the paleoclimatic record provided by eolian deposition in the deep sea the geologic history of wind the mineral component of pelagic sediment is brought to the deep sea by transport in the wind extraction and analysis of this dust allows estimation of the past aridity of the eolian source region via flux determinations and of the intensity of the transporting winds from grain size data these two parameters the grain size and mass flux of dust vary independently there are three significant sources of modern dust eastern and central asia northwest africa and arabia all in the northern hemisphere as the rainfall associated with the intertropical convergence zone is an effective barrier to southerly transport of dust the northern hemisphere is an order of magnitude more dusty than is the southern an asymmetry that has characterized most of the cenozoic eolian flux data show that most of the northern hemisphere was more arid during glacial maxima with 3 to 5 times as much dust transported during glacial stages than during interglacials only northwestern south america varied in the opposite sense the periodicity of quaternary variation in both eolian flux and eolian grain size data is strongly influenced by the milankovitch cycles of orbital variability wind intensities vary on a shorter time scale than the general 100 kyr cycles of glaciation and general aridity eolian grain sizes display forcing at precessional 19 and 23 kyr tilt 41 kyr and at approximately 30 kyr periodicities as a result the generalization that winds are uniformly stronger during glacial times is not valid whole cenozoic records show that the largest change in dust flux an order of magnitude increase occurred in the northern hemisphere and reflects continental drying associated with the late pliocene onset of northern hemisphere glaciation southern hemisphere eolian records show no sign of paleoclimatic changes in the late pliocene the most important change in cenozoic atmospheric circulation was a severalfold reduction in wind intensity that occurred at the time of the paleocene eocene boundary before then latest cretaceous and paleocene winds were essentially as strong as those of the late cenozoic this shift appears to be one of several climatic responses to a change in the nature and amount of global heat transport at about 55 ma

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

quantification of foraminifer and coccolith carbonate in south atlantic surface sediments by means of carbonate grain size distributions this study presents a differentiated carbonate budget for marine surface sediments from the mid atlantic ridge of the south atlantic with results based on carbonate grain size composition upon separation into sand silt and clay sub fractions the silt grain size dis tribution was measured using a sedigraph 5100 we found regionally characteristic grain size distributions with an overall minimum at 8 mm equivalent spherical diameter esd sem observations reveal that the coarse particles 8 mm esd are attributed to planktic for aminifers and their fragments and the fine particles 8 mm esd to coccoliths on the basis of this division the regional variation of the contribution of foraminifers and coccoliths to the carbonate budget of the sediments are calculated foraminifer carbonate dominates the sed iments in mesotropic regions whereas coccoliths contribute most car bonate in oligotrophic regions the grain size of the coccolith share is constant over water depth indicating a lower susceptibility for car bonate dissolution compared to foraminifers finally the characteristic grain size distribution in fine silt 8 mm esd is set into context with the coccolith assemblage counted and biometrically measured using a sem the coccoliths present in the silt fraction are predominantly large species length 4 mm smaller species length 4 mm belong to the clay fraction 2 mm esd the average length of most frequent coccolith species is connected to prominent peaks in grain size distri butions esd with a shape factor the area below gaussian distri butions fitted to these peaks is suggested as a way to quantitatively estimate the carbonate contribution of single coccolith species more precisely compared to conventional volume estimates the quantitative division of carbonate into the fraction produced by coccoliths and that secreted by foraminifers enables a more precise estimate for source sink relations of consumed and released co 2 in the carbon cycle the allocation of coccolith length and grain size esd suggests size windows for the separation or accumulation of distinct coccolith species in investigations that depend on non to slightly mixed signals e g isotopic studies

mediterranean moisture source for an early holocene humid period in the northern red sea paleosalinity and terrigenous sediment input changes reconstructed on two sediment cores from the northernmost red sea were used to infer hydrological changes at the southern margin of the mediterranean climate zone during the holocene between approximately 9 25 and 7 25 thousand years ago about 3 per thousand reduced surface water salinities and enhanced fluvial sediment input suggest substantially higher rainfall and freshwater runoff which thereafter decreased to modern values the northern red sea humid interval is best explained by enhancement and southward extension of rainfall from mediterranean sources possibly involving strengthened early holocene arctic oscillation patterns and a regional monsoon type circulation induced by increased land sea temperature contrasts we conclude that afro asian monsoonal rains did not cross the subtropical desert zone during the early to mid holocene

bottom temperatures in drake passage abstractlong time series of bottom temperatures in the southern ocean are rare the cdrake array with over 40 current and pressure recording inverted echo sounders moored across drake passage to monitor the antarctic circumpolar current acc variability and transport measured temperature at 1 and 50 m above the seafloor at depths 3500 m and at the southern continental margin the 4 yr dataset provided an opportunity to examine the temporal and spatial scales of bottom temperature variability high variability was observed ranges were 0 5 0 9 c in the northern passage and 0 3 0 6 c in the southern passage standard deviations in the two regions were 0 1 0 15 c and 0 05 c respectively meandering of the acc with its deep reaching thermocline accounted for up to 50 of the observed bottom temperature variance northern passage temperatures spaced less than 40 km apart were correlated with each other while those in the southern portion separated by 60 70 km were not a gap in the west scotia

eolian sediment responses to late quaternary climate changes temporal and spatial patterns in the sahara abstract this paper presents a compilation of eolian based records of late quaternary climate changes in the sahara although the data are relatively sparse when viewed as a whole they reveal a general pattern of widespread eolian sediment mobilization prior to 11 000 cal years bp eolian sediment stabilization from 11 000 to 5000 cal years bp and a return to widespread eolian sediment mobilization after 5000 cal years bp furthermore an eolian based record from southern tunisia reveals the existence of millennial scale changes in eolian sediment behavior these millennial scale variations provide examples of eolian sediment responses to climate changes at a scale intermediate between seasonal and orbital milankovitch changes and they are also coincident with abrupt atmospheric and oceanic changes the general synchroneity of the eolian stratigraphic records and their coincidence with various oceanic and atmospheric changes suggest that global forcing mechanisms have influenced late quaternary eolian sediment behavior in the sahara

antifreeze proteins in pelagic fishes from marguerite bay western antarctica abstract the southern ocean is home to two major types of fishes the largely endemic suborder notothenioidei and representatives of oceanic fish families that are widely distributed in the midwater and benthic environments elsewhere e g bathylagids myctophids liparids and zoarcids in most regions of the coastal antarctic e g the ross sea there is a distinct separation in the pelagic communities at the shelf break between the oceanics off shelf and the endemics on shelf coincidentally in much of the coastal antarctic the shelf break also marks the boundary between a water column entirely composed of the very cold 2 c ice shelf water and an oceanic profile that includes warmer circumpolar deep water 2 c at 200 m at intermediate depths the distinct separation in pelagic communities observed in most coastal regions of the antarctic is not seen on the western antarctic peninsula wap where circumpolar deep water intrudes to form a warmer midwater and oceanic species are strongly represented it was hypothesized that the cold ice shelf water lethal to fishes without antifreeze glycoproteins afgps in their blood was excluding the oceanic species from most of the antarctic continental shelf waters to test the hypothesis nine species of fish captured in wap shelf waters were tested for the presence of afgps the oceanic fish families analyzed myctophidae electrona and gymnoscopelus zoarcidae melanostigma gempylidae paradiplospinus paralepididae notolepis and bathylagidae bathylagus showed no antifreeze activity two endemic species captured in the same sampling program did show antifreeze activity the important pelagic species pleuragramma antarcticum nototheniidae and the bathydraconid species vomeridens infuscipinnus the absence of afgps in the blood of antarctic oceanic species makes a strong case for temperature exclusion of oceanic fishes in the coastal antarctic

modelling circumpolar deep water intrusions on the amundsen sea continental shelf antarctica results are presented from an isopycnic coordinate model of ocean circulation in the amundsen sea focusing on the delivery of circumpolar deep water cdw to the inner continental shelf around pine island bay the warmest waters to reach this region are channeled through a submarine trough accessed via bathymetric irregularities along the shelf break temporal variability in the influx of cdw is related to regional wind forcing easterly winds over the shelf edge change to westerlies when the amundsen sea low migrates west and south in winter spring this drives seasonal on shelf flow while inter annual changes in the wind forcing lead to inflow variability on a decadal timescale a modelled period of warming following low cdw influx in the late 1980 s and early 1990 s coincides with a period of observed thinning and acceleration of pine island glacier

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

age models for pelagites and turbidites from the cap timiris canyon off mauritania abstract this study of sediments from the cap timiris canyon demonstrates that geochemical data can provide reliable age depth correlation even of highly turbiditic cores and attempts to improve our understanding of how turbidite emplacement is linked to climatic related sea level changes the canyon incises the continental margin off nw africa and is an active conduit for turbidity currents in sediment cores from levee and intrachannel sites turbidites make up 6 42 of sediment columns age models were fitted to all studied cores by correlating downcore element data to dated reference cores once turbidite beds had been removed from the dataset these age models enabled us to determine turbidite emplacement times the cap timiris canyon has been active at least over the last 245 kyr with turbidite deposition seemingly linked to stage boundaries and glacial stages the highly turbiditic core from the intrachannel site postdates to 15 kyr and comprises holocene and late pleistocene sediments turbidite deposition at this site was associated especially with the rapid sea level rise at the pleistocene holocene transition during the holocene turbidity current activity decreased but did not cease

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

modification of deep waters in marguerite bay western antarctic peninsula caused by topographic overflows circumpolar deep water cdw intrudes from the mid layers of the antarctic circumpolar current onto the shelf of the western antarctic peninsula providing a source of heat and nutrients to the regional ocean it is well known that cdw is modified as it flows across the shelf but the mechanisms responsible for this are not fully known here data from underwater gliders with high spatial resolution are used to demonstrate the importance of detailed bathymetry in inducing multiple local mixing events clear evidence for overflows is observed in the glider data as water flows along a deep channel with multiple transverse ridges the ridges block the densest waters with overflowing water descending several hundred metres to fill subsequent basins this vertical flow leads to entrainment of overlying colder and fresher water in localised mixing events initially this process leads to an increase in bottom temperatures due to the temperature maximum waters descending to greater depths after several ridges however the mixing is sufficient to remove the temperature maximum completely and the entrainment of colder thermocline waters to depth reduces the bottom temperature to approximately the same as in the source region of marguerite trough similarly it is shown that deep waters of palmer deep are warmer than at the same depth at the shelf break the exact details of the transformations observed are heavily dependent on the local bathymetry and water column structure but glacially carved troughs and shallow sills are a common feature of the bathymetry of polar shelves and these types of processes may be a factor in determining the hydrographic conditions close to the coast across a wider area

a fracture loop thermal balance model of black smoker circulation abstract this time dependent model examines the thermal balance between hot rocks and hydrothermal circulation during the formation of volcanogenic ore deposits in order to establish whether the major heat supply to black smoker systems comes from the rock or from an underlying magma chamber flow is modelled as an open thermosyphon in a single fault which is parallel and very close to the mid ocean ridge axis this configuration is based on seafloor and ophiolite evidence of black smoker systems and their structural location the fault is represented in the model by a series of pipes carrying turbulent flow which is driven by the buoyancy difference between upflow and downflow hydraulic resistance to flow is predominantly in the discharge zone the fault is allowed to propagate downwards the horizontal conductive heat flux from the rock is computed analytically with the water flow treated digitally so that the heat exchange at the rock water interface at every position in the loop can be calculated for each time step the model thus provides the exit water temperature and mass flow rate throughout the lifespan of a hot spring enabling the resulting ore mass to be estimated results show that this system cannot obtain sufficient heat from a thin layer of rock such as the 1 2 km thick layer of lavas and dykes overlying an axial magma chamber to sustain long lived ore forming black smokers if the circulation penetrates through a greater volume of hot rock to the base of a recently solidified magma chamber ore masses of a few million tonnes can be formed if conditions are particularly favourable we conclude that systems producing larger ore masses must tap a magmatic heat supply

glacial and interglacial wind regimes over the eastern subtropical atlantic and north west africa analysis of aeolo marine dust deposits in the subtropical eastern atlantic enables the strength of the major wind patterns during the late quaternary to be evaluated and gives an insight into the climate of north africa

circulation and melting beneath george vi ice shelf antarctica introduction george vi ice shelf sandwiched between the western coast of palmer land and the eastern coast of alexander island is the largest and most studied of the west antarctic peninsula ice shelves it covers an area of approximately 25 000 km2 and is underlain by circumpolar deep water cdw with temperatures in excess of 1oc giving rise to rapid basal melting bishop and walton 1981 lennon et al 1982 the maximum ice thickness of about 500 m occurs about 70 km from the southern ice front where a ridge of thick ice extends across george vi sound near 70ow see figure 1 effectively dividing the upper water column into northern and southern regions the northern ice front which faces marguerite bay appears to be near the geographical limit of ice shelf viability and has undergone a gradual retreat in recent decades lucchitta and rosanova 1998 a timeframe over which much of the nearby wordie ice shelf disintegrated doake and vaughan 1991 there is extensive surface melting over the northern parts of the ice shelf and much of the ice column near the northern ice front appears to be temperate paren and cooper 1986 conditions in the south where the ice front faces into ronne entrance are colder and the ice front position appears to be steady the vast majority of the flow into the ice shelf comes from palmer land but basal melting is sufficient to remove most of this so that the ice is derived almost exclusively from local accumulation by the time it reaches the ice fronts potter et al 1984 there is some evidence at the margins of the ice shelf to suggest that it may have disappeared completely during the early holocene before reforming sugden and clapperton 1981 bentley et al 2005

global system of rivers its role in organizing continental land mass and defining land to ocean linkages the spatial organization of the earth s land mass is analyzed using a simulated topological network stn 30p representing potential flow pathways across the entire nonglacierized surface of the globe at 30 min longitude latitude spatial resolution we discuss a semiautomated procedure to develop this topology combining digital elevation models and manual network editing stn 30p was verified against several independent sources including map products and drainage basin statistics although we found substantial inconsistency within the extant literature itself a broad suite of diagnostics is offered that quantitatively describes individual grid cells river segments and complete drainage systems spanning orders 1 through 6 based on the strahler classification scheme continental and global scale summaries of key stn 30p attributes are given summaries are also presented which distinguish basins that potentially deliver discharge to an ocean exorheic from those that potentially empty into an internal receiving body endorheic a total of 59 122 individual grid cells constitutes the global nonglacierized land mass at 30 min spatial resolution the cells are organized into 33 251 distinct river segments which define 6152 drainage basins a global total of 133 1 106 km2 bear stn sop flow paths with a total length of 3 24 106 km the organization of river networks has an important role in linking land mass to ocean from a continental perspective low order river segments orders 1 3 drain the largest fraction of land 90 and thus constitute a primary source area for runoff and constituents from an oceanic perspective however the small number n 101 of large drainage systems orders 4 6 predominates draining 65 of global land area and subsuming a large fraction of the otherwise spatially remote low order rivers along river corridors only 10 of land mass is within 100 km of a coastline 25 is within 250 km and 50 is within 750 km the global mean distance to river mouth is 1050 km with individual continental values from 460 to 1340 km the mediterranean black sea and arctic ocean are the most land dominated of all oceans with land ocean area ratios of 4 4 and 1 2 respectively remaining oceans show ratios from 0 55 to 0 13 we discuss limitations of the stn 30p together with its potential role in future global change studies stn 30p is geographically linked to several hundred river discharge and chemistry monitoring stations to provide a framework for calibrating and validating macroscale hydrology and biogeochemical flux models

downslope convection north of elephant island antarctica influence on deep waters and dependence on enso 1 we present a long time series 1992 2001 of bottom temperature from 1040m on the continental slope north of elephant island antarctic peninsula a strong annual signal is apparent with minimum temperatures during or following the austral winter and a range similar to that found in much shallower waters on the adjacent shelf individual convective events are observable as large amplitude cold spikes that are confined to the period of seasonally coldest temperatures these plumes are dense enough to seasonally displace the circumpolar deep water and directly inject cold water into the deep open ocean the coldest water occurred when anomalous sea ice conditions were prevalent caused by the strong 1997 98 enso event we thus expect repeats of such strong convective events on enso timescales the observed long term warming and reduction in sea ice along the peninsula may however act to suppress this mode of ventilating the deep southern ocean

holocene vegetation zonation in the eastern sahara recent discoveries of fossil bearing holocene lake sediments from the eastern sahara have brought further confirmation and detail to earlier indirect evidence that a major pluvial episode occurred between 9 500 and 4 500 yr bp1 3 the botanical results mainly palynological show that savanna and desert grassland occupied regions that today are plantless hyperarid deserts we present here the first evidence from the entire saharan region of a latitudinal zonation of holocene vegetation extending across a 500 km wide belt in north west sudan we have completed detailed pollen analyses of three of the several sites discovered so far and the salient features from two the northernmost and the southernmost are presented here to demonstrate a steep gradient of past vegetative cover

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

climate change in the western antarctic peninsula since 1945 observations and possible causes temperature records from stations on the west coast of the antarctic peninsula show a very high level of interannual variability and over the last 50 years larger warming trends than are seen elsewhere in antarctica in this paper we investigate the role of atmospheric circulation variability and sea ice extent variations in driving these changes owing to a lack of independent data the reliability of antarctic atmospheric analyses produced in the 1950s and 1960s cannot be readily established but examination of the available data suggests that there has been an increase in the northerly component of the circulation over the peninsula since the late 1950s few observations of sea ice extent are available prior to 1973 but the limited data available indicate that the ice edge to the west of the peninsula lay to the north of recently observed extremes during the very cold conditions prevailing in the late 1950s the ultimate cause of the atmospheric circulation changes remains to be determined and may lie outside the antarctic region

impact of the 1997 98 enso on upper ocean characteristics in marguerite bay western antarctic peninsula 1 a year round sequence of hydrographic casts is used to trace the evolution of the upper ocean waters in marguerite bay western antarctic peninsula wap between 1998 and 2002 winter 1998 was anomalous showing an unusually deep mixed layer that became progressively more saline until spring reaching salinities as high as 34 0 the remnant of this mixed layer the winter water ww was the deepest and most saline observed atmospheric and cryospheric conditions were anomalous during winter 1998 at both local and regional scales locally we observed low sea ice concentrations high air temperatures and a high frequency of northerly winds these are the local manifestations of the strong enso event of 1997 1998 that was then rapidly weakening at the regional scale this enso produced significant anomalies in the sea ice distribution throughout the amundsen bellingshausen sea area and a large scale low pressure anomaly over the southeast pacific was seen to be responsible for the warm northerly winds we use a coupled mixed layer ice production model to investigate the enso driven forcings for the anomalous ocean conditions observed in winter 1998 this reveals that ice production is the main control on upper ocean stratification and that the deep saline mixed layer in 1998 was forced by anomalous sea ice conditions on spatial scales larger than purely local we conclude that the near coastal hydrography along the peninsula shows a profound response to enso with atmospheric and cryospheric forcings both implicated

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

muddled or mixed inferring palaeoclimate from size distributions of deep sea clastics abstract one of the outstanding problems of palaeoclimate reconstruction from physico chemical properties of terrigenous deep sea sediments stems from the fact that most basin fills are mixtures of sediment populations derived from different sources and transported to the site of deposition by different mechanisms conventional approaches to palaeoclimate reconstruction from deep sea sediments do not distinguish between provenance and dispersal related variations and therefore often fail to recognise the true significance of variations in sediment properties we formulate a set of requirements that each proposed palaeo environmental indicator should fulfil and focus on the intrinsic coupling between grain size and chemical composition a critical review of past achievements in grain size analysis serves to introduce a conceptual model of spatio temporal grain size variation based on dynamic populations each dynamic population results from a characteristic combination of production and transport mechanisms that corresponds to a distinct subpopulation in the data analysed the mathematical statistical representation of the conceptual model can be obtained by means of the end member modelling algorithm emma applications of the model to several ocean basins are discussed as well as methods to examine the validity of grain size based palaeoclimate reconstructions palaeoclimate reconstructions of a high and low latitude basin illustrate the common degree of complexity of deep sea grain size records

sediment and pollen evidence for an early to mid holocene humid period in the eastern sahara a major problem in the study of holocene palaeoenvironments of the arid and wind deflated sahara is the low preservation potential of sediments from which a record of past climatic change can be established several indirect and imprecisely dated pieces of evidence have suggested humid episodes in the holocene that may have supported more productive ecosystems and denser human populations than today searches for reliable proxy data on past climates however have failed to yield satisfactory results we report here the discovery of buried lake muds in north west sudan in the hyperarid core of the eastern sahara which yield sedimentological and palynological data clearly interpretable as recording an early to mid holocene humid episode that supported a relatively deep stratified lake surrounded by tropical savanna woodland vegetation

spatial variation in seabed temperatures in the southern ocean implications for benthic ecology and biogeography the antarctic seabed has traditionally been regarded as cold and thermally stable with little spatial or seasonal variation in temperature here we demonstrate marked spatial variations in continental shelf seabed temperature around antarctica with the western antarctic peninsula shelf significantly warmer than shelves around continental antarctica as a result of flooding of the shelf by circumpolar deep water from the antarctic circumpolar current the coldest shelf seabed temperatures are in the weddell sea ross sea and prydz bay as a consequence of seasonal convection associated with strong air sea heat fluxes and sea ice formation these waters constitute the dense precursors of antarctic bottom water and can descend down the adjacent slope to inject cold water into the southern ocean deep sea deep sea seabed temperatures are coldest in the weddell sea and are progressively warmer to the east there is a distinct latitudinal gradient in the difference between seabed temperatures on the shelf and in the deep sea with the deep sea warmer by up to similar to 2 k at high latitudes and colder by similar to 2 k around sub antarctic islands these differences have important consequences for benthic ecology and biogeography understanding the evolutionary history of the antarctic marine biota and the impact of regional climate change

a 300 kyr record of aridity and wind strength in southwestern africa inferences from grain size distributions of sediments on walvis ridge se atlantic abstract the terrigenous fraction of sediments recovered from walvis ridge se atlantic ocean reveals a history of southwestern african climate of the last 300 kyr end member modelling of a data set of grain size distributions n 428 results in three end members the two coarsest end members are interpreted as eolian dust the third end member as hemipelagic mud the ratio of the two eolian end members reflects the eolian grain size and is attributed to the intensity of the se trade winds trade winds were intensified during glacials compared to interglacials changes in the ratio of the two eolian end members over the hemipelagic one are interpreted as variations in southwestern african aridity late quaternary southwestern african climate was relatively arid during the interglacial stages and relatively humid during the glacial stages owing to meridional shifts in the atmospheric circulation system during glacials the polar front shifted equatorward resulting in a northward displacement of the zone of westerlies causing increased rainfall in southwestern africa the equatorward shift of the polar front is coupled with an increase of the meridional pressure gradient leading to enhanced atmospheric circulation and increased trade wind intensity

grain size distribution and depositional processes of the mineral component of abyssal sediments lessons from the north pacific grain size analyses of the mineral component of abyssal pacific surface sediments show two distinctly different size distributions that can be associated with sediments that are dominantly eolian and dominantly hemipelagic respectively in the range of 1 to 30 m eolian grains show a distinct size mode at about 2 m 9 and little material coarser than about 16 m 6 hemipelagic grains are characterized by a much flatter size distribution that has a broad size mode near 2 m 9 and important amounts of material coarser than 16 m 6 this ability to distinguish between hemipelagic and eolian grains will obviate some long standing concerns in the interpretation of deep sea sediments examples of the utility of this technique are given for the north pacific and south atlantic

sortable silt and fine sediment size composition slicing parameters for palaeocurrent speed and palaeoceanography fine sediment size 63 ixm is best measured by a sedimentation technique which records the whole size distribution repeated size measurement with intermediate steps of removal of components by dissolution allows inference of the size distribution of the removed component as well as the residue in this way the size of the biogenic and lithogenic noncarbonate fractions can be determined observations of many size distributions suggest a minimum in grain size frequency curves at 8 to 10 ixm the dynamics of sediment erosion deposition and aggregate breakup suggest that fine sediment behavior is dominantly cohesive below 10 1xm grain size and noncohesive above that size thus silt coarser than 10 ixm displays size sorting in response to hydrodynamlc processes and its properties may be used to infer current speed silt that is f mer than 10 ixm behaves in the same way as clay 2 xm useful parameters of the distribution are the 10 63 ixm mean size and the percentage 10 63 ixm in the fine fraction we cannot use size distributions to distinguish the nature of the currents therefore to infer water mass advection speeds i e the mean kinetic energy of the flow ku regions of high eddy kinetic energy ke must be avoided at the present such abyssal regions lie under the high surface k e of major current systems gulf stream kuroshio agulhas antarctic circmpolar current and brazil falldand currents in the argenthe basin this is probably a satisfactory guide for the pleistocene with regard to the carbonate subfraction of the size spectrum size modes due to both cocco iths and foramlnlferal fragments can be recognized and analyzed with the boundary between them again at about 10 lm the flux of less than 10 lm carbonate at pelagic sites above the lysocline is another candidate for a productivity indicator

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

diversity in deep sea benthic macrofauna the importance of local ecology the larger scale history and the antarctic abstract high diversity in macrobenthos in the deep sea still lacks satisfactory explanation even if this richness may not be exceptional compared to that in coastal soft sediments explanations have assumed a highly ecologically interactive saturated local community with co existence controlled by either niche heterogeneity or spatio temporal heterogeneity embodying disturbance all have failed to provide convincing support local regional scale biodiversity relationships support the idea of local richness in macrobenthos being predominantly dependent on the larger rather local scale local scale ecological interactions seem unlikely to have overriding importance in co existence of species in the deep sea even for relatively abundant core species with wide distributions variety in observed larger scale pattern and the strong inter regional pattern particularly in the poorly known southern hemisphere seem to have a pluralistic causation these include regional scale barriers and extinctions e g arctic and ongoing adaptive zone re colonisation e g mediterranean along with other historical constraints on speciation and migration of species caused by changes in ocean and ocean basin geometry at the global scale lack of knowledge of the antarctic deep sea for example blocks coherent understanding of latitudinal species diversity gradients we need to reconcile emerging understanding of large scale historical variability in the deep sea environment with massive extinctions among microfossil indicators as recently as the pliocene to results from cladistic studies indicating ancient lineages such as asellote isopods that have evolved entirely within the deep sea the degree to which the great age diversity and high degree of endemism in antarctic shelf benthos might have enriched biodiversity in the adjacent deep seas basins remains unclear basin confluence with the atlantic indian and pacific oceans may have encouraged northwards dispersion of species from and into the deep antarctic basins so that any regional identity is superficial interpretation of the antarctic deep sea as a diversity pump for global deep sea biodiversity may simply reflect re colonisation via basin confluence of northern hemisphere areas impoverished by the consequences of rapid environmental change during the quaternary