hydrocarbon sources of cold seeps off the vesteralen coast northern norway abstract we investigated active methane seeps in a water depth of 200 m in the hola area off the coast of vesteralen northern norway to assess 1 hydrocarbon sources 2 migration pathways and 3 the influence of hydrocarbon seepage on sediment pore water and water column chemistry the seepage area is characterised by the presence of gas flares in the water column as revealed by hydro acoustic surveys and elevated methane concentrations of up to 42 nm ca 5 m above the seafloor pore water analyses of three gravity cores from the seepage area show varying depths of the sulphate methane transition zone smtz between 80 cm and 250 cm indicating spatially heterogeneous methane ascent the isotopic composition of methane 13 c from 40 to 63 and 2 h from 191 to 225 and 13 c depth profiles of methane and dissolved inorganic carbon show that the hydrocarbons are predominantly of thermogenic origin consistent with 13 c values of c 2 to c 4 hydrocarbons isotope data also indicate considerable biodegradation of propane seismic profiles from the study area reveal major faults and steeply dipping unconformities between the basement and overlying mesozoic sedimentary rocks we propose that these act as migration pathways for the hydrocarbons from late jurassic to early cretaceous source rocks

the marine based nw fennoscandian ice sheet glacial and deglacial dynamics as reconstructed from submarine landforms abstract the configuration of the marine based nw fennoscandian ice sheet during the last glacial maximum lgm and deglaciation is reconstructed using detailed swath bathymetry and high resolution seismic data the investigated area covers about 10 000 km 2 of the continental shelf off troms northern norway large scale morphology is characterized by cross shelf troughs coast parallel troughs and banks based on mega scale glacial lineations msgl lateral and shear zone moraines and grounding zone systems the extent and dynamics of the ice sheet during the lgm are deduced msgl indicate fast flowing ice streams in the cross shelf troughs while the glacial morphology on the banks indicates more sluggish ice here the marine based part of the fennoscandian ice sheet was sourced from ice domes in the east via fjord and valley systems inshore using a balance flux approach we estimate palaeo ice stream velocities during the lgm to be approximately 350 m year three deglaciation events have been reconstructed i during the torsken 1 event the ice sheet halted or readvanced to form groundings zone wedges gzw and the torsken moraine ii several still stands or readvances characterized the ice behaviour on the shallower banks during the torsken 2 event iii during the flesen event prominent end moraines in the inner parts of the troughs and banks were deposited the locations of the end moraines and gzw in the troughs indicate that the retreat of marine based ice streams in areas of reverse bed slope was episodic probably mainly due to the variation in widths of the cross shelf troughs

potential geologic hazards on the eastern gulf of cadiz slope sw spain abstract geologic hazards resulting from sedimentary oceanographic and tectonic processes affect more than one third of the offshore gulf of cadiz and are identified by interpreting high resolution seismic profiles and sonographs hazards of sedimentary origin include the occurrence of slope instability processes in the form of single or multiple slumps occupying up to 147 km 2 mainly concentrated in the steeper upper slope area besides the presence of steep slopes the triggering of submarine landslides is probably due to seismic activity and favoured by the presence of biogenic gas within the sediment gassy sediments and associated seafloor pockmarks cover more than 240 km 2 in the upper slope hazards from oceanographic processes result from the complex system of bottom currents created by the interaction of the strong mediterranean undercurrent and the rough seafloor physiography the local intensification of bottom currents is responsible for erosive processes along more than 1900 km 2 in the upper slope and in the canyons eroded in the central area of the slope undermining slopes and causing instability the strong bottom currents also create a mobile seafloor containing bedforms in an area of the gulf that extends more than 2500 km 2 mostly in the continental slope terraces hazards of tectonic origin are important because the gulf of cadiz straddles two major tectonic regions the azores gibraltar fracture zone and the betic range which results in diapir uplift over an area of more than 1000 km 2 and in active seismicity with earthquakes of moderate magnitude also tsunamis produced by strong earthquakes occur in the gulf of cadiz and are related to the tectonic activity along the azores gibraltar fracture zone

research on bilateral reverse migration of one group seabed sand waves in a small shallow shelf sea bilateral reverse migration of one group seabed sand waves is found recently in many small sea areas and is a potential threat to the safety of many near seabed engineering structures such as pipelines and cables the mechanism cannot clearly be explained by existing theories this paper focuses on the bilateral reverse migration of one group seabed sand waves around a sand ridge in a small shallow shelf sea of the south china seas and presents a developed numerical model consisting of three dimensional wave current coupling module and sand transport module to simulate this complex phenomenon and perform relative analysis on the characteristics of the current and bilateral reverse migration formation mechanism taking the ocean environment characteristics of the surrounding sea area into account and with sufficient grid resolution this model successfully depicts the process of sand wave bilateral reverse migration bottom friction velocity is taken as the inspecting physical quantity and divided into tidal residual and short term fluctuation components through harmonic analysis with a method based on bedload transport calculation the contributions of the above three parts are determined it is found that long term sand wave migration trend is primarily controlled by both tidal constituents and residual velocity under nonlinear pure tidal impact the sand waves migrate southwards the residual current is northwards and will cancel the tidal impact when the magnitude of the residual velocity is large enough i e in the western part of the sand wave area sand waves migrate northwards when the residual velocity is small i e in the eastern part sand waves migrate southwards and the bilateral reverse migration pattern is formed the residual velocity distribution relates closely to the surrounding topography this paper discloses one kind of mechanisms for bilateral reverse migration of one group seabed sand waves and the model and analysis framework can also be extended and applied to other similar situations to prompt deeper and more comprehensive understanding about sand wave migration c 2016 elsevier b v all rights reserved

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

topographic modification of the florida current by little bahama and great bahama banks abstract the effect of local topography in modifying the structure and variability of the florida current is examined using shipboard acoustic doppler and pegasus acoustic current profiler data pegasus absolute velocity data were obtained during 16 cruises in the florida current at 27 n as part of the subtropical atlantic climate studies stacs program the ensemble average of all pegasus velocity data shows that the effect of the constriction imposed on the mean florida current by little bahama bank can be detected up to 30 km into the straits of florida a simple model is proposed to explain how this effect can produce the subsurface maximum of northward flow commonly observed in the eastern straits pegasus and acoustic doppler data obtained during the march 1984 stacs cruise are used to describe the temporal and spatial variability of the flow it is shown that intermittent southward flow can exist in a band 10 15 km wide off little bahama bank one such event was detected during this cruise the p

the western grand banks of newfoundland transgressive holocene sedimentation under the combined influence of waves and currents abstract the surficial sediments on the western grand banks of newfoundland have been reworked from older deposits primarily late wisconsinan glacial and glaciomarine sediments during the post glacial sea level fall and holocene rise winnowed medium to coarse sands and lag gravels that are generally less than 0 5 m thick chracterize the areas above the lowstand elevation at 110 to 120 m the holocene sediments which have accumulated in the deeper interbank channels isolated basins and on the upper continental slope are thicker and consists of the fine sands and muds that were removed from the banks the deposits below the lowstand elevation coarsen upward because little deposition occured during the sea level rise in general the holocene succession becomes thicker and finer grained to the south active sediment movement occurs at all depths less than about 110 m wave ripples composed of sandy gravel form to depths in excess of 100 m in response to long period 15 s surface waves but the predominant bedforms consist of current generated sand ribbons megaripples and sandwaves which exhibit unidirectional migration azimuths toward the southwest even though the storm current directions are highly variable and the maximum sediment transport rates are directed toward the east or southeast the bedform orientation and resultant transport are instead parallel to the southerly and southwesterly storm currents which occur during the prolonged post storm spin down period and to the weak 1 labrador current and the orbital motion associated with the largestwaaves thus the interaction of the oceanic current and wave orbital motion with the directionally variable strom currents is responsible for the consistent orientation of the current generated bedforms and for the distribution of holocene deposits

sources and turnover of organic carbon and methane in fjord and shelf sediments off northern norway to better understand the present and past carbon cycling and transformation processes in methane influenced fjord and shelf areas of northern norway we compared two sediment cores from the hola trough and from ullsfjorden we investigated 1 the organic matter composition and sedimentological characteristics to study the sources of organic carbon corg and the factors influencing corg burial 2 pore water geochemistry to determine the contribution of organoclastic sulfate reduction and methanogenesis to total organic carbon turnover and 3 the carbon isotopic signature of hydrocarbons to identify the carbon transformation processes and gas sources high sedimentation and corg accumulation rates in ullsfjorden support the notion that fjords are important corg sinks the depth of the sulfate methane transition smt in the fjord is controlled by the supply of predominantly marine organic matter to the sediment organoclastic sulfate reduction accounts for 60 of the total depth integrated sulfate reduction in the fjord in spite of the presence of ethane propane and butane we suggest a purely microbial origin of light hydrocarbons in the sediments based on their low 13c values in the hola trough sedimentation and corg accumulation rates changed during the deglacial to post glacial transition from approximately 80 cm ka 1 to erosion at present thus corg burial in this part of the shelf is presently absent low organic matter content in the sediment and low rates of organoclastic sulfate reduction only 3 of total depth integrated sulfate reduction entail that the shallow depth of the smt is controlled mostly by ascending thermogenic methane from deeper sources this article is protected by copyright all rights reserved

deep water sediment wave fields bottom current sand channels and gravity flow channel lobe systems gulf of cadiz ne atlantic a study of the seafloor of the gulf of cadiz west of the strait of gibraltar using an integrated geophysical and sedimentological data set gives new insights into sediment deposition from downslope thermohaline bottom currents in this area the mediterranean outflow mo begins to mix with north atlantic waters and separates into alongslope geostrophic and downslope ageostrophic components changes in bedform morphology across the study area indicate a decrease in the peak velocity of the mo from 1 m s 1to 0 5 m s 1 the associated sediment waves form a continuum from sand waves to muddy sand waves to mud waves a series of downslope oriented channels formed by the mo are found where the mo starts to descend the continental slope at a water depth of a700 m these channels are up to 40 km long have gradients of 0 5 a fairly constant width of a2 km and a depth of a75 m sand waves move down the channels that have mud wave covered levees similar to those seen in turbidite channel levee systems although the channel size and levee thickness do not decrease downslope as in typical turbidite channel systems the channels terminate abruptly where the mo lifts off the seafloor gravity flow channels with lobes on the basin floor exist downslope from several of the bottom current channels each gravity flow system has a narrow slightly sinuous channel up to 20 m deep feeding a depositional lobe up to 7 km long cores from the lobes recovered up to 8 5 m of massive well sorted fine sand with occasional mud clasts this work provides an insight into the complex facies patterns associated with strong bottom currents and highlights key differences between bottom current and gravity flow channel levee systems the distribution of sand within these systems is of particular interest with applications in understanding the architecture of hydrocarbon reservoirs formed in continental slope settings

bidirectional bedform fields at the head of a submarine canyon ne atlantic abstract submarine canyons are known to force ocean mesoscale circulation and local hydrodynamics alternate up and down canyon near bottom flows have been widely documented along the upper reaches connecting the canyon heads with the contiguous outer shelves and vice versa nonetheless we still miss clear evidence of bedform fields expressing these complex patterns in this study through a multi scale analysis in both space and time we document rare asymmetric bedforms up to 880 m long and 10 m high developing within a depth range of 168 220 m at the head of the whittard canyon ne atlantic one field of well developed sandwaves has an atypical up slope asymmetry with the steeper slope facing the shallower regions of the shelf and contrasting with surrounding down slope sandwaves facing the canyon the bedforms are interpreted to represent both up slope and down slope bottom currents connecting the upper reaches of the canyon to the outer shelf on the southern celtic margin in the bay of biscay the sandwaves were surveyed with shipboard multibeam bathymetry 5 m grid cell resolution auv sidescan sonar 0 15 m grid cell resolution and rov footage and sampled with three rov mounted vibro cores and two box cores sidescan sonar mosaics groundtruthed by sediment samples and rov footage show with unprecedented detail spectacular trains of fresh overprinting megaripples previously undocumented sand peaks and bowl shaped depressions on the crests of the tallest sandwaves differences in sedimentary settings and benthic habitats indicate that these features are currently active in particularly dynamic areas allowing for very slow migration of sandwaves numerical modelling together with concurrent hydrographic observations suggest large amplitude semi diurnal internal tides possibly transitioning to asymmetric internal bores as the main mechanism maintaining the mapped up slope sandwaves this work highlights the importance of uncommon sediment dynamics in canyon head environments and adds insight to the traditional notions of gravity driven processes being dominant in these environments envisaging implications for improving geo hazard assessment of mobile substrates and quantification of offshore sediment and carbon fluxes

a pseudo genetic model of coarse braided river deposits a new method is proposed to produce three dimensional facies models of braided river aquifers based on analog data the algorithm consists of two steps the first step involves building the main geological units the production of the principal inner structures of the aquifer is achieved by stacking multiple point statistics simulations of successive topographies thus mimicking the major successive flooding events responsible for the erosion and deposition of sediments the second step of the algorithm consists of generating fine scale heterogeneity within the main geological units these smaller scale structures are generated by mimicking the trough filling process occurring in braided rivers the imitation of the physical processes relies on the local topography and on a local approximation of the flow this produces realistic cross stratified sediments comparable to what can be observed in outcrops the three main input parameters of the algorithm offer control over the proportions the continuity and the dimensions of the deposits calibration of these parameters does not require invasive field measurements and can rely partly on analog data

potential gain and loss of sand by some sand banks in the southern bight of the north sea abstract a detailed echo sounding and side scan sonar survey has shown that sand banks in the southern bight of the north sea have rounded heads in the approximate upstream direction and tapered tails in the approximate downstream direction of net regional sand transport thus the plan view of sand banks can be used as an indicator of the approximate direction of net sand transport sand wave aprons occur in front of the heads and downstream of the tails of the sand banks the asymmetry of the sand waves implies that sand is being added to sand wave aprons at the heads and to the upstream flanks of these sand banks but is shed from the downstream sand wave aprons there is no evidence for a closed sand circulation on the banks

the cross sectional shape of tidal sandbanks modeling and observations to improve our understanding of tidal sandbank dynamics we have developed a nonlinear morphodynamic model a crucial property of the model is that it fully resolves the dynamics on the fast tidal timescale allowing for asymmetric tidal flow with an m0 m2 and m4 component this approach extending earlier research on the formation of tidal sandbanks leads to equilibrium profiles their heights 60 90 of the water depth and shapes are controlled by the mode of sediment transport and the hydrodynamic conditions bed load transport under symmetrical tidal conditions leads to high spiky banks several mechanisms tend to lower and smooth these profiles such as the relaxation of suspended sediment wind wave stirring and tidal asymmetry this last causes the profiles to be asymmetric as well the morphodynamic equilibrium expresses a tidally averaged balance between a destabilizing flux due to fluid drag and the downslope transport induced by both tidal flow and wind wave stirring the modeled profiles are in fair agreement with observations from the north sea

patterns of shear and turbulence across the florida current abstract measurements by the multi scale profiler msp at seven stations spanning the straits of florida characterize levels and patterns of internal wave activity and mixing in this vertically sheared environment contrasting properties suggest five mixing regimes the largest and most vast is the interior regime where the background flow has an inverse richardson number ri 1 ranging up to 0 55 shear is dominated by fluctuations that are 1 4 times stronger than in the open ocean and turbulent diffusivities are similarly moderate at 1 4 10 5 m2 s 1 the high velocity core of the current near the surface at midchannel is associated with weak mixing to its west is a zone of high mean shear where strong stratification results in background ri 1 of only 0 4 fluctuations are weak and diffusivity is moderate intermittent shear features beneath the core have mean ri 1 1 and strong turbulence two regimes are related to channel topography adjacent to the steep eastern slope finescale shear is

morphology and composition of submarine barchan dunes on the scotian shelf canadian atlantic margin abstract submarine barchan dunes have been mapped on browns bank scotian shelf using multibeam sonar bathymetry and backscatter strength the morphology of subaerial barchans has been studied for almost a century but the advent of multibeam sonar technology now enables a quantitative investigation of their submarine counterparts the browns bank submarine barchans occur at a depth of 60 70 m and are crescentic in planform reaching almost 700 m in horn width and 5 m in height the barchans are convex to the se with steep lee faces to the nw indicating a dominant nw flowing current the barchans overlie a widespread gravel lag covered elsewhere with little or no sand obstacle marks emanate from the lee faces of the barchans and represent a lack of sand deposition and exposure of gravel lag on the sea floor the browns bank submarine barchan sediment texture is gravelly sand or sandy gravel and is primarily composed of subrounded to well rounded quartz grains the allometric relationship between submarine barchan slip face height and distance between horns is markedly different from that of subaerial barchans for the same dune height barchan horn width is about ten times greater in the marine environment the superimposition of megaripples on the stoss slopes of the submarine barchans suggests that the barchans are active and therefore represent an engineering risk to sea floor infrastructure current observations and models indicate that seasonal mean current strength is less than the critical velocity required for barchan migration however the barchans may be active under higher velocity storm induced currents repetitive multibeam sonar mapping is required to detect if barchan migration is occurring over longer time scales

distribution and mixing of intermediate water masses in the channel of sicily mediterranean sea 1 the channel of sicily which represents the only connection between the western and the eastern mediterranean sea can be described as a three layer system where fresher water of atlantic origin modified atlantic water flows eastward in the upper layer levantine intermediate water liw leaves the eastern basin at around 250 350 m and an outflow of transitional ionian deep water tidw is identified below the liw synoptic mesoscale plankton experiment data collected during four surveys in the channel of sicily show that liw and tidw are subject to major modifications when crossing the eastern sill a significant bernoulli aspiration is found there and the bottom water at the sill is composed of modified deep ionian waters coming from 800 m an analysis of the water mass composition proves that liw mixes essentially with tidw and that the tidw flow through the channel must be larger than the expected 0 5 sv the presence of a secondary circulation related to the bottom boundary layer is demonstrated to be the primary factor leading to the dilution and cooling of liw

marine geology of the sodwana bay shelf southeast africa abstract the geostrophic current controlled northern zululand shelf displays a unique assemblage of interesting physical sedimentological and biological phenomena the shelf in this area is extremely narrow 3 km and is characterised by submarine canyons coral reefs and steep gradients on the continental slope three submarine canyons occur in the study area and are classified as mature or youthful phase canyons depending on the degree to which they breach the shelf these canyons originated as mass wasting features which were exploited by palaeo drainage during sea level regressions shelf lithology is dominated by a series of coast parallel patch coral reefs which have colonised beachrock and aeolianite sequences that extend semi continuously from 5 to 95 m and delineate late pleistocene palaeocoastline events the unconsolidated sediment on the shelf is either shelf sand mainly terrigenous quartz grains or bioclastic sediment large scale subaqueous dunes commonly form in the unconsolidated sediment on the outer shelf due to the agulhas current flow these dunes occur as two distinct fields at depths of 35 to 70 m the major sediment transport direction is towards the south but occasional bedload parting zones exist where the bedform migration direction changes from south to north

high frequency internal waves and thick bottom mixed layers observed by gliders in the gulf stream autonomous underwater gliders are conducting high resolution surveys within the gulf stream along the u s east coast glider surveys reveal two mechanisms by which energy is extracted from the gulf stream as it flows over the blake plateau a portion of the outer continental shelf between florida and north carolina where bottom depths are less than 1000 m internal waves with vertical velocities exceeding 0 1 m s 1 and frequencies just below the local buoyancy frequency are routinely found over the blake plateau particularly near the charleston bump a prominent topographic feature these waves are likely internal lee waves generated by the subinertial gulf stream flow over the irregular bathymetry of the outer continental shelf bottom mixed layers with o 100 m thickness are also frequently encountered these thick bottom mixed layers likely form in the lee of topography due to enhanced turbulence generated by o 1 m s 1 near bottom flows

tide and wave dynamics on a sand bank from the deep shelf of the western channel approaches abstract the sedimentary response of deep shelf sediments to the interplay of tides and waves based on the example of the celtic sea sand banks is described grab samples and 1500 km of multibeam and side scan data have been collected from the 300 km2 densely surveyed kaiser bank area the bank is 140 170 m deep 30 m high and 60 km long oriented perpendicular to the shelf edge and nearly parallel to the major axis of the tidal ellipse surficial sediments of the bank consist of medium to gravely biolithoclastic sands they are swept by tidal currents that reach 0 9 m s 1 m above the seabed and under the occasional influence of waves the mobile sands are commonly bedformed and rest on a highly backscattering lag the flanks below 140 m mainly exhibit transverse tidal dunes and sand ribbons whereas the top of the bank mostly displays discontinuous sand patches and wave ripples the sand patches are interpreted as the remnants of tidal bedforms reworked by waves calculations of the threshold of motion of four modal grain sizes under various conditions at the seabed show that tidal bedforms are active during spring tides sometimes associated with waves whereas only the largest annual waves may explain the observed wave ripples as in active tidal banks there is a reversal of tidal bedload transport from one bank side to the other the sedimentation rate is very low on the kaiser bank in addition there is a loss of sediment at both bank extremities and the tidal bedload cross bank transfer is very small probably controlled by shelf residual currents and long term drift of sediment resuspended by waves however the present day action of waves at the bank top is less intense than it was at lower sea levels during the holocene as evidenced by seismic studies

ice sheet dynamics on the lofoten vesteralen shelf north norway from late mis 3 to heinrich stadial 1 abstract we present an account of the glacial dynamics from late marine isotope stage mis 3 to the heinrich stadial hs 1 on the lofoten vesteralen margin northern norway this account is based on an analysis of the sedimentary stratigraphy on the upper slope close to the ice sheet grounding line at the continental shelf edge and an analysis of the glacial morphology on the shelf areas our results point to a dynamic ice sheet that waxed and waned on the lofoten vesteralen continental shelf from the late mis 3 through the last glacial maximum lgm and the hs 1 glacigenic debris flows and deposition of laminated diamicts suggest that the ice sheet reached the shelf edge on several occasions these are dated to 25 5 ka bp hs3 between 21 1 and 21 7 ka bp 19 5 ka bp 15 6 ka bp and 14 4 ka bp uncorrected 14c ages a contouritic sorted sand sediment unit that formed in a relatively warm interval during late mis 3 suggests that a strong norwegian atlantic slope current also developed during glacial interstadials implying that the oceanic circulation in the eastern norwegian sea was similar as today the general pattern is that the ice sheet advanced onto the shelf during cold period with little influx of atlantic water to the norwegian sea and that it retreated toward the coast during intervals with higher temperatures and higher influx of warm atlantic water the last ice advance to the edge of the continental shelf occurred during hs 1

giant sandwaves in the hola glacial trough off vesteralen north norway abstract seabed data acquired from the hola glacial trough off vesteralen north norway reveal the morphology of spectacular sandwave fields and large sandwaves in great detail the sandwave fields are among the deepest known occurring in water depths of 200 260 m multibeam bathymetry and backscatter shallow seismic sediment samples and video data collected by the mareano programme and bottom current modelling results have been used to interpret morphology transport and distribution of the seabed sediments sand supply to the hola trough was initially from wave and current reworking of glacial and glaciomarine deposits during the eustatic sea level lowstand and sea level rise after the last glaciation more recently deposited bioclastic sand dominates the upper part of the sand unit which locally reaches 10 m thickness sand predominantly accumulates behind ridges where current strength decreases and in topographic lows straight crested and barchan type sandwaves up to 3 km long and 7 m high are interpreted to form by tidal and geostrophic currents with bottom current speeds reaching 0 7 0 8 m s 1 a large cluster of coral reefs occur immediately east of the barchan type sandwaves where modelling shows bottom current velocities up to c 1 m s 1 sandwaves occur in four major sandwave fields strong variation in the nature of sandwaves over short distances e g migration direction between neighbouring sandwave fields point to important local variations in current regime and sedimentary processes the present work demonstrates how local topography and interaction of several current systems may produce complex depositional patterns on a local scale detailed multibeam mapping and video inspection of open shelf troughs in other narrow shelf settings worldwide may reveal similar sandwave fields

long term stability in the production of a nw mediterranean posidonia oceanica l delile meadow abstract long term changes in leaf net production of the seagrass posidonia oceanica l delile and in the irradiance arriving at the meadow canopy were investigated in a nw mediterranean meadow using a palaeoecological approach we conducted in situ shade experiments to find the relationships between the carbon stable isotope ratio 13c of plant tissues leaf net production and ambient irradiance the relationships were highly significant and fitted the michaelis menten equation and power functions these functions were used to reconstruct light and net production using the 13c of p oceanica detritus as a palaeoecological proxy along a 150 cm core of p oceanica matte a peat like deposit formed by this endemic seagrass species the 13c values of sheath detritus along the core i e the net leaf production showed a weak but significant decrease towards the present time r 0 308 p 0 02 probably as a result of 1 an increase in dic availability and or 2 a progressive change in the carbon isotopic signature of dic both of which are consistent with a rise in anthropogenic atmospheric co2 the canopy irradiance reconstructed for the last 1200 years showed a mean value of 128 e m 2 s 1 with a range of 12 5 280 e m 2 s 1 the reconstruction of the net leaf production for the same period yielded a mean value of 2 5 mg dw day 1 shoot 1 with a range of 2 0 2 7 mg dw day 1 shoot 1 both ranges are within the values reported in the literature for present day studies the absence of significant fluctuations or sudden changes through time suggests remarkable ecosystem stability during the last millennium

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

architecture of late ordovician glacial valleys in the tassili n ajjer area algeria abstract the architecture of three late ordovician glacial valleys was studied in detail in the tassili n ajjer se algeria outcrops the valleys are oriented south north 2 to 5 km wide and up to 250 m deep the valley fills revealed a very complex sedimentary architecture with significant lateral facies changes several glacial cycles induced the formation of glacial erosion surfaces ges at the base and within the glacial valleys the first type of ges shows a sharp and steep angled contact without striations or associated syn sedimentary deformation suggesting that subglacial meltwater was the dominant erosive agent a second type associated with the deformation of pre glacial and syn glacial sediment suggests that ice was in contact with the valley floor four facies associations are proposed fa1 subglacial tillite fa2 sub to pro glacial ice contact fans fa3 proglacial sub aqueous gravity flows and fa4 outwash fans the stratigraphic architecture of three of the main valleys reveals a complex polyphase infill at least two main cycles of ice sheet advance and retreat can be interpreted from the sedimentary succession of each valley minor glacial cycles by ice oscillations also occur locally ges morphology and the facies sequence suggest that the iherir valleys were initiated by meltwater erosion in subglacial channels whereas the dider and ouarsissen valleys were part of a large ice stream pathway above the valley fill and the interfluves a sand rich unit of stacked lobes and channels is interpreted as submarine outwash fans deposited during final ice retreat a glacial sequence found between two ges comprises fluvio glacial or ice contact fan deposits fluvio glacial eskers and tills these sediments were deposited subglacially or at the glacier front during the ice maximum phase and or the early ice retreat phase during the ice retreat interbedded subaqueous gravity flow deposits and diamictites filled the glacially cut topography as the sea invaded the valleys maximum ice retreat was associated with high water fluxes and sediment discharge causing a sand dominated outwash fan to prograde out over the valleys and interfluves this outwash fan was supplied mainly by flood activity at the ice front involving high density sustained flows the dominant facies consists of giant aggrading climbing dunes filling channels or constructing sandy lobes downstream

effects of western boundary current dynamics on the internal wave field of the southeast florida shelf 1 shelf sea internal wave fields are typically highly variable and dominated by wind and tidal forces however this is not necessarily true for outer shelf regions or very narrow shelves where remote physical processes originating over the slope or deep ocean may exert a strong influence on the internal wave climate during the summers of 2003 and 2004 observational studies were conducted to examine the effects of a western boundary current the florida current tides and wind on the mean currents and internal wave field on the outer southeast florida shelf wavelet analyses of nonstationary shelf currents coupled with altimetry images of the straits of florida reveal the strong influence of florida current fluctuations on outer shelf flows local winds were found to be a small factor in flows on the outer shelf region and tidal forcing diurnal and semidiurnal accounted for less than a third of the total barotropic variance high frequency internal waves dominate the internal wave field and result in bottom intensified currents and strong shoreward mass transport we present evidence that suggests that the florida current plays as large a role in the determination of the high frequency internal wave field as tidal forces these observations and analyses show that it is necessary to include the forcing from the florida current meanders and instabilities in order to predict accurately the episodic nature of the internal wave field on the southeast florida shelf

re advance of the fennoscandian ice sheet during heinrich event 1 abstract five sediment cores bathymetric and 2d seismic reflection data from vestfjorden northern norway were studied in order to constrain the timing and extent of a major ice stream draining the northwestern fennoscandian ice sheet during the last deglaciation 15 10 14 c ka b p the bathymetric and seismic data indicate that vestfjorden traenadjupet was covered by grounded ice reaching the shelf break during the last glacial maximum lgm after the lgm break up and recession towards outer inner vestfjorden at 13 6 14 c ka b p the ice re advanced towards the outer part of vestfjorden prior to 13 1 14 c ka b p this is inferred from glaciomarine sediments overlain by diamicton at the base of the sediment cores interpreted as basal tills and associated contemporaneous build up of grounding zone wedges in the central vestfjorden this sequence of events is coherent with the late weichselian ice sheet dynamics both along the western norwegian continental shelf and in the barents sea possibly indicating a direct response to meltwater injection and accompanied cooling during disruption of the thermohaline circulation associated with heinrich event 1 enhanced supply of suspension load resulted in deposition of laminated sediments at 13 14 c ka b p reflecting the retreat of the ice stream towards the inner fjord in response to global climate amelioration and increased atlantic water inflow two additional ice re advances towards the norwegian coastline are inferred from distinct input of ice rafted debris ird with a typical mineralogical fingerprint derived from neoproterozoic early silurian calcite and dolomite marbles in the ice sheet catchment area thus the re advances may be concurrent with the older dryas 12 2 14 c ka b p skarpnes and younger dryas 10 7 14 c ka b p tromso lyngen cooling events

bottom currents interpreted from iceberg ploughmarks revealed by multibeam data at tromsoflaket barents sea abstract multibeam data have been acquired on eastern tromsoflaket a bank in the southwestern barents sea comprising a plateau detailed examination of the multibeam backscatter data however reveals a high variability of sediments especially in and around iceberg ploughmarks we observe coarser sediments on the levees either side of the ploughmarks than inside the furrows moreover we frequently note asymmetries in the distribution of fine sediments across these ploughmark features drawing inspiration from previous studies of bottom currents which use the distribution of sediments across bedforms e g sand waves we have interpreted the direction of bottom currents on tromsoflaket by examining the local distribution of fine sediments across iceberg ploughmarks which are the most prominent bedform in this area further integration of our results with existing grain size current speed models have allowed estimates of the bottom current speeds to be obtained such interpretations are particularly valuable in this area where availability of bottom current measurements sparse our approach makes valuable and novel use of multibeam data and results show good agreement with the limited set of available current data and models we can identify near bottom pathways for the norwegian atlantic current and norwegian coastal current the two major current systems in this region and the patterns of circulation help to explain the sedimentation regime and identify recent erosion and deposition areas

modelling of the construction of the rhone alluvial plain since 15 000 years bp a numerical model presented here develops a three dimensional image of alluvial media on an elementary scale significant for groundwater flow modelling the model was tested on the alluvial plain of the rhone river france on a scale of several kilometres and from geomorphological observations and dating reproduced the construction of this alluvial plain from 15 000 years bp to the present the history of the alluvial plain during the late glacial and holocene periods is summarized through most of this time the river rhone has maintained a braided pattern with the exception of two incising phases with a meander pattern the model does not use any physically based equations or water representation the main processes governing the construction of the plain are modelled by simple rules chosen according to geometrical or empirical laws taken from the literature or as modelling assumptions using multi agent concepts of distribution and interaction of elementary entities these sedimentary rules are applied to sediment entities or to conceptual erosion entities that simulate local deposition and erosion of sediments the sedimentation model reproduces the various climatic periods during which the sediments were deposited by simulating genetic periods and associated modelled processes for each period the model was constrained by quantitative field data such as altitude of ancient channels and deposits or thickness of sediments the general geometry of the alluvial deposits was satisfactorily reproduced during the simulation characteristic large scale features emerge despite the use of local rules the model results are discussed with reference to other approaches such as geostatistical or boolean models and the applicability of the model to other less documented alluvial plains is outlined

mega deposits and erosive features related to the glacial lake nedre glomsjo outburst flood southeastern norway abstract in this paper we present a suite of erosional remnants mega deposits and subtle bar morphology that we relate to the outburst flood from the glacial lake nedre glomsjo at the end of the last ice age by using large datasets of airborne lidar data implemented in a geographic information system gis we have mapped flood related features along the glomma and vrangselva rivers in southeastern norway the unprecedented overview of the valley reaches obtained by the vegetation free lidar derived digital elevation models dem has revealed a set of hitherto undocumented landforms persisting erosive lines indicators of the uppermost flooded level are carved into surficial deposits in the hillsides and are found as high as 80 90 m above the modern valley floor by using the indicators as an upper flood boundary we have computed cross sectional profiles showing that the flood in some reaches inundated more than 120 000 m2 of the valley large streamlined bed forms which we interpret as flood bars drape sections of the valley floor some several kilometers long the most morphologically striking pendant bars are developed behind flood flow projections such as bedrock knolls or in lee of a valley bend flood bars occur in the entire study area but are more widespread in the north and generally decrease in size moving in a downstream direction kettle holes and ice block obstacle marks from icebergs arrested during the flood are common these features support the theory of a catastrophic drainage event but also indicate a pattern of differential erosion and deposition that allowed us to interpret palaeoflow on individual bars vast aeolian dune fields in the region are interpreted as a secondary product of the flood as deposits related to the event were mobilised by northerly winds momentarily after the flood waned the dune fields cover an excess of 50 km2 and reveal that the region was a highly active periglacial desert after the flood our mapping highlights the outburst flood s role as a landscape defining event morphological evidence determine the southern fringe of the scandinavian ice sheet to c 15 km north of elverum at the time of the outburst flood from calibrating existing 14c dates we postulate an age of c 10 10 4 cal ka bp for the nedre glomsjo drainage event and the contemporaneous margin of the ice sheet

use of sedimentological information for geometric simulation of natural porous media structure a geometric simulation method was used to develop a three dimensional highly detailed synthetic representation of point bar sediments in the wabash river system geometric simulation methods in comparison to well known second order stochastic methods offer the advantage of being more closely related to depositional processes which are often similarly conceptualized i e described in terms of shapes of discrete bed forms trends in grain size and spatial relationships of defined geologic facies multiple scales of geometric variation were defined within a sedimentologically prescribed framework and shapes of discrete geometric elements were established at each scale the selected shapes were based on published field studies including sedimentological bed form studies and trench studies in active point bar sediments the parameterization of the shapes allowed for random variability of the shape descriptors discrete shapes were then generated and assimilated by computer hydraulic conductivity values were assigned to the discrete elements based on reports of observed variations in grain size and field measurements of hydraulic conductivity the synthetic model referred to as a numerical aquifer is being used as the basis for extensive numerical experimentation to study the relationship between natural spatial structure and subsurface flow and transport

sandbanks sandwaves and megaripples on spitsbergenbanken barents sea abstract recently acquired multibeam echosounder data from the shallowest part 26 53 m depth of spitsbergenbanken in the western barents sea reveal a variety of bedforms including megaripples sandwaves and sandbanks the bedforms exhibit varying degrees of superimposition and differ in their age of formation and present depositional regime being either active or moribund these are the first observations of co occurring current induced bedforms in the western barents sea and provide evidence of a high energy environment in the study area the bedforms indicate both sediment erosion and transport and confirm that there is enough sand available in this area to maintain them such conditions are not known to be common in the western barents sea and reflect the unique oceanographic and benthic environment of spitsbergenbanken

riparian wetlands of the middle reach of the seine river france historical development investigation and present hydrologic functioning a case study along the middle reach of the seine river upstream from paris riparian wetlands that have been functioning for centuries first as a result of a succession of sedimentological hydrological processes and later of continuous management practices where the value of the wetlands has been acknowledged as well as their importance for the ecology here we present the results of a multidisciplinary research programme focused on this system with emphasis on its present hydrological functioning and on the tools used to understand and quantify it the major hydrological and geochemical processes that are active at present are reviewed but past processes are also described as well as the potential long term evolution of the system this study uses several innovative tools to investigate the wetlands and analyse them in space thermographic aerial survey and electromagnetic prospecting resulting in a map of clay layer thickness concerning the wetland hydrology water budgets established at different time scales several years one year a season illustrate the exchanges between the watershed the river and the wetland and water storage in the wetlands during flood events the results show how the upstream reservoirs built during the last 20 years presently control the preservation and functioning of downstream riparian wetlands monitoring and modelling of the hydrological functioning of one wetland site have made it possible to identify and quantify exchanges between the wetland especially in the unsaturated upper layer and its environment in situ measurements of denitrification rates in the wetland were used to make initial estimates of its nitrate elimination capacity

contour current driven continental slope situated sandwaves with effects from secondary current processes on the barents sea margin offshore norway abstract seabed data acquired from the southern barents sea continental margin offshore norway reveal detailed morphology of large sandwave fields multibeam echosounder bathymetry and backscatter shallow seismic sediment samples and seabed video data collected by the mareano program have been used to describe and interpret the morphology distribution and transport of the sandwaves the bedforms lie on a slope dominated by relict glacial forms and muddy sandy gravelly sediments sandwave migration across small gravity mass failures of the glacial mud constrains the field initiation as early post glacial or later the contour parallel nature of the fields and crestlines normal to the bathymetry contours and the geostrophic norwegian atlantic current nwac demonstrate that the nnw flowing oceanographic circulation is the primary driving current the fields coincide with the depth range at which a transition between warm saline and underlying cooler less saline waters fluctuate across the seabed statistically rigorous measurements of height width and various parameters of slope and symmetry confirm a tendency to downstream nnw sandwave migration but with significant exceptions anomalous bedform symmetry domains within the fields are tuned to meso scale topography along relict glacial debris flow chutes indicating current focusing upstream and upper slope derived winnowed sand transport eroded from the glacial sediments is the supposed source sandwave flank slope values are comparable to the regional slope such that the gravitational vector would have a cumulative downslope migration affect unless balanced by upslope drivers perpendicular cross cutting of stoss face 3 d ripples by linear 2 d ripples in the sandwave troughs and lee faces is evidence for non synchronous episodic current variations though deep ekman transport and internal wave action are unproven here these could explain chute related tuning of bedform symmetry through funneling in the debris flow chutes and favor sand recycling thus contributing to long term maintenance of the sandwave field

bedforms of the canadian eastern seaboard a comparison with global occurrences this paper presents a compilation of sedimentary bedforms identified from acoustic and seismic records collected over the southeastern canadian continental shelf in order to make the compilation it was found necessary to firstly review and synthesise the various terminologies used to describe the bedforms found and thereafter to relate the subsequent terminology to any diagnostic generic characteristic identifiable from the records sand waves are considered to be those forms that are composite in nature and are generated solely by the action of superimposed megaripples these latter features are identifiable adopting the classification of dalrymple et al 1978 which proves adequate for the acoustic differentiation of 2 d and 3 d types though we observed a greater coherence in bedform spacing of the 2 d types than reported hitherto sand ridges were adequately defined and recognised using the description of swift et al 1978 with respect to storm generated ridges and that of off 1963 and others with respect to tidal current ridges such ridges appear not to be uniquely described by ratios of their metrics the bedforms on the southeastern canadian continental shelf are abundant and wide spread virtually all types of bedforms recognisable on the u s shelf are present to the north the majority of these bedforms appear to be storm generated and are considered intermittently active many of the sand waves s s have no measureable relief though retaining their distinctive spatial geometry both active and moribund sand ridges were found of these ridges both tidal and storm generated types were identified the storm generated ridges were further broken down into shoreface connected ridges and offshore ridges a metrical analysis of the various bedforms show that a differentiation into stable bed phases can be made using height length ratios provided the bedforms are active and fully developed there is some overlap between megaripples and small sand waves and indeed between 2 d and 3 d megaripples yet the distinction is good between sand waves and sand ridges even moribund types sand ribbons in general can be confused with moribund sand waves and megaripples a detailed morphological examination is required to distinguish each feature data from sand ridges of the eastern canadian shelf combined with data from similar features found globally shows that ridge heights tend to cluster into three distinctive modes these modes are at heights of 7 13 and 27 m the first mode is correlated with storm generated ridges the second with tidal current ridges and the third possibly with submerged barrier islands

braided rivers perspectives and problems progress towards a fuller understanding of the dynamics and deposits of braided rivers demands an interdisciplinary approach to a host of unresolved problems although many advances have been made within recent years in interpreting the mechanics of flow transport of sediment and sedimentary architecture of braided rivers many key issues remain to be addressed in particular several areas demand attention the mechanisms of braid bar initiation confluence diffluence dynamics the nature of sedimentary facies over a range of grain sizes and the influence of flow stage and aggradational regime upon the depositional architecture over a range of channel scales this paper focuses upon these issues and highlights several areas of fruitful future interdisciplinary collaboration 1993 the geological society

comparison of genesis and geostatistical methods for characterizing the heterogeneity of alluvial media groundwater flow and transport simulations abstract we describe a new method for generating the sediment heterogeneity of alluvial deposits based on a genesis model which mimics some of the hydraulic rules by which meandering or braided streams deposit or erode sediments the new model is described and compared with other approaches and is then applied to a 5 4 1 4 km 2 reach of the aube river upstream of paris the model is based on data obtained from a single cross section of the alluvial plain with 44 auger holes along the 1 4 km stretch of the plain as the model describes the sediment facies we assign to each facies an assumed hydraulic conductivity flow and transport simulations are then made with the commonly used code modflow in order to compare the results with simulations based on other descriptions of heterogeneous media we then use classical geostatistical tools indicator simulations to generate alternative images of the same medium based on the same initial data and the same assumptions on the hydraulic conductivity of each facies finally an equivalent uniform medium obtained through permeability upscaling is also generated comparing these three approaches on the basis of the flow and transport results we show the intrinsic capacity of the new model to generate more realistic flow and transport processes in heterogeneous alluvial media where permeability barriers and buried channels dominate the spatial distribution of heterogeneities

fluvial responses to climate and sea level change a review and look forward summary fluvial landforms and deposits provide one of the most readily studied quaternary continental records and alluvial strata represent an important component in most ancient continental interior and continental margin successions moreover studies of the long term dynamics of fluvial systems and their responses to external or allogenic controls can play important roles in research concerning both global change and sequence stratigraphy as well as in studies of the dynamic interactions between tectonic activity and surface processes these themes were energized in the final decades of the twentieth century and may become increasingly important in the first decades of this millennium this review paper provides a historical perspective on the development of ideas in the fields of geomorphology quaternary geology vs sedimentary geology and then summarizes key processes that operate to produce alluvial stratigraphic records over time scales of 103 106 years of particular interest are changes in discharge regimes sediment supply and sediment storage en route from source terrains to sedimentary basins as well as changes in sea level and the concept of accommodation late quaternary stratigraphic records from the loire france mississippi usa colorado texas usa and rhine meuse the netherlands rivers are used to illustrate the influences of climate change on continental interior rivers as well as the influence of interacting climate and sea level change on continental margin systems the paper concludes with a look forward to a bright future for studies of fluvial response to climate and sea level change at present empirical field based research on fluvial response to climate and sea level change lags behind a the global change community s understanding of the magnitude and frequency of climate and sea level change b the sequence stratigraphic community s desire to interpret climate and especially sea level change as forcing mechanisms and c the modelling community s ability to generate numerical and physical models of surface processes and their stratigraphic results a major challenge for the future is to catch up which will require the development of more detailed and sophisticated quaternary stratigraphic sedimentological and geochronological frameworks in a variety of continental interior and continental margin settings there is a particular need for studies that seek to document fluvial responses to allogenic forcing over both shorter 102 103 years and longer 104 106 years time scales than has commonly been the case to date as well as in larger river systems from source to sink studies of quaternary systems in depositional basin settings are especially critical because they can provide realistic analogues for interpretation of the pre quaternary rock record

submarine landforms and the reconstruction of fast flowing ice streams within a large quaternary ice sheet the 2500 km long norwegian svalbard margin 57 80 n morphological interpretation of regional and detailed bathymetric data sets on the 2500 km long norwegian shelf from the north sea 57 n to svalbard 80 n has revealed a dynamic ice flow pattern along the western marginof the scandinavian and barents svalbard ice sheets about 20 cross shelf troughs with megascale glacial lineations msgl elongate ridges and grooves oriented parallel to trough long axes are interpreted as former pathways for fast flowing ice streams studies of large scale margin morphology and seismic profiles have identified large submarine fans at the mouths of several major cross shelf troughs less dynamic ice probably existed on shallower banks the two largest paleo ice streams were the norwegian channel ice stream and bear island trough ice stream each 150 200 km wide at the mouth the lengths of individual msgl vary from hundreds of meters to several tens of kilometers and the distance between ridges varies from 0 1 to 3 km msgl amplitudes reach 15 m but are commonly 10 m the onset of msgl and hence fast ice flow is generally close to the outer coast at the border zone between crystalline rocks and softer sedimentary rocks transverse submarine ridges on various scales commonly parallel to the shelf edge reflect either the maximum ice sheet position or the recessional pattern of the ice sheet lateral ice stream moraines several tens of kilometers long have also been mapped along the sides of several cross shelf troughs identifying the border zone between fast ice flow and stagnant or slow flowing ice on intervening banks

glacigenic sediments on a passive continental margin as exemplified by the barents sea abstract reflection seismics data borehole data and gravity cores have been used to map the sedimentary facies of the epicontinental southwestern barents sea its continental margin and adjacent coastal areas an upper unconformity with a glacially eroded morphology extends over most of the continental shelf up to 300 m of stratiform glacigenic sediments overlie this unconformity these sediments can be grouped into seismic units which are separated by smooth or irregular erosional surfaces each sequence may be up to 150 m thick normally with a lateral extent of 100 300 km three dominant seismic signatures may be discerned semi transparent stratified and chaotic reflection patterns these sediments are partly glacifluvial in origin but on the whole they represent more or less tectonized glacimarine deposits the deeper troughs on the continental shelf contain an undeformed postglacial glacimarine marine trough fill the greatest thickness up to 1000 m of glacigenic sediments occurs at the shelf break and below the upper slope the depocentres are situated at trough mouth fans the sediments comprise several prograding sequences with a complex sigmoid oblique character indicating alternating upbuilding and depositional bypass erosion in the topset segment small and large scale slide scars are observed on the upper slope and debris lobes on the lower slope a marked progradation seems to have occurred during the plio pleistocene glacial periods this progradation also resulted in an unstable slope with various types of sediment gravity flows currents flowing along the slope were established during interglacials and gullying by cold dense shelf water and downslope transport occurred maximum net sedimentation rates are 0 2 m ka continental slope 0 4 m ka shelf break 0 1 m ka shelf stratiform diamictons 1 m ka shelf trough fills and 60 m ka fjord trough fills

a benthic front in the straits of florida and its relationship to the structure of the florida current abstract observations from ctd tow yos and microstructure profiles indicate the presence of a benthic front near 450 m depth on the western side of the straits of florida at 27 n the front is at midslope and approximately beneath the axis of the florida current there is a 0 2 kg m 3 jump in density across the front and the bottom boundary layer changes thickness from 10 20 m upslope to 50 70 m downslope of the front axial currents exhibit very strong vertical shear and cyclonic vertical vorticity upslope of the front in contrast to the significantly weaker vertical shears and anticyclonic vertical vorticity observed downslope all these features with the exception of the anticyclonic vorticity downslope of the front are consistent with a model of barotropically forced flow through a stratified channel with a sloping sidewall an arrested boundary layer regime is predicted to develop on the sloping sidewall upslope of a height hp fl n where f is the coriolis parameter l is the width of the flat b

florida current salinity and salinity transport mean and decadal changes the florida current fc contributes to atlantic circulation by carrying the western boundary flow of the subtropical gyre and the upper branch of meridional overturning circulation repeated fc hydrographic velocity salinity and temperature sections during 1982 1987 and 2001 2015 characterize its water mass structure and associated transport variability on average fc volume transport comes from subtropical north atlantic water naw 44 antarctic intermediate water aaiw 14 surface water sw 27 and an indistinct source rem 15 while salinity transport relative to the average salinity along 26 n comes from naw 55 aaiw 0 2 sw 30 and rem 15 from 1982 1987 to 2001 2015 naw aaiw and rem salinified by 0 03 0 16 g kg 1 and increased the salinity anomaly transport by 3 these patterns imply that advective salt transport by the fc 1 is sensitive to subtropical north atlantic variability and 2 is partially decoupled from the volumetric pathway of the upper overturning branch

arctic and antarctic submarine gullies a comparison of high latitude continental margins abstract submarine gullies are common features of high latitude continental slopes and over the last decade have been shown to play a key role in continental margin evolution submarine erosion downslope sediment transport slope deposits and the architecture of petroleum reservoirs however the processes that form these gullies the timescales over which they develop and the environmental controls influencing their morphology remain poorly constrained we present the first systematic and comparative analysis between arctic and antarctic gullies with the aim of identifying differences in slope character from which we infer differences in processes operating in these environments quantitative analysis of multibeam echosounder data along 2441 km of the continental shelf and upper slope and morphometric signatures of over 1450 gullies show that six geomorphically distinct gully types exist on high latitude continental margins we identify distinct differences between arctic and antarctic gully morphologies in the arctic data sets deep relief 30 m gully incision depth at 50 m below the shelf edge and shelf incising gullies are lacking these differences have implications for the timescales over which the gullies were formed and for the magnitude of the flows that formed them we consider two hypotheses for these differences 1 some antarctic gullies developed through several glacial cycles and 2 larger antarctic gullies were formed since the last glacial maximum as a result of erosive flows i e sediment laden subglacial meltwater being more abundant on parts of the antarctic margin over longer timescales a second difference is that unique gully signatures are observed on arctic and on antarctic margins environmental controls such as the oceanographic regime and geotechnical differences may lead to particular styles of gully erosion observed on arctic and antarctic margins

sandwaves and sand transport on the barents sea continental slope offshore northern norway we integrate morphological geological and geophysical observations with oceanographic measurements and modelling results to interpret the processes behind the origin and evolution of sandwaves on the upper continental slope of the southwestern barents sea seven sandwave fields covering c 130 km2 and with a volume of c 72 106 m3 occur in water depths of c 475 800 m individual sandwaves reach heights up to 6 m the sandwaves are sinusoidal with asymmetric stoss lee side relationships and time lapse multibeam bathymetry analysis indicates that some sandwaves have migrated up to 10 m towards the nnw over a 4 year time period this contour current parallel migration is counteracted by currents in several other directions and the sand unit is thus not a simple contourite measurements show prominent diurnal period oscillations and clear spring neap variations of current direction and speed with along slope and cross slope current velocities up to 75 cm s 1 and 65 cm s 1 respectively numerical ocean modelling results produce eddies travelling along the slope positionally stable daily recurring vortices and bottom current velocity up to 100 cm s 1 in the sandwave fields sandwave migration towards the se is also observed and the cross slope currents are capable of transporting sand up and down the slope as evidenced by ripples migrating normal to the sandwaves eroded sand accumulates in the sandwave fields along the boundary between north atlantic water and norwegian sea arctic intermediate water this is a consequence of the combined effect of the nnw directed along slope norwegian atlantic current and tidally induced topographic waves the sandwave fields started to form 11 000 13 000 14c yrs bp or possibly later by current erosion of glacial sediments on the continental shelf and slope

u th chronology and formation controls of methane derived authigenic carbonates from the hola trough seep area northern norway abstract we investigated methane derived authigenic carbonate mdac crusts and nodules from a cold seep site on the northern norwegian continental shelf in ca 220 m water depth to determine the timing and mode of their formation gas bubbling observed during remotely operated vehicle rov assisted sampling of mdac crusts revealed ongoing seep activity authigenic carbonates were present as crusts on the seafloor and as centimetre size carbonate cemented nodules at several intervals within an adjacent sediment core aragonite dominated mineralogy of the mdac crusts suggests formation close to the seafloor at higher rates of sulphate dependent anaerobic oxidation of methane aom in contrast dolomite cemented nodules are consistent with the formation at the sulphate methane transition zone deeper within the sediment at lower rates of aom the 13c carbonate values of bulk rock and of micro drilled aragonite samples vary between 22 2 and 34 6 vpdb we interpret the carbon in aragonite to be mainly derived from the anaerobic oxidation of thermogenic methane with a minor contribution from seawater dissolved inorganic carbon dic aom activity is supported by high concentrations of aom related biomarkers of archaea archaeol and 2 sn hydroxyarchaeol and sulphate reducing bacteria iso and anteiso c15 0 fatty acids in the crusts the dolomite nodules exhibit higher 13c carbonate values 12 vpdb suggesting a smaller amount of methane derived carbon presumably due to the contribution of dic migrating from depth and lower aom rates the latter is supported by orders of magnitude lower concentrations of archaeol and sn 2 hydroxyarchaeol in the sediment interval containing the largest dolomite nodules 18o values of pure aragonite samples and dolomite nodules indicate the precipitation of carbonate close to isotopic equilibrium with seawater and no influence of gas hydrate derived water u th dating of two mdac crusts shows that they formed between 1 61 0 02 and 4 39 1 63 ka bp and between 2 65 0 02 and 4 32 0 08 ka bp we infer both a spatial and temporal change in methane flux and related mdac formation at this seep site these changes might be caused by regional seismic events that can affect pore pressure or re activation of migration pathways thus facilitating fluid flow from deep sources towards the seabed

stream curvature and bank erosion a model based on the momentum equation based on the momentum equation of flow the force per unit area which the flow exerts radially on the outer bank of a stream bend is expressed by an equation in which the force is explicitly related to the curvature ratio r w curve radius channel width a dimensionless curvature coefficient c alpha is defined which is dependent on r w and the coefficient of dynamic bedload friction tan alpha exploring the changes of c alpha with r w reveals a functional relationship which for the known values of tan alpha shows maximal values of c alpha for r w values between 2 and 4 the theoretical results agree with field measurements of the relationship between r w and the rate of channel migration

comparison between predicted and observed sand waves and sand banks in the north sea for the first time a prediction model of regular morphological patterns on the seabed was tested against observations of sand wave and sand bank occurrence in the entire north sea the model which originates from first physical principles predicts this occurrence via two dimensionless parameters on the basis of the water depth the tidal velocity amplitude the level of zero intercept above the seabed z0 and a viscosity variation parameter alt epsilon the latter two quantities were varied in a number of predictions for the entire north sea whereas for the first two local values were used the range of realistic values of alt epsilon and z0 was large enough to let these two parameters distinguish between the possible combinations of bed forms as is shown in the shallower southern bight of the north sea the results were more sensitive to variations in z0 than in alt epsilon a slightly more detailed approach focused on sand waves only in the southern north sea and estimated local values for z0 using depth information quantification of the results showed that the model was able to predict the contours of the sand wave patches but it could not account for the absence of the bed features within this area the type of bed deposit partly explains the smaller scale variation the work confirms the validity of the theoretical bed form prediction model and verifies the hypothesis that the large scale seabed features are formed as free instabilities of tide topography interactions

types of internal structure and external morphology of submarine dunes under the influence of tide and wind driven processes dover strait northern france abstract the expression of tide and wind driven processes in the internal architecture and external morphology of very large submarine dunes is investigated at a dune field located in the central part of the dover strait southern north sea current measurements and sediment transport calculations enable quantification of the dominant tide and wind driven processes and very high resolution seismic recordings are used to analyse internal structure and external morphology of dunes in the study area moderate to storm winds induce temporary currents that can lead to the reversal of sediment transport direction and induced erosive processes on the seabed dune master bedding consists of extended erosive surfaces with 9 11 slopes that bound storm generated sets of cross beds 1 5 4 85 m thick 2 65 m on average three types of architectures are observed within the dune field and occasionally within single dunes these types which display a strong linkage between internal structure and external morphology consist of 1 asymmetric dunes with cosets which dip in the direction of the marked external asymmetry of the dunes 2 dunes with a complex external morphology composed of mega herringbone structures each displaying an opposing progradation direction and 3 dunes with a symmetric megaripple bedding unit at their summit the diversity of the internal architecture and external morphology results from the variable combinations of tide and wind driven processes occurring in the study area the asymmetry of tidal peak current velocity and the relative strength of wind driven currents are the parameters that have the most important influence on dune architecture asymmetric dunes with cosets result from a strong asymmetry in tidal peak current velocity which is reversed only slightly during storms dunes with mega herringbone structures and dunes with symmetric megaripple units at their summit are observed in areas where the asymmetry of the tidal peak current velocity is negligible allowing reversal of the sediment transport and dune migration directions each time moderate winds blow the latter architectural type is specific to sand rich areas the different dune architecture types display strong geometrical similarities to structures observed in dunes from other shelf environments where driving processes are different this implies that a study based only on a geometric description of internal structures will not enable the reconstitution of ancient depositional environments a better discriminant criteria seems to be the thickness of the second order sets of cross beds which is correlated with specific hydro sedimentary processes dune height also probably has an important influence on the thickness of second order sets of cross beds as dune height and set thickness display a clear relationship in the present study