basement controlled multiple slope collapses rockall bank slide complex ne atlantic abstract the eastern flank of the rockall bank is dominated by the scarps of a large slide complex with unusual low aspect ratio 120 km wide and 150 km long with the use of an extensive 2d seismic dataset re processed high resolution multibeam bathymetry a tobi mosaic and one piston core we demonstrate that the headwall of the rockall bank slide complex previously referred to as the rockall bank mass flow may have occurred as a multiphase slope collapse in at least three episodes of instability since 16 ka bp with each of the events generating a slide with a more typical elongate morphology reconstructions of the pre slide seafloor suggest a volume of collapsed sediments in the range265 765 km3 seismic facies investigation indicates that the slope and failed sediments comprise almost exclusively contourites that are prone to failure due to their well sorted character and high water content a systematic examination of the slope morphology seismic facies and sub seafloor structures demonstrates a direct relationship between the rbsc seafloor scarps and the basement morphology focused fluid flow along basement bounding faults and or differential compaction across the scarps are considered to have had an important role in slope failure this study highlights the importance of deeply buried structures in the stability of slopes it also shows how detailed examination of high resolution data across large areas of slope instability can reveal smaller scale multiple events we believe that where contour swept slopes are steep the resulting intensification of near bottom velocities prevents healing slope processes and as a result steep slope gradients remain steep and thus unstable

the 8 2 ka event from greenland ice cores we present a collection of high resolution chemistry and stable isotope records from the plateau of the greenland ice cap during the cold event 8200 yr ago using a composite of four records the cold event is observed as a 160 5 yr period during which decadal mean isotopic values were below average within which there is a central event of 69 yr during which values were consistently more than one standard deviation below the average for the preceding period four cores in north south and central greenland show differences at decadal and shorter timescales it is not yet clear if this represents significant spatial differences in response the results show clear evidence for colder temperatures and a decrease in snow accumulation rate however the changes in chemical concentrations for the ions looked at here are small suggesting only minor changes in atmospheric circulation for this event apart from the decrease in methane concentration greenland ice cores give only weak evidence for effects outside the north atlantic region

origins of the ice contact stratified ridges eskers of ireland abstract an extensive system of ridged landforms composed of ice contact stratified sediment was deposited in the central lowland of ireland during the most recent deglaciation 18 000 bp the ridges have been interpreted by others as deposits of an ice sheet drainage system i e eskers and as such have been used with other data to reconstruct the deglacial history the traditional deglaciation model shows systematic retreat of ice from south to north our study which involves an analysis of the ridged landforms using lithofacies sedimentary structures and paleocurrent data in conjunction with the geomorphology indicates that the pattern and nature of the ridges are not compatible with this model and support a new model of deglaciation warren 1991 in which the esker system fo med in an interlobate area during the simultaneous shrinking of two main glacial outflow centers an extensive lake system developed in the lowland between the two outflow centers ridges formed both perpendicular and parallel to ice margins involved both active and stagnating ice are both continuous and segmented beaded and were deposited in subaqueous and subaerial environments almost all of the ridges are associated with lacustrine sediments ice contact ridges are polygenetic and a genetic classification is proposed those that formed perpendicular to the ice margin are termed eskers those that formed parallel are termed moraines distinction between esker types continuous or beaded subglacial tunnel fills fluvial ice channel fills and subaqueous fans and moraine types s baqueous or subaerial is crucial to a reconstruction of the mode and pattern of deglaciation in the central irish lowlands

final stages in the collapse of the laurentide ice sheet hudson strait canada nwt 14c ams dates seismics stratigraphy and magnetic susceptibility logs hudson strait is a key element in the history of ice sheet ocean interactions during the deglaciation of the laurentide ice sheet data from over 30 giant piston cores supplemented by ca 40 14c ams dates on foraminifers and shells from sites within and adjacent to hudson strait are used to describe the timing and sediment processes associated with the final complex phases of deglaciation in hudson strait each core is placed in a regional seismic architectural context through examination of deep tow huntec profiles most cores sample a thin postglacial unit and extend several meters into glacial marine sediment mainly silty clays 14c dates on cores from the adjacent shelf indicate that deglaciation occurred by 13 ka we have not been unable to confirm that hudson strait was ice free by 11 12 ka although this is a probable scenario whole core magnetic susceptibility wcms data are used as the primary signature of changes in the style of sedimentation and provenance and to correlate between cores wcms indicates that there is a regional differentiation in the ms signal associated with bedrock provenance changes in contribution of detrital carbonate and style of sedimentation one low wcms event is present in cores from both the western and eastern basins it dates from 8 0 ka and provides an important regional stratigraphic marker probably associated with the final deglaciation of hudson bay and hudson strait sediment from a southerly source ungava bay continued to be contributed into the 900 m deep eastern basin of hudson strait 8 and 7 ka and thick postglacial sequences were piled within ungava bay between 6 and 7 ka

suppression of multiple reflections in gloria sidescan sonar imagery the gloria side scan sonar system is an invaluable tool for studying the texture of the sea floor a substantial set of computer algorithms now exist for radiometric and geometric correction of the raw gloria imagery one type of artefact not presently removed is bright lines parallel to the ship track due to multiple reflections between the seabed and the sea surface these are common where the sea floor is flat and can lead to misinterpretation a method is presented here to suppress multiple reflections in gloria and other side scan sonar imagery as a by product of the suppression method a value for the seabed backscattering coefficient can be calculated giving some extra insight regarding sea floor type for sedimentary areas

the early holocene sea level rise the causes anatomy and consequences of the early holocene sea level rise ehslr are reviewed the rise of ca 60m took place over most of the earth as the volume of the oceans increased during deglaciation and is dated at 11 650 7000 cal bp the ehslr was largely driven by meltwater release from decaying ice masses and the break up of coastal ice streams the patterns of ice sheet decay and the evidence for meltwater pulses are reviewed and it is argued that the ehslr was a factor in the ca 8470 bp flood from lake agassiz ojibway patterns of relative sea level changes are examined and it is argued that in addition to regional variations temporal changes are indicated the impact of the ehslr on climate is reviewed and it is maintained that the event was a factor in the 8200 bp cooling event as well as in changes in ocean current patterns and their resultant effects the ehslr may also have enhanced volcanic activity but no clear evidence of a causal link with submarine sliding on continental slopes and shelves can yet be demonstrated the rise probably influenced rates and patterns of human migrations and cultural changes it is concluded that the ehslr was a major event of global significance knowledge of which is relevant to an understanding of the impacts of global climate change in the future

tracking the sea level signature of the 8 2 ka cooling event new constraints from the mississippi delta 1 the ever increasing need for accurate predictions of global environmental change under greenhouse conditions has sparked immense interest in an abrupt century scale cooling around 8200 years ago with a focal point in the north atlantic and with hemispheric teleconnections despite considerable progress in the unraveling of this striking feature including a conceivable driving mechanism rapid drainage of proglacial lake agassiz ojibway and a resulting reduced strength of north atlantic thermohaline circulation several key questions remain unanswered one salient aspect concerns the total amount of freshwater released during this catastrophic event likely echoed by a near instantaneous eustatic sea level rise so far no attempts have been made to perform high resolution sea level studies that explicitly focus on this critical time interval here we present new data from the mississippi delta suggestive of abrupt sea level rise associated with the 8 2 ka event however the amount of sea level rise was likely less than 1 2 m corresponding to a meltwater volume of less than 4 3 1014 m3 values lower than estimates used by several recent studies

holocene climatic instability a prominent widespread event 8200 yr ago the most prominent holocene climatic event in greenland ice core proxies with approximately half the amplitude of the younger dryas occurred 8000 to 8400 yr ago this holocene event affected regions well beyond the north atlantic basin as shown by synchronous increases in windblown chemical indicators together with a significant decrease in methane widespread proxy records from the tropics to the north polar regions show a short lived cool dry or windy event of similar age the spatial pattern of terrestrial and marine changes is similar to that of the younger dryas event suggesting a role for north atlantic thermohaline circulation possible forcings identified thus far for this holocene event are small consistent with recent model results indicating high sensitivity and strong linkages in the climatic system

ground truthing 6 5 khz side scan sonographs what are we really imaging a 1000 km2 area on the distal lobe of monterey fan shows a digitate pattern of juxtaposed high and low backscatter on gloria side scan sonographs this area was investigated using stereo photography high resolution seismic profiles and measurements of physical properties of cores to quantitatively evaluate the causes of backscatter from the 6 5 khz side scan sonar stereo photography and bottom video were used to determine that the sediment water interface typically has a bed roughness less than 10 cm over the entire ground truth area consequently bed roughness is not a significant contributor to the sonar backscatter vertical incidence 3 5 khz profiles reveal that high backscatter areas allow less penetration and have slightly more relief than low backscatter areas closely spaced measurements of p wave velocity density and grain size made on transponder navigated cores are used to investigate the geoacoustic properties of the sediment with the aid of a numerical model the model results demonstrate that the sediment water interface is in most cases acoustically transparent to the sonar energy and that most or all of the energy is refracted into the sediment to depths of at least a few meters rather than scattered from the surface the variability of the dominant lithofacies sands with minor silty clays or silty clay with minor sands accounts for a regional correlation with backscatter when viewed over hundreds to thousands of square kilometers but at the detailed scale of hundreds to a few square meters we have not yet been able to identify a correlation of backscatter intensity with the angle of incidence surface lithology measured acoustic impedance or attenuation however there is a qualitative but unexpected correlation with lithostratigraphic variability in this area thick up to 50 cm sand deposits with thin interbeds of silty clay correlate with lower backscatter than do silty clay deposits with thin interbeds of sand this suggests that volume inhomogeneities and complex constructive and destructive interferences caused by the subsurface volume inhomogeneities within the top few meters of the sediment ultimately modulate the intensity of backscatter the ground truthing effort was further complicated by trying to compare the very large sonar footprint of a pixel 70 000 m2 relative to the area of an individual core 0 006 m2 bottom photograph 5 m2 or seismic profile although 6 5 khz sonographs appear easy to interpret in a conventional and simplistic manner caution should be used when interpreting lithofacies from backscatter intensities

pleistocene glacial history of the nw european continental margin abstract in this paper new and previously published data on the pleistocene glacial impact on the nw european margin from ireland to svalbard between c 48 n 80 n are compiled the morphology of the glaciated part of the european margin strongly reflects repeated occurrence of fast moving ice streams creating numerous glacial troughs channels that are separated by shallow bank areas end moraines have been identified at several locations on the shelf suggesting shelf edge glaciation along the major part of the margin during the last glacial maximum deposition of stacked units of glacigenic debris flows on the continental slope form fans at a number of locations from 55 n and northwards whereas the margin to the south of this is characterised by the presence of submarine canyons glaciation curves based primarily on information from the glacial fed fan systems that depict the pleistocene trends in extent of glaciations along the margin have been compiled these curves suggest that extensive shelf glaciations started around svalbard at 1 6 1 3 ma while repeated periods of shelf edge glaciations on the uk margin started with mis 12 c 0 45 ma the available evidence for mis 2 suggest that shelf edge glaciation for the whole margin was reached between c 28 and 22 14 c ka bp and maximum positions after this were more limited in some regions north sea and lofoten the last glacial advance on the margin has been dated to 15 13 5 14 c ka bp and by c 13 14 c ka bp the shelf areas were completely deglaciated the younger dryas loch lomond advance reached the coastal areas in only a few regions

the innuitian ice sheet configuration dynamics and chronology abstract portrayal of north american ice cover during the last glacial maximum is dominated by the laurentide ice sheet leaving little detail for the adjacent innuitian ice sheet iis four decades of geological fieldwork across the queen elizabeth islands now warrant specific treatment of the iis including its chronology configuration dynamics and retreat this reconstruction is relevant to the sedimentary history of the arctic ocean and to high latitude climate forcing the iis was composed of both an alpine and lowland sector the advance of the alpine sector occurred as recently as 19 14 c ka bp geological evidence configures outflow from alpine and lowland divides that produced several palaeo ice streams one extending northwestward across the canadian arctic archipelago to the polar continental shelf retreat of the iis commenced along its southwest margin 11 6 14 c ka bp however most of the ice sheet remained on the continental shelf during the younger dryas by 10 14 c ka bp marine based ice experienced widespread calving through the western and central archipelago in response to holocene warming and ongoing eustatic sea level rise the sea penetrated the eastern archipelago by 8 5 14 c ka bp gutting the alpine sector of the iis regional isobases record the glacioisostatic signature of the ice sheet and are congruent with the primary geological evidence the delayed buildup of the iis was out of phase with the growth of the laurentide ice sheet that occasioned climatic and glacio eustatic forcing in the innuitian region recent modelling experiments reinforce the hypothesis that growth of the laurentide ice sheet culminated in a split jet stream that temporarily favoured augmented precipitation and growth of the iis

oxygen isotope and palaeotemperature records from six greenland ice core stations camp century dye 3 grip gisp2 renland and northgrip oxygen isotope variations spanning the last glacial cycle and the holocene derived from ice core records for six sites in greenland camp century dye 3 grip gisp2 renland and northgrip show strong similarities this suggests that the dominant influence on oxygen isotope variations reflected in the ice sheet records was regional climatic change differences in detail between the records probably reflect the effects of basal deformation in the ice as well as geographical gradients in atmospheric isotope ratios palaeotemperature estimates have been obtained from the records using three approaches i inferences based on the measured relationship between mean annual 18o of snow and of mean annual surface temperature over greenland ii modelled inversion of the borehole temperature profile constrained either by the dated isotopic profile or iii by using monte carlo simulation techniques the third of these approaches was adopted to reconstruct holocene temperature variations for the dye 3 and grip temperature profiles which yields remarkably compatible results a new record of holocene isotope variations obtained from the northgrip ice core matches the grip short term isotope record and also shows similar long term trends to the dye 3 and grip inverted temperature data the northgrip isotope record reflects i a generally stronger isotopic signal than is found in the grip record ii several short lived temperature fluctuations during the first 1500 yr of the holocene iii a marked cold event at ca 8 2 ka the 8 2 ka event iv optimum temperatures for the holocene between ca 8 6 and 4 3 ka a signal that is 0 6 stronger than for the grip profile v a clear signal for the little ice age and vi a clear signal of climate warming during the last century these data suggest that the northgrip stable isotope record responded in a sensitive manner to temperature fluctuations during the holocene copyright 2001 john wiley sons ltd

dynamics of the coastal north black sea area in late pleistocene and holocene and early human dispersal abstract based on detailed surveys the reconstructions of coastal and marine environments of the north western black sea shelf were carried out for the time spans of ca 25 18 12 10 7 and 7 4 ka bp during the great part of late pleistocene and early holocene the black sea remained a land locked basin of the caspian type which became colonised by early modern humans starting from ca 40 ka the subsequent evolution of coastal landscapes was generally conditioned by the gradual rise of sea level and increasing temperature and precipitation resulting in the formation of coastal wetland landscapes which became the main source of late palaeolithic and mesolithic subsistence the initial indications of farming activities are coeval with the kalamitian stage being conditioned by the increased temperature and precipitation combined with the formation of fertile soils under altithermal conditions the data fail to indicate a catastrophic breakthrough of mediterranean water that could have had any effect on the subsistence or migrations of early farming communities

hydrate occurrence in europe a review of available evidence abstract large national programs in the united states and several asian countries have defined and characterised their marine methane hydrate occurrences in some detail but european hydrate occurrence has received less attention the european union funded project marine gas hydrate an indigenous resource of natural gas for europe migrate aimed to determine the european potential inventory of exploitable gas hydrate to assess current technologies for their production and to evaluate the associated risks we present a synthesis of results from a migrate working group that focused on the definition and assessment of hydrate in europe our review includes the western and eastern margins of greenland the barents sea and onshore and offshore svalbard the atlantic margin of europe extending south to the northwestern margin of morocco the mediterranean sea the sea of marmara and the western and southern margins of the black sea we have not attempted to cover the high arctic the russian ukrainian and georgian sectors of the black sea or overseas territories of european nations following a formalised process we defined a range of indicators of hydrate presence based on geophysical geochemical and geological data our study was framed by the constraint of the hydrate stability field in european seas direct hydrate indicators included sampling of hydrate the presence of bottom simulating reflectors in seismic reflection profiles gas seepage into the ocean and chlorinity anomalies in sediment cores indirect indicators included geophysical survey evidence for seismic velocity and or resistivity anomalies seismic reflectivity anomalies or subsurface gas escape structures various seabed features associated with gas escape and the presence of an underlying conventional petroleum system we used these indicators to develop a database of hydrate occurrence across europe we identified a series of regions where there is substantial evidence for hydrate occurrence some areas offshore greenland offshore west svalbard the barents sea the mid norwegian margin the gulf of cadiz parts of the eastern mediterranean the sea of marmara and the black sea and regions where the evidence is more tenuous other areas offshore greenland and of the eastern mediterranean onshore svalbard offshore ireland and offshore northwest iberia we provide an overview of the evidence for hydrate occurrence in each of these regions we conclude that around europe areas with strong evidence for the presence of hydrate commonly coincide with conventional thermogenic hydrocarbon provinces

paleo redox fronts and their formation in carbonate mound sediments from the rockall trough abstract piston cores from the summits of coral topped carbonate mounds at the south west rockall trough margin reveal that the sediments have undergone significant post depositional modifications affecting the original geochemical signature and mineralogical composition of the sediments this diagenetic imprint provides information about the geochemical processes within cold water coral mounds the most prominent result of diagenetic alteration of the primary sediment composition is the absence or poor preservation of aragonitic coral skeletons in certain depth intervals associated with lithification this study focuses on the enrichment of redox sensitive elements and the dissolution of primary magnetic ferric iron minerals in the depth interval below lithification levels by combining the magnetic susceptibility with the xrf signal of fe and ti specifically intervals with susceptibility variations related to the conversion of strongly magnetic into weakly magnetic iron species can be defined in all three studied cores a succession is recognised with a lithified interval with aragonite dissolution and low mg calcite precipitates that is underlain by an interval of magnetite dissolution and of iron and manganese enrichment for the most recent lithified interval it is demonstrated that initial lithification occurred before an erosional regime was in place most likely near the end of interglacial or at the start of glacial periods

the last glacial termination a major puzzle of paleoclimatology is why after a long interval of cooling climate each late quaternary ice age ended with a relatively short warming leg called a termination we here offer a comprehensive hypothesis of how earth emerged from the last global ice age a prerequisite was the growth of very large northern hemisphere ice sheets whose subsequent collapse created stadial conditions that disrupted global patterns of ocean and atmospheric circulation the southern hemisphere westerlies shifted poleward during each northern stadial producing pulses of ocean upwelling and warming that together accounted for much of the termination in the southern ocean and antarctica rising atmospheric co2 during southern upwelling pulses augmented warming during the last termination in both polar hemispheres

cenozoic sediment accumulation on drifts in the northern north atlantic the northern north atlantic sediment drifts have a much greater areal extent than has previously been indicated the northern north atlantic extends from the east greenland to european continental shelves and from the charlie gibbs fracture zone to the greenland scotland ridge within this region are seven major sediment drifts containing some of the best clues for pre quaternary bottom water circulation in the north atlantic feni drift is the oldest in the region originating near the eocene oligocene boundary it was followed by accumulation of bjorn gardar hatton and snorri drifts from the late early to middle miocene eirik drift may have started to accumulate in the late miocene and gloria drift in the early pliocene through analysis of the cenozoic sediment mass age distribution and the change in area of seafloor for the study area through time it can be concluded that there is a deficit of oligocene to miocene sediment in the region this could be explained by sediment cycling the erosion of older sediment to produce young sediment the mass of pliocene to quaternary sediment is much larger than that of the older sediment and may have been derived primarily through the erosion of eocene and oligocene sediment by deepwater currents the sediment drift portion of the total regional sediment mass increased significantly three times during the cenozoic each of these drift growth phases lasted between 3 and 4 m y the first two growth phases from the late eocene to early oligocene and from the early to middle miocene were accompanied by northward shifts of depocenters to the newly forming sediment drifts the third and largest growth phase occurred between 7 and 3 ma and may have been preceded by the initiation of accumulation on eirik drift 8 7 ma this most recent growth phase was accompanied by high apparent accumulation rates over southern gardar drift as it expanded to the east erosion rates over reykjanes ridge may have been higher during the mid pliocene than in the quaternary there are two main sites where dense water could have formed and flowed south through the rockall trough to start the accumulation of feni drift if the atmosphere were cool enough dense water could have formed in the norwegian sea or on the faeroe shelf dense water may also have formed on the rockall plateau due to salinity increase through evaporation in an arid climate as an alternative to the hypothesis of arctic ocean water overflowing the iceland faeroe ridge in the early middle miocene it is suggested that dense water formed along shallow segments of the iceland faeroe ridge and then flowed into the south iceland basin to begin accumulation of bjorn gardar and snorri drifts the initiation of sediment drift formation in the south iceland basin may have been caused by a decreased rate of deep water production combined with a higher rate of detrital input due to a middle miocene uplift event along the iceland faeroe ridge a second uplift event along the greenland scotland ridge and a decreased production of northern component water in the early pliocene may be responsible for increased sediment accumulation in bjorn gardar and eirik drifts

the rockall bank mass flow collapse of a moated contourite drift onlapping the eastern flank of rockall bank west of ireland abstract the rockall bank mass flow rbmf is a large multi phase submarine slope failure and mass flow complex it is located in an area where the feni drift impinges upon the eastern flank of the rockall bank in the ne atlantic a 6100 km 2 region of slope failure scarps extending over a wide water depth range and with individual scarps reaching up to 22 km long and 150 m high lies upslope of a series of mass flow lobes that cover at least 18 000 km 2 of the base of slope and floor of the rockall trough the downslope lobe complex has a negative topographic relief along much of its northern boundary being inset below the level of the undisplaced contourite drift at the base of slope the southern margin is topographically more subtle but is marked by the sharp termination of sediment waves outside the lobe within the lobe complex the southern margin of the largest lobe shows a positive relief along its southern margin the initial failure is suggested to have occurred along coherent layer parallel detachment surfaces at depths of up to 100 m and this promoted initial downslope block sliding which in turn transformed into debris flows which moved out into the basin the remains of a deep erosional moat linked to the onlapping contourite complex bisects the region of failed slope and post failure thermohaline currents have continued to modify the mass flow in this area differential sedimentation and erosion associated with the moat may have promoted slope instability following the major failure phase continuous readjustments of the slope occurred and resulted in small volume turbidites found in shallow gravity cores collected on the lobes the short term trigger for the failure remains uncertain but earthquake events associated with a deep seated tectonic lineament to the north of the mass flow may have been important a late pleistocene age for the slope failure is likely the rbmf is unusual in that it records large scale collapse of a contourite body that impinged on a sediment undersupplied slope system unlike many other large slope failure complexes along the ne atlantic margin the rbmf occurs in a region where there was little overloading by glacial sediment

deep water geomorphology of the glaciated irish margin from high resolution marine geophysical data abstract the continental margin offshore of western ireland offers an opportunity to study the effects of glacial forcing on the morphology and sediment architecture of a mid latitude margin high resolution multibeam bathymetry and backscatter data combined with shallow seismic and tobi deep towed side scan sonar profiles provide the basis for this study and allow a detailed geomorphological interpretation of the northwest irish continental margin several features including submarine mass failures canyon systems and escarpments are identified in the rockall trough for the first time a new physiographic classification of the irish margin is proposed and linked to the impact of glaciations along the margin correlation of the position and dimensions of moraines on the continental shelf with the level of canyon evolution suggests that the sediment and meltwater delivered by the british irish ice sheet played a fundamental role in shaping the margin including the upslope development of some of the canyon systems the glacial influence is also suggested by the variable extent and backscatter signal of sedimentary lobes associated with the canyons these lobes provide an indirect measurement of the amount of glaciogenic sediment delivered by the ice sheet into the rockall trough during the last glacial maximum none of the sedimentary lobes demonstrates notable relief indicating that the amount of glaciogenic sediment delivered by the british irish ice sheet into the rockall trough was limited their southward disappearance suggests a more restricted biis which did not reach the shelf edge south of 54 23 n the various slope styles observed on the irish margin represent snapshots of the progressive stages of slope development for a glacially influenced passive margin and may provide a predictive model for the evolution of other such margins

carbonate mound development at the sw rockall trough margin based on high resolution tobi and seismic recording abstract in 2002 high resolution sidescan sonar images of a mound area at the sw rockall trough margin were recorded with the tobi deep towed sidescan sonar processed tobi images with a pixel resolution of 6 m provide a unique overview of the carbonate mound distribution and related sedimentary features around the mounds three morphologically distinct areas can be recognised on the tobi mosaic in area i between 500 and 600 m water depth upslope of the mounds giant sediment waves are found with wavelengths up to 500 m with wave crests of more than 10 km long and with an amplitude of up to 30 m reflected with a high backscatter on the tobi image area ii between 600 and 1000 m water depth along the upper margin flank is characterised by clustered and isolated carbonate mounds forming elongated ridges with an orientation perpendicular to the slope sedimentary structures such as flow ridges sediment waves local scouring at the foot of some mounds and draping of sediment around mounds indicate the influence of strong near bed currents oriented in two main current directions parallel to the margin as well as in a similar direction as the mound clusters the mound clusters are several kilometres long can be over 380 m high and are dissected by downslope directed channels on the tobi image the mounds appear as regions of high backscatter caused by the presence of cold water coral colonies on the sediment and by distinct shadows all mounds have their tops at a specific depth level 500 600 m water depth area iii between 900 and 1200 m water depth below the mounds is characterised by disturbed sediments with distinct slump scars and flow ridges 2d high resolution seismic profiling perpendicular and parallel to the slope across the mounds does in general not reveal strong internal reflectors within the mounds in contrast three strong reflectors can be observed in the sedimentary sequence underneath the mounds the upper reflector c10 of early pliocene age is an erosional unconformity above which most of the mounds and sediment waves seem to have developed the second erosional unconformity c20 of middle miocene age is formed by an apparent irregular surface that locally is dissected by some small faults reflectors c10 and c20 have been found in two different mound clusters in the mound area indicating that mound development possibly started after formation of reflector c20 the third reflector forms the acoustic basement which is locally dissected by some small faults that are located below the mounds at least two stages of mound formation are recognised on the seismic profiles

abyssal bedforms as indicators of changing bottom current flow examples from the u s east coast continental rise direct indications of circulation patterns in the deep sea especially along continental rises and sediment drifts can be obtained through the analysis of abyssal bedforms such as furrows and mud waves such direct evidence of current flow will supplement indirect indicators of circulation provide by paleochemical indicators preserved in ocean sediments water that sinks in the northern north atlantic forms part of the western boundary undercurrent along the continental rise of eastern north america and the understanding of temporal changes in the speed and depth of flow along this continental rise can provide important information on how the speed at which the global conveyor system is operating changes with time studies of furrows and mud waves along the blake bahama outer ridges show that they are active sedimentary features today and that bottom flow speeds near the time of the younger dryas were increased over later intervals a more detailed analysis of bedform structure and sediments is needed to more clearly show the timing and reason for this change in sedimentation pattern

sediment accumulation on a cold water carbonate mound at the southwest rockall trough margin abstract cold water corals build isolated and clustered carbonate mound ridges several kilometres long and wide and up to 380 m high at the southwest rockall trough margin photo and video observations as well as coring have shown that mainly the upper mound flanks and summits are covered with a thriving living coral cover and associated fauna box cores prove the presence of living coral colonies on top of a thick layer of coral debris which is to a large extent abraded and bio eroded before being buried the coral framework is filled with sediment from settling pelagic material and parts of the fauna living at the mounds resulting in a 120 cm thick layer of sediments since 10 800 yr component analysis of a piston core from the top of a carbonate mound shows a complete record of cold water coral mound facies comprising many different species living at the mounds an alternation of skeletal and cement dominated intervals was distinguished in the piston core stable isotope analysis of planktonic and benthic foraminifera indicates a continuous sedimentation pattern since the younger dryas the deeper and older part of the core showing mainly intermediate isotope values is dominated by the presence of large hiatuses spanning a total of up to 200 000 yr hiatuses in the core may reflect periods of non deposition or erosion which may be linked to glacial interglacial variability environmental conditions probably change at glacial interglacial timescales influencing the local hydrodynamic regime food supply and sedimentation rate around the carbonate mounds affecting coral growth and therefore carbonate mound development

the feni drift the importance and meaning of slump deposits on the eastern slope of the rockall bank abstract sedimentary facies of cores from the eastern side of the rockall bank and the feni drift 55 58 n have been studied they confirm the existence of slumping which contributes only in small part to drift build up these slumps occurred at different episodes during last glacial and early post glacial periods they are sandwiched in between more important turbidites and are found in places where the drift lies against the slope and so is acted upon by continental rise sedimentary processes the sea level rise periods with ice melting seems to favour gravity flow release

irregular glacial interstadials recorded in a new greenland ice core the greenland ice sheet offers the most favourable conditions in the northern hemisphere for obtaining high resolution continuous time series of climate related parameters profiles of 18o 16o ratio along three previous deep greenland ice cores1 3 seemed to reveal irregular but well defined episodes of relatively mild climate conditions interstadials during the mid and late parts of the last glaciation but there has been some doubt as to whether the shifts in oxygen isotope ratio were genuine representations of changes in climate rather than artefacts due to disturbed stratification here we present results from a new deep ice core drilled at the summit of the greenland ice sheet where the depositional environ ment and the flow pattern of the ice are close to ideal for core recovery and analysis the results reproduce the previous findings to such a degree that the existence of the interstadial episodes can no longer be in doubt according to a preliminary timescale based on stratigraphic studies the interstadials lasted from 500 to 2 000 years and their irregular occurrence suggests complexity in the behaviour of the north atlantic ocean circulation

shelf to slope sedimentation processes and the impact of plio pleistocene glaciations in the northeast atlantic west of ireland abstract the eastern margin of the rockall trough between 54 30 and 56 30 n straddles the southern limit of the glaciated north atlantic margin a complex system of canyons channels and slope failure features on the steep margins of the trough is resolved on the trim tobi rockall irish margins project deep towed sidescan survey the canyons consist of a complex headwall gullied region produced by multiple slope failure events and interconnect to form a cauliform pattern in plan view individual gullies are u shaped in cross section in the upper parts of the margin and are connected downslope to single deeply incised v shaped channels these channels feed extensive aprons of sediment on the lower slope supplied by failure of the headwall regions and sediment transport along the shelf into the canyon headwalls the change in the canyon morphology systems reflects a more abundant supply of glaciomarine sedimentary debris from pleistocene ice sheets whose southerly limit lay along this part of the eastern rockall margin a good correlation between acoustic sidescan backscatter and more deeply penetrating 3 5 khz profiler data suggests that sediment transport pathways were stable through time slope failure escarpments occur in association with the canyon systems along the upper part of the slope canyons are absent in the north of the area where the slope gradient is lowest this is interpreted as the result of progradation of sheet like flows of glaciomarine debris across the southern part of the donegal fan a poor correlation between acoustic backscatter and sub bottom profile data indicates that sediment transport pathways varied through time in the region evidence for strong northeast flowing bottom currents is present along the upper slope and shelf these currents transport and sort sediments along the entire eastern margin of the rockall trough from the south of the porcupine bank to the hebrides terrace seamount

deglacial meltwater pulse 1b and younger dryas sea levels revisited with boreholes at tahiti reconstructing sea level changes during the last deglaciation provides a way of understanding the ice dynamics that can perturb large continental ice sheets the resolution of the few sea level records covering the critical time interval between 14 000 and 9 000 calendar years before the present is still insufficient to draw conclusions about sea level changes associated with the younger dryas cold event and the meltwater pulse 1b mwp 1b we used the uranium thorium method to date shallow living corals from three new cores drilled onshore in the tahiti barrier reef no significant discontinuity can be detected in the sea level rise during the mwp 1b period the new tahiti sea level record shows that the sea level rise slowed down during the younger dryas before accelerating again during the holocene

sedimentation and climate induced sediments on feni ridge northeast atlantic ocean abstract feni ridge has been studied in detail in the area between 54 and 56 n and 12 and 16 w by using high resolution profiling water column sampling with transmissometer and ctd and by piston and box core sampling the crest of feni ridge is absent in the north of the study area but on passing towards the south it begins to take shape and to increase in height its distance from the rockall slope also increases towards the south on the lower southeastern flank a secondary ridge crest has locally developed and sediment waves along the flanks have amplitudes of up to 45 m the highest and steepest waves are found in the south the waves in the north being somewhat lower bottom nepheloid layers of 175 m near the foot of rockall plateau and 125 m near the crest are attributed to a norwegian sea overflow water current with a core at 2100 m bottom nepheloid layers of 75 100 m on the lower flanks of the ridge are thought to have been caused by northeast atlantic deep water piston cores reflect alternations of glacial and interglacial periods interglacials have a high carbonate and corg content a positive eu anomaly and negative ce anomaly and a low rb la ratio they are dominated by a subpolar and transitional foraminiferal assemblage glacials show a low carbonate and corg content a reduced eu anomaly and a high rb la ratio suggesting increasingly continental sediment sources these deposits are dominated by a polar planktonic foraminiferal assemblage characterized by n pachyderma s and g quinqueloba sedimentation rates reflect differential deposition at crestal station na 87 25 rates are 17 1 cm k y for stage 1 14 6 cm k y for stage 2 3 4 cm k y for stage 3 and 5 4 cm k y for stage 4 stage 5a e has a mean sedimentation rate of 6 1 cm k y over the last 125 k y the development of feni ridge has taken place under a relatively low energy current regime

on the timing and nature of the multiple phases of slope instability on eastern rockall bank northeast atlantic one of the most challenging tasks when studying large submarine landslides is determining whether the landslide was initiated as a single large event a chain of events closely spaced in time or multiple events separated by long periods of time as all have implications in risk assessments in this study we combine new multichannel seismic profiles and new sediment cores with bathymetric data to test whether the rockall bank slide complex offshore western ireland is the composite of multiple slope collapse events and if so to differentiate them we conclude that there have been at least three voluminous episodes of slope collapse separated by long periods of slope stability a fourth less voluminous event and possibly a fifth more localized event the oldest event slide a 200 km3 is estimated to be several hundred thousand years old the second event slide b 125 km3 took place at the same location as slide a reactivating the same scar nearly 200 ka ago possibly through retrogression of the scarp slide c 400 km3 took place 22 ka ago and occurred further north from the other slides slide d was a much smaller event that happened 10 ka ago while the most recent event albeit very small scale took place within the last 1 000 years this study highlights the need to thoroughly investigate large slide complexes to evaluate event sequencing as seismic studies may hide multiple small scale events this work also reveals that the same slide scarps can be reactivated and generate slides with different flow behaviors

holocene sediment deposition on a ne atlantic transect including feni drift quantified by radiocarbon and 230thexcess methods radiocarbon and 230thexcess measurements were undertaken on the sediments from fifteen box cores recovered between 48 58 n and 12 22 w at water depths of 1100 4500 m in the northeast atlantic eight of the cores were from feni drift in the rockall trough at depths of 1700 2500 m and the remaining seven formed an approximate north south transect at various water depths from the abyssal plain north on to rockall plateau mean holocene sediment accumulation fluxes for all cores were established from profiles of bulk sediment radiocarbon age against depth to be in the range 3 7 cm ky 1 off the drift and 4 23 cm ky 1 on the drift when compared with these radiocarbon based sediment accumulation rates precision measurements of 230th reveal that the 230thexcess levels present in the sediments match or slightly exceed the potential supply from the vertically overlying water column in a majority of the cores one core from the open abyssal plain that had the lowest radiocarbon based accumulation rate in the entire set also had a close balance between predicted and measured 230thexcess values with the assumption that only 230thexcess produced in the overlying water column is supplied to the sediments the 230thexcess values in this core and the other six containing carbonate ooze sediments all imply a rather constant regionally averaged sedimentation flux of 2 0 0 2 1 n 7 min 1 8 max 2 3 g cm 2 ky 1 rather than matching the 230thexcess fluxes implied by the radiocarbon data this singular value is similar to the mean holocene flux reported for the northeast atlantic by previous work that utilized the 230thexcess method although it is somewhat lower than estimates for the holocene based on oxygen isotope stratigraphy or radiocarbon methods the current transport processes that result in the high sediment accumulation rates on feni drift have also resulted in a compositional fractionation of the sediments all sediments from feni drift have lower caco3 contents than the remainder of the sediments studied and these are accompanied by lower 230thexcess specific activities than are found at comparable water column depths elsewhere on the transect consistently higher and more variable regional sediment accumulation fluxes are therefore calculated from the feni drift 230thexcess data with the constant flux assumption average 2 8 0 4 g cm 2 ky 1 n 8 min 2 3 max 3 3 some 40 higher than the constant value measured in the other cores it seems likely that the mean sediment accumulation flux at feni drift inside rockall trough is consistently higher than on the open ocean margin of the basin so that the regionally averaged sedimentation fluxes indicated for the drift by the measured 230thexcess data are also consistently higher than the singular value measured elsewhere in the northeast atlantic

the intraplate porto dos gauchos seismic zone in the amazon craton brazil abstract the largest earthquake observed in the stable continental interior of the south american plate occurred in serra do tombador mato grosso state brazil on january 31 1955 with a magnitude of 6 2 mb since then no other earthquake has been located near the 1955 epicentre however in porto dos gauchos 100 km northeast of serra do tombador a recurrent seismicity has been observed since 1959 both serra do tombador and porto dos gauchos are located in the phanerozoic parecis basin two magnitude 5 earthquakes occurred in porto dos gauchos in 1998 and 2005 with intensities up to vi and v respectively these two main shocks were followed by aftershock sequences lasting more than three years each local seismic stations have been deployed by the seismological observatory of the university of brasilia since 1998 to study the porto dos gauchos seismic zone pgsz a local seismic refraction survey was carried out with two explosions to help define the seismic velocity model both the 1998 and 2005 earthquake sequences occurred in the same wsw ene oriented fault zone with right lateral strike slip mechanisms the epicentral zone is in the parecis basin near its northern border where there are buried grabens generally trending wnw ese such as the deep mesoproterozoic caiabis graben which lies partly beneath the parecis basin however the epicentral distribution indicates that the 1998 and 2005 sequences are related to a n60 e fault which probably crosses the entire caiabis graben the 1955 earthquake despite the uncertainty in its epicentre does not seem to be directly related to any buried graben either the seismicity in the porto dos gauchos seismic zone therefore is not directly related to rifted crust the probable direction of the maximum horizontal stress near porto dos gauchos is roughly e w consistent with other focal mechanisms further south in the pantanal basin and paraguay but seems to be different from the nw se direction observed further north in the amazon basin the recurrent seismicity observed in porto dos gauchos and the large 1955 earthquake nearby make this area of the parecis basin one of the most important seismic zones of brazil

sedimentation on the feni drift and late glacial bottom water production in the northern rockall trough abstract between the last glacial maximum 18 ka and the present day a variety of contrasting oceanographic conditions including changes in surface and bottom water circulation patterns have existed in the north atlantic evidence for the presence of bottom currents in winnowed silt horizons in the feni drift suggests that deep water may have been produced in this region during glacial deglacial intervals current activity and thus perhaps deep water production was apparently intermittent and may have been controlled by a process analogous to the formation of labrador sea water this involves cooling and deep ocean convection during winter months physical factors controlling this production would be temperature and salinity of upper waters which would govern the density and wind stress necessary for vertical overturn coupled with absence of ice cover evidence of bottom current activity suggests that deep water production occurred in the rockall trough throughout termination 1a which closely corresponds to fairbanks 1989 meltwater pulse 1a mwp 1a production of deep water in this region apparently ceased shortly after the decline in influence of the meltwater influx

the first decade of gloria in 10 years of operation gloria has been used on 26 cruises and has inonified more than 1 of the ocean floor the paper reviews the scientific achievements of this new tool and assesses the contributions which it has made to understanding the geological features and the processes creating them a complete bibliography of gloria papers is included in the references and the best examples of sonographs obtained over different terrain are illustrated

a record of fluctuating bottom currents on the slopes west of the porcupine bank offshore ireland implications for late quaternary climate forcing abstract both local and regional controls on slope sedimentation west of porcupine bank are assessed using an array of 25 gravity cores integrated with shallow seismic tobi side scan and high resolution bathymetry data the cores were retrieved from an area of smooth distally steepened slope between 52 and 53 n in water depths of 950 to 2750 m the slope here is unmodified by gravity failures and is swept by bottom currents that flow from s to n along the margin the cores reveal a coherent shallow stratigraphy that can be traced along and between transects at upper mid and lower slope levels ams 14 c dating oxygen isotopes and carbonate profiles suggest the cored record could extend as far back as 500 ka in the longest cores with most cores providing details of the slope response to the last interglacial last glacial and holocene forcing the facies indicate deposition was dominated by a combination of bottom currents ice rafting and hemipelagic settling with carbonate prone deposits during interglacials and siliciclastic deposits during glacials inferred contourites imply that strong currents operated during interglacials with weaker current reworking during glacial conditions a pair of erosion surfaces record significant mid and upper slope scouring during marine isotope stage mis 3 and in the early holocene the lateral facies distribution implies stronger currents at shallower levels on the slope although there is evidence that the core of the current migrated up and down the slope and that sand might locally have spilt down slope the bathymetry influenced both the wider geometry of the condensed contourite sheet and the local thickness and facies variation across the slope a significant result of the study is the identification of a pair of thin sand mud contourite couplets that record enhanced bottom current reworking corresponding to periods of interstadial warming during mis 3 the couplets can be correlated to the terrestrial records onshore ireland and imply that the ne atlantic margin oceanographic and onshore climate records are strongly coupled at interstadial level

the sea level conundrum case studies from palaeo archives uncertainties in sea level projections for the 21st century have focused ice sheet modelling efforts to include the processes that are thought to be contributing to the recently observed rapid changes at ice sheet margins this effort is still in its infancy however leaving us unable to make reliable predictions of ice sheet responses to a warming climate if such glacier accelerations were to increase in size and frequency the geological record however has long identified examples of nonlinear ice sheet response to climate forcing shackleton nj opdyke nd 1973 oxygen isotope and paleomagnetic stratigraphy of equatorial pacific core v28 239 late pliocene to latest pleistocene geological society of america memoirs145 449 464 fairbanks rg 1989 a 17 000 year glacio eustatic sea level record influence of glacial melting rates on the younger dryas event and deep ocean circulation nature342 637 642 bard e hamelin b arnold m montaggioni l cabioch g faure g rougerie f 1996 sea level record from tahiti corals and the timing of deglacial meltwater discharge nature382 241 244 thus suggesting an alternative strategy for constraining the rate and magnitude of sea level change that we might expect by the end of this century copyright 2009 john wiley sons ltd

low sea level emplacement of a very large late pleistocene megaturbidite in the western mediterranean sea large volume turbidites termed megaturbidites or megabeds result from catastrophic slope failures and the associated downslope transport of enormous quantities of sediment from continental margins to the deep sea such large sediment failures can generate tsunamis and in terrains underlain by gas hydrates clathrates may be associated with the release of substantial amounts of the greenhouse gas methane it has been proposed that the megaturbidite events may be triggered by seismic activity or may result from gas hydrate release itself caused by a lowering of hydrostatic pressure on clathrates as a result of low sea level previous conclusions on the significance of sea level change however have been conditional because of the lack of absolute times of turbidite emplacement here we use accelerator mass spectrometry radiocarbon dating in five widely spaced cores to constrain the date of emplacement of a large volume 500 km3 bed in the balearic basin of the western mediterranean this turbidite is exceptional in its magnitude and represents the main sedimentation event in the balearic basin over the past 100 kyr our data provide an estimate of 22 000 calendar years before present for emplacement of the megabed a time when sea level stood at its lowest level during the last glacial maximum the coincidence of these dates is consistent with emplacement due to clathrate destabilization caused by low sea level although other triggering mechanisms such as seismic shock cannot be ruled out

mid to late cenozoic sedimentation on the continental margin off nw britain the present morphological expression of the continental margin off nw britain is a mid to late cenozoic phenomenon initiated by a major phase of rapid subsidence in the rockall trough and hatton rockall basin during the early late eocene this led to a deepening of the basins and the onset of bottom current activity in this region on seismic reflection profiles this basin subsidence event is manifest in the form of a widespread deep water unconformity caused by bottom current erosion this boundary is particularly enhanced at the basin margins and adjacent to the axial seamounts of rosemary bank and anton dohrn where the downwarped and eroded surface of lower upper eocene and older strata is onlapped by middle to upper cenozoic sediments the latter comprise two megasequences of late eocene to mid miocene and mid miocene to holocene age which consist predominantly of deep marine contourites both in the rockall trough and the hatton rockall basin although a clastic wedge has built out along the hebridean shelf margin since the mid miocene these megasequences reflect a gross two stage depositional history a response to intra plate tectonism which modified sedimentation patterns and palaeoceanographic circulation the development of the bounding unconformities early late eocene and mid miocene was coincident with major phases of regional tectonism

ice sheet uncertainty gravity measurements of the ice mass loss in greenland and antarctica are complicated by glacial isostatic adjustment simultaneous estimates of both signals confirm the negative trends in ice sheet mass balance but not their magnitude

correlation between rate of sea level change and frequency of explosive volcanism in the mediterranean volcanic activity has frequently been linked to quaternary environmental change either by driving climate modification1 2 or in response to environmental changes3 although a link has been established between large explosive eruptions and small 0 5 c brief 1 2 years falls in global temperatures4 both the evidence and mechanisms responsible for longer episodes of eruption induced planetary cooling remain questionable1 2 5 6 in contrast recent research based on ice core data suggests that rapid climate changes during the past 110 000 years increased explosive volcanic activity7 here we present a statistical analysis relating the frequency of explosive activity of mediterranean volcanoes based on dated8 9 10 11 tephra layers in deep sea sediment cores to the rate of late quaternary sea level change the nonlinear correlation between the two is tentatively explained in terms of dynamic responses of the volcanoes to stress related influences on various spatial scales the correlation supports a mechanism or mechanisms by which the climate driven growth and decay of large ice sheets can influence the eruptive chronologies of distant volcanic edifices via changes in global sea level