productivity controlled cold water coral growth periods during the last glacial off mauritania abstract article history received 17 june 2010received in revised form 29 october 2010accepted 11 december 2010available online 21 december 2010communicated by g j de langekeywords lopheliacold water coralscarbonate moundsbanda mound provincenw africaprimary productivity cold water corals are widely distributed along the atlantic continental margin with varying growth patternsinrelationtotheirspeci cenvironment here weinvestigatethelong termdevelopmentofcold watercoralsthat once thrived on a low latitude 17 40 n cold water coral mound in the banda mound province offmauritania duringthelastglacial interglacialcycle u thdatesobtained from20specimensofthecold watercoral lophelia pertusa revealed three distinct periods of coral growth during the last glacial at 65 to 57 kyr bp 45 to 32 kyr bp and 14 kyr bp thus comprising the cool periods of marine isotopic stages mis 2 4 thesecoral growth periods occur during periods of increased productivity in the region emphasizing thatproductivity seems to be the major steering factor for coral growth off mauritania which is one of the majorupwelling regions in the world this pattern differs from the well studied coral mounds off ireland where thecurrent regime predominantly in uences the prosperity of the cold water corals moreover coral growth offireland takes place during rather warm interglacial and interstadial periods whereas off mauritania coralgrowth is restricted to glacial and stadial periods however the on mound sedimentation patterns offmauritania largely resemble the observations reported from the irish mounds the bulk of the preservedsediments derives from periods of coral growth whereas during periods without corals hardly any netsedimentation or mound growth took place 2011 elsevier b v all rights reserved

mean flow and variability in the kuroshio extension from geosat altimetry data using altimeter data from the geosat exact repeat mission erm we investigated the mean flow and temporal and spatial variations of the kuroshio extension in the region of 140 180 e and 30 40 n mean surface height profiles were estimated along individual tracks by assuming the velocity profile of the kuroshio extension to be gaussian shaped and by successively fitting this synthetic current s height profile to the residual height data using the mean profiles from ascending and descending tracks we derived the mean surface height field by an inverse method and obtained the absolute surface height fields for the first 2 5 years of the geosat erm both the mean and the instantaneous height fields thus derived compared well with the available hydrographic data and the sst patterns from the noaa satellites the mean surface height difference across the kuroshio extension attains its maximum around 146 e between the two quasi stationary meanders and its decrease thereafter is mainly due to large scale recirculations on the southern side of the kuroshio extension the ratio of the eddy kinetic energy over the mean kinetic energy has a nearly constant value of 1 5 2 0 along the kuroshio extension path propagation of mesoscale fluctuations in the height fields is generally westward except for the upstream region of the kuroshio extension effects of deep mean flow and baroclinic shear are found to be important in explaining the observed propagation speeds in the upstream region of 141 e and 154 e annual variations in the surface height difference across the kuroshio extension h have a september maximum with an average amplitude of 0 2m for large scale interannual fluctuations anomalies in h are found to be significantly correlated with those of the current axis positions a larger surface height difference corresponds to a more northerly position of the kuroshio extension the interannual changes in h are possibly related to the 86 87 enso event in the low latitude pacific ocean

rapid flooding of the sunda shelf a late glacial sea level record the increase in sea level from the last glacial maximum has been derived from a siliciclastic system on the tectonically stable sunda shelf in southeast asia the time from 21 to 14 thousand calendar years before the present has been poorly covered in other records the record generally confirms sea level reconstructions from coral reefs the rise of sea level during meltwater pulse 1a was as much as 16 meters within 300 years 14 6 to 14 3 thousand years ago

a high resolution simulation of the ocean during the pomme experiment simulation results and comparison with observations 1 a modeling study of physical processes occurring in an area of the northeast atlantic 21 33 15 33 w 38 00 45 00 n that was extensively sampled during the programme ocean multidisciplinaire meso echelle pomme october 2000 september 2001 is carried out the model is a mesoscale version of the ocean general circulation model opa developed at the laboratoire d oceanographie dynamique et de climatologie in paris it is used in a three dimensional limited area domain with a high resolution grid approximately 5 km horizontal spacing 69 vertical levels and realistic boundary conditions initial state air sea fluxes open boundary fluxes and bottom topography the objectives of the study are to properly simulate the upper ocean dynamics particularly mesoscale activity and mixed layer evolution during a key period restratification of the pomme experiment pomme 1 and pomme 2 from february to may 2001 and to compare model results with oceanographic observations collected during the experiment in order to establish confidence in the model some results provided by the high resolution simulation in particular features related to mixed layer depth and vertical velocities are also presented there is no pronounced north south mixed layer depth gradient but strong filament shaped structures associated with stirring at the periphery of eddies are present mixed layer restratification is simulated it is associated with submesoscale mixed layer depth structures and intense vertical velocity filaments in the upper ocean correlated with the relative vorticity gradient field

a general circulation model for upper ocean simulation abstract a general circulation model gcm of the ocean that emphasizes the simulation of the upper ocean has been developed this emphasis is in keeping with its future intent that of an air sea coupled model the basic model is the primitive equation model of bryan and cox with the additions of optional usage of the mellor yamada level 2 5 turbulence closure scheme and horizontal nonlinear viscosity these modifications are intended to improve the upper ocean simulations particularly sea surface temperature and heat content the horizontal grid spacing is 1 latitude 1 longitude and is global in domain the equatorial region between 10 n and 10 s is further refined in the north south direction to resolution there are 12 vertical levels with six levels in the top 70 m the model incorporates varying bottom topography prior to coupling the ocean model to an atmospheric gcm experiments have been carried out to determine the ocean gcm s performance using atmospheric forcing from observed data

n alkane and pollen reconstruction of terrestrial climate and vegetation for n w africa over the last 160 kyr abstract proxy environmental records have been obtained for the last 160 kyr from the well studied odp site 658 ca 200 km west of cap blanc n w africa 20 5 n 18 35 w 2263 m water depth this collective assessment of several terrigenous proxies lithogenic fraction n alkane content and 13 c values and pollen counts provides a better understanding of the climate and vegetation history of the n w african hinterland the results indicate that for this site beneath the main dust pathway of the saharan air layer sal the dust flux increased during dry glacial periods but the amounts of pollen and n alkanes delivered were lower in accord with the decreased density of vegetation cover of their source areas the c 4 pollen was also very low but the heavy 13 c values of the n alkanes accorded with a mainly c 4 plant origin evidently the load of sal pollen was much reduced but not the sal wind strength

paleo sea levels reconsidered from direct observation of paleoshoreline position during glacial maxima for the last 500 000 yr the drastic climatic changes which characterise the cooling trend of the last few million years of earth history led to variations in eustatic sea level that had tremendous impact on the geology and ecology of continental margins reconstructing a sea level curve back in time is not an easy task observations of shoreline positions are always a local measurement of relative sea level that needs to be corrected from the effect of tectonic and thermal subsidence sediment loading compaction and glacio hydro isostasy extensive studies have been done for the last deglaciation and for the last 100 000 yr cycle but very few studies deal with position of sea level during earlier cycles simply because conditions are very rarely favourable for the preservation of such witnesses the shelf of the golfe du lion western mediterranean reveals a unique record of shoreline paleopositions during glacial maxima of at least the last five circa 100 kyr glacial interglacial cycles in fact it is the entire glacial deltaic lobe of up to 50 m thick from delta front or shoreface to prodelta that has been preserved in place and which provides direct and independent constraints for relative sea level minima we measure a relative sea level of 112m 128 134 246 and 262 m for mis 2 6 8 10 and 12 respectively after corrections taking into account postdepositional movement of strata subsidence we find that sea level dropped to a depth of 102 6 m during the last three glaciations mis2 mis6 mis8 but reached exceptionally low values of more than 150 10 m during the preceding glaciations mis10 and mis 12 at about 340 and 434 kyr bp this general time framework and sedimentological interpretation has been confirmed by preliminary results from two deep drillings during the promess cruise july 2004 which validate our methodology however no detailed and absolute datings of such witnesses are available so far so that we cannot prove that these levels are the lowest ever reached during each glacials but they correspond undoubtedly to the last preserved shoreface before rapid sea level rise we also suggest that the abrupt change in sea level maxima might be the overprint of 400 kyr orbital periodicity cycles last but not least these results prove that the golfe du lion is indeed a unique laboratory to study paleoclimates and sea level variations on a larger time scale further work is needed for a complete glacio hydro sedimento isostatic modelling of each sequence and each glacial to further constraint local sea level versus global sea level and quantify in particular the relative effect of glacio hydro isostatic effect which differ according to ice sheet extend but also of erosion sedimentation isostatic effect erosion on land and deposition on the outer shelf and slope

tracking the last sea level cycle seafloor morphology and shallow stratigraphy of the latest quaternary new jersey middle continental shelf seafloor geomorphology and surficial stratigraphy of the new jersey middle continental shelf provide a detailed record of sea level change during the last advance and retreat of the laurentide ice sheet 120 kyr b p to present a nw se oriented corridor on the middle shelf between water depths of 40 m the mid shelf paleo shore and 100 m the franklin paleo shore encompasses 500 line km of 2d huntec boomer profiles 500 3500 hz an embedded 4 6 km2 3d volume and a 490 km2 swath bathymetry map we use these data to develop a relative stratigraphy core samples from published studies also provide some chronological and sedimentological constraints on the upper 5 m of the stratigraphic succession the following stratigraphic units and surfaces occur from bottom to top 1 r a high amplitude reflection that separates sediment 46 5 kyr old by ams 14c dating from overlying sediment wedges 2 the outer shelf wedge a marine unit up to 50 m thick that onlaps r 3 channels a reflection sub parallel to the seafloor that incises r and appears as a dendritic system of channels in map view 4 channels fill the upper portion of which is sampled and known to represent deepening upward marine sediments 12 3 kyr in age 5 the t horizon a seismically discontinuous surface that caps channels fill 6 oblique ridge deposits coarse grained shelly units comprised of km scale shallow shelf bedforms and 7 ribbon floored swales bathymetric depressions parallel to modern shelf currents that truncate the oblique ridges and cut into surficial deposits we interpret this succession of features in light of a global eustatic sea level curve and the consequent migration of the coastline across the middle shelf during the last 120 kyr the morphology of the new jersey middle shelf shows a discrete sequence of stratigraphic elements and reflects the pulsed episodicity of the last sea level cycle r is a complicated marine non marine erosional surface formed during the last regression while the outer shelf wedge represents a shelf wedge emplaced during a minor glacial retreat before maximum wisconsin lowstand i e marine oxygen isotope stage 3 1 channels is a widespread fluvial subarial erosion surface formed at the late wisconsin glacial maximum 22 kyr b p the shoreline migrated back across the mid shelf corridor non uniformly during the period represented by channels fill oblique ridges are relict features on the new jersey middle shelf while the ribbon floored swales represent modern shelf erosion there is no systematic relationship between modern seafloor morphology and the very shallowly buried stratigraphic succession

sea level changes in the tectonically stable malay thai peninsula abstract the malay thai peninsula is part of sundaland a geological region of relative crustal stability therefore the positions of old shorelines of the past 20 000 years or so can be considered to indicate positions of actual past sea levels over 200 shoreline indicators at positions below as well as above present sea level have been radiocarbon dated these dated positions indicate that prior to 6 ka the sea rose from at least 90 m below present sea level at rates that initially amounted to 15 mm year and later became 6 mm year at ca 6 ka the rising sea surpassed its current position and at ca 5 ka it reached 5 m during this maximum marine inundation the peninsula was narrower since the low coastal plains did not yet exist in the period following this maximum transgression the available shoreline data indicate that regional sea level subsided through a series of fluctuations that were characterized by amplitudes of 2 m and periods of about 2000 years the peaks and troughs of the fluctuations became progressively lower the latest peak was at about 1 7 m or 0 7 m above present high tide a few hundred years ago the cause of these fluctuations is not yet known but the trend suggests that in the near future regional sea level may subside at annual rates between 1 5 mm and 2 mm this rate of descent is 25 to 30 of the estimated rate of sea level rise that may result from global warming in other words for the malay thai peninsula the effect of sea level rise on account of manmade greenhouse gases will be partly compensated by actual subsidence of regional sea level

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

aeolian to shallow marine shelf architecture off a major desert since the late pleistocene northern mauritania abstract continental shelves off major desert regions are not expected to host substantial amounts of sediments due to long lasting and unfocused material supply and a high re mobilization potential of aeolian material this study in contrast demonstrates that significant volumes of sediments have accumulated on the northern mauritanian shelf under the arid climate conditions and prevail over consecutive climatic cycles eight late pleistocene to holocene depositional units each formed under contrasting depositional conditions are identified in high resolution seismo acoustic data and dated sediment cores these are 1 a highly differentiated pleistocene paleo landscape older than the past climatic cycle 2 a continental dune complex mis 4 3 a thick regressive shallow water clinoform late mis 3 4 a regressive to lowstand shore deposit latest mis 3 and 5 a local transgressive cover lgm to deglacial additionally 6 an open shelf highstand cover 7 an outer shelf highstand wedge and 8 mid shelf mud depocenters have formed during the holocene sea level highstand the common local offshore formation and preservation of confined stratigraphic units in particular from during mis 3 mark the interplay of a episodes of pronounced arid climatic conditions resulting in enhanced aeolian and coastal sediment input b shelf current patterns focusing sediment deposition locally and c early post depositional sediment stabilization providing protection against erosion prominent internal surfaces at 63 and 115 m modern water depths indicate widespread and intense erosional activity during late mis 3 regression and mis 2 lowstand to post lgm transgression hosting coarse shell sands and gravels from beach and shoreface paleo environments the reasons for the high preservation potential of confined stratigraphic units are a carbonaceous cementation b sediment composition massive widespread shore related gravel and shell beds with subtle minor admixtures of cohesive material and c morphological protection inside seafloor depressions and behind sheltering relief

the senegal river mud belt a high resolution archive of paleoclimatic change and coastal evolution abstract fine grained sediment depocenters on continental shelves are of increased scientific interest since they record environmental changes sensitively a north south elongated mud depocenter extends along the senegalese coast in mid shelf position shallow acoustic profiling was carried out to determine extent geometry and internal structures of this sedimentary body in addition four sediment cores were retrieved with the main aim to identify how paleoclimatic signals and coastal changes have controlled the formation of this mud depocenter a general paleoclimatic pattern in terms of fluvial input appears to be recorded in this depositional archive intervals characterized by high terrigenous input high sedimentation rates and fine grain sizes occur roughly contemporaneously in all cores and are interpreted as corresponding to intensified river discharge related to more humid conditions in the hinterland from 2750 to 1900 and from 1000 to 700 cal a bp wetter conditions are recorded off senegal an observation which is in accordance with other records from nw africa nevertheless the three employed proxies sedimentation rate grain size and elemental distribution do not always display consistent inter core patterns major differences between the individual core records are attributed to sediment remobilization which was linked to local hydrographic variations as well as reorganizations of the coastal system the senegal mud belt is a layered inhomogeneous sedimentary body deposited on an irregular erosive surface early holocene deceleration in the rate of the sea level rise could have enabled initial mud deposition on the shelf these favorable conditions for mud deposition occur coevally with a humid period over nw africa thus high river discharge sedimentation started preferentially in the northern areas of the mud belt during mid holocene a marine incursion led to the formation of an embayment afterwards sedimentation in the north was interrupted in association with a remarkable southward shift in the location of the active depocenter as it is reflected by the sedimentary architecture and confirmed by radiocarbon dates these sub recent shifts in depocenters location are caused by migrations of the senegal river mouth during late holocene times the weakening of river discharge allowed the longshore currents to build up a chain of beach barriers which have forced the river mouth to shift southwards

rapid rise of sea level 19 000 years ago and its global implications evidence from the irish sea basin supports the existence of an abrupt rise in sea level meltwater pulse at 19 000 years before the present b p climate records indicate a large reduction in the strength of north atlantic deep water formation and attendant cooling of the north atlantic at this time indicating a source of the meltwater pulse from one or more northern hemisphere ice sheets warming of the tropical atlantic and pacific oceans and the southern hemisphere also began at 19 000 years b p these responses identify mechanisms responsible for the propagation of deglacial climate signals to the southern hemisphere and tropics while maintaining a cold climate in the northern hemisphere

abrupt post glacial climate events in west asia and north africa monsoon domains regions beyond the present or past penetration of the indian and african monsoons have experienced several large and abrupt climatic fluctuations over the past 13 14c kyr pollen and lake records from west asia western tibet and rajasthan east africa ethiopia and west africa western sahara sahel and subequatorial africa were selected on the basis of chronological control sensitivity of both site and environmental indicators to climate change the continuity of the record and interdisciplinary control of the palaeoclimatic interpretation conditions wetter than those of today prevailed during the early mid holocene period but major dry spells are recorded at all sites during the intervals 11 0 9 5 kyr bp 8 7 kyr bp and 3 4 kyr bp several records also suggest dry events of minor amplitude around 6 kyr bp potential boundary forcings of insolation and sea surface and tropical land surface conditions are discussed the solar radiation accounts for the general envelop of the post glacial monsoon fluctuations but explains neither the timing nor the amplitude of the short term changes in spite of apparent covariation between fluctuations in sea surface conditions in the north atlantic and the monsoon record no direct mechanism could be found relating the intensity of the oceanic thermohaline conveyor belt to the monsoon strength changes in tropical land surface conditions soil moisture negative feedback and changes in ch4 production from wetlands provide a more satisfactory hypothesis for explaining abrupt reversal events

abrupt onset and termination of the african humid period rapid climate responses to gradual insolation forcing a detailed ca 100 yr resolution and well dated 18 ams 14 c dates to 23 cal ka bp record of latest pleistocene holocene variations in terrigenous eolian sediment deposition at odp site 658c off cap blanc mauritania documents very abrupt large scale changes in subtropical north african climate the terrigenous record exhibits a well defined period of low influx between 14 8 and 5 5 cal ka bp associated with the african humid period when the sahara was nearly completely vegetated and supported numerous perennial lakes an arid interval corresponding to the younger dryas chronozone punctuates this humid period the african humid period has been attributed to a strengthening of the african monsoon due to gradual orbital increases in summer season insolation however the onset and termination of this humid period were very abrupt occurring within decades to centuries both transitions occurred when summer season insolation crossed a nearly identical threshold value which was 4 2 greater than present these abrupt climate responses to gradual insolation forcing require strongly non linear feedback processes and current coupled climate model studies invoke vegetation and ocean temperature feedbacks as candidate mechanisms for the non linear climate sensitivity the african monsoon climate system is thus a low latitude corollary to the bi stable behavior of high latitude deep ocean thermohaline circulation which is similarly capable of rapid and large amplitude climate transitions

modeling and analysis of ageostrophic circulation over the azores oceanic front during the semaphore experiment abstract in the conventional quasigeostrophic qg form of the equation developed by hoskins et al the unique forcing of vertical velocity is the geostrophic deformation as the qg or even the semigeostrophic sg hypotheses are not adapted to study the frontal dynamics in the atmospheric boundary layer this paper proposes a generalized expression of the hoskins et al form of the vertical velocity two thermal and three dynamical sources of the vertical velocity are identified these forcings allow for identification of each of the physical processes acting simultaneously on the ageostrophic circulation in the boundary layer this new form of the equation is used to explain wind increase in the atmospheric boundary layer over the warm waters of the sea surface temperature sst front observed during a fair anticyclonic day of the semaphore experiment 1993 and simulated with a nonhydrostatic mesoscale atmospheric model since the sst gradients are weak of the order of 1 5 c 100 km 1 the surfac

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

modern heterozoan carbonates from a eutrophic tropical shelf mauritania abstract heterozoan or foramol production is typical in extratropical carbonate sedimentary systems however under mesotrophic to eutrophic conditions heterozoan carbonates also form in tropical settings but such heterozoan tropical sedimentary systems are poorly understood nevertheless distinction between tropical and extratropical heterozoan carbonates in ancient successions is crucial for accurate paleoenvironmental and paleoclimate reconstructions here surficial holocene and pleistocene sediments of the northern mauritanian shelf are studied as an example of a tropical eutrophic carbonate depositional system 11 g l 1 chl a chlorophyll a upwelling nutrient rich waters push onto the wide mauritanian shelf where they can warm up to in excess of 25 c this condition favors production of heterozoan carbonates dominated by bivalves and foraminifers even in this tropical setting in addition sediments are provided by eolian input from the desertic hinterland the resulting sediments are carbonate and mixed carbonate siliciclastic facies in which the carbonates are characterized by a mixture of tropical and cosmopolitan taxa benthic photosynthetic biota are absent while suspension feeding organisms are dominant this foramol grain association on a shelf scale is reminiscent of cool water carbonates therefore recognition of warm water heterozoan carbonates relies on key taxa related to tropical waters within the biota assemblages associated with a highly productive environment

storm driven bottom sediment transport on a high energy narrow shelf nw iberia and development of mud depocenters bottom sediment transport on the nw iberian shelf was monitored during a downwelling storm in september 2014 collected data was analyzed and fed into a 3d coastal ocean model to understand storm driven sediment transport on the shelf and its impact on mid shelf mud depocenters mdcs a significantly enhanced level of bottom sediment resuspension nearly two orders of magnitude higher than that in the pre storm period was recorded at the mooring site field data analysis reveals that it was induced by a short lasting strong bottom current in combination with enhanced wave current interaction simulation results indicate that this strong current was part of a coastal jet resulted from downwelling an across shelf horizontal density gradient as high as 0 32 g m4 occurred at the interface between the downwelling and the bottom waters forming a remarkable front due to buoyancy effect the downwelling water was mostly confined to the coast with a depth limit of 80 m in the south and 120 m in the north of the region resulting in a northward directed coastal jet simulation results suggest that during the storm local near bottom sediment suspensions with concentrations on the order of 10 kg m3 would be triggered by wave current interaction and flow convergence associated with the front direct impact on the development of mdcs by transport and deposition of concentrated sediment suspensions is indicated by model results the seaward limit of the front coincided with the shoreward edge of the mdc nucleus suggesting the front as a primary control on the deposition of fine grained sediment this article is protected by copyright all rights reserved

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

isopycnal eofs in the azores current region a statistical tool fordynamical analysis and data assimilation abstract an empirical orthogonal function analysis of the vertical space time variability of the thermohaline structure of the ocean in summer and fall 1993 in the azores current region has been performed the analysis used data from ctd casts of the semaphore intensive experiment referred to as a seasonally varying climatology the relative merits of depth coordinate and isopycnal coordinate representations have been examined in regard to the quality of identification of physical processes and to the effectiveness of the extrapolation from a sea level anomaly in an attempt to improve the treatment of the vertical dimension in oceanic assimilation problems the isopycnal eofs consistently proved more efficient in capturing the vertical structure of both dominant processes in the area of investigation the variability linked to the mediterranean water and the coherent physical system made up of the azores front and the azores current the isopycnal analysis also proved more robust when meddies mediterra

post glacial filling of a semi enclosed basin the arguin basin mauritania semi enclosed basins are not very common features in the world and are most frequently the result of tectonic movements studies of their filling are usually based on the micropaleontological analyses of sediment cores torgersen et al 1988 reeves et al 2007 or seismic analyses lykousis et al 2007 cagatay et al 2009 van daele et al 2011 the morphology of semi enclosed basins is generally simple and bowl shaped and their edges are marked by one or more sills their depths range from a few dozen to several thousand meters semi enclosed basins are however present in some regions in the world the semi enclosed basin of the golfe d arguin northwest africa is present on a wide shallow shelf bordering the sahara desert in a stable tectonic context its sedimentary filling took place during the end of the post glacial transgression the current knowledge on sedimentary filling of semi enclosed basins is rather limited and inadequate to fully understand the processes at play three campaigns have allowed the creation of the first morpho bathymetric map of the golfe d arguin shedding new light on its large scale morphology a series of rocky shoals banc d arguin interrupted by two sills in the north and south divides the shelf into two parts the inner and outer the inner part of the golfe d arguin called the arguin basin forms a shallow semi enclosed depression the basin is situated on the stable west african margin and the lack of any vertical tectonic movement provides an ideal situation for studying the progradation regression variations and the sediment depositional conditions caused by the post glacial sea level changes based on the analysis of very high resolution seismic data seven units were identified the sedimentary sequence of deposition of the arguin basin was interpreted in relation to the bedrock morphology the sea level rise and the climatic changes their chronology was established in comparison to the regional sea level curve basin physiography and unit distribution the arguin basin is interpreted as a land locked freshwater lake during the post glacial sea level rise corresponding to wet climatic conditions the inner part was flooded ca 8 7 ka bp when the sea level reached the sills the filling then corresponded to a marine estuarine environment the climatic aridification and the sea level stabilization from 6 5 ka bp onwards allowed the deposition of the last units which are composed of aeolian sand combined with a significant marine biogenic carbonate fraction bedrock morphology plays a major role in determining the depositional sequence architecture it controls the available accommodation space and in conjunction with climate changes influences the environmental processes that shape the deposit geometry

coastal evolution in senegal and mauritania at 103 102 and 101 year scales natural and human records abstract previous works have shown that the range of marine level eustatic changes on the coast of the senegalo mauritanian basin after 5500 bp must have been relatively small on the basis of regional geological mapping this position is confirmed in two littoral regions in mauritania baie de saint jean cape timiris and in senegal saloum delta the evolution of the shoreline is linked mainly to the balance between deposition and reworking by the sea in mauritania the reworking capacity is high before 4000 bp under a more humid climate than the present one it becomes insufficient around 3000 bp as areas of aeolo marine progradation develop in the sebkhas at the inner part and the shorelines of the baie de saint jean the deposits are subjected to a constant sea level and are therefore formed by the rhythm of recurrent aridity crises on the millenary or the centenary scale in the saloum delta the evolution of the shoreline results from several processes involving more varied timescales an essential factor is the positioning of littoral sand barriers shaped like long sandy spits which divert the flow of the river towards the south and are arranged according to a basal surface also adjusted to a constant sea level the development of sand barriers such as the present sangomar spit and also the langue de barbarie occurs at a centennial rhythm higher probability events at the decade scale are likely to perturb this development by inducing gaps in the sand barrier and as the river goes beyond the sand barrier the latter becomes incorporated in the deltaic plain forcing the littoral drift to build a new sand barrier further forward these events are linked to a conjunction of factors involving different timescales aridity crises at the 10 3 or 10 2 year scales sedimentation waves at the 10 year scale seasonal exceptional swells river floods variations of the tidal range determined by modifications in the estuarian morphology

deciphering signals of late holocene fluvial and aeolian supply from a shelf sediment depocentre off senegal north west africa high resolution climatic records of the late holocene along the north west african continental margin are scarce here we combine sediment grain size elemental distribution and mineral assemblage data to trace dust and riverine sources at a shallow marine sediment depocentre in the vicinity of the senegal river mouth the aim is to understand how these terrigenous components reflect climate variability during the late holocene major element contents were measured and mineral identification was performed on three sub fractions of our sediment core i fluvial material 2 m ii aeolian material of 18 63 m and iii a sub fraction of dual origin material of 2 18 m results show that more than 80 of the total al and fe terrigenous bulk content is present in the fluviogenic fraction in contrast ti k and si cannot be considered as proxies for one specific source off senegal the al ca ratio recording the continental river runoff reveals two dry periods from 3010 to 2750 cal a bp and from 1900 to 1000 cal a bp and two main humid periods from 2750 to 1900 cal a bp and from 1000 to 700 cal a bp the match between i intervals of low river runoff inferred by low al ca values ii reduced river discharge inferred by integrated palynological data from offshore senegal and iii periods of enhanced dune reactivation in mali confirms this interpretation copyright 2011 john wiley sons ltd

fluvial system dynamics of sudano sahelian zone during the late holocene the yame river dogon country mali abstract in west africa most palaeoenvironmental studies have focused on the role of climatic change on the environment because of the richness of its deposits the yame river mali a tributary of the niger river offers an opportunity to reconstruct the evolution of this fluvial system in comparison with both climatic and anthropogenic changes during the late holocene to investigate more closely the spatio temporal response of the fluvial system sedimentary analyses were conducted on seven reaches distributed along the 137 km length of the yame valley this approach testifies of the possibility for each time slice and river reach 1 to reconstruct the fluvial style and processes and 2 to estimate the sediment storage volume reflecting sediment distribution patterns results reveal a wide variability of the sedimentary cascade suggesting contrasting responses to external and local controls humid 4200 2900 cal bp 450 24 cal bp or arid 2350 1700 cal bp phases have been recorded in fluvial archives while an intensification in erosion and sediment supplies even during the arid period 2900 2350 cal bp and the 20th century may be associated with an increase in human pressure two other periods are related to both climate and anthropogenic factors during 1700 1400 cal bp phase huge sediment supplies can be explained mainly by the reactivation of both hydrological processes and human occupation after an intense and long arid event dispersed sediment sinks recorded during the 450 24 cal bp period originate from the combination of intensification of colluvial processes and a wet phase context originating from global climatic change thus this study provides evidence that climate is the strongest driver for the fluvial response of these semi arid and tropical rivers while human disturbance appear as a secondary factor due to the high sensitivity of the environment to climate variability in such areas

a 1 year mesoscale simulation of the northeast atlantic mixed layer heat and mass budgets during the pomme experiment 1 a simplified oceanic model is used to perform a realistic yearlong simulation to study fine scale mixed layer processes in an area of the northeast atlantic 21 15 w 38 45 n during the programme ocean multidisciplinaire meso echelle pomme experiment september 2000 october 2001 this simplified three dimensional oceanic model is obtained by degenerating the primitive equations system by prescribing continuously analysis derived geostrophic currents ug called guide into the momentum equation by the substitution of the horizontal pressure gradient the realism of the simulated mesoscale structures was validated by a comparison with in situ mesoscale and submesoscale data and results from the geostrophic adjustment to the guide with this adjustment being applied at all scales this model was able to simulate energetic small horizontal scales taking place around stirring eddies and frontal regions moreover filamentary mixed layer depth structures were simulated and correlated to intense vertical velocities confined in the mixed layer the circulation structure during pomme consisted of three quasi permanent mesoscale eddies and a front the mixed layer heat and mass budgets were thus strongly driven by the horizontal advection and the vertical transport associated with this mesoscale circulation although no effective detrainment or entrainment occurred at the domain scale three regions were found to be the site of intense detrainment around anticyclonic and cyclonic mesoscale eddies and along an axis of strong currents in which the lateral induction and the vertical velocity at the mixed layer base are the major components driving the detrainment respectively

the upwelling area off northwest africa a description of phenomena related to coastal upwelling abstract this paper presents a brief review of the oceanography along the upwelling coast of northwest africa the description contains information on seasonal variations of large scale winds ocean circulation and some gross of features of the hydrography characteristic meso scale patterns of currents during the upwelling season between 16 n and 26 n air and water temperature differences during upwelling season diurnal and semi diurnal variations of temperature atmospheric pressure wind and current regional peculiarities canyons and the banc d arguin

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 simplified 3d oceanic model assimilating geostrophic currents application to the pomme experiment abstract a simplified oceanic model is developed to easily perform cheap and realistic mesoscale simulations on an annual scale this simplified three dimensional oceanic model is obtained by degenerating the primitive equations system by prescribing continuously analysis derived geostrophic currents ug into the momentum equation in substitution of the horizontal pressure gradient simplification is provided by a time sequence of ug called guide which is used as a low resolution and low frequency interpolator this model is thus necessarily coupled to systems providing geostrophic currents that is ocean circulation models analyzed reanalyzed fields or climatologies in this model the mass and currents fields are constrained to adjust to the geostrophic guide at all scales the vertical velocity is deduced from the vorticity equation which ensures the coherence between the vertical motion and the geostrophic structures evolution horizontal and vertical advection are the coupling processes that can b

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

simulated climate and biomes of africa during the late quaternary comparison with pollen and lake status data new compilations of african pollen and lake data are compared with climate ccm1 ncar boulder and vegetation biome 1 2 gsg lund simulations for the last glacial maximum lgm and early to mid holocene emh the simulated lgm climate was ca 4 c colder and drier than present with maximum reduction in precipitation in semi arid regions biome simulations show lowering of montane vegetation belts and expansion of southern xerophytic associations but no change in the distribution of deserts and tropical rain forests the lakes show lgm conditions similar or drier than present throughout northern and tropical africa pollen data indicate lowering of montane vegetation belts the stability of the sahara and a reduction of rain forest the paleoenvironmental data are consistent with the simulated changes in temperature and moisture budgets although they suggest the climate model underestimates equatorial aridity emh simulations show temperatures slightly less than present and increased monsoonal precipitation in the eastern sahara and east africa biome simulations show an upward shift of montane vegetation belts fragmentation of xerophytic vegetation in southern africa and a major northward shift of the southern margin of the eastern sahara the lakes indicate conditions wetter than present across northern africa pollen data show an upward shift of the montane forests the northward shift of the southern margin of the sahara and a major extension of tropical rain forest the lake and pollen data confirm monsoon expansion in eastern africa but the climate model fails to simulate the wet conditions in western africa

late holocene morphodynamics in the littoral zone of the iwik peninsula area banc d arguin mauritania abstract in the littoral zone of the banc d arguin both littoral sedimentary units and man made deposits shell middens abound they were formed during the late holocene after the final onset of the post glacial transgression here a geo archaeological approach is used to study them the geomorphological and sedimentological characteristics of the coastal features define two distinct coastal sedimentary units in the iwik aouatil zone between an ancient shoreline which formed on the tafaritian substratum ca 6 7 5 7 cal ka bp and the present one i very extensive sand flats and ii linear relief features both covered by a thin veneer of anthropogenic anadara senilis shell middens they developed under a stable sea level regime the sand flats expanded at variable rates throughout the period the linear relief features represent beach barriers and coastal dunes which can be dated archaeologically thanks to the superimposed shell middens supplemented by 14 c dating two series of linear sedimentary units are identified first a complex north south system of dunes and hook ended sand barriers developed from 4 8 to 4 1 cal ka bp and secondly two regular and linear beach barriers developed between 3 7 and 3 3 cal ka bp their morphological characteristics suggest that they were generated through littoral drift processes directional changes observed between the first and the second groups demonstrate that the wave refraction pattern was dramatically modified after the closure of the strait between the former iwik island and the land a comparison with the nearby jerf el oustani coastal area in the late holocene shows that both sites shared similarities in depositional patterns but also differences in the chronology of the sedimentary units processes involved in the development of the linear units are indicative of the enhancement of both sand influx and longshore drift due to climatic change

mid to late quaternary environments and stratigraphy of the southern sierra leone shelf west africa the middle to upper quaternary succession of the southern sierra leone shelf appears to have been deposited in response to three glacial sea level cycles the last of which has left the most complete sedimentary record the unconformable base of the succession is deeply incised off larger rivers inshore but has little relief toward the shelf break this difference results from the steep gradient of the inner shelf and the gentle gradient of the valley9s long profile incision was caused by high fluvial discharge during sea level fall decline of discharge and sediment supply during and after glacial maximum resulted in cessation of incision and in the formation of cemented beach ridges at the shelf break the lowstand wedge of the last glacial cycle is well developed beyond the shelf break and where the pre existing shelf was deep where sampled it is deposited below wave base its preservation is apparently the result of protection from wave and shelf current action during regression the subsequent transgressive wedge entirely dominates shelf deposition infilling incised valleys and contains vertically stacked fluvial estuarine littoral and sub littoral elements it is thin or absent beyond the shelf as a result of active oceanic currents hindering sedimentation active oceanic currents also result in only weak development of the highstand wedge energy regime as represented by moderate shelf currents and variable fluvial discharge has been the fundamental control on sediment architecture sediment supply has been of only secondary importance

calibration of the c 14 timescale over the past 30 000 years using mass spectrometric u th ages from barbados corals uranium thorium ages obtained by mass spectrometry from corals raised off the island of barbados confirm the high precision of this technique over at least the past 30 000 years comparison of the u th ages with 14c ages obtained on the holocene samples shows that the u th ages are accurate because they accord with the dendrochronological calibration before 9 000 yr bp the 14c ages are systematically younger than the u th ages with a maximum difference of 3 500 yr at 20 000 yrbp the u th technique thus provides a way of calibrating the radiocarbon timescale beyond the range of dendrochronological calibration

a high resolution simulation of the ocean during the pomme experiment mesoscale variability and near surface processes 1 this article is the second part of a modeling study of upper ocean mesoscale physical processes in an area of the northeast atlantic that was extensively sampled during the programme ocean multidisciplinaire meso echelle pomme experiment october 2000 september 2001 the model is a high resolution regional version of the ocean general circulation model opa integrated from february to may 2001 the mesoscale upper ocean dynamics has been carefully validated in a first paper paci et al 2005 in the present article the simulation is used to analyze the contribution of mesoscale eddies to the mixed layer processes during the period that controls the annual subduction rate and the characteristics of the subducted water mesoscale eddies account for a significant part of the mixed layer heat salt and water budgets through mesoscale and submesoscale structures acting on the horizontal advective terms they represent respectively 52 and 66 of the horizontal advection of temperature and salinity and 70 of the lateral induction they also induce an earlier and more intense restratification the domain averaged detrainment appears to be reduced by more than 15 by mesoscale eddies and reaches about 0 8 sv however these eddies increase the density of the detrained water some dense water likely to be involved in the formation of subpolar mode water is detrained by them in a way that maximizes its chance to be effectively subducted in the area

stratigraphic paleoecology of the valle di manche section crotone basin italy a candidate gssp of the middle pleistocene abstract macro benthic assemblages from the lower to middle pleistocene of the valle di manche vdm section crotone basin southern italy were used to assess the value of merging paleoecological quantitative patterns into a sequence stratigraphic framework two way cluster and detrended correspondence dca analyses were applied to the macrofossil data matrix comprising 22 samples 157 species and 5761 specimens specifically two way cluster analysis allowed for the characterization of five shelf related biofacies and four sub biofacies along section biofacies change along the vdm generally supported previous sequence stratigraphic interpretations and suggests that faunal turnover is the result of habitat tracking dca stratigraphic curves dc1 sample scores calibrated to the depth preferences of extant species provided quantitative estimates of bathymetry and the assessment of related parameters e g grain size sedimentation turbidity when plotted stratigraphically calibrated dc1 scores highlight the magnitude of relative sea level fluctuations at the scale of the section as well as individual sequences and systems tracts additionally dca derived paleobathymetric trends track the oscillations of the 18 o record indicative of global sea level change based on this comparison it is held that macro benthic assemblages once supported by a robust statistical interpretation can be employed successfully as a proxy for 18 o depicting the variability in local water depths the potential of this approach is especially high for studies in shallow water depositional settings where dependable 18 o data are typically difficult to obtain

delta development and channel incision during marine isotope stages 3 and 2 in the western south yellow sea abstract approximately 4100 km of high resolution seismic profiles recently obtained from the western south yellow sea were subdivided into seven seismic units by six major seismic surfaces three sediment cores between 60 and 71 m in length obtained from localities on the seismic profile lines were analyzed for sedimentary characteristics clay mineral components and benthic foraminiferal and ostracod assemblages and were also dated by ams 14 c and optically stimulated luminescence to document the environmental history of the region stratigraphic units in the cores were confidently correlated with seismic units in the profiles stratigraphic correlation among these three cores and the previous well studied core qc 2 was facilitated by the cold water deposits in marine isotope stage mis 5 the three cores document the sedimentary sequence during the period from mis 6 up to mis 1 strata formation and sedimentary environment changes since mis 5 are strongly controlled by sea level fluctuations most of the successions were formed in mis 5 mis 3 and mis 1 with major erosions in mis 4 and mis 2 particularly our data revealed the presence of a large delta active during mis 3 likely from the early stage of mis 3 to about 40 cal kyr bp that was subsequently affected by channel incision the delta succession and incised channel fills have a high concentration of smectite in their clay mineral components suggesting that the paleo yellow river was the main contributor to the delta development and channel incision the delta sequence has a coarsening upward trend and consists of two seismic facies the upper facies shows lateral alternations of chaotic and oblique or very low angle reflectors which correspond to sandy proximal delta front and delta plain facies and the lower facies is characterized by seaward dipping clinoforms which correspond to muddy distal delta front and prodelta facies the top surface of the delta inclines very gently seaward consistent with a deltaic development associated with the sea level fall in mis 3 the incised channel system consists of two main channels with multiple tributaries in a dendritic pattern the incised channel fills begin with fluvial and then estuarine sediments which are truncated by a transgressive ravinement surface and capped by transgressive deposits the main incision channels have a terraced cross sectional morphology which is interpreted to reflect multiple episodes of sea level fall during incision presumably from the late mis 3 to the last glacial maximum

a three dimensional ocean mesoscale simulation using data from the semaphore experiment mixed layer heat budget a primitive equation model is used to simulate the mesoscale circulation associated with a portion of the azores front investigated during the intensive observation period iop of the structure des echanges mer atmosphere proprietes des heterogeneites oceaniques recherche experimentale semaphore experiment in fall 1993 the model is a mesoscale version of the ocean general circulation model ogcm developed at the laboratoire d oceanographie dynamique et de climatologie lodyc in paris and includes open lateral boundaries a 1 5 level order turbulence closure scheme and fine mesh resolution 0 11 for latitude and 0 09 for longitude the atmospheric forcing is provided by satellite data for the solar and infrared fluxes and by analyzed or reanalyzed for the wind atmospheric data from the european centre for medium range weather forecasts ecmwf forecast model the extended data set collected during the iop of semaphore enables a detailed initialization of the model a coupling with the rest of the basin through time dependent open boundaries and a model data comparison for validation the analysis of model outputs indicates that most features are in good agreement with independent available observations the surface front evolution is subject to an intense deformation different from that of the deep front system which evolves only weakly an estimate of the upper layer heat budget is performed during the 22 days of the integration of the model each term of this budget is analyzed according to various atmospheric events that occurred during the experiment such as the passage of a strong storm this facilitates extended estimates of mixed layer or relevant surface processes beyond those which are obtainable directly from observations surface fluxes represent 54 of the heat loss in the mixed layer and 70 in the top 100 m layer while vertical transport at the mixed layer bottom accounts for 31 and three dimensional processes account for 14

late quaternary environmental change in oujiang delta along the northeastern zhe min uplift zone southeast china abstract we present a history of late quaternary coastal evolution based on the study of a 41 9 m long sediment core from the oujiang delta multiple analytical methods are employed including sedimentary facies analysis radiocarbon dating and the investigations of grain size elemental microfossil and pollen compositions two transgression regression sequences separated by a paleosol layer are identified several lines of evidence indicate that the lower sequence was deposited during the mid early mis 3 stage under a relatively warm and wet climate an elevated mis 3 highstand sea level higher than 24 m is observed the upper sequence was created after the paleosol was flooded during a marine transgression at 9 5 cal kyr bp seven weak east asian summer monsoon easm events are identified with central years of 8 4 7 7 6 3 5 7 4 7 2 8 and 1 8 cal kyr bp these coincide with those from the ecs sediment cores and the dongge cave 18o record indicating that easm precipitation is a major controlling factor of sedimentation rates in the region the oujiang delta is also subject to typhoon impacts and east asian winter monsoon eawm induced coastal currents active typhoons and a relatively enhanced eawm occurred during the weak easm period between 4 9 and 6 4 cal kyr bp producing high sedimentation rates in the middle holocene we emphasize the high sensitivity of small asian river deltas as archives of the easm eawm and typhoons such environments merit further as a means to understand past changes as well as future responses to projected climatic and sea level change

holocene evolution of mud depocentres on a high energy low accumulation shelf nw iberia the high energy low accumulation nw iberian shelf features three confined holocene mud depocentres here we show that the evolution of such depocentres follows successive steps the flooding of inner shelf zones and river catchment areas by the late deglacial sea level rise provided the precondition for shelf mud deposition following this the holocene deceleration of the sea level rise caused a rapid refill of the accommodation space within river valleys subsequently the export of major amounts of fines was initiated the initial onset and loci of shelf mud deposition were related to deposition favouring conditions in mid shelf position or to the presence of morphological highs which act as sediment traps by providing protection against stronger hydrodynamic energy the detailed reconstruction of the holocene depocentre evolution shows for the first time that the expansion of such shelf mud deposits cannot only occur by linear growth off the associated sediment source rather they might develop around centres that are fully disconnected from the source of original sediment supply and expand later into specific directions based on these differences and on the connection of the individual mud depocentres to the material source we propose a conceptual subdivision of the group mid shelf mud depocentres

provenance of present day eolian dust collected off nw africa 1 atmospheric dust samples collected along a transect off the west african coast have been investigated for their physical grain size distribution mineralogical and chemical major elements composition on the basis of these data the samples were grouped into sets of samples that most likely originated from the same source area in addition shipboard collected atmospheric meteorological data modeled 4 day back trajectories for each sampling day and location and total ozone mapping spectrometer aerosol index data for the time period of dust collection february march 1998 were combined and used to reconstruct the sources of the groups of dust samples on the basis of these data we were able to determine the provenance of the various dust samples it appears that the bulk of the wind blown sediments that are deposited in the proximal equatorial atlantic ocean are transported in the lower level 900 hpa ne trade wind layer which is a very dominant feature north of the intertropical convergence zone itcz however south of the surface expression of the itcz down to 5 s where surface winds are southwesterly we still collected sediments that originated from the north and east carried there by the ne trade wind layer as well as by easterly winds from higher altitudes the fact that the size of the wind blown dust depends not only on the wind strength of the transporting agent but also on the distance to the source hampers a direct comparison of the dust s size distributions and measured wind strengths however a comparison between eolian dust and terrigenous sediments collected in three submarine sediment traps off the west coast of nw africa shows that knowledge of the composition of eolian dust is a prerequisite for the interpretation of paleorecords obtained from sediment cores in the equatorial atlantic

geometry lateral variation and preservation of downlapping regressive shelf deposits eastern tyrrhenian sea margin italy abstract the shelf of the eastern tyrrhenian sea margin changes substantially in width shelf break depth and sea floor steepness over relatively short distances largely owing to pronounced lateral changes in subsurface geologic structure remnants of late pleistocene prograded coastal deposits are locally preserved on the middle and outer parts of the shelf we recognize two major controls on the distribution lateral extent thickness and preservation potential of these prograded deposits first prograded downlapping deposits formed only where the physiographic shelf break was deeper than the lowstand shoreline thus providing accommodation space for the lowstand deposits second the proximity and relative size of sediment sources and the local coastal dispersal system apparently infl enced the geometry of the deposit midshelf and shelf margin bodies composed of seaward steepening downlapping reflectors were deposited as thin to thick continuous prograding sheets over an irregular eroded shelf surface and onto the shelf edge during the latest fall and lowstand of sea level decreased sedimentation at the end of this lowstand resulted in net erosion during sea level rise shoreface erosion created a major marine erosional ravinement surface landward of the 120 m isobath and much of the downlapping deposits were removed preservation of these downlapping deposits is largely a function of their thickness thick continuous deposits are common on the shelf edge whereas on the midshelf only thin remnants are preserved locally where depressions or morphologic steps were present in the shelf surface

yardang evolution from maturity to demise yardangs are enigmatic wind parallel ridges sculpted by aeolian processes that are found extensively in arid environments on earth and mars no general theory exists to explain the long term evolution of yardangs curtailing modeling of landscape evolution and dynamics of suspended sediment release we present a hypothesis of yardang evolution using relative rates of sediment flux interyardang corridor downcutting yardang denudation substrate erodibility and substrate clast content to develop and sustain yardangs corridor downcutting must exceed yardang vertical denudation and deflation however erosion of substrate yields considerable quantities of sediment that shelters corridors slowing downcutting we model the evolution of yardangs through various combinations of rates and substrate compositions demonstrating the life span suspended sediment release and resulting landscape evolution we find that yardangs have a distinct and predictable evolution with inevitable demise and unexpectedly dynamic and autogenic erosion rates driven by subtle differences in substrate clast composition

shallow pycnoclines and mode water subduction in the eastern north atlantic abstract a lagrangian one dimensional model is used to study the subduction of subpolar mode water in the eastern north atlantic and to analyze recently observed hydrological features related to this process considering a southward moving column of mode water subduction occurs when the upper part of the column starts gaining buoyancy in an annual budget the seasonal pycnocline on top of the column can no longer be completely eroded in winter and it is shown how its remnant forms a shallow secondary pycnocline at about 200 m depth which isolates the lower part of the fluid column from the atmosphere this mechanism for the subduction of subpolar mode water induces a strong meridional gradient in the winter mixed layer depth the various components of the buoyancy input are thoroughly studied horizontal advection in the shallow ekman layer and vertical advection along the column are shown to modify significantly both the thermal and haline contents of the column with magnitudes comparable to the ai

continuous assimilation in an open domain of the northeast atlantic 1 methodology and application to athena 88 a scheme to perform suboptimal intermittent assimilation in an open ocean quasi geostrophic model is presented and applied to the assimilation of altimeter data in a domain of the northeast atlantic west of ireland both simulated and real altimeter data are used three dimensional 3 d synoptic observation fields and error variance fields are derived from synoptic dynamic topography anomaly estimates using an empirical orthogonal mode eof vertical extension technique the 3 d estimates are suboptimally combined with the time dependent part of the forecast fields suboptimality here means that the combination does not directly use spatial correlations of errors however the 3 d synoptic observation estimates used in the combination do contain error spatial statistical information the resulting fields added to a mean model climatology are used as initial conditions for the model which is integrated until a new dynamic topography anomaly estimate is available locally the combination is optimal in the sense that it is based on the observational error variances which reflect the measurement noise and the space time distribution of data and that it minimizes the error variance of the results a similar combination scheme using past as well as future observations is used to update the boundaries during model integration simulated surface topography anomaly maps typical of the northeast atlantic are assimilated every 20 days for a 300 day period with different noise characteristics typical of altimetric sampling errors in a three level version of the model the assimilation fields especially the vorticity at deeper levels converge toward the reference fields the convergence is obtained after o 100 days regardless of the observational noise levels tested up to 2 db using a relevant observational error model seems to matter in particular the error level should not be underestimated in the case of an 80 day gap in the data inflow the model predictability limits the reliability of the forecast beyond 20 days the scheme makes the model converge back as soon as new observations are entered with limited convergence loss due to the gap the case of an unknown model climatology is also studied finally the scheme is applied to geosat altimeter data added to the robinson bauer and schroeder annual climatology in the athena 88 cruise area which is contained in the model box the assimilation results compare favorably with hydrographical data from the cruise and help the synoptic interpretation of the observed phenomena

the giant mauritanian cold water coral mound province oxygen control on coral mound formation abstract the largest coherent cold water coral cwc mound province in the atlantic ocean exists along the mauritanian margin where up to 100 m high mounds extend over a distance of 400 km arranged in two slope parallel chains in 400 550 m water depth additionally cwcs are present in the numerous submarine canyons with isolated coral mounds being developed on some canyon flanks seventy seven uranium series coral ages were assessed to elucidate the timing of cwc colonisation and coral mound development along the mauritanian margin for the last 120 000 years our results show that cwcs were present on the mounds during the last interglacial though in low numbers corresponding to coral mound aggradation rates of 16 cm kyr 1 most prolific periods for cwc growth are identified for the last glacial and deglaciation resulting in enhanced mound aggradation 1000 cm kyr 1 before mound formation stagnated along the entire margin with the onset of the holocene until today the mauritanian mounds are in a dormant state with only scarce cwc growth in the canyons live cwcs are abundant since the late holocene at least thus the canyons may serve as a refuge to cwcs potentially enabling the observed modest re colonisation pulse on the mounds along the open slope the timing and rate of the pre holocene coral mound aggradation and the cessation of mound formation varied between the individual mounds which was likely the consequence of vertical lateral changes in water mass structure that placed the mounds near or out of oxygen depleted waters respectively