sea level rise in the early to middle miocene carbonate shelf margin deposition was characterized by seaward prograding sigmoidal clinoforms that built both laterally and vertically shelf margin sediments were relatively fine grained and rich in dolomite total organics and clay minerals approximately 12 15 m y ago tectonically controlled oceanographic closure of the isthmus of panama and or increased climatic gradients resulted in an intensification of oceanic circulation increased flow velocity of the loop current truncated antecedant clinoforms along the mid miocene shelf margin as the loop current for the first time became a dynamic oceanographic barrier to off shelf sediment transport carbonate deposition along the west florida slope changed dramatically from a series of prograding clinoforms to a pelagic slope front fill system bounded on its landward margin by contour current flow of an invigorated loop current shelf margin sediments presently represented as a condensed sequence became more carbonate rich coarser grained lower in organic content and depleted in dolomite our results suggest that oceanographic events can generate major seismic stratigraphic breaks that result in the succession of depositional sequences and systems along continental margins and that they may be as important as eustatic oscillations our data also suggest that modern loop current gulf stream circulation was initiated in the middle miocene controlled by tectono oceanographic and or climatic processes the challenge now is to identify seismic stratigraphic scale oceanographic events in the rock record differentiate them from eustatic responses and evaluate their paleoenvironmental significance

episodic folding inferred from syntectonic carbonate sedimentation the santaren anticline bahamas foreland abstract sedimentation coeval with growth of the santaren anticline provides an excellent opportunity to study the relationships between sedimentation and anticline uplift through time the santaren anticline is a kilometre scale nw se trending fold offshore of cuba in the bahamas foreland of the cuban fold and thrust belt the growth strata associated with this anticline consist of a thick package of carbonate sediments that were deposited without major interruptions from neogene and perhaps before to present day the excellent seismic resolution and age control of a number of seismic horizons within the growth strata allowed us to define 25 growth beds each of them representing between 0 1 and 3 2 ma an analysis of the thickness of these beds allowed us to determine accurate quantitative values of cumulative decompacted thickness and crestal structural relief at the time of their deposition in addition for the same periods sedimentation and fold uplift rates were calculated moreover some information on relationships between sedimentation and fold uplift rates was inferred from the growth stratal geometry growth beds that overlap the fold crest and thin over it indicate that sedimentation rates outpaced fold growth rates during their deposition some overlapping beds have constant thickness indicating that no fold uplift occurred during their sedimentation the rest of the growth beds exhibit onlap offlap geometries that do not indicate a unique sedimentation fold uplift rate relationship only in those cases in which the geometry of the underlying bed at the end of its deposition is known is it possible to infer a specific sedimentation fold uplift rate relationship as a result of this analysis we have been able to 1 illustrate that the geometry of the growth strata associated with the santaren anticline results from competition between sedimentation and tectonic fold uplift 2 document the episodic and non steady nature of fold growth and 3 show that short term rates at the scale of hundreds of thousands years provide much insight into the interplay between sedimentation and tectonic fold uplift that control the growth stratal patterns

sea level related resedimentation processes on the northern slope of little bahama bank middle pleistocene to holocene middle pleistocene to holocene sediment variations observed in a 26 metre long core taken during a cruise of the rv marion dufresne are presented core md992202 was retrieved from the northern slope of little bahama bank and provides an excellent example for sedimentation processes in a mid slope depositional environment the sediment composition indicates sea level related deposition processes for the past 375 000 years marine isotope stages 1 to 11 the sediments consist of i periplatform ooze fine grained particles of shallow water and pelagic origin with moderate variations in carbonate content carbonate mineralogy and grain size and ii coarser intervals with cemented debris consisting of massive poorly sorted mud supported or clast supported deposits with an increased high magnesium calcite content during interglacial stages marine isotope stages 1 5 7 9 and 11 periplatform oozes i are characterized by higher aragonite contents finer grain size and higher organic contents whereas during glacial stages marine isotope stages 2 to 4 6 8 and 10 increased low magnesium and high magnesium calcite values coarser grain size and lower organic contents are recorded these glacial to interglacial differences in mineralogy grain size distribution and organic content clearly show the impact of climatically controlled sea level fluctuations on the sedimentation patterns of the northern slope of little bahama bank the coarser deposits ii occur mainly at the transitions from glacial to interglacial and interglacial to glacial stages and are interpreted as redeposition events indicating a direct link between sediment properties changes in mineralogy grain size distribution variations in organic contents and sea level fluctuations changes in hydrostatic pressure and the wave base position during sea level changes are proposed to have triggered these large scale sediment redepositions

highstand versus lowstand deposition on carbonate platform margins insight from quaternary foreslopes in the bahamas abstract examples of both highstand and lowstand deposition have been identified on the upper platform slopes of the tongue of the ocean bahamas highstand deposits consist of a thick wedge of fine grained skeletal and non skeletal sands and carbonate muds derived predominantly from the platform and deposited on slopes of 25 28 these sediments were deposited subsequent to platform flooding and may be more than 30 m thick along open leeward margins in contrast lowstand deposits consists of coarse skeletal sands gravels and boulders deposited on primary depositional slopes of 35 45 these sediments were derived from fringing reefs along the marginal escarpment and deposited during the initial rise of sea level following the last lowstand these variations in depositional character result in a distinctive signature of highstand versus lowstand deposition for steep sided platforms and further elucidate the response of carbonate platforms to high magnitude 5th order scale 20 000 yrs fluctuations in sea level

genesis and morphology of cold water coral ridges in a unidirectional current regime abstract cold water coral ridges with waveform morphology are commonly observed in mound surfaces in the northeast atlantic ocean and interpreted as sediment waves overgrown by corals this work documents the first cold water coral ridges that are waveform but lack sand dune cores these coral ridges are situated at the base of the miami terrace in the straits of florida depths of 630 to 870 m they are entirely biogenic in origin and are unrelated to antecedent topography five lines of evidence based on 27 km2 of high resolution geophysical oceanographic and ground truthing data support this interpretation first the waveform coral ridges are aligned perpendicular to a southward flowing bottom current and juxtaposed to an active sand dune field devoid of corals both the coral ridges and sand dunes are asymmetric in profile with respect to the current however the ridges have their steep sides facing the current whereas the dunes exhibit a steep leeside second the coral ridge matrix consists of very fine sand to mud particles with no bedload sand transport on the ridge flanks or troughs in contrast active sand ripples superimpose the juxtaposed sand dunes indicating that coarse sand is transported via traction in this domain third bedload sand transport occurs in linear streaks at the shallowest part of the surveyed area these streaks orthogonally crosscut some coral ridges but coral colonies are not aligned along these streaks instead the bedload sand smothers the coral colonies and thereby segments the ridges fourth the sand dune and coral ridge acoustic responses are different below the dunes there is no sub bottom reflection due to energy attenuation in contrast a continuous reflection below the coral ridges proves the absence of attenuating sand cores fifth this continuous sub bottom reflection is not correlated with the coral ridge topography indicating that antecedent topography does not control the waveform morphology and spatial distribution of the ridges the differences in depositional profiles sediment dynamics and features as well as acoustic facies between the coral ridges and the juxtaposed dunes show that the coral ridges are entirely bioconstructional and not a veneer of corals covering paleo sand dunes this field of biogenic ridges at the miami terrace and the distribution of sand streaks as well as active sand dunes provide new insights how the lateral variability of current velocities and sediment transport conditions control the morphology distribution and genesis of cold water coral features

paleogene neogene depositional history of the middle u s atlantic continental rise mixed turbidite and contourite depositional systems abstract the construction of the middle u s atlantic continental rise from late paleogene to late pliocene time included downslope sediment gravity flows and alongslope contourite deposition some previous studies have suggested that contour following bottom currents were the dominant control on depositional patterns depositional relief from two large contourite drifts largely built in late miocene to late pliocene time continue to dominate present day lower rise morphology detailed seismic sequence mapping presented here suggests that significant turbidite fan systems also were active and are preserved in the stratigraphic section a single thick depositional sequence is mapped for oligocene to middle miocene sediments deposited when inferred fan systems off the norfolk washington canyons were reworked by bottom currents along the lower rise to form the proto hatteras outer ridge detailed mapping of three late miocene to late pliocene depositional sequences indicates that fan channel systems were active at the same time large contourite drifts were being constructed sediment delivered to the lower rise by these elongate turbidite systems undoubtedly was an important local source for construction of the hatteras outer ridge and through entrainment and resuspension was an important source for regional contourite deposition controlled by the deep western boundary undercurrent we suggest that submarine fans and current controlled sediment drifts may develop simultaneously as companion systems types of fan drift interaction include current only modification of fans transitional fan to drift and adjacent or overlapping submarine fan and sediment drift deposition

late cenozoic sediment drift complex northeast rockall trough north atlantic seismic reflection profiles and two shallow cores have revealed a sediment drift complex in the northeastern rockall trough the drift complex consists of an elongate drift with associated sediment waves a broad sheeted drift with a larger field of sediment waves and smaller moat related isolated drifts the internal reflection configuration of these features indicates consistent upslope migration throughout the late cenozoic the larger field of sediment waves has been described previously with a southerly migration a reevalution of these waves suggests a migration direction to the east conforming with the other drift complex features the most vigorous current activity probably occurred during the miocene with a more reduced current flow prevailing during the pliocene to holocene interval core evidence from the elongate drift and the moat has revealed upper pleistocene and lower holocene glaciomarine sediments reworked to produce muddy silty and sandy contourites sedimentation rates for the early holocene pre 7 5 ka are up to 4 times greater on the drift compared to the moat it is suggested that the sediment drift complex has formed through the interaction of a northward flowing slope current of north atlantic deep water origin with an area of complex bathymetry at the northeastern end of the rockall trough where the wyville thomson ridge intersects the hebridean margin

high resolution simulation of the circulation in the bahamas and turks and caicos archipelagos abstract we present the first high resolution simulation of the bahamas and turks and caicos archipelagos implemented with the regional oceanic modeling system roms our model resolves tides and our simulation consists of two nested grids of respectively 4 and 2 km horizontal resolution this oceanic region is constituted of the juxtaposition of very large and shallow banks and very deep basins our modeling study shows that the dominant influence is the salinity from the sub tropical gyre which varies latitudinally with seasons less salty surface waters entered the archipelago from september to november from the north and through the old bahama obc and the north west providence nwpc channels as the western atlantic saline waters moved south saltier atlantic waters arrived from the south in december and moved north till the next fall this salinity flux is mitigated by the atmospheric forcing on the banks as well as the water temperature which varied diurnally what modified the water density causing this water to be unique to the banks yielding its own seasonality as seen in their sst the banks were colder in winter and warmer in summer than their surrounding waters fronts were formed at every ebb tide in winter spring and fall however the influence of bank waters on the surrounding waters was shown to be limited because of the dominant influence of the large scale flow and of the local production of water masses by wind and cyclone driven upwelling bank waters were contained at the front where they mixed with the surrounding waters as shown by the short time presence of the fronts local wind in addition to the tide was also shown to be a significant forcing which is responsible for some of the short scale variability 5 30 days in florida current transport the nwpc the obc the windward passage wp and the antilles current ac finally the study also confirmed the mesoscale eddy influence at seasonal scales 100 days on the ac transport as well as on the fc through a possible leakage of eddy flux through the nwpc a similar influence was found south of the archipelago where mesoscale eddy could also influence wp transport

the geology of the darien panama and the late miocene pliocene collision of the panama arc with northwestern south america the geology of the darien province of eastern panama is presented through a new geologic map and detailed biostratigraphic and paleobathymetric analysis of its upper cretaceous to upper miocene sediments the sequence of events inferred from the stratigraphic record includes the collision of the panama arc the southwestern margin of the caribbean plate and south american continent three tectonostratigraphic units underlie the darien region 1 precollisional upper cretaceous eocene crystalline basement rocks of the san blas complex form a series of structurally complex topographic massifs along the northeastern and southwestern margins of the darien province these rocks formed part of a 20 m y submarine volcanic arc developed in a pacific setting distant from the continental margin of northwestern south america the northerly basement rocks are quartz diorites granodiorites and basaltic andesites through dacites to rhyolites indicating the presence of a magmatic arc the southerly basement rocks are an accreted suite of diabase pillow basalt and radiolarian chert deposited at abyssal depths precollisional arc related rocks of eocene to lower miocene age consist of 4000 m of pillow basalts and volcaniclastics and biogenic calcareous and siliceous deep water sediments they consist of the eocene oligocene darien formation the oligocene porcona formation and the lower middle miocene clarita formation postcollisional deposits are mostly coarse to fine grained siliciclastic sedimentary rocks and turbiditic sandstone of upper middle to latest miocene age this 3000 m thick sedimentary sequence is deformed as part of a complexly folded and faulted synclinorium that forms the central chucunaque tuira basin of the darien the sedimentary package reveals general shallowing of the basin from bathyal to inner neritic depths during the 12 8 7 1 ma collision of the panama arc with south america the sediments are divided into the upper middle miocene tapaliza formation the lower upper miocene tuira and membrillo formations the middle upper miocene yaviza formation and the middle to upper miocene chucunaque formation the precollisional open marine units of late cretaceous middle miocene age are separated from the overlying postcollisional sequence of middle to late miocene age by a regional unconformity at 14 8 12 8 ma this unconformity marks the disappearance of radiolarians the changeover of predominantly silica deposition from the atlantic to the pacific the initiation of the uplift of the isthmus of panama and the onset of shallowing upward coarser clastic deposition this pattern is also recorded from the southern limon basin of caribbean costa rica to the atrato basin of northwestern colombia by the middle late miocene neritic depths were widespread throughout the darien region and a regional unconformity suggests completion of the central american arc collision with south america by 7 1 ma no pliocene deposits are recorded from either the darien or the panama canal basin and no sediments younger than 4 8 ma have been identified in the atrato basin of colombia suggesting rapid uplift and extensive emergence of the central american isthmus in the latest miocene northward movement of the eastern segment of the panama arc along a now quiescent panama canal zone fault during eocene oligocene time may have dislocated the pre collision arc since collision the portion west of this fault chorotega block has remained stable without rotation to the east in the darien region compression has been accommodated through formation of a panama microplate with convergent boundaries to the north north panama deformed belt and south south panama deformed belt and suturing with south america along the atrato valley deformation within the microplate has been accommodated in the darien province by several major left lateral strike slip faults that were active until the early pliocene since when the plate has behaved rigidly

fluvial processes and vegetation glimpses of the past the present and perhaps the future abstract most research before 1960 into interactions among fluvial processes resulting landforms and vegetation was descriptive since then however research has become more detailed and quantitative permitting numerical modeling and applications including agricultural erosion abatement and rehabilitation of altered bottomlands although progress was largely observational the empiricism increasingly yielded to objective recognition of how vegetation interacts with and influences geomorphic process a review of advances relating fluvial processes and vegetation during the last 50 years centers on hydrologic reconstructions from tree rings plant indicators of flow and flood frequency parameters hydrologic controls on plant species regulation of sediment movement by vegetation vegetative controls on mass movement and relations between plant cover and sediment movement extension of present studies of vegetation as a regulator of bottomland hydrologic and geomorphic processes may become markedly more sophisticated and widespread than at present research emphases that are likely to continue include vegetative considerations for erosion modeling response of riparian zone forests to disturbance such as dams and water diversion the effect of vegetation on channel and bottomland dynamics and rehabilitation of stream corridors research topics that presently are receiving attention are the effect of woody vegetation on the roughness of stream corridors and hence processes of flood conveyance and flood plain sedimentation the development of a theoretical basis for rehabilitation projects as opposed to fully empirical approaches the effect of invasive plant species on the dynamics of bottomland vegetation the quantification of below surface biomass and related soil stability factors for use in erosion prediction models and the effect of impoundments on downstream narrowing of channels and accompanying encroachment of vegetation bottomland vegetation partially controls and is controlled by fluvial geomorphic processes the purposes of this paper are to identify and review investigations that have related vegetation to bottomland features and processes to distinguish the present status of these investigations and to anticipate future research into how hydrologic and fluvial geomorphic processes of bottomlands interact with vegetation

intermediate water links to deep western boundary current variability in the subtropical nw atlantic during marine isotope stages 5 and 4 records from ocean drilling program sites 1057 and 1059 2584 m and 2985 m water depth respectively have been used to reconstruct the behavior of the deep western boundary current dwbc on the blake outer ridge bor from 130 to 60 kyr b p marine isotope stage mis 5 and the 5 4 transition site 1057 lies within labrador sea water lsw but close to the present day boundary with lower north atlantic deep water lnadw while site 1059 lies within lnadw high resolution sortable silt mean inline equation grain size and benthic 13c records were obtained and changes in the dwbc intensity and spatial variability were inferred comparisons are made with similar proxy records generated for the holocene from equivalent depth cores on the bor during mis 5e inline equation evidence at site 1057 suggests slower relative flow speeds consistent with a weakening and a possible shoaling of the lsw sourced shallower limb of the dwbc that occupies these depths today in contrast the paleocurrent record from the deeper site suggests that the fast flowing deep core of the dwbc was located close to its modern depth below 3500 m during this interval the benthic 13c suggests little chemical stratification of the water column and the presence of a near uniform lnadw dominated water mass after 111 kyr b p the inline equation record at site 1057 increases to reach values similar to site 1059 for the rest of mis 5 the strengthening of flow speeds at the shallow site may correspond to the initiation of glacial north atlantic intermediate water formation also suggested by a divergence in the benthic 13c records with site 1057 values increasing to 1 2 coupled suborbital oscillations in dwbc flow variability and paleohydrography persisted throughout mis 5 comparison of these data with planktonic 18o records from the sites and alkenone derived sea surface temperature sst estimates from the nearby bermuda rise suggest a hitherto unrecognized degree of linkage between oscillations in subtropical north atlantic sst and dwbc flow

modern deep water coral mounds north of little bahama bank criteria for recognition of deep water coral bioherms in the rock record abstract deep water ahermatypic coral mounds are present at water depths of 1 000 1 300 m on the lower slope north of little bahama bank the mounds are patchily distributed over a minimum area of 2 500 km2 and typically display 5 40 m of relief above the surrounding sea bottom a diverse benthic community exists on these apparently unlithified mounds including l 1 genera and 16 species of ahermatypic coral bathypsammia caryophyllia deltocyathus desmophyllum enallopsammia javania madrepora polymyces solenosmilia stephanocyathus and a previously undescribed genus alcyonaceans gorgonians antipatharians hydroids ophiuroids crinoids barnacles galatheid crabs polychaetes gastropods bivalves and sponges conspicuously absent from the coral fauna are lop elia and dendrophyllia common deep water corals in other parts of the atlantic radiocarbon dates on fresh coral and gorgonian fragments of 940 40 and 860 50 years indicate the mounds are at least in part recent and are probably actively forming today bored and stained corals date at around 22 10314c years b p which establishes a minimum age for these mounds we speculate that the mounds develop on sea floor perturbations in areas where strong bottom currents provide needed oxygen and nutrients to the fauna the mounds may have undergone multi stage evolution from colony to thicket to coppice to bank this evolution may be accomplished through the in situ contribution of skeletal material along with the baffling and trapping of fine grained carbonate sediment winnowed from adjacent areas by bottom currents if found in the rock record the coral mounds north of little bahama bank would probably be considered bioherms geologists should thus be aware that scleractinian bioherms are not uniquely shallow water in origin the distinction between ancient deep and shallow water coral buildups involves using multiple recognition criteria including 1 presence or absence of algae 2 diversity of corals 3 coral morphology and microstructure 4 abundance of planktonic benthonic components 5 microborings 6 surrounding litho and biofacies 7 stable isotopes of carbon and oxygen and 8 trace element geochemistry particularly sr and u concentrations the distinction between deep and shallow water bioherms is crucial to regional paleoenvironmental and stratigraphic interpretations

the early neogene continental rise off the eastern united states abstract single and multichannel seismic data were used to investigate depositional components comprising the late oligocene middle miocene constructional phase of the united states atlantic continental rise in the georges bank blake plateau region the seismic sequence studied was bounded by the eocene oligocene age horizon a u unconformity at the base and a newly defined late middle miocene age horizon g at the top late oligocene middle miocene accretion onlapping the paleocontinental slope and downlapping onto horizon a u basinward was derived primarily from erosion of the uplifted central appalachian mountains this accretion up to 3 2 km thick was characterized by seven depocenters the lower paleocontinental slope upper rise depocenters situated off southern new england and the delmarva peninsula were interpreted as deep sea fan complexes or a broadly overlapping series of sedimentary blankets a whereas the closely associated oceanographer and wilmington depocenters were inferred to be levee complexes immediately north of the developing blake bahama outer ridge two n ne oriented sediment ridges that were probably built by gulf stream flow during an early miocene sea level lowstand were observed on the lower continental rise the proto hatteras and mytilus outer ridges from 0 4 to 0 8 km thick were probably formed from the wilmington and oceanographer levee complexes in early middle miocene time through the interaction of the growing levee complex with the sw moving abyssal current late middle miocene 12 11 5 ma bottom current erosion created the horizon g unconformity and brought to an end this initial stage of continental rise construction nevertheless the basic morphological framework that controlled late neogene and quaternary continental rise evolution was now in place

the maldives a giant isolated carbonate platform dominated by bottom currents abstract the maldives a 900 km north south trending paired chain of atolls that surround a ca 100 km wide central basin the inner sea represent a giant isolated carbonate platform in the indian ocean new reflection seismic multibeam and acoustic doppler current profiler data show that its development exhibits a distinct change from sea level to mainly bottom current controlled sedimentation at the end of the middle miocene the post middle miocene intra basinal fill consists of 9 mega units that have been formed and shaped by strong bottom currents entering the inner sea between the inter atoll passages at the end of the middle miocene the maldives represent a rimmed platform with the inner sea forming an empty bucket a connection to the indian ocean existed only in the northeastern part of the platform namely the kardiva channel probably concomitant with the onset of the indian monsoon the current pattern around the maldives changed significantly resulting in the formation of channels in the rimmed platform margin the latter process was encouraged by local drowning of large platform parts and additionally by bottom current forced submarine erosion that widened the channels to passages disintegrating the platform into separated atolls of different sizes the reworked material was dumped into the inner sea where it formed large contourite fans along slope bottom currents entering the inner sea from the north further redistributed the material leading to a southward shift of these sedimentary bodies in this way the inner sea was filled from west to east mega units 1 5 starting with mega unit 6 the opening of a southern gateway introduced a prominent northward flow of bottom waters in the inner sea it leads to the deposition of giant elongated drifts at the eastern flank of the basin filling it from east to west because the current swept away most of the material around the atolls the system was not able to prograde and the steady subsidence was compensated by aggradation accordingly a fundamental characteristic of a current controlled carbonate system is that aggradation occurs irrespective of platform sediment export or differently worded the rate of change in accommodation is always positive and greater than the rate of sediment supply adcp measurements in the winter monsoon season show two water masses in the inner sea one at the surface 0 150 m and a second below it that reaches its sea floor by the use of the grain size distribution of odp leg 716 located in the inner sea as proxy for the flow intensity of the bottom water we recognized an inverse proportional relationship to the monsoon strength from late upper miocene to present some of the current controlled deposits form sigmoidal prograding clinoforms preferably at the edges of the atolls adjacent to the passages where the velocity of the current entering the inner sea is high the anatomy of these prograding wedges is similar to sea level controlled clinoforms described for large isolated carbonate platforms in the sequence stratigraphic concept however our results document that they are not influenced by sea level fluctuations because of the high current velocities involved in their deposition they predominately consist of coarse sand sized carbonate fragments exhibiting large pore space and high permeability therefore these drift deposits comprise a possible exploration target

current meter observations in the old bahama channel current meter observations were made at 50 250 and 435 m in 495 m of water at the center of the old bahama channel between november 1990 and november 1991 the mean speed at those depths was 2 6 50 and 26 cm s 1 toward the straits of florida a maximum speed of 193 cm s 1 was found at the 250 m level there was evidence of a speed maximum between 50 and 250 m large internal diurnal tidal currents were observed that produced high shears in the water column under some conditions transport calculated from the single mooring using simple assumptions varied from 2 4 to 6 6 sv with a mean of 1 9 sv toward the straits of florida these values while based on limited data are very significant compared to the mean flow in the straits of florida thus flow through the old bahama channel may be an important component of heat and salt transport in the straits

autonomous underwater vehicle auv mapping reveals coral mound distribution morphology and oceanography in deep water of the straits of florida 1 to make progress in understanding the distribution and genesis of coral mounds in cold and dark water maps of morphology and oceanographic conditions resolving features at the 1 10 m scale are needed an autonomous underwater vehicle auv cruising 40 m above the seafloor surveyed a 48 km2 coral mound field in 600 800 m water depth at the base of slope of great bahama bank the auv acquired 1 3 meter resolution acoustic backscatter and bathymetry together with current vectors salinity and temperature the multibeam bathymetry resolved more than 200 coral mounds reaching up to 90 m height mound morphology is surprisingly diverse and mound distribution follows e w oriented off bank ridges bottom currents reverse every 6 hours indicating tidal flow decoupled from the north flowing surface current the auv data fill the gap between low resolution surface based mapping and visual observations on the seafloor revealing the dynamic environment and spatial relationships of an entire coral mound field

anatomy of a modern open ocean carbonate slope northern little bahama bank the open ocean carbonate slope north of little bahama bank consists of a relatively steep 4 upper slope between water depths of 200 and 900 m and a more gentle 1 2 lower slope between depths of 900 and 1300 m the upper slope is dissected by numerous small submarine canyons 50 150 m in relief that act as a line source for the downslope transport of coarse grained carbonate debris the lower slope is devoid of any well defined canyons but does contain numerous small 1 5 m hummocks of uncertain origin and numerous larger 5 40 m patchily distributed ahermatypic coral mounds sediments along the upper slope have prograded seaward during the cenozoic as a slope front fill seismic facies of fine grained peri platform ooze surface sediments show lateral gradation of both grain size and carbonate mineralogy with the fine fraction derived largely from the adjacent shallow water platform near surface sedimentary facies along the upper slope display a gradual downslope decrease in the degree of submarine cementation from well lithified hardgrounds to patchily cemented nodular ooze to unlithified peri platform ooze controlled by lateral variations in diagenetic potential and or winnowing by bottom currents submarine cementation stabilizes the upper part of the slope allowing upbuilding of the platform margin and controls the distribution of submarine slides as well as the headward extent of submarine canyons where unlithified sediments are heavily bioturbated and are locally undergoing dolomitization upper slope sediments are also conditioned eustatically resulting in vertical cyclic sequences of diagenetically unstable aragonite and magnesian calcite rich and stable calcite rich carbonates that may explain the well bedded nature of ancient peri platform ooze sequences lower slope sediments have prograded seaward during the cenozoic as a chaotic fill seismic facies of coarse grained carbonate turbidites and debris flow deposits with subordinate amounts of peri platform ooze coarse clasts are internally derived from fine grained upper slope sediments via incipient cementation submarine sliding and the generation of sediment gravity flows gravity flows bypass the upper slope via a multitude of canyons and are deposited along the lower slope as a wedge shaped apron of debris parallel to the adjacent shelf edge consisting of a complex spatial arrangement of localized turbidites and debris flow deposits a proximal apron facies of thick mud supported debris flow deposits plus thick coarse grained ta turbidites grades seaward into a distal apron facies of thinner grain supported debris flow deposits and thinner finer grained ta b turbidites with increasing proportions of peri platform ooze both the geomorphology and sedimentary facies relationships of the carbonate apron north of little bahama bank differ significantly from the classic submarine fan model as such a carbonate apron model offers an alternative to the fan model for palaeoenvironmental analysis of ancient open ocean carbonate slope sequences

density cascading off shelf sediment transport evidence and implications bahama banks abstract suspended sediment concentrations in shelf waters are often appreciable but sediments on their own rarely contribute enough density to cause suspension currents to transport them offshelf however sediment charged hyperpycnal high density waters are often generated over continental shelves because of the same set of air sea interactions intense heat and moisture flux responsible for thermohaline production and circulation of deep ocean water masses we propose the term density cascading to describe the processes by which hyperpycnal waters are released from shelf areas and together with entrained sediments sink to their compensation level in the adjacent ocean density cascading probably involves a range of physical processes from pelagic draping of slopes and basins with shelf muds to initiating turbidity currents capable of scouring canyons and plunge pools sediment bypassing of slopes and deposition of sands in deep basins thermodynamic air sea interactions can generate hyperpycnal water at all latitudes therefore density cascading has important implications not only to the sedimentology of modern carbonate platforms such as the florida shelf and bahama banks and stratigraphy in ancient limestones but also to offshelf sediment flux in siliciclastic environments

sedimentary dynamics along carbonate slopes bahamas archipelago hydroacoustic and sedimentological data in the santaren channel covering both the leeward slope of great bahama bank and the windward slope of cay sal bank allow new insights into carbonate platform slope sedimentation the data document the interplay between depositional and erosive processes on both slopes through time and provide information on the current regime and its influence on the slope sedimentary processes this study emphasizes the diversity and complexity of the slope morphology and the sediment distribution of the youngest high frequency sequence which has developed since the last glacial maximum the processes triggering slope failures and the formation of channels and gullies differ on both slopes at the leeward slope of the great bahama bank extensive slope failures occurred primarily during sea level lowering following an interglacial these slope failures created a slope morphology that channelizes the exported platform sediments during the subsequent highstand at the windward slope of cay sal bank contour currents and the local tectonic regime are responsible for slope failures during sea level lowstands downwelling induces turbidity currents the interaction of turbidity and contour currents leads to the formation of a system of furrows and slope parallel sediment ridges the discovered heterogeneities in slope sedimentation improve the understanding of carbonate slope sedimentation and provide implications for sequence stratigraphic interpretation of carbonate platform slopes

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

deposition of a muddy sediment drift in the southern straits of florida during the late quaternary abstract bottom currents associated with the florida current in large part control deposition of muddy biogenic sediments in the constricted deep sea passage of the southern straits of florida the unit is similar to muddy contourite drifts based on sediment structures changing thickness of biostratigraphic zones and distribution of biogenic sands in twenty piston cores current activity affected deposition more during the last glacial period than during the present and penultimate interglacial periods of the last 127 000 years based on the presence of an erosional regional unconformity that ended in the upper part of the glacial interval and on the change in pattern of silt and clay accumulation rates in the glacial interval relative to interglacial intervals

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

carbonate mounds in the santaren channel bahamas a current dominated periplatform depositional regime abstract new hydroacoustic data high resolution multichannel seismic reflection data ctd casts and sampling document the dominance of bottom currents and the occurrence of carbonate mounds in the santaren channel off great bahama bank and cay sal bank based on these data sets three types of carbonate mounds have been identified in water depths between 285 and 685 m their distribution is strongly related to the present current regime bank driven sediments are caught up by the currents and redeposited mainly in the channel center resulting in the accumulation of giant confined drifts flanked by moats the moats at the banks toe of slope are formed by a southward and northward flowing water mass respectively mainly antarctic intermediate water and subtropical underwater the observed carbonate mounds are concentrated in these moats mound type 1 has an elongated shape parallel to the prevailing bottom current direction a gently inclined flank facing the water flow and a steep flank in its current shadow its upstream flank is covered by a thin sediment wedge whereas the lee side is marked by a parabolic scour type 1 occurs at both bank margins but mainly off great bahama bank consisting of cold water coral rubble floating in a sediment matrix and is thus interpreted as cold water coral mound by contrast type 2 mounds are restricted to the cay sal margin and have a predominately conical outline bottom samples including living sessile crinoids show that its surface must be lithified and colonized by macroepibenthos except corals the mounds are situated in pockmark like depressions that are underlain by fluid escape structures like pipes and chimneys these fluid pathways are generated by a network of fractures mound type 2 is probably formed by abiotic carbonate precipitation however a chemoherm nature cannot be ruled out mound type 3 has a conical to polygonal shape and occurs off cay sal bank and great bahama bank it is surrounded by a scour which is elongated in downcurrent and crescent in upcurrent direction a sediment bulge formed at its front this type is consistent with blocks and boulders colonized by macroepibenthos and related to gravity controlled mass movements at the steep bank slopes principally two habitats can be distinguished the great bahama bank periplatform deep water province dominated by mound type 1 and the cay sal bank slope to basin province occupied by mound type 2 that is associated with pockmark like structures mound type 3 occurs in both habitats physical water mass properties for the existence of cold water corals are limited to a depth range of ca 430 to 685 m with seawater densities of 26 93 to 27 29 kg m and temperatures below 13 c their dominance off great bahama bank might be addressed to a more effective food supply induced by local downwelling

effect of the formation of the isthmus of panama on atlantic ocean thermohaline circulation the late cenozoic closure of the seaway between the north andsouth american continents is thought to have caused extensive changes in ocean circulation and northern hemisphere climate1 2 but the timing and consequences of the emergence of the isthmus of panama which closed the seaway remain controversial1 2 3 4 5 here we present stable isotope and carbonate sand fraction records from caribbean sediments which when compared to atlantic and pacific palaeoceanographic records indicate that the closure caused a marked reorganization of ocean circulation starting 4 6 million years ago shallowing of the seaway intensified the gulf stream and introduced warm and saline water masses to high northern latitudes these changes strengthened deep water formation in the labrador sea over the next million years as indicated by an increased deep water ventilation and carbonate preservation in the caribbean sea and favoured early pliocene warming of the northern hemisphere the evaporative cooling of surface waters during north atlantic deep water formation would have introduced moisture to the northern hemisphere although the pronounced intensification of northern hemisphere glaciation between 3 1 and 2 5 million years ago substantially lagged the full development of north atlantic deep water formation we propose that the increased atmospheric moisture content was a necessary precondition for ice sheet growth which was then triggered by the incremental changes in the earth s orbital obliquity

sea level and deep water temperature changes derived from benthic foraminifera isotopic records we show that robust regressions can be established between relative sea level rsl data and benthic foraminifera oxygen isotopic ratios from the north atlantic and equatorial pacific ocean over the last climatic cycle we then apply these regressions to long benthic isotopic records retrieved at one north atlantic and one equatorial pacific site to build a composite rsl curve as well as the associated confidence interval over the last four climatic cycles our proposed reconstruction of rsl is in good agreement with the sparse rsl data available prior to the last climatic cycle we compute bottom water temperature changes at the two sites and at one southern indian ocean site taking into account potential variations in north atlantic local deep water 18o our results indicate that a last glacial maximum lgm enrichment of the ocean mean oxygen isotopic ratio of 0 95 is the lowest value compatible with unfrozen deep waters in the southern indian ocean if local deep water 18o did not increase during glacials with respect to present such a value of the lgm mean ocean isotopic enrichment would impose a maximum decrease in local bottom water 18o at the north atlantic site of 0 30 during glacials

changes in caribbean surface hydrography during the pliocene shoaling of the central american seaway pliocene d18o records of shallow and deep dwelling planktonic foraminifers from the caribbean ocean drilling program sites 999 and 1000 the tropical east pacific sites 1241 and 851 and the atlantic site 925 ceara rise and site 1006 western great bahama bank were used to examine atlantic caribbean pacific atmospheric and oceanic linkages associated with the progressive closure of the central american seaway 5 5 3 ma comparisons indicate the development of an inner caribbean salinity gradient in the mixed layer and salinity changes on precessional periodicities after 4 4 ma site 1000 when the pacific caribbean throughflow became significantly restricted the high amplitude variability in salinity is also observed at site 1006 monitoring the caribbean outflow into the atlantic comparisons of caribbean and atlantic planktonic d18o records suggest the north atlantic subtropical gyre as a major source for high salinity surface waters precession induced variations in the volume transport of pacific surface water masses through the panamanian seaway are considered a main factor to explain the caribbean salinity minima results from a coupled climate model point to changes in the el nin o southern oscillation state as a potential trigger for changes in the amount of pacific inflow into the caribbean

from the great bahama bank into the straits of florida a margin architecture controlled by sea level fluctuations and ocean currents high resolution multichannel seismic data collected across the great bahama bank margin and the adjacent straits of florida indicate that the deposition of neogene quaternary strata in this transect are controlled by two sedimentation mechanisms 1 west dipping layers of the platform margin which are a product of sea level controlled platform derived downslope sedimentation and 2 east or north dipping drift deposits in the basinal areas which are deposited by ocean currents these two sediment systems are active simultaneously and interfinger at the toe of slope the prograding system consists of sigmoidal clinoforms that advanced the margin some 25 km into the straits of florida the foresets of the clinoforms are approximately 600 m high with variable slope angles that steepen significantly in the pleistocene section the seismic facies of the prograding clinoforms on the slope is characterized by dominant partly chaotic cut and fill geometries caused by submarine canyons that are oriented downslope in the basin axis seismic geometries and facies document deposition from and by currents most impressive is an 800 m thick drift deposit at the confluence of the santaren channel and the straits of florida this santaren drift is slightly asymmetric thinning to the north the drift displays a highly coherent seismic facies characterized by a continuous succession of reflections indicating very regular sedimentation leg 166 of the ocean drilling program odp drilled a transect of five deep holes between 2 and 30 km from the modern platform margin and retrieved the sediments from both the slope and basin systems the neogene slope sediments consist of peri platform oozes intercalated with turbidites whereas the basinal drift deposits consist of more homogeneous fine grained carbonates that were deposited without major hiatuses by the florida current starting at approximately 12 4 ma sea level fluctuations which controlled the carbonate production on great bahama bank by repeated exposure of the platform top controlled lithologic alternations and hiatuses in sedimentation across the transect both sedimentary systems are contained in 17 seismic sequences that were identified in the neogene quaternary section seismic sequence boundaries were identified based on geometric unconformities beneath the great bahama bank all the sequence boundaries could be traced across the entire transect into the straits of florida biostratigraphic age determinations of seismic reflections indicate that the seismic reflections of sequence boundaries have chronostratigraphic significance across both depositional environments

gulf stream structure transport and recirculation near 68 w an analysis of the structure and transport of the gulf stream is undertaken using direct current meter observations from a 13 mooring array deployed near 68 w from june 1988 to august 1990 the analysis is based on a stream coordinate approach in which velocities are rotated into a local downstream coordinate frame and averaged according to their relative cross stream location within the current the picture so obtained represents the average synoptic structure of the gulf stream rather than the eulerian averaged structure in which the current is weakened and broadened by lateral meandering of the current and adjacent recirculations many familiar features of the gulf stream are reproduced in the analysis including an asymmetric velocity profile with larger shear on the cyclonic shoreward side of the current an offshore displacement of the velocity core with depth and a subsurface velocity maximum on the offshore side of the current westward recirculations are also seen on both sides of the gulf stream maximum downstream speeds at the axis of the gulf stream reach approximately 2 0 m s at the surface and 0 7 m s at 1000 m roughly twice the corresponding eulerian averaged values the analysis also reveals a deep extension of the gulf stream at 3500 m depth with a width of 130 km and average speeds of 3 4 cm s the transport of the gulf stream in the stream coordinate frame is 113 8 sv approximately 30 larger than the eulerian averaged transport of 88 sv on the basis of these results and other recent studies the downstream transport increase of the gulf stream and the inflow structure to the gulf stream are reconsidered it is concluded that approximately 30 sv or over half of the transport increase between cape hatteras and 68 w is fed by inflow from the northern side of the gulf stream and that this inflow is concentrated near cape hatteras and 68 w where the gulf stream flows steeply across isobaths converging from the north