the importance of bioturbation to continental slope sediment structure and benthic processes off cape hatteras north carolina abstract even though the continental slope off cape hatteras has sediment accumulation rates on the order of 1 cm year large areas of soft sediment are intensively reworked by infaunal organisms primary sedimentary structures have been completely replaced with biogenic structures surface sedimentary structures are dominated by the bioturbational activities of a deep burrowing infauna to at least 30 cm the layer actively mixed by the benthos as evidenced by sediment profile and x ray images is estimated to range from 5 to 20 cm with the residence time for particles within the surface mixed layer ranging from about 4 5 to 18 years the biological mixing parameter g ranges from 0 4 to 5 5 which indicates moderate to strong biological mixing relative to accumulation and strata formation bioturbation contributes to the dynamic forces affecting the surface sediments by decreasing compaction of sediment layers and dilating sediment fabrics by sediment mixing and introducing large water filled burrows and voids to subsurface sediments the sediment profile images captured numerous subsurface feeding voids and worms in the process of making deep burrows many of which extended below the 5 to 10 cm average depth of the apparent color redoxpotential discontinuity layer high rates of accumulation of organic rich sediment lead to high standing stocks of benthos and intensive feeding burrowing activity that result in organic rich stratagraphic sequences that are thoroughly mixed cape hatteras is a apparent focusing point for the transport of shallow water sediments to the deep sea sediments across other areas of the continental slope just 100 s of kilometers south of cape hatteras are not as thoroughly mixed or biologically active

biogenic sedimentation and geochemical properties of deep sea sediments of the demerara slope abyssal plain influence of the amazon river plume abstract the amazon river plume has a major impact on sedimentation at abyssal depths throughout a broad area 450 000 km2 and far 500 1000 km from the river mouth as the plume mixes into the north atlantic ocean it may support ecological niches favorable for n2 fixation driven by the symbiosis between diatoms and diazotrophic cyanobacteria this proposed diatom diazotroph association dda should result in increased carbon export from a limited region of plume influence here we used 32 multicores collected throughout the demerara abyssal slope and plain at depths ranging from 3000 to 5000m to assess patterns and rates of export and accumulation of organic carbon corg or poc and biogenic sio2 bsi sediment accumulation rates were determined from 14c ages of foraminifera obtained at 2 5 depths from 9 cores and were further constrained by 230th measurements the sedimentation rate 1 5 cm ky 1 representing the average accumulation over the past 8000 years was combined with corg and bsi sediment concentrations to establish biogenic matter burial fluxes burial of poc represents 4 16 of the corg raining to the deep sea floor which is high for water depths 4000 m burial of bsi represents 1 9 of the total bsi rain clastic sedimentation dominates regions underlying the plume while carbonate sedimentation dominated otherwise sediment mixing bioturbation was studied in 13 cores using 210pb as a tracer and showed no spatial patterns diffusional mixing coefficients ranged from 0 03 to 3 2 cm year 1 values consistent with other deep sea mixing rates however sites with the highest mixing rates did not have the highest corg content sediment focusing was studied in at 7 sites using 230th as a tracer surficial sediments had focusing factors that ranged from 1 to 2 indicating little sediment redistribution biogenic sedimentation in general and bsi deposition in particular is not concentrated in the region of dda abundance the nw axis of primary plume trajectory is coincident with the maximum deposition and burial of poc and bsi but not caco3

in situ studies of megafaunal mounds indicate rapid sediment turnover and community response at the deep sea floor sediment mounds composed of faecal material from megafaunal deposit feeders1 2 occur in virtually all deep sea environments forming the dominant decimetre scale topography on the ocean floor3 4 mounds are often zones of deep convective bioturbation1 2 and their formation is thought to disturb infaunal benthos5 6 we are studying mounds in the deep sea to evaluate rates and patterns of megafaunal sediment mixing and to assess the disturbance effects of mound building on the infaunal community with the goal of using mounds to investigate natural processes of succession at the deep sea floor7 our results indicate that sediment turnover associated with mound building by echiuran worms can be very rapid and that bathyal infauna can respond quickly to this type of disturbance these findings suggest that spatially rare but dynamic elements in the sediment fabric such as mounds may be important in controlling microstratigraphy chemical diagenesis and community structure at the deep sea floor

the removal rates of234th and228th from waters of the new york bight abstract the mean residence time of thorium with respect to non radioactive removal from water was determined as a function of location in three traverses of the new york bight using both 234 th 238 u and 228 th 228 ra correlates well with salinity increasing from about ten days near shore to 70 days at the shelf break it shows a much weaker correlation with suspended matter concentrations both because suspended matter varies in its scavenging efficiency and because salinity is a longer term integrator of scavenging potential s computed from 228 th 228 ra were often higher than those computed from 234 th 238 u either because of the detrital matter present as reflected in the 232 th concentrations or because the water parcels had recently arrived from environments of lower scavenging efficiency anomalous isotopic concentrations found in three adjacent samples can best be explained as the result of an episodic release of 228 ra from bottom sediments at a rate 200 times the normal one

origin of shore normal channels from the shoreface of sable island canada a suite of modern shore normal channels was discovered on the southern shoreface of sable island canada repetitive multibeam and sidescan surveys of these features as well as box coring and sampling showed that the depressions were perennial features of the shoreface they occurred in fine and medium sand and were most abundant in depths less than 20 m they were up to 1 m deep and 50 m wide increased in size seawards and generally followed the seabed slope the channels dominated the inner shoreface becoming wider and deeper seawards to depths in excess of 40 m there is a continuum of scales in width and depth of these channels the smallest of which are equated with gutters preserved in the geological record the internal structure of the shoreface sediments where the gutters are found is characterised by normally graded tempestites composed of cut and fill structures overlying an erosion surface thus the environmental setting internal structure and scale are similar to ancient counterparts gutters have been interpreted to result from downwelling over palaeo shorefaces during ancient storms myrow 1992b the composition of gutter infill as well as the stability evolution associated bedforms and hydrodynamic conditions under which they formed were examined to verify genetic interpretations of the ancient counterparts the multi sensor benthic lander ralph was deployed within a field of shore normal channels for 17 days during late winter 1998 ralph burst sampled flow at hourly intervals whilst imaging the seabed resulting sequential imagery revealed the genesis of a group of five gutters during storm spin down that was coincident with the formation of large scale wave ripples this was followed by infilling of the gutters and wave ripple destruction within 20 hours by longshore sand transport the gutters were located within and parallel to a larger channel 50 m wide and 0 50 m deep they were formed by coastal set up and subsequent downwelling that was triggered by the rotation of storm winds from seawards onshore wind to landwards offshore wind the gutters formed within 2 to 3 hours during spin down of a winter storm under live bed conditions of sand transport wave height and mean tidal flows were well below those of the storm peak and so they were not directly responsible for creating the gutters the mechanism we propose for the evolution of gutters supports the geological interpretations that is a storm driven downwelling event as described by swift and niedoroda 1985 however the downwelling event appeared to take place suddenly rapid acceleration in flow briefly few hours duration and within narrow corridors guided by wider and deeper channels of the shoreface high turbidity during the time of gutter formation leads us to believe that the phenomenon of inner shelf turbidity currents as suggested by walker 1985 exists and may be instrumental in channel formation

removal of thorium 234 by scavenging in the bottom nepheloid layer of the ocean abstract concentrations of234th were measured in deep sea bottom waters to assess the extent of chemical scavenging near the ocean floor at five stations in the eastern tropical pacific where no appreciable bottom nepheloid layer bnl was observed total234th was in secular equilibrium with its parent238u confirming earlier published results in contrast samples from a well developed bnl in the western north atlantic showed a significant depletion of234th due to its removal by scavenging the largest depletions were observed in the two samples closest to the bottom 25 and 64 m above bottom and amounted to 19 of the equilibrium value the results can be used to test models that have been proposed to explain the existence of a bnl it is concluded that only a model which includes local or nearby resuspension of sediment can account for the observations from a steady state model that assumes local resuspension the average residence time of resuspended particles in the bnl is estimated to be 25 days

palaeogeography of atlantic canada 13 0 kyr abstract we combine isobase maps with a digital terrain model of atlantic canada to map coastlines from 13 14c kyr bp to the present at 13 14c kyr bp there are ridges of high relative sea level rsl values over newfoundland and the maritime provinces and a re entrant of low values in the gulf of st lawrence this pattern persists well into the holocene and reflects crustal response to the slow wasting of ice caps that persisted in newfoundland and the maritime provinces for up to five millennia after the removal of ice from the gulf of st lawrence by a migrating calving embayment the palaeogeographic reconstructions reveal an archipelago on the outer shelf from grand bank to the continent that persisted from 13 14c kyr bp until ca 8 14c kyr bp much of the magdalen shelf was exposed but the magdalen islands were never connected to the mainland prince edward island was initially separated from the mainland became connected after 11 14c kyr bp and was separated again just before 6 14c kyr bp when northumberland strait formed the reconstructions are highly sensitive to relatively small changes in isobase values especially on the shallow banks on the continental shelf

210pb in the sediments and water column of the northeast atlantic from 47 to 59 n along 20 w abstract over much of the deep north atlantic ocean the water column and atmospheric input deficit of the radionuclide 210 pb is not matched by the inventories found in the underlying sediments this implies that an unidentified extensive or intense sink or sinks for 210 pb must exist in the atlantic to counterbalance the large area of open ocean in deficit one likely sink area which has been suggested is the atlantic north of 50 n because of its moderately high surface productivity and the high inventories observed in the limited number of cores available this work tests the hypothesis by investigating the water column at 48 n 20 w and a set of cores from 47 to 59 n along 20 w the water column deficit is 21 dpm 210 pb cm 2 in a water column of 4500 m depth and a 226 ra content of 54 dpm cm 2 the important unmeasured parameter is the atmospheric input of 210 pb to the surface ocean but the 100 yr average fluxes for the majority of the cores eight out of eleven can be modelled to within 40 of their observed values with a value of 0 30 dpm cm 2 y 1 for the atmospheric input added to the flux implied by the water column deficit this leads to best estimate values of the characteristic water column parameters scavenging effectiveness and scav of 48 and 35 yrs respectively which are typical north atlantic values it is concluded that removal from the water column and steady state supply to the sediments are in approximate balance along this transect and that this part of the north atlantic is not a preferential sink for 210 pb two cores from a sediment drift area do show active enhanced 210 pb excess accumulation but this is ascribed to current driven fine sediment redistribution rather than to productivity driven enhanced removal from the water column

surficial sediments of the outer banks scotian shelf canada the distribution thickness and mean grain size of surficial sediments on sable island bank middle bank and banquereau scotian shelf are used to re evaluate interpretations made by others on mechanisms controlling long term stability and net transport pathways of surface sand a holocene sand ridge complex the sable island catena extends 300 km across sable island bank and banquereau this sand ridge complex which is up to 50 m thick formed as a result of holocene sediment transport on the outer banks and controls the modern day distribution of bedforms and sediment size a clockwise circulation of sand centred around sable island which had been proposed as the mechanism maintaining the island does not take place sand is transported from southwest to northeast across sable island bank towards the sable island catena thereafter sand is dispersed eastward spillover of sand from the banks to the gully is not significant sand is trapped in depths less than 100 m the processes proposed to ha

chlorophyll a and pheopigments as tracers of labile organic carbon at the central equatorial pacific seafloor high rates of primary production and fast sinking rates lead to the deposition of phytodetritus and highly reactive organic matter to the sediments of the central equatorial pacific these substances are responsible for driving important chemical fluxes and fueling benthic organisms chloropigments have proven useful as tracers of similar highly reactive organic carbon components in lacustrine and nearshore marine sediments in this study we investigate the degradation of chlorophyll a and pheopigments a at four abyssal sites on the jgofs equatorial pacific transect along 140 w and explore the usefulness of these chloropigments as tracers of the most reactive component of the deep sea particulate organic carbon poc rain first order reaction rate constants k 1 75 yr 1 half life 3 250 d and relative reactivities chlorophyll a allomer and pheophorbide a pheophytin a derived from most of the sedimentary profiles are similar to those found in laboratory and other field lake and coastal marine studies however in some profiles the rate constants determined by fitting data below 0 5 cm are slower by up to 3 orders of magnitude despite an apparent abundance of bacteria macrofauna and porewater oxygen model results assuming multi g kinetics suggest that these chloropigments degrade as two components one which accounts for at least 99 of the degradation and 11 57 of the sediment inventory degrades with a half life of 4 120 days the other component degrades with a half life of up to 440 years these results suggest that some otherwise labile poc may be protected and escape rapid degradation near the sediment water interface if phytodetritus is deposited continuously throughout the year our model calculated chlorophyll a fluxes indicate that it could account for 25 100 of the annual poc flux at sites close to the equator

rapid bioturbation in equatorial pacific sediments evidence from excess 234th measurements in order to evaluate bioturbation in equatorial pacific sediments profiles of excess 234th were measured in cores collected during november and december 1992 at six abyssal sites along the jgofs equatorial pacific 140 w transect from 5 s to 9 n excess 234th half life 24 1 days was detected at all sites mean inventories ranged from a low of 0 25 dpm cm 2 at 9 n to a high of 2 6 dpm cm 2 at 2 n eddy diffusive models of the excess 234th profiles suggest that on 100 day timescales particle mixing is slow at 5 s and 2 s eddy diffusion coefficient db 0 4 cm2 year 1 faster at 0 and 2 n db 0 34 4 8 cm2 year 1 and fastest at 5 n db 4 7 11 cm2 year 1 at 9 n 234th is found almost entirely in the upper 5 mm of the seabed and db values are 0 1 cm2 year 1 db values generally increase as poc flux increases although the linear relationship r2 0 334 p 0 365 displays considerable scatter however if the only sites included in the regression are those where 234th profiles were measured in at least two core sets i e 0 2 n 5 n and 9 n the linear regression improves substantially r2 0 754 p 0 0012 these results suggest that measurements on a single core may not fully characterize the sediment mixing regime in a particular region and highlight the need for multiple samples from an individual site thorium 234 db values at 5 s 2 s and 9 n are in rough agreement with published db values calculated from the profiles of intermediate half life radioisotopes such as 210pb and 137cs in open ocean abyssal sediments however at 0 2 n and 5 n 234th db values are as much as an order of magnitude greater than published 210pb db values this phenomenon has been observed in other marine environments and may result from age dependent mixing non steady state mixing or a combination of the two processes particle mixing in cores collected at 5 s and 5 n appears to be best described as a largely eddy diffusive process however in cores collected at 2 s 0 2 n and 9 n non local mixing processes appear to affect 15 30 of the excess 234th inventory at these sites non local mixing appears to transport recently deposited material at least 3 4 cm below the sediment water interface within 100 days of its arrival at the seafloor the combination of rapid biodiffusive mixing and non local transport allows recently deposited reactive biogenic debris to be rapidly mixed into the seabed this may affect a wide range of diagenetic processes including oxygen consumption dissolution of biogenic debris and organic carbon remineralization models of early diagenesis in deep sea sediments may need to incorporate both non diffusive transport terms and rapid biodiffusive mixing of recently deposited particles

effects of benthic transport processes on abrupt climatic changes recorded in deep sea sediments a time dependent modeling approach 1 a one dimensional time dependent model is developed to characterize qualitatively and quantitatively the combined effects of bioturbation and burial on sedimentary signals related to abrupt climatic changes this model is based on the widely used advection diffusion equation and simulates rapid climatic transitions by representing the deposition flux at the sediment water interface by a steep ramp function of time this model is applied to both planktonic foraminifera abundance curves and 18o distributions output signals obtained at the bottom of the bioturbated layer for these rapid transitions have been generated for a broad range of sedimentation rates 5 50 cm kyr 1 biodiffusion coefficients 10 3 1 cm2 yr 1 and mixed layer thicknesses 2 15 cm numerical results clearly demonstrate the importance of a dynamic description of sediment mixed layers properties of the sedimentary response to fast transitions appear to be highly and nonlinearly dependent upon the sedimentation rate and the relative values of the original transition length and the time required to bury the signal below the mixed layer moreover for low sedimentation rates 10 cm kyr 1 we show that signals occurring over a 50 to a 250 year timescale are indistinguishable to quantify the distortion of sedimentary signals we determined the transition length of the bioturbated simulated signals and compare it to the initial transition length for given sets of bioturbation and sedimentation rates and mixed layer thicknesses comparison between model predictions and experimental results reveals a good agreement showing that transition timescales are compatible with ice core derived transitions 100 years for environments in which bioturbation can be approximated by a diffusive process tests performed on 18o distributions have illustrated the effect of bioturbation on the stratigraphic offsets between isotopic compositions of cold and warm species of foraminifera during climatic transitions

surficial sedimentology of the middelkerke bank southern north sea detailed surficial investigations over the middelkerke bank a tidal sand bank in the southern north sea revealed the relationship between morphology surficial structures and grain size parameters data from 85 grab samples all over the bank show that on a bank normal profile the coarser caco3 rich and badly sorted sediments are generally located near the highest point of the bank seaward at the northern end and landward at the southern end sedimentary structures were studied from 239 boxcores sampled on all the morphological units of the bank crest flanks and adjacent channels in the shallower parts foreset beds are preserved while in the deeper zones intense bioturbation arises and destroys any structure the combination between these data and the virtual absence of wave induced structures indicates that the main agents responsible for the bank shaping are the tidal currents

the use of234th 238 disequilibrium to examine the fate of particle reactive species on the yangtze continental shelf measurements of234th t1 2 24 1d in water column and seabed samples from an area near the mouth of the yangtze river people s republic of china provide the following information 1 most of the234th in waters near the mouth of the yangtze river is in particulate form and horizontal transport and sedimentation of234th bearing particles dominate the234th budget in this area a model incorporating processes in at least two dimensions is required to understand the fate of particle reactive species on the yangtze continental shelf 2 the residence times of particle reactive species with respect to scavenging range from 0 3 days nearshore to 4 0 days offshore the residence times with respect to removal to the seabed range from 0 5 day nearshore to 11 0 days offshore 3 scavenging residence times for234th decrease as the concentration of suspended particles increases the time for removal of234th to the seabed increases with distance from shore a variable which integrates such factors as scavenging rate bathymetry and frequency of resuspension

evolution of sediment fluxes from the middle miocene to present at odp site 817 townsville basin northeastern australia as a record of regional paleogeographic events abstract investigation of the middle miocene pleistocene succession in cores at odp site 817a leg 133 drilled on the slope south of the queensland plateau identified the various material fluxes contributing to sedimentation and has determined thereby the paleogeographic events which occurred close to the studied area and influenced these fluxes to determine proportions of platform origin and of plankton origin of carbonate mud two reference sediments were collected 1 back reef carbonate mud from the young reef area great barrier reef and 2 late miocene chalk from the loyalty basin off new caledonia through their biofacies and mineralogical and geochemical characters these reference sediments were used to distinguish the proportions of platform and basin components in carbonate muds of 25 core samples from hole 817a two origin indexes i 1 and i 2 relate the proportion in platform and basin materials the relative sedimentation rate is inferred from the high frequency cycles determined by redox intervals in the cores bulk carbonate deposited in each core has been calculated in two ways with close results 1 from calcimetric data available in the leg 133 preliminary reports davies et al 1991 and 2 from average magnetic susceptibility of cores a value negatively correlated to the average carbonate content vertical changes in sedimentation rates in carbonate content in origin indexes and in linear fluxes document the evolution of sediment origins from platform carbonates planktonic carbonates and insoluble material through time these data are augmented with the variations in organic matter content through the 817a succession the observed changes and their interpretation are not modified by compaction and are compatible with major paleogeographic events including drowning of the queensland plateau middle miocene early pliocene and the renewal of shallow carbonate production 1 during the late pliocene and 2 from the early pleistocene the birth and growth of the great barrier reef is also recorded from 0 5 ma by a strengthening of detrital carbonate deposition and possibly by a lack of clay minerals in the 4 upper cores a response to trapping of terrigenous material behind this barrier in addition a maximum of biological silica production is displayed in the middle miocene these changes constrain the time of events and the sequence stratigraphy framework some components of which are transgression surface maximum flooding surface and low stand turbidites sedimentation rates and material fluxes show cycles lasting 1 75 myr whatever their origin climatic and or eustatic these cycles affected the planktonic production primarily the changes also show that major carbonate variations in the deposits are due to a dilution effect by insoluble material clay biogenic silica and volcanic glasses and that plankton productivity controlling the major fraction of carbonate sedimentation depends principally on terrigenous supplies but also on deep water upwelling accuracy of the method is reduced by redeposition reworking and probable occurrence of hiatuses

calcium carbonate dissolution in deep sea sediments reconciling microelectrode pore water and benthic flux chamber results abstract we report the benthic fluxes of o2 titration alkalinity ta ca2 no3 po43 and si oh 4 from in situ benthic flux chamber incubations on the ceara rise and cape verde plateau and compare them to previously published results we find within analytical uncertainty that the ta flux is twice the calcium flux suggesting that dissolution precipitation of caco3 is the principal mechanism controlling benthic ta and ca2 fluxes at sites where the sediments contain significant 35 caco3 and the overlying waters are supersaturated with respect to caco3 the ratios of the total dissolution rate to the remineralization rate are significantly less than at all other study sites we propose that these observations can be explained by precipitation of fresh caco3 at the supersaturated sediment surface followed by redissolution deeper in the sediments because of metabolically produced co2 a numerical simulation is presented to demonstrate the feasibility of this explanation in addition surface exchange reactions in high caco3 sediments coupled with high rates of particle mixing may also impact rates of metabolic dissolution and depress chamber derived estimates of carbonate alkalinity and calcium benthic fluxes these results suggest that at supersaturated high caco3 locations previous models of sediment diagenesis may have overestimated the impact of metabolic dissolution on the preservation of caco3 deposited on the sea floor

seabed processes on the northeastern grand banks of newfoundland modern reworking of relict sediments abstract on the northeastern edge of the grand banks of newfoundland between 140 and 70 m depth a thin holocene sediment veneer unconfortmably overlies tertiary siltstones and sandstones these surficial sediments show a depth controlled facies distribution below 90 m the facies range from continuous fine sand through lag gravel and sand ribbons with arcuate sand waves and megaripples normal to the axis of the ribbons above 90 m depth alternating sand ridges up to 3 m in height and lag gravels occur megaripples and arcuate sand waves 400 m wave length with megaripples on their stoss side are developed on the terrace the regional geology suggests that many of the surface sediments were inherited from pleistocene deposits subsequently reworked in coastal environments by an early holocene transgression side scan sonar surveys bottom photography and submersible observations demonstrate intermittent sediment mobilization of recent material in particular photographs show asymmetrical ripples in well sorted sand after seasonal storms side scan sonar coverage in the hibernia area shows fields of two and three dimensional megaripples 5 m length which appear to be migrating across trawl marks left by recent fishing operations these sedimentary bedforms are attributed to unidirectional storm driven currents ocean currents and extreme waves the larger bedforms may reflect paleoceanographic conditions while the small bedforms appear to be modern

plutonium and210pb distributions in northeast atlantic sediments subsurface anomalies caused by non local mixing abstract the depth distributions of 210 pb and 239 240 pu measured in a suite of box cores collected from water depths of 4000 5000 m in the northeast atlantic ocean exhibit pronounced subsurface maxima caused by sediment reworking by benthic infauna small scale spatial heterogeneity in bioturbation rates is indicated by large differences in tracer profiles from duplicate cores separated only by a few centimeters 210 pb and 239 240 pu activity distributions from each subcore exhibit a high degree of correlation and most tracer profiles exhibit one or more subsurface maxima one dimensional biodiffusion analogue models do not adequately simulate the principal features of this data set however an inverse conveyer belt mixing model which simulates subsurface egestion or a functionally equivalent process of surficial material which is enriched both in organic debris and radioactive tracers can reproduce the subsurface tracer maxima single event and continuous subsurface egestion models have been formulated and solved for different feeding rates and background biodiffusive fields the single event model provides a better fit to the data and in particular ensures the observed high degree of correlation between the 210 pb and 239 240 pu activity profiles regardless of the different tracer input functions the most likely candidate responsible for subsurface tracer egestion is a large infaunal worm of the phylum sipunculida which dominates the biomass below a depth of 3 cm

morphology lateral migration and internal structures of shoreface connected ridges sable island bank nova scotia canada an extensive complex of shoreface connected ridges is present around sable island near the outer edge of the canadian east coast shelf two orders or ridges occur the larger first order ridges are longer wider and higher than ridges along the east coast of the us whereas the superimposed second order ridges are smaller all of the ridges converge eastward with the shoreface at angles up to 90 sup 0 sediment grain size becomes finer eastward over each ridge the finest sediment occurring at the base of the eastern flank landward facing straight crested megaripples and sandwaves are present in the troughs whereas hummocky megaripples occur on the ridge crests eastward migration of the ridges will produce a 3 20 m thick upward coarsening sequence that rests on an upward fining basal lag cross bedding predominates throughout the sequence although the upper part may contain hummocky cross stratification the differences between the us and the canadian ridges are due to a more intense storm climate and to obliquely onshore rather than obliquely offshore geostrophic flows in the sable island area nevertheless the sedimentological patterns over the ridges in both areas are similar 17 references 5 figures

fallout 210pb as a soil and sediment tracer in catchment sediment budget investigations a review increasing anthropogenic pressures coupled with climate change impacts on natural resources have promoted a quest for innovative tracing techniques for understanding soil redistribution processes and assessing the environmental status of soil resources among the different existing tracers the fallout component of the radioisotope lead 210 also termed unsupported or excess lead 210 pb 210 ex when referring to its presence in soil or sediment arguably offers the broadest potential for environmental applications due to its origin and relatively long half life for more than five decades pb 210 ex has been widely used for dating sediments to investigate sedimentation processes and since the 1990s to provide information on the magnitude of soil and sediment redistribution the aim of this review is to provide a comprehensive evaluation and discussion of the various applications of pb 210 ex as a tracer in terrestrial and aquatic environments with particular emphasis on catchment sediment budget investigations this paper summarizes the state of the art related to the use of this tracer the main assumptions the requirements including the need for accurate analytical measurements and for parallel validation and the limitations which must be recognised when using this fallout radionuclide as a soil and sediment tracer lessons learned and current and future research needs in the environmental and radiochronological application of pb 210 ex are also presented and discussed c 2014 elsevier b v all rights reserved

rates of sediment reworking at the hebble site based on measurements of th 234 cs 137 and pb 210 abstract th 234 cs 137 and pb 210 measurements have been made on ten cores from the hebble site on the nova scotian continental rise th 234 mixing coefficients from hebble sediments show substantial lateral variability with values ranging from 1 to 33 cm 2 yr 1 these mixing coefficients are one to two orders of magnitude greater than values from typical continental rise and abyssal sediments because of high densities of benthic macrofauna th 234 data from hebble cores indicate that particles at the sediment water interface are mixed to depths of 1 5 cm on a 100 day time scale cs 137 and pb 210 data indicate that on time scales of 30 100 yrs surface sediments are reworked to depths ranging from 1 to 12 cm based on th 234 profiles from two hebble cores collected less than 200 m apart during consecutive years no temporal variability in mixing rate could be resolved in some hebble sediments there is good agreement between the th 234 mixing coefficient 100 day time scale and the pb 210 mixing coefficient 100 yr time scale in other hebble cores however th 234 data indicate that the pb 210 profile has not reached steady state if steady state solutions are used to model these non steady state profiles the calculated pb 210 mixing coefficient may underestimate the true mixing coefficient by as much as an order of magnitude

bioturbation in the abyssal arabian sea influence of fauna and food supply abstract in order to evaluate bioturbation in abyssal arabian sea sediments of the indus fan profiles of 210 pb half life 22 3 yr and 234 th half life 24 1 d were measured in cores collected during september and october 1995 and april 1997 respectively the density and composition of epibenthic megafauna and lebensspuren were determined in vertical seafloor photographs during april 1997 mean eddy diffusive mixing coefficients according to the distribution of excess 210 pb 210 pb db were 0 072 0 028 0 068 0 055 0 373 0 119 0 037 0 009 and 0 079 0 119 cm2 yr 1 in the northern western central eastern and southern abyssal arabian sea respectively mean eddy diffusive mixing coefficients according to the distribution of excess 234 th 234 th db were 0 53 1 64 and 0 47 cm2 yr 1 in the northern western and central abyssal arabian sea respectively mobile epibenthic megafauna at the western northern central and southern study sites were dominated by ophiuroids holothurians ophiuroids and natant decapods the respective densities were 100 82 29 and 6 individuals 1000 m 2 the northern study site was characterized by a high abundance of spoke traces and fecal casts the central site showed spoke traces and many tracks the southern site displayed the highest abundance of spoke traces whereas at the western site hardly any lebensspuren were observed there is evidence for at least two functional endmember communities in the arabian sea in the northwestern arabian sea wast vertical particle displacement seems to be dominated by macrofauna and primarily eddy diffusive in the southern arabian sea sast non local and incidental mixing due to spoke trace producers might become more important and superimpose reduced eddy diffusive mixing with respect to biological data cast is an intermediate location given the biological data average 210 pb db is higher and decimeter scale variability of 210 pb db smaller at cast than expected these findings indicate that in a mixture of both endmember communities the organisms may interact in way that increases values of biodiffusivity as reflected by 210 pb db and reduces decimeter scale 210 pb db heterogeneity in comparison to the simple sum of the isolated effects of the endmembers for time scales 210 pb db and 234 th db bioturbation intensity should be controlled primarily by the composition of the benthic fauna its specific adaptation to the environmental setting and the abundance of each species of the benthic community food supply can have only an indirect influence on bioturbation intensity in certain parts of the ocean the a priori overall positive relationship between poc flux and biodiffusivity might include restricted intervals displaying no or even negative relations

analysis of a longitudinal ripple from the nova scotian continental rise abstract a longitudinal ripple was obtained in a box core taken from the nova scotian continental rise in july 1982 soft brown mud comprising 1 5 10 sand 60 silt and 35 of m clay forms the 5 cm high and 40 cm wide ripple a maximum thickness of 8 cm of this mud under the crest thinning to 2 cm on the ripple s flank overlies stiff muddy foram ooze the vane shear strength of the brown mud is 0 4 kpa whereas that of the ooze is 4 kpa x radiographs show the mud to contain many fine burrows and a few larger ones as well as winnowed horizons rich in foraminifera of both benthonic and planktonic origin gentle wet sieving of the sand fraction on board ship showed the sand fraction to contain very few faecal pellets thus it appears unlikely that bedload movement played a large part in formation of the ripple rapid initial deposition from a concentrated suspension is suggested to have formed the structure but this was followed by periods of erosion to yield winnowed horizons and further rebuilding with material deposited from suspension radiochemical data 234 th and 210 pb and x radiographs suggest intense particle mixing d b 90 cm 2 yr 1 and rapid sediment deposition 1 5 cm month 1 thus it is possible that the bedform and associated structures are destroyed and recreated on a time scale of a few months

fluxes and budget of on the continental margin of the bay of biscay northeastern atlantic abstract as part of the ecofer experiment ecomarge program jgofs france concentrations of the natural radionuclide 210 pb were measured in particulate matter collected by sediment traps deployed for a 14 month period june 1990 august 1991 on two mooring lines within the cap ferret canyon on the french continental slope of the bay of biscay northeastern atlantic 210 pb concentrations range 63 218 dpm g 1 and fluxes range 0 1 11 2 dpm cm 2 yr 1 displayed typical gradients increasing with depth at both mooring sites also at equivalent trap depths 210 pb activities were systematically higher on the deep slope mooring 3000 m compared to the shallow one 2300 m fluxes of 210 pb generally decreased seaward and exhibited temporal variations but with no marked seasonal trend essentially matching variations in total mass flux various correlations between 210 pb activities and major constituents of trapped particles were examined 210 pb activities were independent of the particle composition indicating that in this area scavenging of this nuclide from the water column was not directly tied to the quality of settling particles the 210 pb budget as determined by the balance of local supply flux and fluxes to sediment traps and sediments implies important lateral inputs to both mooring sites 65 89 of the flux measured by traps a feature typical of continental margin environments on the basis of annual mean 210 pb activity in traps of the two mooring sites two particle sources are hypothesized the first source located in shallow waters of the shelf and upper slope supplies the entire slope region the second source probably composed of resuspended particles from upper slope sediments feeds essentially the upper and intermediate slope region maximum sedimentation rates determined on seven box cores decreased with bottom depth from 0 3 to 0 4 cm yr 1 at the canyon heads down to 0 06 cm yr 1 at 3000 m a good agreement was observed at both mooring sites between 210 pb fluxes registered by the near bottom traps and excess 210 pb inventories within the underlying sediments suggesting that nuclide fluxes measured by traps during this experiment are representative of fluxes on a longer time scale 10 100 yr nevertheless total mass fluxes measured by traps were 3 4 times lower than 210 pb determined sediment accumulation rates a discrepancy that may be more apparent than real given the possible errors in the latter due to bioturbation

biological pumping upwards of the coarse fraction of deep sea sediments abstract in a restricted part of an area intensively sampled by box coring approximately 50 samples from a 2 4 km rectangle a stiff muddy glacial gravel till is encountered at a depth of 30 40 cm x radiographs of the upper 15 to 20 cm of the cores reveal gravel in overlying foraminiferal mud only from the area where there is the deeper till layer the localization strongly suggests that the material in the top of the cores has its source in the deeper layer i postulate that burrowing organisms move gravel upward by pushing aside the particles that are too large to be eaten because the strength of the enclosing mud decreases upwards the gravel particles move preferentially upward the upward decrease in shear strength of he sediment is rapid at an interface occurring 4 cm below the surface on average thus hydrostatic pressures of 20 to 200 g cm2 exerted by vermiform burrowing animals act within an environment of asymmetrical sediment property permitting an asymmetrical response to the uniformly directed body pressure this asymmetrical response a pumping mechanism affecting material that does not pass through the gut is contrasted with the mixing of finer sediments that are eaten and excreted documented by many other workers dimensional arguments suggest the flux of material is related to the ratio of biological mixing rate to sedimentation rate the mechanism is also applicable to maintenance of mn nodules and their nuclei at the surface if macrofossils have behaved like gravel some s ell beds may have been generated and some stratigraphic anomalies caused by this means

210pb scavenging in the north atlantic and north pacific oceans the radionuclide210pb shows significant geographic variations in the extent of its removal from the open ocean water column this texture of scavenging is defined by mapping 1 the integrated deficiency of210pb in the water column relative to its supply from the atmosphere and from in situ decay of dissolved226ra and 2 inventories of excess210pb in deep sea sediments the ratio of210pb deficiency to its supply termed the scavenging effectiveness is 20 in the north equatorial pacific and 50 in the north atlantic this variation is related to the combined effects of uptake of210pb onto sinking particles and lateral transport of210pb to areas of more intense removal sediment inventories of excess210pb normalized to the210pb deficiency in the overlying water column permit evaluation of the relative importance of these effects in the north equatorial pacific virtually all of the210pb removed from the water column is present in the underlying sediments but in the mid latitude north atlantic the sediments comprise only about 50 of the210pb removed the deficiencies of210pb in the mid latitude north atlantic sediments south of 50 n are qualitatively offset by surpluses in high latitude sediments north of 50 n higher primary productivity and new production in the surface waters of the high latitude north atlantic and north equatorial pacific relative to the oligotrophic central north atlantic may account for the greater fluxes of210pb to bottom sediments in those areas

particle mixing rates in sediments of the northeast tropical atlantic evidence from 210pbxs 137cs 228thxs and 234thxs downcore distributions particle mixing rates were estimated from distribution of 137cs and excess activities of 210pb 228th and 234th measured in nine sediment cores taken from the northeast tropical atlantic i e at the three sites of the french eumeli eutrophic mesotrophic and oligotrophic programme at the oligotrophic site activities of 137cs and excess 210pb decrease smoothly with depth on the other hand at the mesotrophic and eutrophic sites 210pbxs and 137cs profiles suggest a mixing process with three different regimes with subsurface sediments being mixed more rapidly than those at the surface bioturbation rates were found to be slowest at the oligotrophic site intermediate at the mesotrophic site and fastest at the eutrophic site these mixing intensities appear closely linked to benthic fauna abundance organic carbon flux to the sediments and primary production in overlying surface waters the use of a multitracer approach for studying sediment particle reworking has allowed bioturbation rates to be better constrained and an assessment of whether radionuclide distributions in surface sediments are in steady state for only one core from the mesotrophic site were surface sediments not in steady state perhaps because of the occurrence of a benthic animal i e a xenophyophore at the surface of this core

palaeocurrent reconstruction sediment and thorium focussing on the iberian margin over the last 140 ka sedimentological data in particular sediment grain size from core md95 2040 2465 m water depth and a nearby core omii 9k 1833 m water depth presently under the influence of lower north atlantic deep water and deep intermediate water respectively have been used to reconstruct changes in the vigour of near bottom water flow on the iberian margin the pattern of the palaeocurrent index indicates faster flow during the warm periods and slower in cold this also holds in the younger dryas but there is an added effect of brief collapse of the circulation in termination 1b due to meltwater input the cold periods are generally characterised by elevated clay percentage terrigenous material 2 m diameter and overall sediment accumulation fluxes in comparison warm periods of higher flow have lower depositional flux and a tendency to silt terrigenous material 2 63 m dominance consistent with the higher flow speed and suppression of clay deposition this is in contrast with a recent 230thxs based study of two cores md95 2040 and 2039 j thomson et al earth planet sci lett 165 1999 255 270 which concluded that increased glacial sediment and th focussing was caused by stronger currents the crucial factor is that the controlling speed is that of the flow at the depositional site rather than that of the erosional or transporting flow to the site high glacial maximum fluxes of carbonate 6 g cm 2 ka 1 are largely composed of fine 63 m non coccolith material suggesting a significant detrital component it is proposed that the sediment and th arrive on the lower slope via different routes sediment down slope in bottom nepheloid layers and turbidity currents th scavenged from the water column by organic matter and are rapidly mixed on the bed removal of this material from a large area and its deposition in a small one adjacent to oporto seamount together with a small amount of direct down slope supply accounts for the properties of the deposits allowance for erroneous fluxes due to possible coring expansion stretching of the upper part of md95 2040 does not invalidate these conclusions

developing sediment geochronologies for high latitude continental shelf deposits a radiochemical approach during january and february of 1986 47 box cores and 55 piston cores were collected from the bransfield strait gerlache strait and marguerite bay antarctica apparent sediment accumulation rates established for 100 yr time scales with excess 210pb chronologies range from 0 08 to 0 34 cm yr in the bransfield strait 0 15 to 0 51 cm yr in gerlache strait and from 0 10 to 0 27 cm yr in marguerite bay for the antarctic peninsula continental shelf 137cs appears to have a limited usefulness in assessing the effects of deep sediment mixing by benthic fauna on seabed excess 210pb profiles because the observed activities for the regions examined are close to counter detection limits conventional 14c ages of surface sediments from the peninsula region range from 1370 to 2820 yr the spatial variations in the conventional 14c surface ages are thought to be influenced predominantly by inputs of older eroded sediment from lateral transport current winnowing mass wasting and or a variable carbon source on a 1000 yr time scale 14c measurements of five piston cores from the bransfield strait yielded sediment accumulation rates ranging from 0 06 to 0 49 cm yr these data support the assertion by nelson 1988 that the excess 210pb sediment accumulation rates range of 0 08 to 0 34 cm yr for this region represent true rates the two sediment accumulation rates of 0 01 and 0 08 cm yr for the gerlache strait are significantly lower than the observed apparent 210pb accumulation rates mean 0 27 0 12 cm yr for this region likewise two piston cores from marguerite bay yielded 14c sediment accumulation rates of 0 02 and 0 05 cm yr which also are considerably lower than the excess 210pb apparent accumulation rates mean 0 17 0 06 cm yr for this area the one dimensional advection diffusion equation of goldberg and koide geochim cosmochim acta vol 51 1962 was solved simulatneously with 210pb and 14c data for bc18 and bc113 to resolved the effects of sediment mixing and accumulation in bransfield strait and marguerite bay box core samples respectively this model indicates that 210pb and 14c sediment accumulation rates agree within a factor of 2 in the bransfield strait whereas in marguerite bay the apparent 210pb accumulation rates may be as much as an order of magnitude too high in the southern high latitude environments of the antarctic peninsula radiocarbon measurements in the seabed are a better approach than 210pb for establishing accurate rates of sediment accumulation the 14c chronologies when coupled with 210pb data provide more reliable assessments of biological deep mixing and sediment accumulation rates than do excess 210pb data coupled with measurements of bomb produced 137cs the sediment accumulation rates for both 100 yr and 1000 yr time scales tend to decrease southward from the bransfield strait to marguerite bay which is consistent with glacial marine sedimentation processes as modulated by climate for the western antarctic peninsula seabed measurements of 210pb 137cs and 14c also provides valuable insights on water circulation and upwelling atmospheric inputs and sediment depositional processes for the antarctic peninsula region

comparative evolution of material flux through two dsdp successions from central atlantic and gulf of mexico preliminary results of a new approach to sedimentary dynamics in pelagic environments abstract a method for gaining knowledge of sedimentary fluxes is applied to early cretaceous sequences containing high frequency sedimentary cycles the cycles are used to determine the relative fluctuations of 1 the sedimentation rate and 2 carbonate and silicate terrigenous fluxes expressed as linear values proportional to the real fluxes visual and automatic image analysis of black and white core photographs have been used to calculate the masses of different materials deposited in equal time intervals and to determine the distribution of the material within lithological units of cores according to a scale of grey intensities characters of these distributions allow definition of a flux variability index assumed to be negatively correlated to ocean continent distance related itself to sea level at dsdp sites 534 western atlantic off florida and 535 gulf of mexico the valanginian hauterivian interval has been chosen for determining the relative variations of carbonate and silicate terrigenous linear fluxes their variability and their distribution within lithological cycles these data provide information about the relative importance of carbonate sources lands platforms and plankton and their evolution and of other forcings upon planktonic productivity such as supply of nutrients from the continents and upwelling by comparing the data in the two sites it is possible to detect common climate and eustasy and local factors tectonics and upwelling which have controlled the material fluxes compared with site 534 site 535 was supplied by more numerous active and close carbonate sources and less active but closer terrigenous sources at the two sites carbonate flux was controlled by common factors sea level being the most prominent one terrigenous flux was submitted to more local controls this new method for investigating pelagic deposits should be tested on other deep sea sedimentary sequences

current response at the shelf break to transient wind forcing an analysis of current meter and hydrographic data collected from an array of moorings situated at the shelf break off nova scotia is presented in particular the focus is on the ocean response to moderate 10 to 20m s 1 transient winds of two or more days duration blowing parallel to the local bathymetry under these conditions upwelling occurs at the shelf break from depths of 400 m and with peak vertical velocities of about 2 mm s 1 the upwelling appears to be confined to within 10 km of the slope acceleration coriolis and pressure gradient terms are important for the along and cross slope momentum balances local topographic variations contribute to the anomalously large bottom currents on the shoreward side of the shelf break both coastal sea level and the shelf break circulation respond coherently to synoptic wind forcing but quantitative agreement with two layer shelf models is poor

seasonal circulation on the western and central scotian shelf abstract a realistic representation of 3d seasonal circulation and hydrography on the western and central scotian shelf has been obtained from historical observations and a combination of diagnostic and prognostic numerical models with forcing by tides wind stress and baroclinic and barotropic pressure gradients the major current features the southwestward nova scotian and shelf edge currents and partial gyres around browns and sable island banks are found to persist year round but with significant seasonal changes comparison with current meter observations shows good agreement for the browns bank southwest nova scotia and inner shelf regions and poorer agreement in the sable island bank and shelf edge regions where current and density observations are sparser and tidal influences weaker there is significant spatial structure in the seasonal circulation and hydrography and in the underlying dynamical processes on the shelf scale there are substantial changes in stratification potential energy

origin of longitudinal triangular ripples on the nova scotian continental rise since the recognition of current produced bedforms in the deep ocean attempts have been made to predict direction intensity and variability of abyssal flow from the nature of sea floor irregularities see refs 1 3 longitudinal triangular ripples ltrs are inter mediate scale bedforms 20 cm high metres long that are becoming recognized as important and geographically widespread predictors of bottom current flow i describe here recent sea floor photographs of ltr fields on the lower continental rise off nova scotia which show that the bedforms develop in fine grained cohesive sediment with an alignment parallel to the regional contours and to mean current direction an observational model for ltr formation is proposed wherein ltrs are deposited initially as tails behind large sea floor biological mounds and are subsequently constructed and propagate in the direction of mean current by sediment transport oblique to the bedform axis this transport is induced by short term currents that deviate from mean current direction and cause formation of a separation bubble on one side or the other of the ltr crest

labile organic carbon dynamics in continental shelf sediments after the recent collapse of the larsen ice shelves off the eastern antarctic peninsula a radiochemical approach abstract labile organic carbon loc dynamics i e of recently produced planktonic material and sediment dynamics were studied in the seabed using naturally occurring 14corg and 210pb measurements in the region where the larsen ice shelves a and b were floating almost two decades ago a non steady state diagenetic model was used to estimate sediment mixing coefficients as well as loc fluxes to the seabed and loc turnover times i e mean residence times in a suite of 14 sediment cores from the continental shelf including a glacial trough at four of the stations cores were collected during 2007 and 2011 cruises enabling a time series approach for understanding the evolution of sedimentary processes and loc dynamics in the deposits below a collapsed ice shelf sediment mixing coefficients based on non steady state 210pb profiles varied between 0 01 cm2 y 1 and 1 6 cm2 y 1 in these post ice shelf sediments these values were similar to those found in polar deep sea environments where sedimentary conditions are less dynamic than in shallower provinces loc whose abundance decreased uniformly with depth was detected to depths ranging from 2 to 16 cm with loc seabed inventories varying from 1 5 to 22 mg loc cm 2 excess 210pb and loc fluxes were relatively uniform across the study area suggesting that similar particle fluxes have taken place within the larsen system since the disintegration of the various ice shelves the loc mean residence time at the different stations varied from 3 y to 60 y the 14corg approach calculating loc content based on a two end member model with planktonic 14corg as the labile carbon end member most closely correlated with the lipid content of the sediment which has been considered one of the best descriptors of reactive organic matter readily available to benthic consumers we suggest that the irregular combination of sea ice coverage organic matter production and supply to the sea floor introduce scatter in the determination of sediment and loc dynamics such that short term temporal

wave current bedform scales orientation and migration on sable island bank 1 field observations of wave current bedforms on sable island bank show that medium to large bedforms were generally aligned with the wave direction and did not follow the rotating tidal current normalized bedform heights and wavelengths were larger than predictions by nielsen 1992 but agreed well with predictions by khelifa and ouellet 2000 which includes current effects maximum observed bedform wavelengths of 1 9 m were larger than those predicted for bedforms in wave dominated nearshore conditions but this may be expected as the water depths are larger 20 42 m and currents are present measured bedform migration rates had higher vector correlation amplitudes when compared to significant wave velocity than with current velocity or skewness migration rate predictions from three presently available models were not able to predict net migration rate and direction in all cases

foraminifera of petroleum platforms louisiana shelf gulf of mexico abstract species of benthic foraminifera presumably colonized the bacteria and diatom rich biofilms on the metallic surfaces of louisiana shelf petroleum platforms soon after their emplacement at present however most underwater platform legs are covered by a thick growth of macrofoulers predominantly barnacles and bivalves scraped samples from five such platforms water depth 16 61 m yielded 104 species of benthic foraminifera 44 of them with attached individuals among the obligate sessile species dyocibicides biserialis and planorbulina mediterranensis were especially widespread this group of epibionts also includes calcituba polymorpha carpenteria candei cornuspiramia antillarum lepidodeuterammina ochracea nubeculinita inhaerens planorbulina acervalis planorbulinoides retinaculatus sagenina divaricans webbina bonairensis and webbina rugosa the platforms are habitat islands the surrounding soft clastic substrate does not support the growth of most sessile species the putative source areas of these colonists of louisiana platforms are carbonate hardgrounds to the west banks or reefs formed over salt diapirs and other platforms the foraminiferal species diversity on the platforms is most likely controlled by their distances from these sources and the dimensions of the settlement surfaces the eastward transport of propagules in summer by the louisiana texas coastal current facilitates the species migration

estimates of particle flux and reworking at the deep sea floor using234th 238u disequilibrium abstract because of high specific activities of excess234th t1 2 24 1 days on suspended particles in the deep sea this nuclide is potentially an extremely sensitive indicator of particle inputs and dynamics at the seafloor measurements were made at two deep sea sites in order to examine this potential inventories of excess234th at a low current hemipelagic mud site 3990 m in the panama basin were 1 5 september 81 and 2 5 june 82 dpm cm2 the steady state fluxes to the seafloor calculated from these inventories are in rough agreement with radionuclide fluxes measured in sediment traps small scale 100m spatial variability in inventories implies biologically significant heterogeneity in particle inputs sediment from the continental rise site in the northwest atlantic 2800 m a site with higher current velocities than the panama basin had an inventory of 1 9dpm cm2 this inventory is also in rough agreement with predictions made on the basis of nearby sediment trap data particle mixing coefficients of 30cm2 yr calculated at the pacific and atlantic sites are similar to those in shallow water deposits but could reflect disturbance during handling based on210pb data from the panama basin sediment from below 6cm is mixed at a rate 10 slower than the near surface sediment to a depth of at least 20 cm agreement between234th predicted mixing rates at the panama basin site with210pb profiles and in situ experiments with glass bead tracers implies that these rates are real although the diffusion of dissolved234th into deep sea sediments complicates interpretations 234thxs distributions in bottom sediments offer a useful adjunct to sediment traps for investigation of particle dynamics near the deep sea floor

glacial tunnel valleys and quaternary history of the outer scotian shelf high resolution seismic reflection data indicate the presence of huge sub surface channel networks on the outer nova scotian continental shelf channel axes extend to over 450 m below present sea level b s l channel walls average 2 3 km in width mechanisms capable of producing such channels include fluvial submarine canyon or glacial erosion considerable debate has focused on the positions of the tertiary quaternary t q and pleistocene holocene p h boundaries1 4 in this region and the relationship of the t q boundary to the unconformity at the base of the channel networks uncertainty also surrounds the extent of pleistocene ice sheets on the south east canadian continental margin4 7 here we present a new stratigraphy for the sable island region fig 1 based on seismic profiles with lithologic and biostratigraphic control provided by two strategically placed boreholes this strati graphic analysis establishes the positions of the t q and p h boundaries outside the valleys at 51 m and 220 m b s l respectively our analysis also implies that the large channel systems are tunnel valleys cut by a sub ice meltwater process under a pre to early wisconsinan ice sheet which extended close to the shelf edge

drifting buoy trajectories in the north atlantic current despite intensive research there are still large gaps in our knowledge of the north atlantic current which is considered to be the link between the gulf stream off the east coast of north america and the anomalously warm surface waters off northwestern europe in the classical scheme given in fig 1 which was constructed by dietrich1 on the basis of data obtained during 1957 58 the current is seen to comprise several branches two important difficulties arise from this interpretation first the data for the area east of newfoundland are not sufficient to define the causes of why the branching of the north atlantic current is stationary in its source area over the slope off grand banks2 second rapid surveys of the oceanic temperature structure by means of expendable sondes and increasingly of long term moored instrumentation and satellite tracked drifting buoys3 have revealed that the oceanic current field is dominated by mesoscale eddies and meanders rather than by large scale steady flows we report here two experiments using satellite tracked buoys which address these difficulties our results clarify the pathway of the north atlantic current and confirm that the current indeed consists of mesoscale eddies and meanders

bioturbation into a recently emplaced deep sea turbidite surface as revealed by210pbexcess 230thexcess and planktonic foraminifera distributions abstract this study concerns the surface of a single distal turbidite emplaced on the madeira abyssal plain a few hundred years ago the vertical and horizontal homogeneity of the unit and the low pelagic sediment accumulation rate enable a ready identification of the post emplacement signals of near surface bioturbation data have been gathered to define 210 pb excess 230 th excess and pelagic foraminifera profiles with depth these are used as constant input flux tracers and treated by models conventionally used to determine bioturbation parameters the mixed layer depths 7 11 cm bioturbation coefficients 0 08 0 18 cm 2 y 1 and inventory data 10 23 dpm cm 2 derived from the 210 pb excess data in five out of six cores are similar to those previously observed elsewhere with the same model in the deep sea under steady state sediment accumulation conditions although the mixed layer depths are similar for all three tracers the 210 pb excess bioturbation intensities are larger by a factor of 3 than those estimated from 230 th excess profiles and about an order of magnitude larger than those from the foraminifera profiles it is suggested that the conventional 210 pb excess model and those for other short lived tracers probably over estimate the intensity of biomixing of foraminifera for palaeontological and related geochemical purposes

accumulation rates and mixing processes in near surface north atlantic sediments evidence from c 14 and pu 239 240 downcore profiles abstract the vertical distributions of c 14 and pu 239 240 activities and c 14 ages of sediments collected in n atlantic areas are analyzed in ne atlantic sediments radiocarbon mixed layer ages and depths range from 2500 to 5500 years and from 6 5 to 7 3 cm respectively while the vertical profiles of pu 239 240 indicate for the nw atlantic sites mixing depths of only 1 2cm in the ne atlantic sites accumulation rates over a period of 10 kyr range from 1 4 to 3 7 cm kyr 1 and fluxes of material from 9 2 to 24 1 g m 2 yr 1 with a marked difference between samples collected from plain highest values and hill lowest values sites related to downslope transport of sediment the inventories of pu range from 9 0 to 25 2 bq m 2 for ne atlantic sites and from 0 3 to 1 1 bq m 2 for nw atlantic sites probably reflecting different primary production rates in the two areas numerical simulations of the mixing processes of plutonium have been performed by using both a continuous and a pulse input flux the estimated values of d b range from 0 1 to 1 0 cm 2 yr 1 for the ne atlantic sites while they are in the order of 0 01 cm 2 yr 1 for the nw atlantic sites

particle mixing rates in deep sea sediments determined from excess 210pb and 32si profiles particle mixing rates have been determined for 5 south atlantic antarctic and 3 equatorial pacific deep sea cores using excess 210 pb and 32 si measurements radionuclide profiles from these siliceous calcareous and clay rich sediments have been evaluated using a steady state vertical advection diffusion model in antarctic siliceous sediments 210 pb mixing coefficients 0 04 0 16 cm 2 y are in reasonable agreement with the 32 si mixing coefficient 0 2 or 0 4 cm 2 y depending on 32 si half life in an equatorial pacific sediment core however the 210 pb mixing coefficient 0 22 cm 2 y is 3 7 times greater than the 32 si mixing coefficient 0 03 or 0 07 cm 2 y the difference in 210 pb and 32 si mixing rates in the pacific sediments results from 1 non steady state mixing and differences in characteristic time and depth scales of the two radionuclides 2 preferential mixing of fine grained clay particles containing most of the 210 pb activity relative to coarser particles large radiolaria containing the 32 si activity or 3 the supply of 222 rn from the bottom of manganese nodules which increases the measured excess 210 pb activity relative to 226 ra at depth and artificially increases the 210 pb mixing coefficient based on 32 si data and pore water silica profiles dissolution of biogenic silica in the sediment column appears to have a minor effect on the 32 si profile in the mixed layer deep sea particle mixing rates reported in this study and the literature do not correlate with sediment type sediment accumulation rate or surface productivity based on differences in mixing rate among three antarctic cores collected within 50 km of each other local variability in the intensity of deep sea mixing appears to be as important as regional differences in sediment properties

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

the utility of estuarine settling basins for constructing multi decadal high resolution records of sedimentation abstract obtaining long term records of sediment flux and sources to estuaries is a major challenge in coastal and estuarine research due to the dynamic processes that actively work to remove the high resolution long term sediment record examining cores from sediment settling basins directly adjacent to or within estuarine systems provides a potential alternative means of establishing a history of estuarine sedimentation as a recently established settling basin with high rates of sedimentation cape lookout bight nc presents an ideal coastal environment to capture a long term record of sedimentation in 2010 a 4 6 m long core was extracted from the deepest portion of the basin through lithologic description grain size water content and radioisotope analysis excess 210 pb a high resolution geochronology of sedimentation has been established for the basin the record begins with spit accretion that captured part of the marine shelf and formed the basin a transition in sediment type from dominantly marine through storm induced overwash to dominantly estuarine was identified in the basin this transition reflects the increased trapping efficiency of fine estuarine mud due to the changing geomorphology of the settling basin an abrupt increase in the rate of estuarine sedimentation occurred 1983 1985 after careful reconstruction of basin geomorphology this increased rate of sedimentation is not attributed to internal dynamics within the basin but instead is interpreted to be the result of increased estuarine sediment flux through barden s inlet from the upper estuary this work underscores the importance of considering changing basin physiography and sediment dynamics before interpreting the paleo record as those mechanisms can hide or eliminate allogenic signatures from the sediment profile

sediment mixing and accumulation rates in the sulu and south china seas implications for organic carbon preservation in deep sea environments abstract recent rates of carbon burial in two deep sea basins the sulu and south china seas are contrasted to determine factors influencing organic carbon preservation in deep sea sediments the sulu sea is an oxygen deficient warm water basin whereas the adjacent south china sea is more typical of open ocean conditions high resolution sea bed profiles of 14c reveal that accumulation rates for the sulu sea are 2 3 times higher than the south china sea at any given water depth e g 1 3 g cm 2 kyr 1 versus 4 3 g cm 1 kyr 1 respectively at 3000 m higher accumulation rates for the sulu sea are attributed to higher terrigenous input and better carbonate preservation in the warmer bottom waters benthic mixing rates determined using 210pb geochronology generally are lower in the sulu sea 0 004 0 254 cm2 yr 1 as compared with the south china sea 0 17 1 57 cm2 yr 1 especially at the 3000 m and 4000 m sites in the sulu sea where benthic mixing is negligible despite the higher bottom water oxygen levels in the south china sea organic carbon concentration profiles are strikingly similar for the two basins at any given water depth with similar concentration profiles higher accumulation rates in the sulu sea result in equivalently higher carbon preservation rates as primary production is thought to be similar for both basins if anything slightly higher for the south china sea the production rate can not explain the observed differences in carbon preservation in addition benthic mixing rates apparently do not play a major role in carbon preservation as extremely low rates at the deeper sulu sea would predict relatively low carbon preservation contrary to our observations we conclude that sediment accumulation rate is the dominant control on carbon preservation in this area of the western equatorial pacific

the quaternary sediments of banquereau scotian shelf the acoustic stratigraphy of quaternary sediments on banquereau scotian shelf interpreted from regional and site survey seismic lines is correlated with sedimentary units determined from undisturbed borehole samples the informal litho stratigraphic formations of the quaternary sequence on the scotian shelf are subdivided into members and litho chronozones diagnostic of depositional environments a late wisconsinan glacial advance dated by c 14 analysis of shell material took place ca 18 000 to 26 000 yr b p it formed a widespread gravel rich unit barren of fauna and capped by an erosional surface the unit occurs within stratified pro glacial marine sediments known locally as the emerald silt formation the minimum stand of sea level 110 m below present sea level occurred ca 14 000 yr b p and is confirmed by the presence of channel deposits at the top of the pleistocene section a marine transgression across banquereau took place ca 8 000 yr b p and is defined by a widespread erosion surface at the base of the holocene section this section known locally as the sable island sand and gravel formation comprises as much as 40 m of reworked sand which is divided into 3 members diagnostic of sand ridge generation eastern shoal an offshore storm ridge situated on banquereau was formed during holocene time created by modern day storm driven and tidal currents the intensity of these currents is the result of water mass acceleration over the shallow banks of the scotian shelf eastern shoal appears to have moved 30 km across the continental shelf at a rate of 3 5 m yr the regional setting of this sand body is similar to that postulated for the shannon and tocito sandstones of the western interior of the united states and shows that sand transport and large scale ridge development can take place in a shelf edge setting on present day storm dominated continental shelves

rates of particle scavenging sediment reworking and longitudinal ripple formation at the hebble site based on measurements of 234th and 210pb between 1982 and 1986 seabed and water column samples were collected on five cruises to the hebble site two particle reactive radionuclides 234th and 210pb have been used to examine rates of particle scavenging in bottom waters sediment reworking in the surface seabed and formation of longitudinal ripples settling particles at the hebble site which have been resuspended by strong bottom currents and benthic storm activity scavenge approximately 60 of the 234th half life of 24 days produced in the bottom 100 m of the water column corresponding to removal of 15 dpm cm2 234th inventories from cores collected during a single cruise show substantial variability e g 1 18 dpm cm2 over the 8 km2 study area indicating that resuspended particles are not deposited uniformly across the seabed after storm events the activity of benthic fauna and non uniform deposition over a topographically variable seabed are important processes affecting boundary layer sedimentation and the development of radionuclide inventories in surface sediments the mean 234th seabed inventory from individual cruises to the hebble area varies to a greater extent 2 to 38 dpm cm2 than the mean 210pb inventory 47 to 82 dpm cm2 because 234th inventories are dominated by sedimentary processes associated with an individual storm event whereas 210pb inventories integrate these processes over several cycles of deposition and erosion sediments at the hebble site are biologically reworked very rapidly on a 100 day time scale with 234th mixing coefficients db 1 to 33 cm2 yr comparable to values observed in nearshore environments on time scales of months to years 210pb data indicate that strong currents and benthic storm activity have eroded and redeposited the upper 5 10 cm in 17 of the 20 cores examined reoccupying field sites during the 4 year field program provided the unique opportunity to evaluate steady state models commonly used to determine rates of sediment reworking 210pb profiles and inventories showed significant temporal variations at hebble sites occupied between 1982 and 1986 steady state models should not be used to evaluate profiles of 210pb or other tracers with comparable half life in a dynamic environment like the hebble area because steady state assumptions are not justified and rates of sediment reworking can be underestimated by nearly an order of magnitude 234th and 210pb analyses of core samples from two longitudinal ripples suggest that these sedimentary features form on a time scale of months and consist of a mixture of recently deposited material from the water column and older sediment from depth the absence of 234th activity in one of the ripples indicates that these bedforms may be stable for periods of months without significant growth

multibeam bathymetric investigations of the morphology of sand ridges and associated bedforms and their relation to storm processes sable island bank scotian shelf abstract storm formed sand ridges are important morphologic features occurring on many continental shelves of the world our understanding of the morphology migration and the controlling processes over this type of sand ridges under different environmental settings is still limited in the present study an integrated approach of combining multibeam bathymetry survey seabed sampling sidescan seismic surveys and seabed instrumentation measurements was used to characterize the surficial geology and morphology of offshore sand ridges on sable island bank scotian shelf and to evaluate the relationship between sand ridge morphology migration and the storm processes on this storm dominated outer shelf bank multibeam data obtained at selected sites demonstrated the series of ne sw oriented sand ridges with an average height of 4 3 m and wavelength of 1 5 km sand ridges attain maximum spacing height and steepness in the intermediate depths sand ridge size and steepness also decrease from west to east and this decrease is correlated with decreasing dynamic condition sediment grain size and depth sand ridge profiles show weak easterly asymmetry this morphological asymmetry augments the sediment texture and grain size asymmetry to suggest eastward long term sand ridge migration and net sediment transport in the study area sand ridge asymmetry also shows strong local variation and the number of sand ridges asymmetric to the west is nearly equal to that asymmetric to the east the likely cause is the nearly equal number of major storms passing to the south versus to the north of the island so that the westerly and easterly peak storm driven currents associated with these storm path scenarios affect equally the sand ridges on sable island bank a suite of smaller bedforms were found superimposed on the sand ridges sand waves widely superimposed on both flanks of sand ridges mixed 2d and 3d megaripples commonly occurring in the sand ridge troughs and on the lower western flanks and large wave ripples ubiquitously found in the troughs of all megaripples surficial sediment is coarsest coarse sand with shell fragments in the sand ridge trough and becomes progressively finer eastward toward the sand ridge crest and down the eastern flank while best sorting occurs near the sand ridge crest box cores collected across sand ridges reveal characteristic storm event bed structure that reflects the development and migration of bedforms with the evolution of storms

the quaternary stratigraphy of southwest sable island bank eastern canada three boreholes ten vibrocores and high resolution seismic reflection records were analyzed to reconstruct the stratigraphy of the topmost 50 m of the sedimentary column of southwest sable island bank the sequence is complex owing to the effects of two low stands of sea level two periods of ice influence and two periods of open water during the past 40 000 yr a low stand at 98 m occurred at 28 000 to 32 000 yr b p a second low stand occurred at 110 m between 13 000 and 15 000 yr b p ice covered the region prior to 32 000 yr b p and from 11 000 to 10 000 yr b p open water prevailed at other times the sequence of sediments follows the formational subdivision of king and the member subdivision of amos and knoll the sable island sand and gravel formation is subdivided into three members trough bedded sand cross bedded sand and conformably bedded sand the conformably bedded sand formed under heavy sea ice and represents the younger dryas event the uppermost two members result from shelf sediment reworking the emerald silt formation is subdivided into four members channel gravel upper stratified sand fines barren gravelly sand and lower stratified sand fines and two regional discontinuities r1 and r2 the coarser grained facies of the channel gravel and barren gravelly sand members are the result of reworking during late and mid wisconsinan transgressions of sable island bank the upper stratified sand fines member is interpreted as an open water marine deposit formed during the scotian stade the lower stratified sand fines member is interpreted as a glacio marine deposit formed during the digby stade

biological activity and associated sedimentary structures in hebble area deposits western north atlantic abstract biological and sedimentologic samples along with x radiographs of sediment slabs were taken from box cores in the hebble area 4700 m on the lower continental rise of nova scotia these data suggest that this bottom region is influenced by periods of strong near bottom currents which erode and physically transport sediments these high periods alternate with periods of weaker flow biogenic reworking of sediments by an active benthic infauna dominates particle transport during low flow conditions