the transition of the monsoonal and the n atlantic climate system off nw africa during the holocene 1 the northward extent of the influence of the w african monsoon during humid periods of interglacials is subject to investigations highlighting feedback mechanisms such as vegetation to detect this regional variation and the climate system acting farther to the north will be the aim of this paper focussing on the holocene we present two very high resolution holocene sediment records off nw africa located at 31 n and 27 n the well known mid holocene climate change from the african humid period to present arid conditions is well reflected by the terrigenous input in the southern core in contrast in the northern core spectral and wavelet analyses indicate a periodic oscillation of about 900 years of the terrigenous input throughout the last 9000 years b p we conclude that the w african monsoonal influence characterized by the abrupt climatic change at 5000 years b p can be separated from the influence of the n atlantic climate system reflected by a periodic oscillation throughout the holocene

a 300 kyr high resolution aridity record of the north african continent new oxygen isotope data and a high resolution carbonate record of a core from the northwest african margin are presented in combination with geochemical zr rb ratio al ba organic carbon and biogenic opal fluxes and sedimentological median grain size parameters to show variations in african continental aridity wind strength and upwelling productivity the carbonate record is diluted by the supply of aeolian dust from the arid saharan region of north africa and is seen to vary with a characteristic sawtooth pattern indicating rapid increases in dust flux followed by gradual declines spectral analysis of the carbonate profile and comparison with specmap stacked oxygen isotope and etp solar radiation profiles shows that the precessional 23 kyr insolation induced african climate system is decoupled from global ice volume climate changes operating on a 100 kyr frequency detailed studies of glacial terminations i ii and iii confirm the presence of rapid short timescale climatic variations in aridity

assessment of geochemical and micropaleontological sedimentary parameters as proxies of surface water properties in the canary islands region abstract the canary islands region occupies a key position with respect to biogeochemical cycles with the zonal transition from oligotrophic to nutrient rich waters and the contribution of saharan dust to the particle flux we present the distribution of geochemical proxies toc carbonate 15n 13corg c n ratio and micropaleontological parameters diatoms dinoflagellates foraminifera pteropods in 80 surface sediment samples in order to characterise the influence of coastally upwelled water on the domain of the subtropical gyre results of the surface sediment analyses confirmed the high biomass gradient from the coast to the open ocean inferred from satellite data of surface chlorophyll or sst the distribution of total dinoflagellate cysts the planktic foraminifera species globigerina bulloides the diatom resting spore chaetoceros spp and toc concentration coincided well with the areas of strong filament production off cape ghir and cape yubi the warm water planktic foraminifera globigerinoides ruber white the diatom nitzschia spp and the 15n values showed the opposite trend with high values in the open ocean factor analyses on the planktic foraminifera species distribution indicated three major assemblages in the canary islands region that represent the present surface water conditions from the upwelling influenced region via a mixing area towards the subtropical gyre

from source to sink a new perspective on the past dynamics of the murray canyon group from benthic foraminiferal communities abstract we present fossil benthic foraminiferal assemblage data from marine sediment core ss02 06 gc2 located in the abyssal plain of the murray canyon group offshore south australia the sedimentological characteristics indicate the presence of turbidite deposits showing classical bouma like sequences dated between 40 and 12 cal ka bp these results confirm the previous interpretation of the observed large deep water holes in the abyssal area where the core was sampled as being gouged by surges of high energy turbidity currents the presence of good indicator taxa and unique assemblages occupying specific bathymetric depths allows the determination of the source origin of the sediments making the turbidites three distinct faunal groups are found 1 mostly shelf species 2 mostly bathyal species and 3 mostly abyssal species in the sediment core these groups present a quasi systematic succession with nearly all bouma like sequences starting with the dominance of bathyal species in the coarse grained base followed by the dominance of shallow species in the silty part and finally with abyssal species in the clays to explain such phenomena turbidites triggered by mixed hyperpycnal hypopycnal flow processes and turbidity currents during periods of river floods are considered for the first time within the murray canyon group they are mostly related to periods of increased fluvial discharges during wet phases in the murray darling basin

gas venting that bypasses the feather edge of marine hydrate offshore mauritania methane can be released from the vast marine hydrate reservoirs that surround continents into oceans and perhaps the atmosphere but how these pathways work within the global carbon cycle now and during a warmer world is only partially understood here we use 3 d seismic data to identify what we interpret to be a gas venting system that bypasses the hydrate stability zone hsz offshore of mauritania this venting is manifested by the presence of the acoustic wipe out awo across a densely faulted succession above a salt diapir and a set of morphological features including a substantial 260 m wide and 32 m deep pockmark at the seabed the base of the hsz is marked by a bottom simulating reflector bsr which deflects upwards above the diapir rather than mimicking the seabed we use a numerical modelling to show that this deflection is caused by the underlying salt diapir it creates a trapping geometry for gas sealed by hydrate clogged sediment after entering the hsz some methane accumulated as hydrate in the levees of a buried canyon venting in this locality probably reduces the flux of gas to the landward limit of feather edge of hydrate reducing the volume of gas that would be susceptible for release during a warmer world

late quaternary sedimentation within a submarine channel levee system offshore cap timiris mauritania aeolian and fluvial sediment transport to the atlantic ocean offshore mauritania were reconstructed based on grain size distributions of the carbonate free silt fraction of three marine sediment records of cap timiris canyon to monitor the climatic evolution of present day arid north western africa during the late pleistocene predominantly coarse grained particles which are interpreted as windborne dust characterise glacial dry climate conditions with a low sea level and extended sand seas that reach onto the exposed continental shelf off mauritania subsequent particle fining and the abrupt decrease in terrigenous supply are attributed to humid climate conditions and dune stabilisation on the adjacent african continent with the onset of the holocene humid period indications for an ancient drainage system which was discharging fluvial mud offshore via cap timiris canyon are provided by the finest end member for early to mid holocene times however in comparison to the senegal and niger river further south the river system connecting cap timiris canyon with the mauritanian hinterland was starved during the late holocene and is non discharging under present day arid climate conditions

onset of submarine debris flow deposition far from original giant landslide submarine landslides can generate sediment laden flows whose scale is impressive individual flow deposits have been mapped that extend for 1 500 km offshore from northwest africa these are the longest run out sediment density flow deposits yet documented on earth this contribution analyses one of these deposits which contains ten times the mass of sediment transported annually by all of the world s rivers understanding how this type of submarine flow evolves is a significant problem because they are extremely difficult to monitor directly previous work has shown how progressive disintegration of landslide blocks can generate debris flow the deposit of which extends downslope from the original landslide we provide evidence that submarine flows can produce giant debris flow deposits that start several hundred kilometres from the original landslide encased within deposits of a more dilute flow type called turbidity current very little sediment was deposited across the intervening large expanse of sea floor where the flow was locally very erosive sediment deposition was finally triggered by a remarkably small but abrupt decrease in sea floor gradient from 0 05 degrees to 0 01 degrees this debris flow was probably generated by flow transformation from the decelerating turbidity current the alternative is that non channelized debris flow left almost no trace of its passage across one hundred kilometres of flat 0 2 degrees to 0 05 degrees sea floor our work shows that initially well mixed and highly erosive submarine flows can produce extensive debris flow deposits beyond subtle slope breaks located far out in the deep ocean

climate and sea level induced turbidite activity in a canyon system offshore the hyperarid western sahara mauritania the timiris canyon a major outcome from studying the sedimentary archives of the timiris canyon off mauritania is that climate shifts in the saharan hinterland not only influenced offshore pelagic sedimentation processes but also controlled turbidite activity in the canyon system sediment supply to the slope predominantly by eolian dust and remobilised eolian sands was greatest during hyperarid glacial and deglacial periods and due to overall increased trade wind strengths upwelling induced pelagic productivity attained highest magnitudes turbidite activity in the timiris canyon responded rapidly to these shifts in the surrounding sedimentary regime on the continental slope during high glacial and early deglacial times the tributary gullies on the uppermost slope were rapidly and repetitively filled subsequently turbidity currents were released and large turbulent flows passed through the lower timiris canyon leading to overspill sequences on its levees in addition to this scenario partial flooding of the exposed outer shelf during successive sea level rises in mis 3 may have caused re mobilization of huge aeolian dune fields that had expanded close to the shelf edge during glacial exposure during the holocene no turbidite activity is recorded in the lower timiris canyon whereas a core from a widened area in the upper timiris canyon reveals an interesting climate controlled 900 year cyclicity of carbonate and mud turbidite events throughout the holocene

planktonic foraminiferal assemblages in surface sediments from the southwest african continental margin abstract the relative abundance and distribution of planktonic foraminifera species on the southwest african continental margin have been determined from 152 surface sediment samples of the 23 recorded taxa only 8 make a significant contribution to the foraminiferal assemblages individual species proportions are closely related to the surface water hydrography of the benguela system and the associated upwelling phenomena three foraminiferal assemblages can be distinguished by correlation analysis on the abundances of the major taxa species composition of these assemblages range from polar taxa in the upwelling area left coiling neogloboquadrina pachyderma and globigerina quinqueloba to transitional taxa in the offshore oligotrophic environment globorotalia inflata and globorotalia truncatulinoides globigerina bulloides and right coiling neogloboquadrina pachyderma are preferentially distributed in the highly productive intermediate zone of the benguela system where upwelled and oligotrophic offshore waters mix variability of the hydrological processes upwelling intensity strength of the frontal features may be inferred from frequency variations of individual species and diversity patterns

recurrent decadal scale dust events over holocene western africa and their control on canyon turbidite activity mauritania abstract sediment records from continental shelves and slopes might provide paleoenvironmental information in the highest temporal resolution but are often hampered due to strong erosional and reworking processes here we present a holocene sedimentary record from an exceptional shelf mud belt depocenter off northern mauritania compared to a second sediment core located inside a large canyon system at the adjacent continental slope both records are of outstandingly continuous and highest temporal resolution 9 a cm and are investigated by sedimentological and geochemical methods a series of sharply defined recurrent dust peaks is preserved in the shelf archive each event has lasted for a single decade only and seems to coincide with an individual turbidite bed in the canyon a joint mechanism should thus be responsible for both of these deposits and we suggest a regional atmospheric trigger only short lasting trade wind strengthening would cause such pronounced aridity over western saharan africa the effect would be massive dust export to shelf and slope recently developed high resolution aridity and humidity records from western africa support the existence of these events over the holocene and identify them as being controlled by the atlantic system as far south as 19 n

paleoclimates off northwest africa 28 35 n about 18 000 yr b p based on continental eolian deposits abstract most conceptual models of atmospheric circulation deduced from bottom sediments or isopollen maps off nw africa assume the occurrence of intensified nne trade winds about 18 000 yr b p in latitudes of 28 35 n and oversimplify the glacial atmospheric circulation over africa an alternative method for reconstructing paleowinds of the last glacial maximum in these latitudes was recently put forward and uses sedimentological records from the canary islands and coastal regions of morocco the continental data do not agree with the previous models and show the prevalence of westerlies all the data from deep sea cores reduction of sea surface temperature increase of biogenic opal accumulation distribution patterns of pollen or dinoflagellate cysts and xeric conditions on the adjacent continent can be explained without increased activity of the trade winds but with a discharge current of cold meltwater from the european and north american ice sheets the model is backed up by a comparison with the present day humboldt current off subtropical south america

an irregular feather edge and potential outcrop of marine gas hydrate along the mauritanian margin the dissociation of marine hydrate that surrounds continental margins is thought to be an agent for past and future climate change as the water depth decreases landwards the base of the hydrate stability zone progressively shallows until hydrate can occur at or immediately below the seabed where an increase in bottom water temperature can cause dissociation but the true extent of these most vulnerable hydrate deposits is unknown here we use exceptional quality three dimensional 3 d seismic reflection imagery offshore of mauritania that reveals a rare example of a bottom simulating reflection bsr that intersects the seabed and delineates the feather edge of hydrate the bsr intersects the seabed at the 636 m isobath but along the 32 km of the margin analysed the intersection is highly irregular intersections and seismic evidence for hydrate less than 4 3 m below the seabed occur in seven small localised areas that are 0 02 0 45 km2 in extent we propose gas flux below the dipping base of the hydrate to these places has been particularly effective the intersections are separated by recessions in the bsr where it terminates below the seabed seawards of the 636 m isobath recessions are areas where the concentration of hydrate is very low or hydrate is absent they are regions that have been bypassed by gas that has migrated landwards along the base of the hydrate or via hydraulic fractures that pass vertically through the hydrate stability zone and terminate at pockmarks at the seabed an irregular feather edge of marine hydrate may be typical of other margins

upwelling intensity and ocean productivity changes off cape blanc northwest africa during the last 70 000 years geochemical and micropalaeontological evidence abstract the accumulation of biogeochemical organic carbon calcium carbonates molybdenum and iodine micropalaeontological benthic foraminifera and terrigenous markers grain size si al zr al k al over the last 70 000 years in one core sedorqua 11k on the northwest african margin off cap blanc has been used to reconstruct past variations of local upwelling intensity and oceanic productivity the study demonstrates that productivity in this area increased during stage 3 particularly between 40 000 and 50 000 yr b p and during the last deglaciation between 6000 and 15 000 yr b p during most of isotopic stage 2 and particularly during the last glacial maximum productivity was much lower these variations can be attributed to changes in local wind stress and seasonality that are related to variability in monsoon pressure intensity because of the establishment of upwelling cells over the shelf due to high sea level the conditions of sedimentation during the last 5 6 thousend years on the upper slope site 11k are largely dominated by advection from the shelf leading to strong sorting prior to deposition advection seems to have been minor during the other periods of enhanced productivity a conceptual model is proposed to link the productivity variations to atmospheric circulation in particular to the wind stress direction and seasonality

late pleistocene and holocene dune activity and wind regimes in the western sahara desert of mauritania the western sahara desert in mauritania is dominated by extensive sand seas consisting largely of linear dunes analyses of landsat images geomorphic and stratigraphic studies and optically stimulated luminescence dating of dunes in the azefal agneitir and akchar sand seas provide evidence that three main generations of dunes were formed during the periods 25 15 ka centered around the last glacial maximum 10 13 ka spanning the younger dryas event and after 5 ka the wind regimes that occurred during each of these periods were significantly different leading to the formation of dunes on three distinct superimposed trends northeast north northeast and north and the development of the sand seas as composite geomorphic features

a late quaternary sedimentary shelf system under hyperarid conditions unravelling climatic oceanographic and sea level controls golfe d arguin mauritania nw africa abstract the sedimentation on the mauritanian shelf off the western saharan desert is strongly influenced by dust supply and is investigated in this study by combining surface mapping shallow acoustic profiling and the analysis of sediment cores coarse grained carbonates dominate wide areas of the open shelf at places where the shelf is exposed to the atlantic ocean the sheltered golfe d arguin in contrast is characterized by muddy shoals in its inner area and locally restricted and wedge shaped muddy depocenters which extend contour parallel in mid shelf settings in both cases cross shelf currents control sediment distribution and accumulation not location or amount of primary eolian input a major discontinuity on the outer shelf is located in 85 110 m water plus sediment depth and is refilled by a sedimentary facies succession of shoreface tidal and open shelf deposits this succession has formed during temporary lower sea levels along the last overall regression mis 4 and mis 3 the last glacial maximum and during the past deglacial sea level rise significant sediment deposition and preservation is limited to very restricted areas concerning the overall shelf architecture sea level was the main factor enabling or suppressing deposition in terms of 1 creating local erosional swales which have favored subsequent trapping and preservation of sediments 2 determining the type of sedimentary facies refilling these depressions under a variety of nearshore conditions 3 interacting with the seafloor morphology and by this controlling timing of initiation and growth geometry of mud wedges these mud wedges have formed since ca 9 cal ka under a continuous sedimentation regime these bodies must have formed in hydraulic equilibrium but they are detached from a defined sediment point source which is in contrast to common mudbelt formation on shelves in general sediment storage on shelves under hyperarid conditions seems to be very limited due to comparably low sediment input strong ravinement activity during changing sea levels significant bypassing and winnowing processes and a low preservation potential of the non cohesive materials these conditions lead to the development of almost sediment starved systems with low subsidence rates over longer geological timescales

late quaternary climatic events and sea level changes recorded by turbidite activity dakar canyon nw africa the relationship of sea level changes and short term climatic changes with turbidite deposition is poorly documented although the mechanisms of gravity driven sediment transport in submarine canyons during sea level changes have been reported from many regions this study focuses on the activity of the dakar canyon off southern senegal in response to major glacial interglacial sea level shifts and variability in the nw african continental climate the sedimentary record from the canyon allows us to determine the timing of turbidite events and on the basis of xrf scanning element data we have identified the climate signal at a sub millennial time scale from the surrounding hemipelagic sediments over the late quaternary the highest frequency in turbidite activity in the dakar canyon is confined to major climatic terminations when remobilisation of sediments from the shelf was triggered by the eustatic sea level rise however episodic turbidite events coincide with the timing of heinrich events in the north atlantic during these times continental climate has changed rapidly with evidence for higher dust supply over nw africa which has fed turbidity currents increased aridity and enhanced wind strength in the southern saharan sahelian zone may have provided a source for this dust

relationship between infrasound derived and buoyancy derived eruption plume volume estimates volume estimations for discrete plumes accompanying short lived eruptions are potentially possible using infrasound signals plumes emitted during short lived discrete eruptions have been modeled as discrete thermals or rooted thermals based on the one dimensional model for the rise of a discrete thermal the initial buoyancy f0 of a spherical thermal can theoretically be estimated from the maximum plume height and a vertical profile of ambient air density we here examine the relationship between infrasound derived volume vinf and the buoyancy derived volume vb as derived from f0 which depends on gravity acceleration and the difference in density between the thermal and surrounding air to understand how vinf relates to the dynamics of the eruption plume we analyze infrasound data accompanying vulcanian and phreatic eruptions at aso shinmoedake and lokon empung volcanoes to estimate vinf and consider vinf from other volcanoes we obtain a representative ratio of vb vinf of 16 by examining 53 events because the analyzed infrasound signals share a prominent pressure pulse at the onset we regard vinf as the volume of the plume at initiation i e as a jet instead we consider vb as the thermal volume when it has entrained a sufficient amount of the surrounding air and obtains f0 comparison of the bulk density of the jet and the discrete thermal yields a rate of volume change between both regimes by a factor of 1 8 32 which is consistent with the ratio of vb vinf our result provides an effective index to constrain erupted plume volume using infrasound data with real time monitoring systems

terrigenous sedimentation processes along the continental margin off nw africa implications from grain size analysis of seabed sediments the terrigenous fraction of seabed sediments recovered along the north west african continental margin illustrates spatial variability in grain size attributed to different transport mechanisms three subpopulations are determined from the grain size analyses n 78 of the carbonate free silt fraction applying an end member modelling algorithm g j weltje 1997 the two coarsest endmembers are interpreted as representing aeolian dust and the fine grained end member is related to fluvial supply the end member model thus allows aeolian fallout to be distinguished from fluvial sourced mud in this area the relative contributions of the end members show distinct regional variations that can be related to different transport processes and pathways understanding present day sediment dispersal and mixing is important for a better understanding of older sedimentary records and palaeoclimate reconstructions in the region

the northwest african slope apron a modern analogue for deep water systems with complex seafloor topography the northwest african slope apron is an interesting modern analogue for deep water systems with complex seafloor topography a sediment process map of the northwest african continental margin illustrates the relative roles of different sedimentary processes acting across the entire margin fine grained pelagic and hemipelagic sedimentation is dominant across a large area of the margin and is considered to result from background sedimentary processes alongslope bottom currents smooth and mould the seafloor sediments and produce bedforms such as erosional furrows sediment waves and contourite drifts downslope gravity flows debris avalanches debris flows and turbidity currents are infrequent but important events on the margin and are the dominant processes shaping the morphology of the slope and rise the overall distribution of sedimentary facies and morphological elements on the northwest african margin is characteristic of a fine grained clastic slope apron however the presence of numerous volcanic islands and seamounts along the margin leads to a more complex distribution of sedimentary facies than is accounted for by slope apron models in particular the distribution and thickness of turbidite sands are controlled by the location of the break of slope which is itself controlled by the pre existing submarine topography

testing the astronomical time scale for oceanic anoxic event 2 and its extension into cenomanian strata of the western interior basin usa the development of integrated astronomical and radioisotopic time scales from rhythmic strata of the western interior basin wib has played a fundamental role in the refi nement of late cretaceous chronostratigraphy in this study x ray fl uorescence xrf core scanning is utilized to develop a new elemental data set for cyclostratigraphic investigation of cenomanianturonian strata in the wib using material from the aristocrat angus 12 8 core northcentral colorado the xrf data set yields the fi rst continuous 5 mm resolution analysis of lithogenic biogenic and syngeneticauthigenic proxies through the uppermost lincoln limestone member the hartland shale member and the bridge creek limestone member including oceanic anoxic event 2 oae 2 the 40 ar 39 ar ages from ashes in three biozones including a new age from the dunveganoceras pondi biozone uppermost lincoln limestone member provide geochronologic constraints for the cyclostratigraphic analysis astrochronologic testing of the 5 mm resolution xrf weight percent caco3 data via average spectral misfi t analysis yields strong evidence for astronomical infl uence on climate and sedimentation results from the bridge creek limestone member are consistent with the previously published astrochronology from the u s geological survey 1 portland core central colorado and identifi cation of an astronomical signal in the underlying hartland shale member now permits extension of the wib astrochronology into the earlier cenomanian prior to oae 2 high rates of sedimentation in the angus core during the interval of oae 2 initiation as compared to the portland core allow recognition of a strong precessional control on bedding development as a consequence the new results provide a rare high resolution chronometer for the onset of oae 2 and the timing of proposed hydrothermal trace metal enrichment as observed in the 5 mm xrf data

hydraulic fractures how far can they go the maximum reported height of an upward propagating hydraulic fracture from several thousand fracturing operations in the marcellus barnett woodford eagle ford and niobrara shale usa is 588 m of the 1170 natural hydraulic fracture pipes imaged with three dimensional seismic data offshore of west africa and mid norway it is 1106 m based on these empirical data the probability of a stimulated and natural hydraulic fracture extending vertically 350 m is 1 and 33 respectively constraining the probability of stimulating unusually tall hydraulic fractures in sedimentary rocks is extremely important as an evidence base for decisions on the safe vertical separation between the depth of stimulation and rock strata not intended for penetration

age dating and the orbital theory of the ice ages development of a high resolution 0 to 300 000 year chronostratigraphy using the concept of orbital tuning a continuous high resolution deep sea chronostratigraphy has been developed spanning the last 300 000 yr the chronology is developed using a stacked oxygen isotope stratigraphy and four different orbital tuning approaches each of which is based upon a different assumption concerning the response of the orbital signal recorded in the data each approach yields a separate chronology the error measured by the standard deviation about the average of these four results which represents the best chronology has an average magnitude of only 2500 yr this small value indicates that the chronology produced is insensitive to the specific orbital tuning technique used excellent convergence between chronologies developed using each of five different paleoclimatological indicators from a single core is also obtained the resultant chronology is also insensitive to the specific indicator used the error associated with each tuning approach is estimated independently and propagated through to the average result the resulting error estimate is independent of that associated with the degree of convergence and has an average magnitude of 3500 yr in excellent agreement with the 2500 yr estimate transfer of the final chronology to the stacked record leads to an estimated error of 1500 yr thus the final chronology has an average error of 5000 yr