monsoon related variations in zaire congo sediment load and influence of fluvial silicate supply on marine productivity in the east equatorial atlantic during the last 200 000 years two sediment cores from the zaire fan and the angola margin have been investigated for their composition of terrigenous and biogenic constituents respectively for the late quaternary kaolinite feldspar ratios and variations of terrigenous element ratios of zr ti k rb and al reveal that the composition of zaire river sediment load has fluctuated in tune with precessional variations of boreal summer insolation in particular the correspondence of high kaolinite feldspar and al k ratios with low latitude insolation maxima strongly corroborates the assumption that west african monsoonal precipitation and chemical weathering was enhanced during periods of increased central african heating the most striking feauture observed is that opal accumulation has been 2 to 10 times higher in zaire fan sediments than outside in continental margin sediments off angola although biogenic ba and corg fluctuations from both areas indicate that changes of total paleoproductivity were of the same magnitude in the zaire river plume and off angola from this we infer that the contribution of biogenic opal production to total paleoproductivity has been significantly higher within the zaire river plume than in the oceanic upwelling regime farther to the south off angola over the last 200 000 years the pattern of opal accumulation rates with respect to that of marine organic carbon implies that enhanced opal production off the zaire river to a great extent was the result of additional fluvial supply of dissolved silica during humid climates characterized by more intense chemical weathering on the continent while total paleoproductivity created by oceanic upwelling was high in periods of increased zonal trade wind intensity at precessional insolation minima and during cold more arid glacial climate conditions we presume that paleoproductivity off the zaire was controlled by the following two sources of nutrients 1 marine nutrients including nitrate and phosphate as well as the uptake of ba on particulate corg caused by upwelling and 2 silica mainly delivered by the zaire river dissolved silicate hence our results underline the importance of dissolved silicate in large rivers for marine silicate cycling in the tropical to subtropical atlantic at a millenial timescale

glacial interglacial variablity in the benguela upwelling system spatial distribution and budgets of organic carbon accumulation 1 modern sedimentary total organic carbon toc content as a proxy for surface water export production was mapped on the shelf and on the upper continental slope of the benguela upwelling system using 137 core tops shelf maxima in toc can be correlated with maxima in surface water productivity on the slope high toc contents are observed offshore from sites of strong modern upwelling estimates of modern toc mass accumulation rates mar show that approximately 85 of the total is accumulating on the shelf toc mar were calculated mapped and budgeted for the holocene and for the last glacial maximum lgm using 19 sediment cores from the continental slope during the lgm centers of deposition and production have migrated offshore with respect to their holocene positions toc accumulation on the continental slope was approximately 84 higher during the lgm than during the holocene possibly reflecting enhanced productivity the toc distribution patterns and sediment echo sounding data suggest that undercurrents strongly influence the sedimentation off namibia winnowing and focusing result in great lateral heterogeneity of sedimentation rates and sediment properties individual cores therefore do not necessarily reflect general changes in export production these results highlight the need for detailed regional studies based on a large number of sediment cores for highly heterogeneous high productivity areas in order to derive general statements on total fluxes

dinoflagellate cyst assemblages as tracers of sea surface conditions in the northern north atlantic arctic and sub arctic seas the new n 677 data base and its application for quantitative palaeoceanographic reconstruction the distribution of dinoflagellate cyst dinocyst assemblages in surface sediment samples from 677 sites of the northern north atlantic arctic and sub arctic seas is discussed with emphasis on the relationships with sea surface parameters including sea ice cover salinity and temperature of the coldest and warmest months difficulties in developing a circum arctic data base include the morphological variation within taxa e g operculodinium centrocarpum islandinium cezare and polykrikos sp which probably relate to phenotypic adaptations to cold and or low salinity environments sparse hydrographical data together with large interannual variations of temperature and salinity in surface waters of arctic seas constitute additional limitations nevertheless the use of the best analogue technique with this new dinocyst data base including 677 samples permits quantitative reconstruction of sea surface conditions at the scale of the northern north atlantic and the arctic domain the error of prediction calculated from modern assemblages is 1 3 ca nd 1 8 c for the temperature of february and august respectively 1 8 for the salinity and 1 5 months yr 1 for the sea ice cover application to late quaternary sequences from the western and eastern subpolar north atlantic labrador sea and barents sea provide reconstructions compatible with those obtained using the previous dinocyst data base n 371 which mainly included modern data from the northern north atlantic copyright 2001 john wiley sons ltd journal of quaternary science

methodology of the last climatic cycle reconstruction in france from pollen data abstract a numerical method has been devised to reconstruct the climate of the last climatic cycle from pollen data reliable annual reconstructions have been published recently for the two french long sequences of la grande pile and les echets covering the last 140 millennia this paper gives more details on the method used it proposes new reconstructions based on an extended modern pollen data base these reconstructions are consistent with the previous ones especially detailed information is obtained on the seasonal characteristics of climate computations of gcm on the climate variations from 125 kyr to 115 kyr b p are confirmed for western europe the insolation variations have induced at that time a decrease of seasonality accompanied by an increased cyclonic activity in winter this has been the beginning of the first ice growth after the last interglacial other glacier growth phases are detected at the end of substage 5c and particularly at the end of substage 5a

living planktonic foraminifera in the eastern south atlantic during spring indicators of water masses upwelling and the congo zaire river plume abstract planktonic foraminifera were collected from pumped surface water samples and net tows over the upper 150 m of the water column in the eastern south atlantic ocean during october and november 1989 of the 24 recorded species 12 occur abundantly and form 6 foraminiferal assemblages the assemblages correlate with the physico chemical properties of the 6 vapor surface water masses the correlation of species with low salinity is due to properties co variant with salinity at several locations high concentrations of planktonic foraminifera were observed caused by frontal mixing or reproduction the g sacculifer g ruber white assemblage marks the south equatorial under current minor contribution of g menardii and n dutertrei to this assemblage reflects the subsurface equatorial under current the n dutertrei g siphonifera assemblage reflects the south equatorial counter current and equatorial under current the n pachyderma g bulloides assemblage reflects the coastal upwelling over the shelf and the filaments of mixed oceanic and upwelled water in the coastal of the benguela current bc the angola benguela front at the interface of the warm currents and the bc is reflected by the g inflata g bulloides assemblage an unusual coincidence of g sacculifer g bulloides in one assemblage results from the interaction of the subtropical gyre and the oceanic branch of the bc the g rubber pink n dutertrei assemblage is attributed to the warm low saline plume in front of the congo zaire river

land sea correlation by means of terrestrial and marine palynomorphs from the equatorial east atlantic phasing of se trade winds and the oceanic productivity abstract sporomorphs and dinoflagellate cysts from site gik16867 in the northern angola basin record the vegetation history of the west african forest during the last 700 ka in relation to changes in salinity and productivity of the eastern gulf of guinea during most cool and cold periods the afromontane forest rather than the open grass rich dry forest expanded to lower altitudes partly replacing the lowland rain forest of the borderlands east of the gulf of guinea except in stage 3 when oceanic productivity was high during a period of decreased atmospheric circulation high oceanic productivity is correlated to strong winds the response of marine productivity in the course of a climatic cycle however is earlier than that of wind vigour and makes wind stress induced oceanic upwelling in the area less likely monsoon variation is well illustrated by the pollen record of increased lowland rain forest that is paired to the dinoflagellate cyst record of decreased salinity forced by increased precipitation and run off

insolation values for the climate of the last 10 million years new values for the astronomical parameters of the earth s orbit and rotation eccentricity obliquity and precession are proposed for paleoclimatic research related to the late miocene the pliocene and the quaternary they have been obtained from a numerical solution of the lagrangian system of the planetary point masses and from an analytical solution of the poisson equations of the earth moon system the analytical expansion allows the direct determination of the main frequencies with their phase and amplitude numerical and analytical comparisons with the former astronomical solution ber78 are performed so that the accuracy and the interval of time over which the new solution is valid can be estimated the corresponding insolation values have also been computed this analysis leads to the conclusion that the new values are expected to be reliable over the last 5 ma in the time domain and at least over the last 10 ma in the frequency domain

vegetation change in equatorial west africa time slices for the last 150 ka abstract drawing on marine and terrestrial palynological data vegetation maps of equatorial west africa are presented for eleven time slices over the last 150 000 years long marine records are used as a basis for the regional picture and provide a chronology for the last glacial cycle much shorter terrestrial records help fill in the picture for the most recent periods and facilitate interpretation of regional patterns for the longer timescale temporal and spatial variation is revealed in relation to global patterns of climate change rain forest was widespread during oxygen isotope stages 1 and 5 but strongly reduced during stages 3 and 4 and especially during stages 2 and 6 when open grass rich vegetation prevailed glacial rain forest refuges are found in the southwest of the guinean mountains and along the eastern coast of the gulf of guinea miombo woodland of the zambezian vegetation zone expanded during stage 5 especially during the first half podocarpus forest had its largest expansion during substages 5d 115 to 105 ka and 5b 95 to 85 ka the last occurrence of podocarpus in the guinean mountains is during stage 5a the distribution of mangrove swamps was extremely reduced during glacial times during stage 6 savanna and open dry forest covered large areas along the northern coast of the gulf of guinea the southern saharan desert reached far to the south and the namib desert far to the north the area of rain forest was restricted mangrove swamps were strongly reduced and the area of podocarpus forest fluctuated during stage 5 large changes in the area of afromontane forest rain forest dry open forest and savanna occurred rain forest was widespread and mangroves were extensive along the coast during the last interglacial substage 5e podocarpus forest area strongly extended during substages 5d and 5b in substages 5c and 5a rain forest reclaimed areas it had lost in the previous substages 5d and 5b respectively mangrove swamps were less widespread in the later substages of stage 5 than during substage 5e during stage 4 the rain forest area was again strongly reduced and recovered only slightly during the following stage 3 also the mangrove swamp area was reduced except along the ivory coast and along the northwestern coast of the gulf of guinea podocarpus forest only occurred in angola and may be in congo during stage 4 again forest was much reduced during stage 2 and open vegetation types covered large parts of equatorial west africa mangrove swamps must have been rare at the beginning of the holocene mangrove swamps had recovered and reached their largest extension also the rain forests area increased in the early holocene and guinean and congolian rain forests were probably not separated by a savanna corridor now existing in togo and benin

oceanographic control of diatom abundances and species distributions in surface sediments of the tropical and southeast atlantic abstract this paper is a descriptive account of the diatomaceous component of pelagic sediments from the tropical and southeast atlantic ocean geographic patterns of diatom abundances whole and fragment forms per gram major species relative abundances and diatom assemblages resolved by q mode factor analysis are presented comparisons drawn to sea surface temperature salinity and productivity distributions suggest that i diatom abundances primarily reflect surface productivity with no apparent dissolution effects due to water depth ii relative abundances of major species vary in their ecologic dependence but do reflect major oceanographic features of the surface waters and iii of the five diatom assemblages resolved four finely delineate the complexity of the low latitude oceanography these assemblages in decreasing order of statistical importance are associated with 1 warm waters of moderate to high primary productivity 2 the oceanward margin of the benguela current a region of high primary production with a maximum in austral summer 3 low salinity waters off the mouths of the zaire and niger rivers and 4 subtropical to tropical waters of lower productivity mostly in the western equatorial atlantic the fifth factor represents the equatorward advection of subantarctic to antarctic diatoms by deep ocean currents

quaternary forest associations in lowland tropical west africa terrestrial fossil pollen records are frequently used to reveal the response of vegetation to changes in both regional and global climate here we present a fossil pollen record from sediment cores extracted from lake bosumtwi west africa this record covers the last c 520 thousand years ka and represents the longest terrestrial pollen record from africa published to date the fossil pollen assemblages reveal dynamic vegetation change which can be broadly characterized as indicative of shifts between savannah and forest savannah formations are heavily dominated by grass poaceae pollen 55 typically associated with cyperaceae chenopodiaceae amaranthaceae and caryophyllaceae forest formations are palynologically more diverse than the savannah with the key taxa occurring in multiple forest zones being moraceae celtis uapaca macaranga and trema the fossil pollen data indicate that over the last c 520 ka the vegetation of lowland tropical west africa has mainly been savannah however six periods of forest expansion are evident which most likely correspond to global interglacial periods a comparison of the forest assemblage composition within each interglacial suggests that the holocene 11 0 ka forest occurred under the wettest climate while the forest which occurred at the time of marine isotope stage 7 probably occurred under the driest climate

a 30 000 yr record of land ocean interaction in the eastern gulf of guinea abstract a 30 000 yr dinocyst and pollen record from the eastern equatorial atlantic off cameroon has been investigated in order to identify land ocean linkages during the last deglacial transition a strong correlation between the abundance of brigantedinium spp and the ca fe ratio during the last glacial period suggests enhanced marine productivity in association with cool seawater temperatures and nutrient input linked to coastal upwelling and or a proximal river mouth dry conditions are recorded on the adjacent continent with a significant representation of open vegetation indicators and the afromontane taxon podocarpus after 17 cal ka bp these indicators register a sharp decline as a result of a climatic transition from the dry cooler conditions of the last glacial period to the wetter warmer conditions of the deglaciation simultaneously dinocysts show a significant shift from dominant heterotrophs to an increasing abundance of autotrophs reflecting warmer conditions significant changes are observed during the younger dryas with a return to drier conditions and higher salinities the start of the holocene is marked by very low salinity conditions reflecting optimal monsoonal conditions over west equatorial africa the end of the african humid period is observed between 6 and 5 cal ka bp followed by significant fluctuations in both terrestrial and oceanic proxies

high resolution last deglaciation record from the congo fan reveals significance of mangrove pollen and biomarkers as indicators of shelf transgression abstract high abundances of mangrove pollen have been associated with transgressive cycles on tropical margins but the detailed relations between systems tracts and the taphonomy of the pollen are unclear we report here the occurrence and high abundance of rhizophora pollen in association with taraxerol a rhizophora sourced biomarker from a high resolution congo fan core covering the last deglaciation an age model based on 14c dates enables the temporal changes in taraxerol content and the percentage frequencies and flux pollen grains pg cm 2 103 yr 1 of mangrove pollen to be compared quantitatively with the lateral rate of transgression across the flooding surface derived from glacio hydro isostatic model output and the bathymetry of the margin rhizophora pollen concentrations and taraxerol content of the sediment are very strongly positively correlated with the lateral rate of transgression and indicate independently of any sequence stratigraphic context that mangrove pollen spikes are associated with the transgressive systems tract rather than the highstand systems tract or maximum flooding surface lower resolution longer term records from this margin indicate an association between taraxerol concentrations and transgressive rather than regressive phases the flux of these materials to the congo fan is interpreted as a function of the erosion of flooded mangrove swamp on the shelf and less importantly changing extent of mangrove habitat during sea level rise congo river palaeoflood events also result in reworking of mangrove pollen and supply to the fan but this mechanism is subdominant rhizophora pollen has been underestimated in many palynological studies undertaken on cores from the african margin because of inappropriate sieve mesh size used during laboratory preparation

rapid palaeoceanographic changes in the benguela upwelling system for the last 160 000 years as indicated by abundances of planktonic foraminifera abstract two sediment cores retrieved from the continental slope in the benguela upwelling system geob 1706 19 33 7 s 11 10 5 e and geob 1711 23 18 9 s 12 22 6 e reveal striking variations in planktonic foraminiferal abundances during the last 160 000 years these fluctuations are investigated to assess changes in the intensity and position of the upwelling centres off namibia four species make up over 95 of the variation within the core and enable the record to be divided into episodes characterized by particular planktonic foraminiferal assemblages the fossil assemblages have meaningful ecological significance when compared to those of the modern day and the relationship to their environment the cold water planktonic foraminifer neogloboquadrina pachyderma sinistral n pachyderma s dominates the modern day coastal upwelling centres and neogloboquadrina pachyderma dextral and globigerina bulloides characterize the fringes of the upwelling cells globorotalia inflata is representative of the offshore boundary between newly upwelled waters and the transitional reduced nutrient levels of the subtropical waters in the fossil record episodes of high n pachyderma s abundances are interpreted as evidence of increased upwelling intensity and the associated increase in nutrients the n pachyderma s record suggests temporal shifts in the intensity of upwelling and corresponding trophic domains that do not follow the typical glacial interglacial pattern periods of high n pachyderma s abundance describe rapid discrete events dominating isotope stages 3 and 2 the timing of these events correlates to the temporal shifts of the angola benguela front jansen et al 1997 situated to the north of the walvis ridge absence of high abundances of n pachyderma s from the continental slope of the southern cape basin indicates that southern ocean surface water advection has not exerted a major influence on the benguela current system the coincidence of increased upwelling intensity with the movement of the angola benguela front can be interpreted mainly by changes in strength and zonality of the trade wind system

dinoflagellate cyst distribution in high latitude marine environments and quantitative reconstruction of sea surface salinity temperature and seasonality a data base of 179 reference sites documents the relations between the assemblages of organic walled dinoflagellate cysts and sea surface temperature salinity and seasonality throughout the north atlantic adjacent subpolar basins labrador sea baffin bay irminger and iceland basins and epicontinental environments off eastern canada estuary and gulf of st lawrence hudson bay principal component analyses show close relationships between dinoflagellate cyst data and sea surface conditions the first component 71 1 of the variance correlates with the winter temperature salinity and seasonal duration of sea ice cover whereas the second component 11 3 of the variance appears mainly related to summer temperature transfer functions using the best analogue method were tested by reconstructing modern sea surface conditions on the basis of the reference dinoflagellate cyst assemblages the correlation coefficient between instrumental averages and reconstructed values ranges from 0 87 august tem

reconstruction of sea surface conditions at middle to high latitudes of the northern hemisphere during the last glacial maximum lgm based on dinoflagellate cyst assemblages abstract a new calibration database of census counts of organic walled dinoflagellate cyst dinocyst assemblages has been developed from the analyses of surface sediment samples collected at middle to high latitudes of the northern hemisphere after standardisation of taxonomy and laboratory procedures the database comprises 940 reference data points from the north atlantic arctic and north pacific oceans and their adjacent seas including the mediterranean sea as well as epicontinental environments such as the estuary and gulf of st lawrence the bering sea and the hudson bay the relative abundance of taxa was analysed to describe the distribution of assemblages the best analogue technique was used for the reconstruction of last glacial maximum lgm sea surface temperature and salinity during summer and winter in addition to sea ice cover extent at sites from the north atlantic n 63 mediterranean sea n 1 and eastern north pacific n 1 three of the north atlantic cores from the continental margin of eastern canada revealed a barren lgm interval probably because of quasi permanent sea ice six other cores from the greenland and norwegian seas were excluded from the compilation because of too sparse assemblages and poor analogue situation at the remaining sites n 54 relatively close modern analogues were found for most lgm samples which allowed reconstructions the new lgm results are consistent with previous reconstructions based on dinocyst data which show much cooler conditions than at present along the continental margins of canada and europe but sharp gradients of increasing temperature offshore the results also suggest low salinity and larger than present contrasts in seasonal temperatures with colder winters and more extensive sea ice cover whereas relatively warm conditions may have prevailed offshore in summer from these data we hypothesise low thermal inertia in a shallow and low density surface water layer

marine production in the congo influenced se atlantic over the past 30 000 years a novel dinoflagellate cyst based transfer function approach abstract the sediments from the congo deep sea fan contain valuable information about past environmental conditions of the east equatorial atlantic during the transition from the last glacial maximum lgm to present day climatic conditions the high resolution marine organic walled dinoflagellate cysts dinocysts and terrestrial pollen palynological records from two cores off equatorial west africa covering the last 30 000 years document three major phases in surface productivity 1 during the lgm relative sea level was low nutrient enrichment due to seasonal coastal upwelling prevailed off the congo mouth and high aridity prevailed in the catchment area 2 at around 15 2 cal ka bp monsoonal precipitation strengthened over the congo basin generating high river discharges river induced upwelling and increased nutrient flux to the ocean in parallel erosion of shelf sediments during shelf transgression further enhanced nutrient flux 3 at around 9 8 cal ka bp rainforest vegetation inhibited soil erosion depleted nutrient supply and restricted marine productivity to its modern levels the study presents the application of modern analogues and a dinocyst transfer function to the reconstruction of primary palaeoproductivity pp in a region of freshwater influence a database of recent dinocyst assemblages comprising of 208 sites in the equatorial atlantic enabled the reconstruction of sea surface annual pp 1 3 times higher during the lgm and also during the deglaciation between 15 2 and 13 2 cal ka bp the reconstruction is independent of and thus provides corroboration for other biological and geochemical proxies

sea ice cover sea surface salinity and halo thermocline structure of the northwest north atlantic modern versus full glacial conditions abstract seasonal sea ice develops along the eastern continental margins in the northern north atlantic where freshwater and or meltwater outflow are responsible for relatively low salinity in surface waters and very pronounced water mass stratification sea ice constitutes a major parameter in the marine ecosystem since the duration and extent of its seasonal spreading constrain the plankton distribution and the related microfossil assemblages on the sea floor organic walled dinoflagellate cysts that are highly resistant to dissolution were recovered from surface sediments of the northern north atlantic and used to develop transfer functions best analogue method for the reconstruction of the seasonal spreading and duration of sea ice cover in addition to salinity and temperature of the warmest month of the year application of the best analogue approach to cores from the labrador sea reveals large variations in sea ice cover and sea surface conditions throughout the last glacial stage and during the early holocene isotopic analyses in epipelagic and mesopelagic planktonic foraminifers also suggest important changes in salinity and temperature gradients between the surface and sub surface water masses specific study of the last glacial maximum lgm time slice 16 20 ka on a 14 c time scale in the northwestern north atlantic shows much more extensive sea ice than at present with perennial sea ice lying along the continental margins of eastern canada seasonal spreading of the cover of sea ice offshore was accompanied by large seasonal contrasts in temperature with very cold winters but relatively warm summers a pattern linked to strong stratification between a buoyant low saline surface layer having a low thermal inertia and the underlying intermediate oceanic waters

african monsoonal precipitation modulated by interhemispheric temperature gradients the deglacial pattern of a high resolution alkenone derived temperature record from the southeastern tropical atlantic was compared with one from the northeastern subtropical atlantic during termination 1b surface waters in the eastern tropical atlantic 6 s reached modern temperatures whereas those in the subtropical northeastern atlantic 21 n were still considerably colder than today this asymmetrical warming may reflect meltwater induced reductions in the northward heat transport and helps to explain rapid changes in monsoonal precipitation over africa during the early holocene

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

nw african climate variations during the last 47 000years evidence from organic walled dinoflagellate cysts nw african climate shows orbital and millennial scale variations which are tightly connected to changes in marine productivity we present an organic walled dinoflagellate cyst dinocyst record from a sediment core off cape yubi at about 27 n in the canary basin covering the time period from 47 to 3 ka before present bp the dinocyst record reflects differences in upwelling intensity and seasonality as well as the influence of fluvial input sea level changes play an important role for the upwelling pattern and productivity signals at the core site within the studied time interval four main phases were distinguished 1 from 45 to 24 ka bp when sea level was mostly about 75 m lower than today high relative abundances of cysts of heterotrophic taxa point to enhanced upwelling activity especially during heinrich events while relatively low dinocyst accumulation rates indicate that filament activity at the core location was strongly reduced 2 at sea level lowstand during the lgm to h1 dinocyst accumulation rates suggest that local filament formation was even more inhibited 3 from the early holocene to about 8 ka bp extraordinary high accumulation rates of most dinocyst species especially of lingulodinium machaerophorum suggest that nutrient supply via fluvial input increased and rising sea level promoted filament formation at the same time the upwelling season prolongated 4 a relative increase in cysts of photoautotrophic taxa from about 8 ka bp onwards indicates more stratified conditions while fluvial input decreased our study shows that productivity records can be very sensitive to regional features from the dinocyst data we infer that marine surface productivity off cape yubi during glacial times was within the scale of modern times but extremely enhanced during deglaciation

major and trace elements of river borne material the congo basin the congo river basin is the second largest drainage basin in the world after the amazon the materials carried by its main rivers provide the opportunity to study the products of denudation of a large fraction of the upper continental crust of the african continent this paper presents the chemical composition of the different phases carried in the congo rivers and is followed by a companion paper devoted to the modelling of major and trace elements the congo river between bangui and brazzaville as well as its main tributaries including a few organic rich rivers also called black rivers were sampled during the 1989 high water stage the three main phases suspended load dissolved load and bedload were analysed for twenty five major and trace elements concentrations normalized to the upper continental crust show that in each river suspended sediments and dissolved load are chemical complements for the most soluble elements ca na sr k ba rb and u while these elements are enriched in the dissolved loads they are considerably depleted in the corresponding suspended sediments this is consistent with their high mobility during weathering another type of complementarity is observed for zr and hf between suspended sediments and bedload related to the differential velocity of suspended sediments and zircons which are concentrated in bedloads compared to other rivers absolute dissolved concentrations of ca na sr k ba rb and u are remarkably low surprisingly high dissolved concentrations are found in the congo waters for other trace elements e g rees especially in the black rivers on a world scale these concentrations are among the highest measured in rivers and are shown to be ph dependent for a number of dissolved trace elements the dissolved loads are systematically normalized to the suspended loads for each river in order to remove the variations of the element abundances owing to source rock variations normalized diagrams for rees are presented and extended to the other elements they strongly support the argument that the apparent higher solubility of trace elements in the congo waters is due to the presence in the dissolved load of a colloidal phase as a result of 0 2 m filtration an important result is that these colloids are strongly depleted in fe and al with respect to the other elements finally the comparison of the dissolved suspended and sandy transport fluxes of each element in the congo basin rivers shows that although the proportions of for example the rees in the dissolved loads of the majority of the congo basin rivers is close to 10 of the total transport flux up to 80 of the rees are transported by the so called dissolved load in the black rivers

environmental control of pollen grain distribution patterns in the gulf of guinea and offshore nw africa the areas of marine pollen deposition are related to the pollen source areas by aeolian and fluvial transport regimes whereas wind transport is much more important than river transport pollen distribution patterns ofpinus artemisia chenopodiaceae amaranthaceae and asteraceae tubuliflorae trace atmospheric transport by the northeast trades pollen transport by the african easterly jet is reflected in the pollen distribution patterns of chenopodiaceae amaranthaceae asteraceae tubuliflorae andmitracarpus grass pollen distribution registers the latitudinal extension of sahel savannas and dry open forests marine pollen distribution patterns of combretaceae melastomataceae alchornea andelaeis reflect the extension of wooded grasslands and transitional forests pollen from the guinean congolian zambezian forest and from the sudanian guinean vegetation zones mark the northernmost extension of the tropical rain forest rhizophora pollen in marine sediments traces the distribution of mangrove swamps only near the continent pollen ofrhizophora mitracarpus chenopodiaceae amaranthaceae and pollen from the sudanian and guinean vegetation zones are transported by the upwelling under current and the equatorial under current where those currents act as bottom currents the distribution of pollen in marine sediments reflecting the position of major climatic zones desert dry tropics humid tropics can be used in tracing climatic changes in the past

global variations of surface ocean productivity in low and mid latitudes influence on co2 reservoirs of the deep ocean and atmosphere during the last 21 000 years based on detailed reconstructions of global distribution patterns both paleoproductivity and the benthic 13c record of co2 which is dissolved in the deep ocean strongly differed between the last glacial maximum and the holocene with the onset of termination i about 15 000 years ago the new export production of low and mid latitude upwelling cells started to decline by more than 2 4 gt carbon year this reduction is regarded as a main factor leading to both the simultaneous rise in atmospheric co2 as recorded in ice cores and with a slight delay of more than 1000 years to a large scale gradual co2 depletion of the deep ocean by about 650 gt c this estimate is based on an average increase in benthic 13c by 0 4 0 5 the decrease in new production also matches a clear 13c depletion of organic matter possibly recording an end of extreme nutrient utilization in upwelling cells as shown by sarnthein et al 1987 the productivity reversal appears to be triggered by a rapid reduction in the strength of meridional trades which in turn was linked via a shrinking extent of sea ice to a massive increase in high latitude insolation i e to orbital forcing as primary cause

increased upwelling off portugal during the last glaciation diatom evidence abstract quantitative studies have been carried out on opaline microfossils and diatom assemblage composition and preservation in two latest quarternary 100 ka to p d cores from the continental slope of portugal the sites are located in an area of modern summer coastal upwelling created by northerly trade winds the spatial patterns and intensity of this upwelling system are accurately recorded by the diatom abundances and taxonomic composition in the surface sediments the diatom record in these sequences indicates that coastal upwelling intensity was similar to present day during late holocene interglacial stage 3 and late stage 5 higher during stage 2 and lower in the early holocene paleoproductivity estimates are consistent with the diatom accumulation rates from stage 3 to the recent this indicates that both the diatom and the organic carbon content of the sediments mainly reflect the biological productivity in the euphotic zone

land sea comparisons during the last glacial interglacial transition pollen records from west tropical africa abstract land sea correlation off nw africa is investigated from both modern and late pleistocene holocene pollen records pollen spectra from modern soil surface samples from senegal and mauritania and from modern bottom sediment samples from the offshore eastern atlantic 21 12 n record accurately the latitudinal vegetation zonation of west tropical africa by showing the maximum percentages of the phytogeographical groups and the same characteristic taxa as the adjacent vegetation zones a strong correlation between marine core m 16017 2 near cap blanc and continental sebkha of chemchane mauritania pollen signals is also observed for the evolution of the late glacial holocene vegetation zonation at 21 n the differing sensitivities of each sedimentary environment for recording local and or subcontinental vegetation changes are discussed

marine dinocysts acritarchs and less well known npp tintinnids ostracod and foraminiferal linings copepod and worm remains

integrated marine and terrestrial evidence for abrupt congo river palaeodischarge fluctuations during the last deglaciation we present a high resolution reconstruction of tropical palaeoenvironmental changes for the last deglacial transition 18 to 9 cal kyr bp based on integrated oceanic and terrestrial proxies from a congo fan core pollen grass cuticle pediastrum and dinoflagellate cyst fluxes sedimentation rates and planktonic foraminiferal 18o ratios u37k sea surface temperature and alkane alkenone ratio data highlight a series of abrupt changes in congo river palaeodischarge a major discharge pulse is registered at around 13 0 cal kyr bp which we attribute to latitudinal migration of the intertropical convergence zone itcz during deglaciation the data indicate abrupt and short lived changes in the equatorial precipitation regime within a system of monsoonal dynamics forced by precessional cycles the phases of enhanced congo discharge stimulated river induced upwelling and enhanced productivity in the adjacent ocean copyright 2001 john wiley sons ltd

reconstructing pathways of aeolian pollen transport to the marine sediments along the coastline of sw africa the distribution of pollen in marine sediments is used to reconstruct pathways of terrigenous input to the oceans and provides a record of vegetation change on adjacent continents the wind transport routes of aeolian pollen is comprehensively illustrated by clusters of trajectories isobaric 4 day backward trajectories are calculated using the modelled wind field of echam3 and are clustered on a seasonal basis to estimate the main pathways of aeolian particles to sites of marine cores in the south eastern atlantic trajectories and clusters based on the modelled wind field of the last glacial maximum hardly differ from those of the present day trajectory clusters show three regional and two seasonal patterns determining the pathways of aeolian pollen transport into the south eastern atlantic ocean mainly transport out of the continent occurs during austral fall and winter when easterly and south easterly winds prevail south of 25 s winds blow mostly from the west and southwest and aeolian terrestrial input is very low generally a good latitudinal correspondence exists between the distribution patterns of pollen in marine surface sediments and the occurrence of the source plants on the adjacent continent the northern angola basin receives pollen and spores from the congolian and zambezian forests mainly through river discharge the zambezian vegetation zone is the main source area for wind blown pollen in sediments of the angola basin while the semi desert and desert areas are the main sources for pollen in sediments of the walvis basin and on the walvis ridge a transect of six marine pollen records along the south western african coast indicates considerable changes in the vegetation of southern africa between glacial and interglacial periods important changes in the vegetation are the decline of forests in equatorial africa and the north of southern africa and a northward shift of winter rain vegetation along the western escarpment

distribution of dinoflagellate cysts in surface sediments of the northern north atlantic in relation to nutrient content and productivity in surface waters abstract analyses of dinoflagellate cyst assemblages from the surface sediments of 371 sites from the north atlantic ocean were performed in order to define relationships with nitrate phosphate silica and productivity in the upper water layer statistical analyses reveal close links between dinocyst assemblages and nitrate content in february that provide a measurement of nitrate availability amongst the taxa in the assemblages nematosphaeropsis labyrinthus which positively correlates with nutrients shows an eutrophic relationship whereas impagidinium aculeatum impagidinium patulum spiniferites ramosus spiniferites mirabilis and lingulodinium machaerophorum have a rather oligotrophic relationship on these grounds transfer functions were tested using multiple regressions and the best analogue technique to reconstruct nitrate content both methods yielded reasonable estimates and allow the reconstruction of the nitrate concentrations in february with an accuracy better than 1 45 mol m 3 at the scale of the study area there are also tenuous links between the dinocyst assemblages and productivity transfer functions were tentatively tested for the reconstruction of annual productivity and yield estimates with an accuracy of about 26 5 gc m 2 year 1 these results demonstrate that dinocyst assemblages can be used for the reconstruction of productivity and nitrates

an overview and brief description of common marine organic walled dinoflagellate cyst taxa occurring in surface sediments of the northern hemisphere abstract organic walled resting cysts of planktonic dinoflagellates occur commonly in modern marine sediment where they represent with rare exceptions the only geologically preservable part of the life cycle although many species do not produce fossilizable resting cysts upper quaternary sediments contain a diverse cyst record that is used frequently for paleoenvironmental reconstruction and stratigraphic analysis reconstructions of past sea surface conditions rely on an understanding of the distributions of dinoflagellate cysts in modern sediments linked to their respective environmental parameters underpinned by sound taxonomy and standardized nomenclature stratigraphic studies additionally require knowledge of morphological details to distinguish extant from similar extinct taxa here 51 dinoflagellate cyst species and two informal cyst morphotypes that are commonly encountered during routine palynological analysis of upper quaternary marine sediments from the northern hemisphere are briefly described taxonomically and illustrated in addition their lowest stratigraphic occurrences are provided

anomalous occurrences of neogloboquadrina pachyderma left in a 420 ky upwelling record from walvis ridge se atlantic abstract core t89 40 eastern walvis ridge between the subtropical gyre and benguela coastal upwelling system contains three types of levels of abundant left coiled neogloboquadrina pachyderma a cold eutrophic species next to subtropical species type a peaks 362 110 and 53 43 ky bp are accompanied with high percentages of other eutrophic species they are attributed to intensified upwelling in the northern benguela region type b peaks 129 and 92 ky bp are accompanied by moderate the highest c peaks up to 38 are associated with strongly reduced percentages of other eutrophic species and with abundant subtropical species marine isotopic stage 11 3 401 ky and 9 3 326 ky the subtropical species preceeded the c peaks by ca 8 ky we argue that the c peaks were not produced by local reproduction but expatriated from the northern benguela upwelling cells here more nutrient rich waters may have produced a mono specific neogloboquadrina pachyderma left fauna during strong polewards shifts of the frontal systems in the south atlantic which could have been transported 700 km offshore to the core location unadmixed with eutrophic species from the surrounding waters we propose meandering shelf edge jets strong contour jets as a mechanism for the transport the timing of the c peaks and associated subtropical peaks agrees with the known precessional cyclicity of the se atlantic front movements and zonality of the trade winds which supports the shelf edge jet hypothesis

use of selective degradation to separate preservation from productivity the assessment of diagenetic influences on the sedimentary record is problematic despite its crucial importance for accurate environmental reconstruction and understanding of biochemical cycles we propose a general applicable method that uses differences in degradation rates of organic components to separate degradation and productivity we demonstrate this method on a southeastern atlantic sediment core covering the past 145 k y the new method solves discrepancies between existing organic matter silica and carbonate based productivity reconstructions and emphasizes the importance of bottom and pore water characteristics for transformation of the sedimentary record

late quaternary history of continental climate and the benguela current off south west africa abstract late quaternary sediments from the southwest african continental slope were analyzed for their coarse fraction diatom and clay mineral content and composition changes in the flow pattern of the benguela current were inferred from the abundance and preservation of opaline microfossils and diatom species composition the benguela current turned westward in the general latitude of 24 s during oxygen isotope stages 1 and 3 this flow pattern caused transportation of opaline microorganisms from the near shore area to the continental slope area or caused increased upwelling phenomena further offshore during oxygen isotope stage 2 on the other hand the westward deflection of the benguela current was moved further to the north two sources of terrigenous material were determined one is local with supply from the namib desert the other one is distant with supply from the orange river mouth material from the orange river mouth might have been transported to the north by the benguela current the local terrigenous supply from the namib desert increased during oxygen isotope stage 2 as evidenced by the increased amounts of terrigenous matter by an increase of illite and by greater grain sizes occurrence of wood fibres in oxygen isotope stage 3 indicates a denser continental vegetation over the present namib desert

late quaternary environmental dynamics inferred from marine sediment core geob6211 2 off southern brazil abstract vegetation and climate changes in southern brazil as well as the dynamics of the adjacent south atlantic were investigated through the analyses of pollen spores and dinocysts from marine sediment core geob6211 2 that covers the last 19 3 kyr the pollen record indicates the dominance of grassland campos in southeastern south america sesa reflecting cold and or dry conditions during the last glacial maximum forests mainly gallery forests expanded slightly during heinrich stadial 1 suggesting slightly wetter conditions a stronger expansion of the atlantic lowland rainforest is noticed in the record after ca 5 5 cal kyr bp likely due to wetter conditions the relatively high amount of exotic nothofagus pollen transported by wind rivers and then by oceanic currents northwards to the study site as well as the dinocyst brigantedinium spp indicate a noticeable influence of the brazilian coastal current from the south between 19 3 and 14 8 cal kyr bp after that the decrease in nothofagus and brigantedinium spp together with the increase in dinocyst operculodinium centrocarpum indicate that the brazil current from the north dominated the coring site the abundance of freshwater algae between ca 19 3 and 17 0 cal kyr bp suggests that the rio de la plata mouth was located close to the coring site during this period and its discharge of nutrient rich freshwaters strongly affected the upper water column sea level rise decreased this impact during the late glacial phase by moving the coastline further away from the core site the presence of the brazil current at the core site became stronger after ca 15 cal kyr bp and strongest after 9 cal kyr bp in summary the pollen spores and dinocyst records from core geob6211 2 provide important climatic records to reconstruct the environmental changes in sesa

late quaternary sea surface conditions at dsdp hole 594 in the southwest pacific ocean based on dinoflagellate cyst assemblages investigation of organic walled dinoflagellate cyst distribution in 118 surface sediments from the southern ocean documents the relatively high diversity of cyst taxa 46 species and distinct cyst assemblages in relation to surface water masses and temperature gradient the antarctic water is characterised by the dominance of selenopemphix antarctica accompanied by brigantedinium spp and impagidinium pallidum the subantarctic water association consists mainly of nematosphaeropsis labyrinthus impagidinium paradoxum and impagidinium sphaericum whereas the subtropical water association is dominated by operculodinium centrocarpum and spiniferites membranaceus multivariate analyses non metric multidimensional scaling demonstrate the relationships with sea surface conditions and enable the application of the best analogue method on the southern ocean dinoflagellate cyst data base for reconstructing past sea surface conditions the dinoflagellate cyst record from dsdp 594 45 31 41 s 174 56 88 e 1204 m southern margin of the chatham rise shows important fluctuations in species composition from glacial to interglacial over the last 140 000 yr reconstruction of winter and summer ssts reveals much lower temperature than present by 6 to 10 c during marine oxygen isotope stages 6 4 and 2 and higher temperature by 2 3 c during substage 5e copyright 2001 john wiley sons ltd journal of quaternary science

dinoflagellate cysts as environmental indicators in surface sediments from the congo deep sea fan and adjacent regions abstract forty nine surface sediment samples from the congo deep sea fan and adjacent hydrographic environments were prepared palynologically and analyzed for fossilizable dinoflagellate cysts the main objective was to investigate the extent to which cysts reflect present day hydrography and therefore may be used downcore as paleoceanographic indicators the region sampled covers a range of marine environments including the congo river plume the area of pronounced upwelling off west central africa and a nearshore offshore gradient from coastal to oceanic waters this is the first study of the effect of a major tropical river plume on associated cyst assemblages cyst counts were low particularly near the river mouth and furthest offshore a combination of statistical treatments was used to analyze the surface cyst assemblages including cluster analysis correspondence analysis and canonical correspondence analysis this helped to identify five distinctive sample groupings representing the main ecological signals in the cyst assemblage data group a dominated by spherical brown protoperidinioid cysts is an upwelling signal group b dominated by the cosmopolitan species o centrocarpum is the main signal for the river plume effect interpreted as reflecting the environmental instability this creates group c dominated by s delicatus with accessory o centrocarpum characterizes the outer plume somehow reflecting the mixing of progressively diluted river plume water and the surrounding oceanic waters group d with high proportions of impagidinium and nematosphaeropsis together with o centrocarpum and s delicatus characterizes the location of the angola benguela front group e with mainly impagidinium species is an oceanic signal the identification of nutrients and water stability as main determining factors influencing cyst signals has implications for cyst based paleoenvironmental interpretations using transfer functions which are developed almost exclusively from comparisons with more easily available data for sea surface temperature salinity and sea ice

minor element and ca isotope composition of calcareous dinoflagellate cysts of cultured thoracosphaera heimii abstract we investigated the influences of temperature salinity and ph on the calcium isotope as well as trace and minor element uranium strontium magnesium to ca ratios on calcium carbonate cysts of the calcareous dinoflagellate species thoracosphaera heimii grown in laboratory cultures the natural habitat of this species is the photic zone preferentially at the chlorophyll maximum depth of temperate to tropical oceans and it is abundant in deep sea sediments over the entire cenozoic in our experiments temperatures ranged from 12 to 30 c salinity from 36 5 to 38 8 and ph from 7 9 to 8 4 the 44 40ca of t heimii cysts resembles that of other marine calcifiers including coccolithophores foraminifers and corals however its temperature sensitivity is considerably smaller and statistically insignificant and t heimii might serve as a recorder of changes in seawater 44 40ca over geologic time the sr ca ratios of t heimii cysts show a pronounced temperature sensitivity 0 016 mmol mol c 1 and have the potential to serve as a palaeo sea surface temperature proxy no clear temperature and ph dependences were observed for mg ca u ca seems to be influenced by temperature and ph but the correlations change sign at 23 c and ph 8 2 respectively

summer monsoon intensity controls c4 c3 plant abundance during the last 35 ka in the chinese loess plateau carbon isotope evidence from bulk organic matter and individual leaf waxes the natural abundance of c3 and c4 plants is affected by multiple environmental factors including temperature moisture balance and atmospheric pco2 the relative importance of these factors is a subject of considerable debate and may vary in different natural ecosystems previous studies generally focus on single loess sequences in the chinese loess plateau and conflicting conclusions on c4 c3 have been reached when studying carbon isotope ratios in carbonate and organic matter in this paper we report a comprehensive carbon isotopic characterization of total organic carbon toc and individual higher plant leaf waxes from five loess sequences spanning the last 35 ka from the chinese loess plateau clp the five coring sites encompass large gradients of annual mean temperature 9 2 13 9 c and precipitation 402 673 mm allowing us to assess the controlling mechanisms on c4 c3 plant ratios glacial interglacial sequences provide carbon isotope data for comparison with other climatic and environmental proxies such as lithology and magnetic susceptibility our results demonstrate that increased c4 c3 ratios are positively correlated with higher temperature and increased summer rainfall which characterize stronger summer monsoon in all five sites we conclude c4 abundance increases from the last glacial to the holocene in response to greater monsoon activity and that the c4 expression is suppressed in the cold and drier intervals

evidence for congo river freshwater load in late quaternary sediments of odp site 1077 5 s 10 e abstract late quaternary fluctuations in the intensity of congo river freshwater load were reconstructed using three different proxies marine and freshwater diatoms and the 18o record of globigerinoides ruber preserved in the sediments of ocean drilling program odp site 1077 located at the northern rim of the congo river fan 5 10 s 10 26 e an abrupt change in the diatom assemblage is evident at termination ii a two to four fold increase in a the relative abundance of a marine planktonic diatom tolerant of low salinity conditions cyclotella litoralis and b in the concentration of freshwater diatoms the microfossil data suggest a change in the environmental conditions surrounding site 1077 from predominantly marine to mixed marine brackish fresh the 18o record of the planktic foraminifera g ruber pink revealed negative deviations from the global oxygen isotope signal since termination ii which occurred during warm stage 1 and substages 3 2 5 1 5 3 and 5 5 comparison of the isotopic signal of odp site 1077 with the record from a pelagic location core geob1041 at 3 48 s 7 05 w confirms these results the construction of an artificial 18o curve using alkenone derived sea surface temperature sst data from a nearby core geob1008 at 6 s 10 e allowed us to estimate salinity and temperature effects on the odp site 1077 isotopic signal although increased ssts may account for lighter 18o values during warmer periods they do not explain the extremely light values documented in the sediments of site 1077 we used the oxygen isotope difference 18o between our site and geob1041 as a proxy for freshwater input a general trend in the 18o was observed with more negative values since termination ii in addition conspicuous 18o negative pulses coincided with periods of northern hemisphere summer insolation maxima over the african continent suggesting an increase in the freshwater discharge from the congo river due to enhanced precipitation on the hinterland here we propose that the abrupt change in environmental conditions at site 1077 since termination ii is a consequence of a major reorganization in the depositional environment of the congo river delta this reorganization involved sustained equatorward displacement of the angola benguela front causing a northward deflection of the congo river plume thus moving plume waters further north than normal and over site 1077

pleistocene variations in dust input and marine productivity in the northern benguela current evidence of evolution of global glacial interglacial cycles abstract we infer variations in paleoproductivity and eolian input at odp site 1082 in the walvis basin from stable oxygen isotope compositions of the planktonic foraminifera globorotalia inflata total organic carbon mass accumulation rates toc mar and x ray fluorescence analyses of fe content the most pronounced paleoclimatic changes correspond to the time at about 0 9 ma when glacial conditions in the northern hemisphere nh led to the onset of pronounced 100 kyr glacial interglacial cycles we used fe intensity as a proxy for eolian terrigenous input and toc mar as a paleoproductivity indicator paleoproductivity and eolian input show generally higher amplitude variations of glacial interglacial cyclicity from 1 5 to 0 58 ma indicating pronounced variations in upwelling favorable winds in this area at 0 58 ma paleoproductivity and eolian input shifted abruptly to lower amplitude variations with a periodicity of 100 kyr while 18o values show a trend toward more negative isotope values for the past 0 65 myr especially during glacial periods oxygen isotope values indicate increasingly warmer sea surface temperatures toward the end of the pleistocene to evaluate the relative influences of nh glaciation and southern hemisphere sh insolation as potential forcing mechanisms for variations of eolian input and productivity in the northern benguela system we filtered our proxy records at orbital frequencies the filtered records of fe intensity and toc mar indicate a strong influence of the 100 kyr and 41 kyr frequency bands supporting our assumption that strong ice buildup in the nh is the dominant trigger for climate changes on the continent and probably in trade wind intensity sh insolation and low latitude precession related insolation changes were important for paleoproductivity variations in the northern benguela system modifying the nutrient supply by southern ocean intermediate waters and the zonal direction of upwelling inducing trades by the african monsoon system respectively

late quaternary paleoproductivity changes off the congo deduced from stable carbon isotopes of planktonic foraminifera a detailed record 2 kyr intervals of the difference in stable carbon isotopes 13c between globigerina bulloides and globigerinoides ruber pink is used to reconstruct changes in upwelling intensity off the congo river for the last 190 000 yr comparisons of the oxygen and carbon isotope data from this core with records from the niger fan and from pelagic cores in the eastern equatorial south atlantic indicate that the congo fan isotope records do not contain a strong freshwater signal as is described off other major rivers the temporal pattern of the 13c signal correlates with the marine organic carbon record from the congo fan thus the planktonic 13c record reflecting past changes in upwelling intensity and nutrient content corroborates the signal provided by sedimentary organic carbon which is presumed to indicate changes in the amount of biological productivity and export flux to the seafloor the planktonic 13c signal is characterized by a dominant 23 kyr periodicity which provides evidence for a strong response of upwelling fluctuations off the congo to precessional forcing minima in the 13c record are aligned with periods of minimum boreal summer insolation over central africa reflecting an increase of upwelling and biological productivity off the congo at periods of enhanced zonal intensity of southeast trades and corresponding weak southeast monsoon over the eastern south atlantic a strong response to changes in river discharge probably did not occur indicating that fertilization by river derived nutrients has played only a minor role with respect to late quaternary changes in the total amount of primary productivity off the congo

a 220 000 year climatic record for the east equatorial atlantic ocean and equatorial africa evidence from diatoms and opal phytoliths in the zaire congo deep sea fan accumulation rates of microfossil associations are independent of each other and in addition to percentages give indispensable information about the significance of micropaleontological data marine diatoms reveal that coastal upwelling in the zaire fan region was strong during glacial stages and weaker during interglacial stages oceanic upwelling was intense in stages 6 and 2 and at the stage 5d 5c transition but not in stage 4 the benguela current did not penetrate into the gulf of guinea during the last 220 000 years the phfd index based on the contribution of freshwater diatoms and phytoliths is a measure of aridity in equatorial africa the production in the low salinity river plume was high in the most humid periods on land and low when the continent was arid on the long term nutrient supply of the zaire river to the ocean was probably larger in humid than in arid periods periods of strong seasonal variability in river fluxes and coastal upwelling coincide with arid intervals except in the very humid interval with strong seasonal contrasts 35 000 20 000 years b p

radiocarbon calibration curve spanning 0 to 50 000 years bp based on paired 230 th 234 u 238 u and 14 c dates on pristine corals abstract radiocarbon dating is the most widely used dating technique in the world recent advances in accelerator mass spectrometry ams and sample preparation techniques have reduced the sample size requirements by a factor of 1000 and decreased the measurement time from weeks to minutes today it is estimated that more than 90 percent of all measurements made on accelerator mass spectrometers are for radiocarbon age dates the production of 14 c in the atmosphere varies through time due to changes in the earth s geomagnetic field intensity and in its concentration which is regulated by the carbon cycle as a result of these two variables a radiocarbon age is not equivalent to a calendar age four decades of joint research by the dendrochronology and radiocarbon communities have produced a radiocarbon calibration data set of remarkable precision and accuracy extending from the present to approximately 12 000 calendar years before present this paper presents high precision paired 230 th 234 u 238 u and 14 c age determinations on pristine coral samples that enable us to extend the radiocarbon calibration curve from 12 000 to 50 000 years before present we developed a statistical model to properly estimate sample age conversion from radiocarbon years to calendar years taking full account of combined errors in input ages and calibration uncertainties our radiocarbon calibration program is publicly accessible at http www radiocarbon ldeo columbia edu along with full documentation of the samples data and our statistical calibration model

diagenesis of late proterozoic carbonates the beck spring dolomite of eastern california abstract petrographic and compositional attributes of limestone and dolostone in the beck spring dolomite of eastern california indicate that late proterozoic mineralogies isotopic compositions and diagenetic processes were similar to those inferred for younger phanerozoic marine carbonate sequences allochems in limestones include strontium rich ooids with well preserved concentric banding and cortices of equant neomorphic calcite that commonly exhibit dissolution and compaction features cements include microdolomite rich columnar marine calcite and equant spar calcites are depleted relative to dolomites and become enriched with stratigraphic depth acicular and columnar marine cement and allochems in dolostones display fabrics identical to those in limestones but are isotopically distinct dolomite ooids columnar and acicular cement crystal silt and early luminescent spar cement from four stratigraphic sections define overlapping positive oxygen carbon covariant fields that trend toward depleted calcite compositions early luminescent dolomite spar cement is unconformably overlain by depleted ferroan dolospar fabric data suggest that marine cement and ooids were precipitated as magnesian calcite and aragonite isotopic data suggest that metastable components locally stabilized to calcite during subaerial exposure while laterally dolomitization took place in mixed marine meteoric waters early dolospar precipitation began during replacive dolomitization and late ferroan dolospar precipitated during burial lack of evidence for hypersaline conditions suggests that late proterozoic normal marine carbonate isotopic composition was 0 5 2 018o and 5 3 0 5913c

surface water conditions in the northern benguela region se atlantic during the last 450 ky reconstructed from assemblages of planktonic foraminifera planktonic foraminiferal records from site 1083 odp leg 175 were used to investigate changes in surface water conditions in the northern benguela region over the past 450 ky the assemblages of planktonic foraminifera are dominated by four species sinistral coiling neogloboquadrina pachyderma dextral coiling n pachyderma globigerina bulloides and globorotalia inflata besides tropical species deliver a small contribution to the assemblage the most prominent temporal variations displayed by n pachyderma s d represent changes in the coastal upwelling and the presence of cold nutrient rich waters over the core site neogloboquadrina pachyderma s d shows cyclic variability in the eccentricity and to a less extent precession frequencies the changes indicate increased upwelling intensity in glacial maxima and precession maxima and correlate well with the wind strength record of stuut et al 2002 during glacial maxima steep temperature gradients over the southern hemisphere caused strong se trade winds and strong upwelling precession maxima cause a weak monsoonal circulation more zonal se trade winds strong coastal upwelling and nutrient rich surface waters over odp site 1083 advection of angola current ac surface water into the walvis basin indicated by the tropical species occurs when the angola benguela front abf is positioned southward occasionally this happened during glacial maxima as can be explained with the reconstructed and predicted meridional movements of the abf the amount of ac water was never sufficient to suppress the marine biological production at the core site the contribution of benguela current bc water reflected by globorotalia inflata is greatly determined by the upwelling in periods of strong upwelling the bc influence is suppressed in several glacial substages the temperature of the upwelling south atlantic central water sacw may have been increased as suggested by the dominance of neogloboquadrina pachyderma d in the upwelling record this phenomenon may be due to intensified subduction in the central south atlantic that induces the formation of sacw or to larger contributions of eastern sacw to the upwelling water around 250 200 ky bp a long term shift to higher productivity occurred that is absent in the upwelling record it was accompanied with a transition from a precession and obliquity variability to an eccentricity dominated variability in the

diatom record of late quaternary climatic change in the eastern equatorial atlantic and tropical africa over the last 160 000 years diatom productivity in the equatorial atlantic has been high during glacial stages 2 4 and 6 and moderate during parts of substages 5b and 5d approximately 96 000 and 117 000 years b p productivity of diatoms is particularly low in substage 5e and in the middle of stage 1 higher productivity implies more vigorous trade winds during glacial intervals leading to stronger upwelling and enhanced lateral advection of surface waters the influence of the benguela current near the equator was strongest during stage 2 a separate regime of coastal upwelling may have existed in the gulf of guinea at this time upwelling and advection may not always respond to the same forcing as sea surface temperature results support the hypothesis that ethmodiscus rex oozes result from high diatom productivity runoff from the zaire and niger rivers was highest at the stage 2 1 transition and just prior to the stage 6 5 transition due to intensification of the african monsoon several other times of increased runoff are indicated periods of increased surface water circulation in the eastern equatorial atlantic appear to correspond to times of aridity in tropical africa conversely intervals of decreased circulation in the eastern equatorial atlantic coincide with times of humidity in tropical africa precessional forcing of continental climate is suggested however precessional forcing of surface ocean circulation can be neither confirmed nor refuted based on the records examined here influx of antarctic bottom water to the eastern equatorial atlantic has been insignificant throughout the last 160 000 years