magnetic petrology of equatorial atlantic sediments electron microscopy results and their implications for environmental magnetic interpretation 1 the magnetic microparticle and nanoparticle inventories of marine sediments from equatorial atlantic sites were investigated by scanning and transmission electron microscopy to classify all present detrital and authigenic magnetic mineral species and to investigate their regional distribution origin transport and preservation this information is used to establish source to sink relations and to constrain environmental magnetic proxy interpretations for this area magnetic extracts were prepared from sediments of three supralysoclinal open ocean gravity cores located at the ceara rise geob 1523 1 3 49 9 n 41 37 3 w the mid atlantic ridge geob 4313 2 4 02 8 n 33 26 3 w and the sierra leone rise geob 2910 1 4 50 7 n 21 03 2 w sediments from two depths corresponding to marine isotope stages 4 and 5 5 were processed this selection represents glacial and interglacial conditions of sedimentation for the western central and eastern equatorial atlantic and avoids interferences from subsurface and anoxic processes crystallographic elemental morphological and granulometric data of more than 2000 magnetic particles were collected by scanning and transmission electron microscopy on basis of these properties nine particle classes could be defined detrital magnetite titanomagnetite fragmental and euhedral titanomagnetite hemoilmentite intergrowths silicates with magnetic inclusions microcrystalline hematite magnetite spherules bacterial magnetite goethite needles and nanoparticle clusters each class can be associated with fluvial eolian subaeric and submarine volcanic biogenic or chemogenic sources large scale sedimentation patterns are delineated as well detrital magnetite is typical of amazon discharge fragmental titanomagnetite is a submarine weathering product of mid ocean ridge basalts and titanomagnetite hemoilmenite intergrowths are common magnetic particles in west african dust this clear regionalization underlines that magnetic petrology is an excellent indicator of source to sink relations hematite encrustations magnetic spherules and nanoparticle clusters were found at all investigated sites while bacterial magnetite and authigenic hematite were only detected at the more oxic western site at the eastern site surface pits and crevices were seen on the crystal faces indicating subtle early diagenetic reductive dissolution it was observed that paleoclimatic signatures of magnetogranulometric parameters such as the ratio of anhysteretic and isothermal remanent magnetizations can be formed either by mixing of multiple sources with separate relatively narrow grain size ranges western site or by variable sorting of a single source with a broad grain size distribution eastern site hematite goethite and possibly ferrihydrite nanoparticles occur in all sediment cores studied and have either high coercive or superparamagnetic properties depending on their partly ultrafine grain sizes these two magnetic fractions are generally discussed as separate fractions but we suggest that they could actually be genetically linked

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

paleoclimate and paleoceanography over the past 20 000 yr in the mediterranean sea basins as indicated by sediment elemental proxies abstract marine sediments record paleoenvironmental changes over time through variations in major and trace element concentrations the main objective of this paper is to review such changes in the mediterranean sea basins over the last 20 ka using the inorganic chemistry and mineralogy of marine sediment records elemental ratio proxies that are mostly used are discussed and the uncertainties involved in using them for paleoclimate and paleoceanographic reconstructions are evaluated the focus on the mediterranean region is based on the sensitivity of this region to global climate changes due to its semi enclosed nature the elemental ratios that have been particularly useful for reconstructing terrigenous inputs into the mediterranean have been ti al and zr al ratios as proxies for eolian dust input and mg al k al and rb al ratios as proxies for fluvial input redox sensitive elements e g u mo v co ni cr have provided reliable reconstructions of oxygen conditions at the time of deposition some of these elements are also particularly susceptible to post depositional remobilization and record diagenetic processes instead of the original environmental signatures regarding productivity fluctuations most of the paleoproductivity reconstructions are based on the abundance of barite and ba excess algorithms the biogeochemistry of ba is however not fully understood and mechanisms for barite precipitation in the water column are not yet known two case studies are presented the eastern mediterranean sapropel s1 deposited between 10 8 and 6 1 cal ka bp and the westernmost mediterranean paleoclimate record over the last 20 ka

orbital forcing of dust supply to the north canary basin over the last 250kyr abstract the canary basin lies in a region of strong interaction between the atmospheric and ocean circulation systems trade winds drive seasonal coastal upwelling and dust storm outbreaks from the neighbouring sahara desert are the major source of terrigenous sediment to investigate the forcing mechanisms for dust input and wind strength in the north canary basin the temporal pattern of variability of sedimentological and geochemical proxy records has been analysed in two sediment cores between latitudes 30 30 n and 31 40 n spectral analysis of the dust proxy records indicates that insolation changes related to eccentricity and precession are the main periods of temporal variation in the record si al and grain size of the terrigenous fraction show an increase in glacial interglacial transitions while al concentration and fe al ratio are both in phase with minima in the precessional index hence the results obtained show that the wind strength was intensified at terminations at times of maxima of northern hemisphere seasonal insolation when the african monsoon was enhanced the north canary basin also received higher dust input this result suggests that the moisture brought by the monsoon may have increased the availability of dust in the source region

late miocene to early pliocene climate variability off nw africa odp site 659 abstract a high resolution 18 o record obtained on benthic foraminifera cibicides wuellerstorfi combined with clay mineralogy major element concentrations and sr nd isotopes have been investigated for the odp site 659 located off north africa in order to reconstruct african monsoon intensity between 6 2 and 4 9 myr mineralogical and geochemical results indicate that sedimentation on the cape verde plateau results from a mixture of sediments deriving from the neighbouring saharan arid region and the wet african equatorial zone sahelian band characterised by a low and a high state of weathering respectively past variations in terrigenous inputs from these areas allow us to track past extension and displacement of the west african monsoon itcz system maximal summer insolation induced a northward migration of the inter tropical convergence zone itcz wetter conditions in the sahel and a diminishing of wind erosion over the south sahara and sahel in addition there was an increase in river input to the cape verde plateau of sediments derived from southern sedimentary sources which are characterised by high kaolinite contents and cia values and low k al and ti al ratios on the other hand minimal summer insolation induced a southward movement of the itcz drier climate in the sahel and stronger easterly winds resulting in an increase in dust transport from the sahara to the cape verde plateau dust particles are characterised by high illite and smectite content and by a low chemical state of weathering low cia values and high k al and ti al ratios finally our results provide new clues regarding the re flooding of the mediterranean sea at the end of the msc 5 33 myr this event was associated with enhanced aridity in the sahara implying a reorganisation of the atmospheric circulation and a southward migration of the itcz

comparative studies of the spreading of mediterranean water through the gulf of cadiz abstract a comparative study is made of observations and model results of the spreading of mediterranean water mw from the strait of gibraltar through the gulf of cadiz past cape st vincent into the eastern north atlantic the observations include a set of hydrographic sections from west of gibraltar to offshore of lisbon the tracking of rafos floats and current meter data the model is a very fine resolution limited area model of the region around cape st vincent and includes model floats both the observational data and the model results show the mw descending into two distinct layers around 800 and 1200 m depth in the gulf of cadiz as the mw leaves the slope meddies are generated and have been tracked by the rafos floats and can be seen in the paths of the model floats

grain size control on sr nd isotope provenance studies and impact on paleoclimate reconstructions an example from deep sea sediments offshore nw africa five sediment cores from offshore nw africa were analyzed for strontium and neodymium isotope ratios to reconstruct temporal variations in continental weathering regimes sediments were taken from three time slices with well known and distinctive environmental conditions present day dry and warm similar to 6 ka wet and warm and similar to 12 ka dry and cold terrigenous sediment samples were split into two size fractions to distinguish between the two dominant transport mechanisms offshore nw africa fluvial 0 10 mu m and aeolian 10 40 mu m sr isotope data record evidence of marked grain size control with higher isotopic ratios in the fine fraction in contrast epsilon nd values are largely unaffected by grain size minor variability in nd isotope data at each sampling site indicates near constant sources of terrigenous matter over the last similar to 12 ka variations in sr isotope ratios are interpreted to reflect major changes in the evaporation precipitation balance we suggest that the sr nd isotope data record a latitudinal shift of the northern limit of the african rainbelt and associated wind systems causing changes in the humidity and rate of chemical weathering over nw africa while hyperarid conditions prevailed similar to 12 ka more humid conditions and intensified monsoonal rainfall at similar to 6 ka resulted in greater breakdown of easily weathered k bearing phases and increased sr 87 sr 86 in the detritus in late holocene times the monsoonal circulation diminished resulting in a return to arid conditions our results clearly show that it is of vital importance in paleoenvironmental studies to carry out isotopic analyses on individual sediment fractions that were carried to the studied deposition site by distinct sediment transport mechanisms if isotopic analyses are carried out on bulk sediments the observed variability in isotopic values most likely represents changes in the particle size and mixing proportions of the sediment subpopulations

estimating the hydrogen isotopic composition of past precipitation using leaf waxes from western africa abstract the hydrogen isotopic composition of plant leaf wax n alkanes d wax is a novel proxy for estimating d of past precipitation d p however vegetation life form and relative humidity exert secondary effects on d wax preventing quantitative estimates of past d p here we present an approach for removing the effect of vegetation type and relative humidity from d wax and thus for directly estimating past d p we test this approach on modern day late holocene 0 3 ka sediments from a transect of 9 marine cores spanning 21 n 23 s off the western coast of africa we estimate vegetation type c 3 tree versus c 4 grass using 13 c of leaf wax n alkanes and correct d wax for vegetation type with previously derived apparent fractionation factors for each vegetation type late holocene vegetation corrected d wax d vc displays a good fit with modern day d p suggesting that the effects of vegetation type and relative humidity have both been removed and thus that d vc is a good estimate of d p we find that the magnitude of the effect of c 3 tree c 4 grass changes on d wax is small compared to d p changes we go on to estimate d vc for the mid holocene 6 8 ka the last glacial maximum lgm 19 23 ka and heinrich stadial 1 hs1 16 18 5 ka in terms of past hydrological changes our leaf wax based estimates of d p mostly reflect changes in wet season intensity which is complementary to estimates of wet season length based on leaf wax 13 c

distribution and geochemistry of aerosols in the tropical north atlantic troposphere relationship to saharan dust the spatial distribution and water soluble chemical composition of aerosols in the tropical north atlantic troposphere were examined during the nasa global tropospheric experiment atlantic boundary layer experiment gte able 1 conducted in the vicinity of barbados west indies during the june 1984 study period the atmospheric aerosol at 1 5 3 5 km altitudes the region composing the so called sahara air layer consisted primarily of saharan dust the aerosol mass concentration reached 400 g m 3 stp x in the mainstream of a massive saharan dust outbreak which passed through the longitude of barbados between june 13 and 18 1984 measurements of the particle size distribution of the saharan dust indicated that about 85 of the mass was contained in the 1 to 8 m diameter range a distribution that was consistent among the three dust outbreaks observed during able enrichment of water soluble aerosol chemical species in sahara air compared to clean tropospheric air at 4 km altitude ranged from about tenfold for nh4 to more than a factor of 400 for no3 calculations suggest that saharan dust represented a significant source of airborne no3 non sea salt so42 and po 3 in boundary layer air over the tropical north atlantic estimates of the annual input of saharan dust related chemical species to the equatorial atlantic suggested that deposition of mineral aerosol to oligotrophic surface waters may be an important source of the nutrient species no 3 and po 3 this atmospheric input probably occurs principally in the form of dry deposition aerosols over the world s oceans originate from both natural and anthropogenic sources various estimates of global aerosol production although based on limited data especially for natural sources strongly suggest that mobilization of natural eolian material greatly exceeds anthropogenic related emissions see e g prospero et el 1983 additional evidence demonstrating the importance of natural eolian sources has come from geochemical studies of deep sea sediments these studies suggest that soil material transported from arid regions by wind is primarily responsible for the distribution of certain clay minerals in oceanic sediments griffin and goldberg 1968 windom 1975 chester et al 1979 prospero 1981a it is important therefore to determine the spatial and temporal variability of major eolian sources and the subsequent long range geochemical transport in the global atmosphere the first direct measurements of soil dust in the atmosphere over the tropical north atlantic were obtained by delany et al 1967 it was hypothesized from studies of soil dust mineral aerosol at barbados west indies that saharan dust originating from north africa was a major source of natural eolian material to the tropical north atlantic troposphere over the past two decades extensive documentation of spatial and temporal variability of saharan mineral aerosol over the tropical north atlantic has been obtained prospero and carlson 1970 1972 prospero et al 1970 1981 chester et al

influence of anthropogenic combustion emissions on the deposition of soluble aerosol iron to the ocean empirical estimates for island sites in the north atlantic the results of several recent studies challenge the reigning paradigm that continental soil dust provides the only significant atmospheric source of dissolved iron to the surface ocean this evidence includes correlations between the operational solubility of aerosol iron and atmospheric loadings of black carbon and aluminum normalized vanadium and nickel each of which are associated with emissions from the combustion of fossil fuel oil these observations suggest that the relative solubility of aerosol iron hence the eolian flux of soluble iron to the surface ocean may be significantly impacted by anthropogenic oil combustion products using recent field data from the bermuda region we have developed an empirical method to estimate the solubility of aerosol iron using bulk aerosol concentrations of fe v and al we apply this method to a large body of published data from the aeroce program for north atlantic island sites on tenerife barbados bermuda and ireland where the relative proportions of anthropogenic aerosols range from minor to major respectively our aerosol iron solubility estimates suggest that anthropogenic emissions contribute approximately 70 and 85 of the annual dry deposition of soluble iron to the surface ocean near bermuda and ireland respectively implying that human activities have profoundly affected the iron budget of the north atlantic region the annual mean dry deposition of soluble iron at barbados and izana is dominated by soil dust the anthropogenic contribution at these two sites ranges from 12 to 30 and is highly dependent on the soil dust solubility of fe employed in the model the low end 12 estimate appears to be more representative of these high dust sites

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

the role of north brazil current transport in the paleoclimate of the brazilian nordeste margin and paleoceanography of the western tropical atlantic during the late quaternary abstract reconstructions of surface paleoceanographic conditions of the western equatorial atlantic and past climates of the adjacent northeast brazilian the nordeste continental margin were undertaken by analyzing sediments from a piston core and associated gravity and box cores recovered from 3107 meter water depth at 0 20 n on the equatorial brazilian continental slope the record is dated by radiocarbon analysis and oxygen isotopic stratigraphy of planktonic foraminifers and spans from near modern to approximately 110 ka high resolution xrf analysis provides insight into the paleoclimate history of the nordeste during the last glacial interval several large amplitude and abrupt peaks are observed in the time series of ti ca and are usually accompanied by peaks of fe k together these record periods of increased precipitation and intense weathering on the adjacent continent and increased terrestrial sediment discharge from nordeste rivers into the atlantic within the limits of dating accuracy most ti ca peaks correlate with heinrich events in the north atlantic this record thus corroborates and extends back in time the previous record of arz et al 1998 determined on sediment cores from farther southeast along the nordeste margin stable oxygen isotopic analysis and mg ca paleothermometry on the near surface dwelling planktonic foraminiferal species globierinoides ruber find that mean sea surface temperature sst during glacial time 20 55 ka n 97 was 23 89 0 79 c and the mean sst during the late holocene 0 5 ka n 14 was 26 89 0 33 c ssts were 0 5 2 c higher and inferred sea surface salinities were lower during most of the periods of elevated ti ca thus as observed in previous studies the western equatorial atlantic was warm at least locally and the adjacent southern tropical continent was wet at the same time that the high latitude north atlantic was cold using the syntrace ccsm3 fully coupled climate model with transient forcing for the period 22 ka to present we find that decreased transport of the north brazil current co occurs with reduced atlantic meridional overturning circulation and colder than normal ssts in the north atlantic region these simulated conditions are invariably associated with significantly increased precipitation in the nordeste region

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

late quaternary climate changes in central africa as inferred from terrigenous input to the niger fan time series of terrigenous source elements al k ti zr from core geob4901 8 recovered from the deep sea fan of the niger river record variations in riverine sediment discharge over the past 245 000 yr although the flux rates of all the elements depend on physical erosion which is mainly controlled by the extent of vegetation coverage in central africa element al ratios reflect conditions for chemical weathering in the river basin maximum sediment input to the ocean occurs during cold and arid periods when precipitation intensity and associated freshwater runoff are reduced high carbonate contents during the same periods indicate that the sediment supply has a positive effect on river induced marine productivity in general variations in the terrestrial signals contain a strong precessional component in tune with changes in low latitude solar radiation however the terrestrial signal lags the insolation signal by several thousand years k al ti al and zr al records reveal that african monsoonal precipitation depends on high latitude forcing we attribute the shift between insolation cycle and river discharge to the frequently reported nonlinear response of african climate to primary orbital configurations which may be caused by a complex interaction of the secondary control parameters such as surface albedo and or thermohaline circulation

bulk composition of northern african dust and its source sediments a compilation abstract this paper presents a review of bulk compositional data of northern african dust and its potential source sediments and includes elemental isotope and mineralogical data northern african dust represents about one half of the total global atmospheric mineral dust burden and its uplift transport and deposition have strong impacts on climate and various terrestrial and marine ecosystems the chemical data set shows that an average northern african dust exhibits comparable si fe and mn contents with respect to the average composition of the upper continental crust is slightly depleted in the alkali metals k and na and enriched in ti and p however the complete data set yields clear evidence that northern african dust and its source sediments are compositionally heterogeneous on a regional scale and that this heterogeneity can be used to differentiate between major potential source areas on the basis of so called source markers an evaluation of these compositional fingerprints shows that the following parameters and especially their combination are effective in the discrimination of the most active source areas in northern africa ratio of ca mg fe wt calcite or carbonate content palygorskite occurrence and abundance illite kaolinite ratio nd 0 value and 87sr 86sr ratio for example the data set corroborates previous ideas which assign carbonate illite and palygorskite rich mineral dusts to north west ern source regions because most of the above listed source markers do not change substantially during transport even far traveled dusts may be assigned to specific potential source areas in northern africa some limitations of the presented data set are also discussed our compilation reveals some substantial gaps in the knowledge of the composition of source sediments and mineral dusts from important potential source areas that should be filled in the future the here compiled data set can be used as a reference frame when incorporating the composition of source sediments e g mineralogy into global or regional dust transport models and can be compared with source analysis by remote sensing or back trajectory analysis however source apportionment studies supported by our data set will not only be useful for actual dust samples but will also be helpful for the understanding of paleo wind directions and hence paleo climatological conditions through the investigation of quaternary eolian sediments deposited in and around northern africa

detrital events and hydroclimate variability in the romanian carpathians during the mid to late holocene the romanian carpathians are located at the confluence of three major atmospheric pressure fields the north atlantic the mediterranean and the siberian despite its importance for understanding past human impact and climate change high resolution palaeoenvironmental reconstructions of holocene hydroclimate variability and in particular records of extreme precipitation events in the area are rare here we present a 7500 year long high resolution record of past climatic change and human impact recorded in a peatbog from the southern carpathians integrating palynological geochemical and sedimentological proxies natural climate fluctuations appear to be dominant until 4500 years before present yr bp followed by increasing importance of human impact sedimentological and geochemical analyses document regular minerogenic deposition within the bog linked to periods of high precipitation such minerogenic depositional events began 4000 yr bp with increased depositional rates during the medieval warm period mwp the little ice age lia and during periods of societal upheaval e g the roman conquest of dacia the timing of minerogenic events appears to indicate a teleconnection between major shifts in north atlantic oscillation nao and hydroclimate variability in southeastern europe with increased minerogenic deposition correlating to low nao index values by linking the minerogenic deposition to precipitation variability we state that this link persists throughout the mid to late holocene

distribution and provenance of wind blown se pacific surface sediments article i nfo the reconstruction of low latitude ocean atmosphere interactions is one of the major issues of paleo environmental studies the trade winds extending over 20 to 30 of latitude in both hemispheres between the subtropical highs and the intertropical convergence zone are major components of the atmospheric circulation and little is known about their long term variability on geological time scales in particular in the pacific sector we present the modern spatial pattern of eolian derived marine sediments in the eastern equatorial and subtropical pacific 10 n to 25 s as a reference data set for the interpretation of se pacific paleo dust records the terrigenous silt and clay fractions of 75 surface sediment samples have been investigated for their grain size distribution and clay mineral compositions respectively to identify their provenances and transport agents dust delivered to the southeast pacific from the semi to hyper arid areas of peru and chile is rather fine grained 4 8 m due to low level transport within the southeast trade winds nevertheless wind is the dominant transport agent and eolian material is the dominant terrigenous component west of the peru chile trench south of 5 s grain size distributions alone are insufficient to identify the eolian signal in marine sediments due to authigenic particle formation on the sub oceanic ridges and abundant volcanic glass around the galapagos islands together with the clay mineral compositions of the clay fraction we have identified the dust lobe extending from the coasts of peru and chile onto galapagos rise as well as across the equator into the doldrums illite is a very useful parameter to identify source areas of dust in this smectite dominated study area

geochemical evidence of saharan dust parent material for soils developed on quaternary limestones of caribbean and western atlantic islands abstract most previous workers have regarded the insoluble residues of high purity quaternary limestones coral reefs and oolites as the most important parent material for well developed clay rich soils on caribbean and western atlantic islands but this genetic mechanism requires unreasonable amounts of limestone solution in quaternary time other possible parent materials from external sources are volcanic ash from the lesser antilles island arc and saharan dust carried across the atlantic ocean on the northeast trade winds soils on quaternary coral terraces and carbonate eolianites on barbados jamaica the florida keys united states and new providence island bahamas were studied to determine which if either external source was important caribbean volcanic ashes and saharan dust can be clearly distinguished using ratios of relatively immobile elements al 2 o 3 tio 2 ti y ti zr and ti th comparison of these ratios in 25 soils where estimated ages range from 125 000 to about 870 000 yr shows that saharan dust is the most important parent material for soils on all islands these results indicate that the northeast trade winds have been an important component of the regional climatology for much of the quaterary saharan dust may also be an important parent material for caribbean island bauxites of much greater age

sources and modes of terrigenous sediment input to the chilean continental slope abstract physical chemical and mineralogical properties of a set of surface sediment samples collected along the chilean continental slope 21 44 s are used to characterise present day sedimentation patterns and sediment provenance on the chilean margin despite the presence of several exceptional latitudinal gradients in relief oceanography tectonic evolution volcanic activity and onshore geology the present day input of terrigenous sediments to the chilean continental margin appears to be mainly controlled by precipitation gradients and source rock composition in the hinterland general trends in grain size denote a southward decrease in median grain size of the terrigenous corganic caco3 and opal free fraction which is interpreted as a shift from aeolian to fluvial sedimentation this interpretation is supported by previous observations of southward increasing bulk sedimentation rates north south trends in sediment bulk chemistry are best recognised in the iron fe and titanium ti vs potassium k and aluminium al ratios of the sediments that most likely reflect the contribution of source rocks from the andean volcanic arc these ratios are high in the northernmost part abruptly decrease at 25 s and then more or less constantly increase southwards to a maximum at 40 s

a 550 ka record of aeolian activity near north west cape australia inferences from grain size distributions and bulk chemistry of se indian ocean deep sea sediments abstract the terrigenous fraction of sediments from a deep sea sediment core recovered from the northwestern western australian continental slope offshore north west cape se indian ocean reveals a history of western australian climate throughout the last 550 ka end member modelling of a data set of grain size distributions n 438 of the terrigenous sediment fraction allows to interpret the record in terms of aeolian and fluvial sediment deposition both related to palaeo environmental conditions in the north west cape area the data set can be best described by two aeolian end members and one fluvial one changes in the ratio of the two aeolian end members over the fluvial one are interpreted as aridity variations in northwestern western australia these grain size data are compared with bulk geochemical data obtained by xrf scans of the core log ratios of the elements zr fe and ti ca which indicate a terrigenous origin corroborate the grain size data we postulate that the mid to late quaternary near north west cape climate was relatively arid during the glacial and relatively humid during the interglacial stages owing to meridional shifts in the atmospheric circulation system opposite to published palaeo environmental records from the same latitude 20 s offshore chile and offshore namibia the australian aridity record does not show the typical southern hemisphere climate variability of humid glacials and dry interglacials which we interpret to be the result of the relatively large land mass of the australian continent which emphasises a strong monsoonal climatic system

significance of the sedimentary al ti ratio as an indicator for variations in the circulation patterns of the equatorial north atlantic this paper presents results of geochemical investigations conducted on three gravity cores recovered along an e w transect 3 5 n in the equatorial atlantic considering the high terrigenous content of the sediments approximately 12 74 wt al and ti concentrations are predominantly associated with the input of lithogenous phases the terrigenous fraction of western equatorial atlantic sediments is more influenced by material derived from the amazon river than by deposition of african dust particles as a possible route of transport the north equatorial countercurrent is considered fluctuations in al ti records generally reflect changes in the low latitude insolation cycle 19 23 kyr and thus indicate a close coupling to variations of the trade wind systems and continental climate changes however characteristics of the terrigenous signals in the eastern and western equatorial atlantic differ significantly while the precessional signal in the al ti record and low latitude portion in the 18o record are in phase in the eastern equatorial atlantic variations in the terrigenous fraction westward of the mid atlantic ridge precede the marine signal by 2 3 kyr which we ascribe to a shift between trade wind systems of the northern and southern hemispheres

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

paleolimnological evidence for the onset and termination of glacial aridity from lake tanganyika tropical east africa geochemical and sedimentological data in a continuous 60 000 year sediment core record from the kalya horst region of central lake tanganyika provide a detailed history of paleoclimate mediated weathering and overflow events from upstream lake kivu univariate elemental profiles bivariate elemental ratios and multivariate analyses of chemical trends show variations between the dry late pleistocene 32 18 ka cal yr bp and the wetter conditions that both preceded and post date that interval this record places important new constraints on the timing of aridity in east africa during the high latitude last glacial maximum lgm based on significant decreases in magnetic susceptibility and soluble cation concentrations coinciding with biogenic silica elemental indicatorsintheoldestportionofthesedimentaryrecord 60 50kacalyrbp characterizethisintervalasacomparatively wetperiod similar to modern conditions our record demonstrates that the ensuing transition toward arid conditions in tropical africa during high latitude glaciation was a two staged event with intermediate levels of aridity occurring from 50 32 ka cal yr bp followed by intense aridity from 32 18 ka cal yr bp the initiation of inflow from upstream lake kivu into lake tanganyika is evidenced at 10 6 ka cal yr bp through its influence on both elemental profiles mg ca and through its effect on 87 sr 86 sr increases in elemental mg ca sr concentrations coincide with the timing of the lake kivu overflow metal geochemistry suggests that the overflow from lake kivu into lake tanganyika may have ceased between 8 and 6 ka cal yr bp suggesting a period of middle holocene aridity in east africa 2007 elsevier b v all rights reserved

geochemical and mineralogical evidence for sahara and sahel dust additions to quaternary soils on lanzarote eastern canary islands spain africa is the most important source of dust in the world today and dust storms are frequent on the nearby canary islands previous workers have inferred that the sahara is the most important source of dust to canary islands soils with little contribution from the sahel region soils overlying a late quaternary basalt flow on lanzarote canary islands contain in addition to volcanic minerals quartz and mica exotic to the island s bedrock kaolinite in the soils also likely has an exotic origin trace element geochemistry shows that the soils are derived from varying proportions of locally derived basalt and african dust major element geochemistry clay mineralogy and interpretation of satellite imagery suggest that dust additions to the canary islands come not only from the sahara desert but also from the sahel region

societal need for improved understanding of climate change anthropogenic impacts and geo hazard warning drive development of ocean observatories in european seas society s needs for a network of in situ ocean observing systems cross many areas of earth and marine science here we review the science themes that benefit from data supplied from ocean observatories understanding from existing studies is fragmented to the extent that it lacks the coherent long term monitoring needed to address questions at the scales essential to understand climate change and improve geo hazard early warning data sets from the deep sea are particularly rare with long term data available from only a few locations worldwide these science areas have impacts on societal health and well being and our awareness of ocean function in a shifting climate substantial efforts are underway to realise a network of open ocean observatories around european seas that will operate over multiple decades some systems are already collecting high resolution data from surface water column seafloor and sub seafloor sensors linked to shore by satellite or cable connection in real or near real time along with samples and other data collected in a delayed mode we expect that such observatories will contribute to answering major ocean science questions including how can monitoring of factors such as seismic activity pore fluid chemistry and pressure and gas hydrate stability improve seismic slope failure and tsunami warning what aspects of physical oceanography biogeochemical cycling and ecosystems will be most sensitive to climatic and anthropogenic change what are natural versus anthropogenic changes most fundamentally how are marine processes that occur at differing scales related the development of ocean observatories provides a substantial opportunity for ocean science to evolve in europe here we also describe some basic attributes of network design observatory networks provide the means to coordinate and integrate the collection of standardised data capable of bridging measurement scales across a dispersed area in european seas adding needed certainty to estimates of future oceanic conditions observatory data can be analysed along with other data such as those from satellites drifting floats autonomous underwater vehicles model analysis and the known distribution and abundances of marine fauna in order to address some of the questions posed above standardised methods for information management are also becoming established to ensure better accessibility and traceability of these data sets and ultimately to increase their use for societal benefit the connection of ocean observatory effort into larger frameworks including the global earth observation system of systems geoss and the global monitoring of environment and security gmes is integral to its success it is in a greater integrated framework that the full potential of the component systems will be realised highlights societies increasingly depend on timely information on ecosystems and natural hazards data is needed to improve climate related uncertainty and geo hazard early warning observatory networks coordinate and integrate the collection of standardised data ocean observatories provide opportunity for ocean science to evolve

framework of hydrocarbon generation and destruction in eastern smackover trend laminated lime mudstones of the lower member of the jurassic smackover formation are significant source rocks for crude oil across mississippi alabama and florida the source facies was deposited in an anoxic and perhaps hypersaline environment that preserved algal derived kerogen the distribution of kerogen along laminations of depositional origin and along stylolites of diagenetic origin resulted in efficient expulsion of crude oil with increasing thermal maturity crude oil initially emplaced in reservoirs was cracked to yield gas condensate and then methane rich in nonhydrocarbon gases such as hydrogen sulfide carbon dioxide and nitrogen early destruction of methane was driven by thermochemical sulfate reduction the thermal maturity framework of the smackover rend explains the distribution of hydrocarbon discoveries and suggests areas previously overlooked by exploration

temporal variability of fluxes of eolian transported freshwater diatoms phytoliths and pollen grains off cape blanc as reflection of land atmosphere ocean interactions in northwest africa 1 fluxes of airborne freshwater diatoms fd phytoliths ph and pollen grains po collected with sediment traps off cape blanc northwest africa from 1988 till 1991 are presented both continental rainfall variations and wind mean strength and direction play a key role in the temporal fluctuations of the fluxes of eolian traces in the pelagic realm drier conditions in northern africa in 1987 could have preceded the high lithogenic input and moderate fd flux in 1988 the ph peak in summer 1988 was probably caused by increased wind velocity wetter rainy seasons of 1988 89 might have promoted a significant pollen production in summer 1989 and fd in late 1989 and early 1990 as well as contributed to the reduction of the lithogenic flux in 1989 90 decreased fluxes of fd ph and po and higher contribution of the 6 11 m lithogenic fraction in 1991 would mainly reflect minor intensity and decreased amount of continental trade winds air mass backward trajectories confirm that the saharan air layer is predominantly involved in the spring summer transport trade winds play a decisive role in the fall winter months but also contribute to the transport during late spring summer origin of wind trajectories does not support a direct relationship between transporting wind layers and material source areas in northern africa high winter fluxes of eolian tracers and high amount of trade winds with continental origin in summer warn against a simplistic interpretation of the seasonal eolian signal preserved in the sediments off cape blanc and the wind layer involved in its transport

impact of anthropogenic combustion emissions on the fractional solubility of aerosol iron evidence from the sargasso sea we report empirical estimates of the fractional solubility of aerosol iron over the sargasso sea during periods characterized by high concentrations of saharan dust summer 2003 and by low concentrations of aerosols in north american maritime north atlantic air masses spring 2004 and early summer 2004 we observed a strong inverse relationship between the operational solubility of aerosol iron defined using a flow through deionized water leaching protocol and the total concentration of aerosol iron whereby the operational solubility of aerosol iron was elevated when total aerosol iron loadings were low this relationship is consistent with source dependent differences in the solubility characteristics of our aerosol samples and can be described by a simple mixing model wherein bulk aerosols represent a conservative mixture of two air mass end members that carry different aerosol types saharan air which contains a relatively high loading of aerosol iron 27 8 nmol fe m 3 that has a low fractional solubility 0 44 and north american air which contains a relatively low concentration of aerosol iron 0 5 nmol fe m 3 that has a high fractional solubility 19 historical data for aerosols collected on bermuda indicate that the low iron loadings associated with north american air masses are typically accompanied by elevated v al fe al and v mn mass ratios in the bulk aerosol relative to saharan dust which are indicative of anthropogenic fuel combustion products the identification of similar compositional trends in our sargasso sea aerosol samples leads us to suggest that the elevated solubility of iron in the aerosols associated with north american air masses reflects the presence of anthropogenic combustion products which contain iron that is readily soluble relative to iron in saharan soil dust we thus propose that the source dependent composition of aerosol particles specifically the relative proportion of anthropogenic combustion products is a primary determinant for the fractional solubility of aerosol iron over the sargasso sea this hypothesis implies that anthropogenic combustion emissions could play a significant role in determining the atmospheric input of soluble iron to the surface ocean

geochemical fingerprinting of trans atlantic african dust based on radiogenic sr nd hf isotopes and rare earth element anomalies mineral dust is an important component of earth s climate system and biogeochemical cycles on a global scale in order to understand the relationship between climate processes in the source areas and the properties of aerosols at distant receptor sites we must be able to identify the source provenance of dust here we present a multiproxy study that characterizes the temporal variability in the geochemical composition of long range african dust lrad collected between 2003 and 2011 in the trade winds on the caribbean island of barbados we find systematic differences between sr nd hf isotopic composition and rare earth element anomalies of individual dust events and evidence of seasonal shifts in dust source activity and transport these results indicate that coherent geochemical source signatures of lrad can be preserved even after transport across thousands of kilometers we investigated the possibility of identifying the potential source areas through comparisons with literature data however these data are almost entirely based on measurements of soil and sediment samples this could lead to biases because of soil aerosol particle size and composition differences nonetheless our data suggest that many samples are linked to sources in mali and sub saharan regions radiogenic nd hf composition of aerosols can potentially be a useful proxy to study the proximity of mineral dust sources to depositional sites in order to establish firmer links between lrad and dust source areas however we require much more data on the geochemical composition of aerosols from potential source areas in north africa

how environmental magnetism can enhance the interpretational value of grain size analysis a time slice study on sediment export to the nw african margin in heinrich stadial 1 and mid holocene abstract sediment dynamics in limnic fluvial and marine environments can be assessed by granulometric and rock magnetic methodologies while classical grain size analysis by sieving or settling mainly bears information on composition and transport the magnetic mineral assemblages reflect to a larger extent the petrology and weathering conditions in the sediment source areas here we combine both methods to investigate late quaternary marine sediments from five cores along a transect across the continental slope off senegal this region near the modern summer intertropical convergence zone is particularly sensitive to climate change and receives sediments from several aeolian fluvial and marine sources from each of the investigated five geob sediment cores 494 2956 m water depth two time slices were processed which represent contrasting climatic conditions the arid heinrich stadial 1 15 kyr bp and the humid mid holocene 6 kyr bp each sediment sample was split into 16 grain size fractions ranging from 1 6 to 500 m concentration and grain size indicative magnetic parameters susceptibility sirm hirm arm and arm irm were determined at room temperature for each of these fractions the joint consideration of whole sediment and magnetic mineral grain size distributions allows to address several important issues i distinction of two aeolian sediment fractions one carried by the north easterly trade winds 40 63 m and the other by the overlying easterly harmattan wind 10 20 m as well as a fluvial fraction assigned to the senegal river

orbital and millennial scale changes in the hydrologic cycle and vegetation in the western african sahel insights from individual plant wax δd and δ 13 c to reconstruct variability of the west african monsoon and associated vegetation changes on precessional and millennial time scales we analyzed a marine sediment core from the continental slope off senegal spanning the past 44 000 years 44 ka we used the stable hydrogen isotopic composition dd of individual terrestrial plant wax n alkanes as a proxy for past rainfall variability the abundance and stable carbon isotopic composition d 13 c of the same compounds were analyzed to assess changes in vegetation composition c3 c4 plants and density the dd record reveals two wet periods that coincide with local maximum summer insolation from 38 to 28 ka and 15 to 4 ka and that are separated by a less wet period during minimum summer insolation our data indicate that rainfall intensity during the rainy season throughout both wet humid periods was similar whereas the length of the rainy season was presumably shorter during the last glacial than during the holocene additional dry intervals are identified that coincide with north atlantic heinrich stadials and the younger dryas interval indicating that the west african monsoon over tropical northwest africa is linked to both insolation forcing and highlatitude climate variability the d 13 c record indicates that vegetation of the western sahel was consistently dominated by c4 plants during the past 44 ka whereas c3 type vegetation increased during the holocene moreover we observe a gradual ending of the holocene humid period together with unchanging ratio of c3 to c4 plants indicating that an abrupt aridification due to vegetation feedbacks is not a general characteristic of this time interval

astronomical forcing of northwest african climate and glacial history during the late messinian 6 5 5 5 ma abstract high resolution physical chemical and biological based climate proxies from the ain el beida aeb section in atlantic morocco reveal the imprint of the three main orbital parameters precession 19 23 kyr obliquity 41 kyr and eccentricity 95 125 and 400 kyr between 6 5 and 5 5 million years before present ma the precession related variations are most prominently reflected in the color reflectance and chemical composition of the sediment showing that precession minimum configurations lead to more humid climate conditions in northwest africa probably related to the atlantic system while more arid climate conditions prevailed during precession maxima in addition precession bound changes in planktonic foraminifera and calcareous nannofossil assemblages indicate that sea surface temperature sst increased during the humid phases while productivity conditions increased during the dry periods the clear imprint of the short and long term eccentricity cycles is explained by a non linear climatic response to the precession forcing with overall more humid climate conditions during eccentricity maxima the obliquity controlled variations in sst and aridity conditions concur with glacial interglacial variability that is conspicuously recorded in both planktonic and benthic 18 o records of aeb the observed warm and wet climates during interglacial periods most likely reflect the direct ice driven thermal response to the reduced ice sheets and more active atlantic depressions similar as during the pliocene and pleistocene obliquity controlled variations in the planktonic and benthic carbon isotope 13 c records of aeb are inversely related to the oxygen isotope records with more depleted values during glacial stages although with a small lag the close correspondence between the 13 c records of aeb and other open ocean sites i e sites 982 and 926 may therefore point to large scale glacial controlled variations in the deep sea carbon reservoir superimposed on the orbital variations the imprint of the onset of the messinian salinity crisis can be traced by a marked 1 0 negative excursion in planktonic 13 c at 6 0 ma followed by a prominent peak in ti al at 5 97 ma

on the formation of desert loess sequences of quartz rich coarse 20 63 m silt occur in many low and midlatitude unglaciated arid and semiarid areas and have been termed desert loess the processes by which these deposits are generated have been debated for decades all hypotheses to explain their origin seek to provide mechanisms for the generation of silt sized material without glacial grinding which is the main process involved in the production of coarse silt at high latitudes possible mechanisms for the formation of coarse silt in arid regions include derivation from preexisting siltstones mechanical weathering of silicate rocks and abrasion of sand grains in active dune environments during intense transport events examination of the characteristics of desert loess and field and laboratory experiments to assess the role of dune areas as a source of coarse silt by abrasion and or resuspension of residual fines suggests that many loess sequences are dominated by locally derived coarse silt improvements in the characterization of desert loess particle size mineralogy and geochemistry are needed however to identify sources and sinks of coarse silt especially when combined with climatic back trajectory analysis properly scaled experiments and modeling of particle collisions will also help to better quantify the effectiveness of abrasion in the generation of coarse silt in support of field observations

glacial cold water coral growth in the gulf of cadiz implications of increased palaeo productivity a set of 40 uranium series datings obtained on the reef forming scleractinian cold water corals lophelia pertusa and madrepora oculata revealed that during the past 400 kyr their occurrence in the gulf of cadiz goc was almost exclusively restricted to glacial periods this result strengthens the outcomes of former studies that coral growth in the temperate ne atlantic encompassing the french iberian and moroccan margins dominated during glacial periods whereas in the higher latitudes irish and norwegian margins extended coral growth prevailed during interglacial periods thus it appears that the biogeographical limits for sustained cold water coral growth along the ne atlantic margin are strongly related to climate change by focussing on the last glacial interglacial cycle this study shows that palaeo productivity was increased during the last glacial this was likely driven by the fertilisation effect of an increased input of aeolian dust and locally intensified upwelling after the younger dryas cold event the input of aeolian dust and productivity significantly decreased concurrent with an increase in water temperatures in the goc this primarily resulted in reduced food availability and caused a widespread demise of the formerly thriving coral ecosystems moreover these climate induced changes most likely caused a latitudinal shift of areas with optimum coral growth conditions towards the northern ne atlantic where more suitable environmental conditions established with the onset of the holocene

influence of the water content on x ray fluorescence core scanning measurements in soft marine sediments the x ray fluorescence xrf core scanner provides bulk sediment chemistry data measured nondestructively at the split core sediment surface although this method is widely accepted there is little known about the effects of physical properties such as density and water content on xrf core scanner data comparison of xrf scanner measurements from the sediment surface and dry powder samples of sediment core geob7920 indicates strongly reduced element intensities for the lighter elements al and si we relate the lower element intensities of the measurements taken at the sediment surface to the amount of water in the sample volume analyzed by the xrf core scanner the heavier elements k ca ti and fe remain relatively unaffected by the variation of any physical property within sediment core geob7920 additionally we successfully use the elemental intensity of cl as a proxy for the seawater content in the sample volume analyzed by the xrf core scanner this enables the establishment of a correction function for the elements al and si that corrects for the radiation absorption of the water content in sediment core geob7920 off cape blanc nw africa

modelling the joint variability of grain size and chemical composition in sediments abstract the geochemical composition of siliciclastic sediments correlates strongly with grain size hence geochemical composition may serve as a grain size proxy in the absence of grain size variations geochemical data of siliciclastic sediments may be used to characterise size independent processes i e sediment provenance weathering mixing shape density sorting and diagenesis in general however geochemical data sets contain both types of information in order to formalise interpretation of geochemical data we propose a mathematical method to decompose the total geochemical variability of a series of genetically related specimens into a grain size dependent the shared signal and a grain size independent part the residual signal the former may serve as a proxy for grain size whereas the latter represents geochemical variability that would have been observed if all sediments would have had the same grain size distribution the two data sets are jointly decomposed by means of partial least squares pls and orthogonal projection subsequently the presence of significant grain size independent geochemical variability in the residual signal is determined in a statistically rigorous manner using a 2 test using a synthetic example we show that the residual record effectively reveals an imposed provenance signal which could not have been resolved from the geochemical or grain size data sets individually we analysed the relation between grain size and geochemical composition in three quaternary marine sediment cores located offshore west africa and south america geob7920 2 geob9508 5 and geob7139 2 both sites are characterised by biogenic sediment input in addition to fluvial and aeolian sediment input from the continent it was found that all cores show a strong but different correlation between the mean grain size and the bulk geochemical composition these results demonstrate that geochemical grain size proxies are empirical and site specific it was also found that the geochemical and grain size data in cores geob7920 2 and geob7139 2 do not contain unique information whereas in core geob9508 5 ti varies independently from the grain size this residual ti signal correlates with the transport mechanism as demonstrated by statistically different values of aeolian and fluvial dominated sediments however a unique interpretation of this residual signal in terms of the postulated grain size independent mechanisms could not be provided without additional information we conclude that the proposed model facilitates identification and validation of different element ratios as grain size proxies and more importantly as proxies for size independent processes for this reason the model paves the way for rigorous analysis of multi proxy data which are widely used in palaeoceanographic and palaeoclimatic research

north african paleodrainage discharges to the central mediterranean during the last 18 000 years a multiproxy characterization using elemental geochemistry clay mineralogy grain size end member modeling and planktonic foraminiferal 18o we characterize the provenance of central mediterranean sediments over the past 18 ka the provenance is dust dominated before and after the african humid period ahp by contrast during the ahp 11 5 ka largely concurrent with organic rich sapropel s1 formation it is predominantly riverine from north african sources such fluvial supply is suggested to come from paleodrainage networks that were reactivated by intensified monsoon precipitation during the ahp the supply is characterized by high mg al and smectite contents and has been accompanied by considerable freshwater influx as indicated by the enhanced grain size and lighter foraminiferal 18o the clay mineral assemblages in our core and in nearby cores correspond with a provenance from the libyan tunisian margin mainly via the paleo river irharhar the inferred fluvial discharge is strongest during the late ahp 8 5 5 ka coinciding with reported enhanced fluvial dynamics and wettest conditions over western libya and tunisia algeria this period is not only synchronous with the largest extension of open water bodies in north africa and lowest saharan dust inputs but also consistent with precipitation records of the west african monsoon moreover our records show a remarkable correspondence with that of a paleodrainage system towards the atlantic west african margin inferring a common headwater region in the central saharan mountains and a similar climate mechanism taken together we suggest a dominant control of north african humid surfaces on the paleodrainage delivery modulated by groundwater level in response to the insolation driven west african monsoon precipitation

tracing dust input to the mid atlantic ridge between 14 45 n and 36 14 n geochemical and sr isotope study abstract a geochemical and sr isotope study was carried out on sediment cores collected from the logatchev hydrothermal vent site 14 45 n on the mid atlantic ridge with the aim of identifying aeolian inputs into the overlying water column the logatchev vent site lies beneath the ne trade winds which transport large amounts of detrital material across the north atlantic sequential dissolution of the sediments has been used to discriminate the detrital fraction from other types of sediment ree patterns for the acetic acid fraction at the logatchev site record contributions from biogenic carbonate seawater like ree pattern and oxy hydroxides which exhibit positive eu and negative ce anomalies typical of hydrothermal origin the logatchev site leach fractions have similar major element compositions to terrigenous al fe mn oxides whose ree patterns show strong positive ce and negative eu anomalies that are comparable to those of preformed fe mn oxides of continental origin based on major element and ree data and 87sr 86sr ratios 0 7197 0 7236 the residue fractions at the logatchev site contain a significant contribution from aeolian sources whose chemical composition is virtually identical to oligotrophic settling particles in the equatorial atlantic ocean in comparison residues from the rainbow hydrothermal site 36 14 n have geochemical composition controlled by hydrothermal anhydrite chavagnac v german c r milton j a palmer m r 2005 sources of ree in sediment cores from the rainbow vent site 36 14 n mar chem geol 216 329 352 they are characterised by 87sr 86sr ratios of 0 7087 0 7093 which are significantly less radiogenic than those of the saharan dust hence the data suggest 1 a significant contribution of aeolian material to the sediments at 14 45 n that is not present at 36 14 n 2 that this material is most likely of north african origin and 3 that this source of aeolian material has remained relatively constant over the last 13 kyr

bipolar seesaw in the northeastern tropical atlantic during heinrich stadials two sst records based on mg ca of g ruber pink from the continental slope off west africa at 15 n and 12 n shed new light on the thermal bipolar seesaw pattern in the northeastern tropical atlantic during periods of reduced atlantic meridional overturning circulation amoc associated with heinrich stadials h1 to h6 the two records indicate that the latitudinal position of the bipolar seesaw s zero anomaly line between cooling in the north and warming in the south gradually shifted southward from h6 to h1 a conceptual model is presented that aims to provide a physically consistent mechanism for the southward migration of the seesaw s fulcrum the conceptual model suggests latitudinal movements of the intertropical convergence zone driven by a combination of orbital forced changes in the meridional temperature gradient within the realm of the hadley cell and the expansion of the northern hemisphere cryosphere as a major factor

comparison of laser grain size analysis with pipette and sieve analysis a solution for the underestimation of the clay fraction classically the grain size of soil and sediment samples is determined by the sieve method for the coarse fractions and by the pipette method based on the stokes sedimentation rates for the fine fractions results from the two methods are compared with results from laser diffraction size analysis which is based on the forward scattering of monochromatic coherent light from a point of view of laboratory efficiency the laser sizing technique is far superior accuracy and reproducibility are shown by measurements on certified materials it appears that laser grain size measurements of certified materials correspond very well with the certificated measurements tests were also done on a set of randomly selected sediments of fluvial aeolian and lacustrine origin except for the 2 m clay fraction there is a coarsening of the mean diameter of one to two size classes 0 25 caused by the non sphericity of the particles the platy form of the clay particles induces considerable differences eight size classes between pipette and laser measurements the 2 m grain size defined by the pipette method corresponds with a grain size of 8 m defined by the laser particle sizer for the studied sediments using a higher grain size level for the clay fraction when laser analysis is applied enables workers in the geological and environmental field to compare classical pipette analysis with a laser sizing technique

variability in terrigenous sedimentation processes off northwest africa and its relation to climate changes inferences from grain size distributions of a holocene marine sediment record variations in deposition of terrigenous fine sediments and their grain size distributions from a high resolution marine sediment record offshore northwest africa 30 51 0 n 10 16 1 w document climate changes on the african continent during the holocene end member grain size distributions of the terrigenous silt fraction which are related to fluvial and aeolian dust transport indicate millennial scale variability in the dominant transport processes at the investigation site off northwest africa as well as recurring periods of dry conditions in northwest africa during the holocene the terrigenous record from the subtropical north atlantic reflects generally humid conditions before the younger dryas during the early to mid holocene as well as after 1 3 kyr bp by contrast continental runoff was reduced and arid conditions were prevalent at the beginning of the younger dryas and during the mid and late holocene a comparison with high and low latitude holocene climate records reveals a strong link between northwest african climate and northern hemisphere atmospheric circulation throughout the holocene due to its proximal position close to an ephemeral river system draining the atlas mountains as well as the adjacent saharan desert this detailed marine sediment record which has a temporal resolution between 15 and 120 years is ideally suited to enhance our understanding of ocean continent atmosphere interactions in african climates and the hydrological cycle of northern africa after the last deglaciation

aeolian transport and deposition of plant wax n alkanes across the tropical north atlantic ocean long chain n alkanes are terrestrial higher plant biomarkers analysed in marine sedimentary archives to reconstruct continental palaeoclimatic and palaeohydrological conditions latitudinal variation in their concentration and distribution in marine sediments relatively close to the continent has been widely studied but little is known on the extent to which this continental signal extends to the ocean furthermore no studies have examined the seasonal variation in the deposition of these biomarkers in marine sediments here we studied longitudinal variation in the composition of long chain n alkanes and two other terrestrial higher plant biomarkers long chain n alkanols and long chain fatty acids in atmospheric particles as well as longitudinal and seasonal variation in long chain n alkanes in sinking particles in the ocean at different water depths and in surface sediments all collected along a 12 n transect across the tropical north atlantic ocean the highest abundance of all three biomarker classes was closest to the african coast as expected because they are transported with saharan dust and the largest part of the dust is deposited close to the source at this proximal location the seasonal variability in long chain n alkane flux and the chain length distribution of the n alkanes in sinking particles was most pronounced due to seasonal change in the dust source or to change in vegetation composition in the source area related to the position of the intertropical convergence zone itcz in contrast in the open ocean the seasonal variability in both the long chain n alkane flux and chain length distribution of the n alkanes was low the abundance of the alkanes was also lower as expected because of the larger source to sink distance at the western part of the transect close to south america we found an additional source of the alkanes in the sinking particles during spring and autumn in the year 2013 the 13c values of the alkanes in the surface sediment closest to the south american continent indicated that the isotope signal was likely derived from c3 vegetation from the amazon implying an input from the amazon river as there is no significant aeolian input from south america there since the prevailing wind direction is from the east finally the concentration of the alkanes was similar in the material collected from the atmosphere the particles collected while settling through the marine water column and in the surface sediments providing evidence that degradation of long chain n alkanes from the atmosphere to settling at the sediment water interface at deep open ocean sites is minimal

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

the formation of silt material for terrestrial sediments particularly loess and dust abstract there are certain controls which operate to demarcate the nature and size range of quartz particles in terrestrial detrital sediments the particles fall into discrete and definable populations which are separated by tanner gaps quartz sand nature is determined largely by a eutectic like reaction which takes place in the original igneous rock this reaction delivers a fine mix of quartz and feldspar units which on weathering deliver quartz sand the formation of quartz silt is more contentious it appears that silt production is essentially controlled by the presence in larger quartz particles of moss defects crystalline defects introduced into the quartz structure by earlier events e g the high low displacive transformation in cooling quartz there appear to be two modes in airborne quartz particles which have been called large dust and small dust the larger mode is the loess mode around 20 60 m and the smaller mode falls into the classification with high level long travel dust mode around 2 6 m the quartz in small dust is around the same size as the clay mineral agglomerate particles cma which form a considerable proportion of high level dust clouds the cma particles form from lake deposits the size is controlled by the openness of the packing in the lake sediments this can be demonstrated by a simple monte carlo model

influence of the inter tropical discontinuity on harmattan dust deposition in ghana the harmattan is a dry dust laden continental wind and in the boreal winter harmattan dust plumes affects many west african countries including ghana when the harmattan is strongest the southern part of ghana is affected by the inter tropical discontinuity itd in this study we investigate if the itd functions as a barrier preventing long transported harmattan dust to settle south of and below it this is done by analyzing a harmattan dust outbreak mapped using earth observation eo data from the spinning enhanced visible and infrared imager seviri onboard the meteosat second generation msg platform coupled with data from west african aeronet stations and comparing these observations with wind data from noaa s air resources laboratory arl program and the mineral suite of samples from seasonal dust deposits in north and south ghana in northern ghana traces of minerals indicate a weak influence of particles from an arid environment which is found consistent with the mapped dust plumes and ne wind directions in southern ghana the mineral composition show no sediments of an arid origin the mapped dust plumes is less intense and the surface wind directions and wind mass trajectories are more varying with lower wind speeds based on the results of this study it is concluded that dust deposited or measured near ground in the harmattan period under the itd and south of it does not contain material from the chad basin due to the local winds conditions less

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

ground satellite observations and atmospheric modeling of dust storms originating in the high puna altiplano deserts south america implications for the interpretation of paleo climatic archives 1 this study provides a detailed description of the sources transport dispersion and deposition of two major dust events originating from the high altitude subtropical puna altiplano plateau 15 26 s 65 69 w in south america a long and severe drought provided the right conditions for the onset of both events in july 2009 and 2010 dust was transported se and deposited over the pampas region and was observed to continue to the atlantic ocean dust monitoring stations located downwind recorded both events and samples were characterized through chemical and textural analysis through a combination of meteorological data and satellite observations calipso and modis detectors we estimate the emission flux for the 2010 event this estimate was used to constrain the hybrid single particle lagrangian integrated trajectory hysplit transport model and simulate the dust event both satellite imagery and model results agree in the location and extension of the dust cloud calipso detected dust between 6000 and 8500 m a s l which remained at this height during most of its trajectory the dust cloud mixed with a strong convective system in the region and the associated precipitation brought down significant amounts of dust to the ground dust particle size analysis for both events indicates that near the sources dust samples show median modes of 12 4 14 1 m similar to modes observed 1300 km away chemical composition of sediments from potential dust sources shows distinct signatures within the puna altiplano plateau the puna sector being clearly different from the altiplano area in addition both sources are markedly different from the patagonian chemical fingerprint these results have important implications to improve the interpretation of paleo environmental archives preserved on the argentine loess antarctic ice cores and southern ocean marine sediments

magnetic mineral inventory of equatorial atlantic ocean marine sediments off senegal glacial and interglacial contrast summary in complex sedimentary environments with different sources of the magnetic particles it is vital that all factors contributing to and affecting the magnetic signals be well understood for a better constrained interpretation of palaeomagnetic and climatic data palaeoclimatically driven signals may be convolved with volcanic and cosmogenic sources and diagenesis may alter the primary magnetic mineral assemblage rock magnetic methods and electron microscopy provide suitable means to characterize magnetic assemblages in this study of core retrieved sediments from the last 150 kyr off the coast of senegal nw africa the occurrence of magnetic spherules of volcanic origin and cosmogenic particles appeared to be insignificant in glacial stages ferrimagnetic iron and iron titanium oxides magnetically dominate with few signs of a diagenetic overprint low and room temperature magnetic results reveal evidence for haematite and goethite while haemoilmenite and titanomagnetite were identified from low temperature ac susceptibility measurements and sem observations in contrast the interglacial samples appeared to have been severely affected by diagenesis with pyrite making up over 50 per cent of the iron bearing phases the verwey transition observed at 110 k confirms the presence of magnetite the glacial and interglacial contrast reveals the close link between northwest african climate and the magnetic mineralogy of the sediments

astronomically forced variations in western african rainfall 21 n 20 s during the last interglacial period this study documents the long term evolution of western african precipitation during the last interglacial lig we compare geochemical records obtained on nine sediment cores from the western african margin to a transient simulation 130 115 ka performed with an ocean atmosphere general circulation model and insolation as sole forcing good agreement between proxy records and model outputs indicates that long term changes in western african precipitation largely responded to insolation variations during most of the lig after an early lig dry phase related to high latitude iceberg melting or dating uncertainties boreal summer insolation controlled the intensification of the north african monsoon between 127 and 122 ka perhaps facilitating human migrations out of africa equatorial african rainfall slightly increased throughout the lig in response to increasing annual insolation east west contrasting rainfall evolutions at 10 20 s illustrate the complex southern african response in contrast to more direct responses of north and equatorial western africa to insolation forcing

dirtmap the geological record of dust atmospheric dust is an important feedback in the climate system potentially affecting the radiative balance and chemical composition of the atmosphere and providing nutrients to terrestrial and marine ecosystems yet the potential impact of dust on the climate system both in the anthropogenically disturbed future and the naturally varying past remains to be quantified the geologic record of dust provides the opportunity to test earth system models designed to simulate dust records of dust can be obtained from ice cores marine sediments and terrestrial loess deposits although rarely unequivocal these records document a variety of processes source transport and deposition in the dust cycle stored in each archive as changes in clay mineralogy isotopes grain size and concentration of terrigenous materials although the extraction of information from each type of archive is slightly different the basic controls on these dust indicators are the same changes in the dust flux and particle size might be controlled by a combination of a source area extent b dust emission efficiency wind speed and atmospheric transport c atmospheric residence time of dust and or d relative contributions of dry settling and rainout of dust similarly changes in mineralogy reflect a source area mineralogy and weathering and b shifts in atmospheric transport the combination of these geological data with process based forward modelling schemes in global earth system models provides an excellent means of achieving a comprehensive picture of the global pattern of dust accumulation rates their controlling mechanisms and how those mechanisms may vary regionally the dust indicators and records of terrestrial and marine palaeoenvironments dirtmap data base has been established to provide a global palaeoenvironmental data set that can be used to validate earth system model simulations of the dust cycle over the past 150 000 years

dinoflagellate cyst distribution in marine surface sediments off west africa 17 6 n in relation to sea surface conditions freshwater input and seasonal coastal upwelling abstract an organic walled dinoflagellate cyst analysis was carried out on 53 surface sediment samples from west africa 17 6 n to obtain insight in the relationship between their spatial distribution and hydrological conditions in the upper water column as well as marine productivity in the study area multivariate analysis of the dinoflagellate cyst relative abundances and environmental parameters of the water column shows that sea surface temperature salinity marine productivity and bottom water oxygen are the factors that relate significantly to the distribution patterns of individual species in the region the composition of cyst assemblages and dinoflagellate cyst concentrations allows the identification of four hydrographic regimes 1 the northern regime between 17 and 14 n characterized by high productivity associated with seasonal coastal upwelling 2 the southern regime between 12 and 6 n associated with high nutrient waters influenced by river discharge 3 the intermediate regime between 14 and 12 n influenced mainly by seasonal coastal upwelling additionally associated with fluvial input of terrestrial nutrients and 4 the offshore regime characterized by low chlorophyll a concentrations in upper waters and high bottom water oxygen concentrations our data show that cysts of polykrikos kofoidii selenopemphix quanta dubridinium spp echinidinium species cysts of protoperidinium monospinum and spiniferites pachydermus are the best proxies to reconstruct the boundary between the ne trade winds and the monsoon winds in the subtropical eastern atlantic ocean the association of bitectatodinium spongium lejeunecysta oliva quinquecuspis concreta selenopemphix nephroides trinovantedinium applanatum can be used to reconstruct past river outflow variations within this region