production and destruction control of biogenous sedimentation in the tropical atlantic 0 300 000 years b p late pleistocene signals of calcium carbonate organic carbon and opaline silica concentration and accumulation are documented in a series of cores from a zonal meridional depth transect in the equatorial atlantic ocean to reconstruct the regional sedimentary history spectral analysis reveals that maxima and minima in biogenous sedimentation occur with glacial interglacial cyclicity as a function of both 1 primary production at the sea surface modulated by orbitally forced variation in trade wind zonality and 2 destruction at the seafloor by variation in the chemical character of advected intermediate and deep water from high latitudes modulated by high latitude ice volume from these results a pattern emerges in which the relative proportion of signal variance from the productivity signal centered on the precessional 23 kyr band decreases while that of the destruction signal centered on the obliquity 41 kyr and eccentricity 100 kyr periods increases below 3600 m ocean depth

phytoliths indicators of grassland dynamics during the late holocene in intertropical africa abstract the reconstruction of african tropical grassland history during the late holocene can be carried out using phytolith analysis fossil phytolith assemblages from lake guiers in the sahelian region of senegal and from lake sinnda in the guineo congolian region of congo were investigated the results are interpreted on the basis of modern phytolith assemblages from the same regions and compared to pollen data previously obtained tall or short grass associations are discriminated by their phytolith index iph saddle cross dumbel saddle while the density of shrubs and trees is indicated by relative proportions of the dicotyledon phytoliths the phytolith data emphasize that in the guineo congolian region around lake sinnda the driest phase of the late holocene occurred between 4000 and 1200 yr b p commencing with the opening of the dense forest and its replacement by a short grass savanna from ca 1000 yr b p wetter climatic conditions developed as represented by the setting up of a tall grass savanna woodland the modern shrub and tall grass savanna was developed ca 700 yr b p in the sahelian region around lake guiers the driest phase occurred after about 2000 yr b p and has not been followed by moister conditions a tall grass savanna woodland was gradually replaced by a shrub and short grass savanna which still occurs a short period of development of swampy vegetation which can be related to a lake level change interrupted the semi arid adaptation of the vegetation between about 2000 yr b p and the present

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

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

impacts of rapid sea level rise on mangrove deposit erosion application of taraxerol and rhizophora records we investigated a well dated marine sediment core from the tropical se atlantic covering the last 25 kyr applying taraxerol and rhizophora pollen as organic geochemical and palynological proxies for mangrove respectively taraxerol records are positively correlated with rhizophora pollen records showing an enhanced supply of mangrove materials into deep sea environments during the last deglaciation termination i sedimentation rates peaked during meltwater pulses 1a and 1b which were associated with the maxima of taraxerol and rhizophora pollen this study supports the view that mangrove input was dominantly controlled by erosion of mangrove rich shelf sediments during the transgressions whether reworked materials were penecontemporaneous or from much older deposits formed during previous sea level cycles is discussed copyright 2005 john wiley sons ltd

woody vegetation and channel morphogenesis in low gradient gravel bed streams in the ozark plateaus missouri and arkansas woody vegetation affects channel morphogenesis in ozark streams of missouri and arkansas by increasing local roughness increasing bank strength providing sedimentation sites and creating obstructions to flow variations in physiographic controls on channel morphology result in systematic changes in vegetation patterns and geomorphic functions with increasing drainage basin area in upstream reaches streams have abundant bedrock control and bank heights that typically are less than or equal to the rooting depth of trees in downstream reaches where valleys are wider and alluvial banks are higher vegetation has different geomorphic functions at drainage areas of greater than 100 200 km2 ozarks streams are characterized by longitudinally juxtaposed reaches of high and low lateral channel migration rates referred to as disturbance reaches and stable reaches respectively whereas stable reaches can develop stable forested floodplains if they are not farmed disturbance reaches are characterized by dynamic vegetation communities that interact with erosion and deposition processes disturbance reaches can be subdivided into low gradient and high gradient longitudinal zones low energy zones are characterized by incremental unidirectional lateral channel migration and deposition of gravel and sand bars the bars are characterized by prominent bands of woody vegetation and ridge and swale topography channel monitoring data indicate that densely vegetated bands of woody vegetation formed depositional sites during bedload transporting events the same floods caused up to 20 m of erosion of adjacent cutbanks scoured non vegetated areas between vegetation bands and increased thalweg depth and definition in high energy or riffle zones channel movement is dominantly by avulsion in these zones vegetation creates areas of erosional resistance that become temporary islands as the channel avulses around or through them woody vegetation on islands creates steep root defended banks that contribute to narrow channels with high velocities calculation of hydraulic roughness from density and average diameter of woody vegetation groups of different ages indicates that flow resistance provided by vegetation decreases systematically with group age mainly through decreasing stem density if all other factors remain constant the stabilizing effect of a group of woody vegetation on a gravel bar decreases with vegetation age

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

west african monsoon variability during the last deglaciation and the holocene evidence from fresh water algae pollen and isotope data from core kw31 gulf of guinea abstract the study of aquatic and mangrove pollen grains and algae from freshwater environments from core kw31 recovered off the mouth of the niger river 3 31 1 n 05 34 1 e 1181 m water depth documents large scale changes in river discharge linked to variations of atlantic monsoon rainfall during the last glacial interglacial transition and the holocene two main taxa dominate the algal record pediastrum which records the strongest river input from 14 500 to 7100 cal b p and botryococcus which indicates surficial runoff in more arid periods particularly at the onset of the deglaciation and during the younger dryas several dry events punctuated the first and more intense phase of the so called african humid period 14 500 cal b p 9800 cal b p during the younger dryas 13 400 12 100 cal b p then around 11 400 11 200 cal b p indicating major climate instability freshwater discharges decreased after 7100 cal b p and definitely dropped to modern values after 3500 cal b p in agreement with land data the microfossil record of core kw31 shows that the increase in freshwater input to the ocean and correlative rainfall over western tropical africa were gradual in response to insolation forcing at the transition from glacial to interglacial situations

200 000 yr diatom records from atlantic upwelling sites reveal maximum productivity during lgm and a shift in phytoplankton community structure at 185 000 yr abstract time series of diatom accumulation rates dar and assemblage composition for the last 200 000 yr from the atlantic major coastal upwelling loci of the eastern boundary currents and eastern equator are compared together with independent proxies of export production to upper ocean productivity and to nutrient concentration and utilization fluctuations in dar are concomitant at all sites and in good correspondence to the other independent productivity proxies indicating glacial periods as the most productive times of the past 200 000 yr in both hemispheres of the eastern atlantic maximum productivity about one order of magnitude higher than at present occurred at the last glacial maximum lgm despite the atlantic similarities in both coastal and equatorial upwelling types important regional and site specific variations emerge in the southern hemisphere site a contradiction of proxies diatoms and corg is noted at the base of stage 6 185 000 yr the good correlation found between the corg and the benthic foraminifera fluxes is interpreted as a reflection of a shift in the phytoplankton community structure related to either a change in the n p si ratio or similar nutrient conditions associated with increased water column stratification

paleo environmental change in amazonian and african rainforest during the lgm abstract the paper provides new and comparative insight into the ecological history of the two largest continental tropical forest areas during the last glacial maximum lgm the tropical forest regions are of particular interest because they present a large source of heat and have been shown to have significant impact on the extra tropical atmospheric circulation they are also the most intense land based convective centers thus especially from the tropics paleoecological information is needed as benchmarks for climate modeling the african data for lgm climates were published earlier including the reconstructed paleoprecipitation patterns deduced from ssts the tropical south american lgm data were interpreted from pollen geochemical and 18o stable oxygen isotope data from brazil and selected surrounding areas the available terrestrial data are consistent with the sst derived precipitation data for the tropical forests in brazil and for africa however the impact of lgm climate extremes was less severe in the amazon than in the congo basin the lgm humid forest area including evergreen and semi deciduous forest types in africa was probably reduced by 84 in contrast the amazon humid forest area probably shrank to 54 of their present day extension still there are different interpretations with respect to the amount of reduction of the amazon forest area during the lgm although direct information about lgm climates in amazonia is still limited the more detailed map obtained in the present work however allows a more reliable characterization of the last glacial tropical environment than previously published for the amazon region

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

thermoluminescence age determinations for the mungo iii human burial lake mungo southeastern australia abstract thermoluminescence dating work was carried out on quartz from unburnt sediment associated with the mungo iii human burial site in southeastern australia using selective bleach and total bleach methods a comparison of the techniques showed that in some cases incomplete removal of the thermoluminescence tl signal used for dating sediments had occurred and in these cases selective bleach was the more reliable of the two methods the selective bleach results indicate the age of the burial to be older than 24 6 2 4 ka and younger than 43 3 3 8 ka while the total bleach results give ages for the burial between 34 0 3 9 ka and 43 1 6 7 ka the ages place the mungo iii human remains with the same population or older than the previously discovered human remains of mungo i

selective preservation of organic walled dinoflagellate cysts as a tool to quantify past net primary production and bottom water oxygen concentrations to understand the role of the ocean within the global carbon cycle detailed information is required on key processes within the marine carbon cycle bio production in the upper ocean export of the produced material to the deep ocean and the storage of carbon in oceanic sediments quantification of these processes requires the separation of signals of net primary production and the rate of organic matter decay as reflected in fossil sediments this study examines the large differences in degradation rates of organic walled dinoflagellate cyst species to separate these degradation and productivity signals for this accumulation rates of cyst species known to be resistant r cysts or sensitive s cysts to aerobic degradation of 62 sites are compared to mean annual chlorophyll a sea surface temperature sea surface salinity nitrate and phosphate concentrations of the upper waters and deepwater oxygen concentrations furthermore the degradation of sensitive cysts as expressed by the degradation constant k and reaction time t has been related to bottom water o2 the studied sediments were taken from the arabian sea north western african margin north atlantic western equatorial atlantic ocean caraibic south western african margin south atlantic and southern ocean atlantic sector significant relationships are observed between a accumulation rates of r cysts and upper water chlorophyll a concentrations b accumulation rates of s cysts and bottom water o2 and c degradation rates of s cysts kt and bottom water o2 relationships that are extremely weak or are clearly insignificant on all confidence intervals are between 1 s cyst accumulation rates and chlorophyll a concentrations sea surface temperature sst sea surface salinity sss phosphate concentrations p and nitrate concentrations n 2 between r cyst accumulation rates and bottom water o2 sst sss p and n and between 3 kt and water depth co variance is present between the parameters n and p n p and chlorophyll a oxygen and water depth correcting for this co variance does not influence the significance of the relationship given above the possible applicability of dinoflagellate cyst degradation to estimate past net primary production and deep ocean ventilation is discussed 2007 published by elsevier b v

a 26 000 year integrated record of marine and terrestrial environmental change off gabon west equatorial africa abstract dinoflagellate cysts pollen and charred cuticle fragments from two sediment cores giant piston calypso core md03 2708 and giant gravity casq core md03 2708cq collected off the ogooue river mouth gabon 01 10 33 s 08 19 01 e 920 m water depth have been analysed to identify the direction and timing of marine terrestrial environmental changes over western equatorial africa during the last 26 000 yr changes in the proxy records indicate that both terrestrial and oceanic domains off gabon were impacted synchronously by significant climate changes during the last glacial interglacial transition and holocene leading into and during the last glacial maximum lgm heterotrophic dinoflagellate cysts and ca x ray fluorescence intensity suggest nutrient enrichment off the river mouth while pollen records indicate expansions of open forest savannah woodland and the afromontane forest in the catchment area from the deglaciation to mid holocene however a marked decrease in brigantedinium spp as well as in poaceae cyperaceae and podocarpus pollen reflects a reduction in nutrient supply to the coastal ocean in parallel with a reduction in grassland herbaceous communities and afromontane forest within the catchment a sharp decline or even disappearance of heterotrophic species during this period is almost contemporaneous with an appearance of operculodinium aguinawense reflecting enhanced river influence in the study area a marked increase of rhizophora mangrove pollen during this transition also indicates eustatic sea level rise after the lgm which forced a major expansion of the mangrove ecosystem across the gradually submerging shelf the combination during the late holocene of a reoccurrence of heterotrophic dinoflagellate cysts with increased poaceae cyperaceae and podocarpus pollen indicates enhanced nutrient supply to the ocean concordant with a shift to cooler drier conditions over the gabon basin this appears to be linked to a deterioration of the monsoon system induced by the low sea surface temperatures in the tropical atlantic ocean in association with decreased summer insolation

interhemispheric comparison of deglacial sea surface temperature patterns in atlantic eastern boundary currents abstract sediment core geob 1023 5 from the eastern south atlantic was investigated at high temporal resolution for variations of sea surface temperature sst during the past 22 kyr using the alkenone u k 37 method ssts increased by 3 5 c from about 18 c during the last ice age 21 2 cal kyr bp to about 21 5 c at 14 5 cal kyr bp this warming trend associated with the deglaciation phase was followed by a cooling event with lowest ssts near 20 c persisting for about 1000 years between 13 and 12 cal kyr bp the ssts then continued to increase to about 22 5 c at the holocene climatic optimum at 7 cal kyr bp and decreased again during the late holocene to a core top value of 19 8 c that is comparable to modern annual mean sst values when compared with alkenone sst records from the eastern north atlantic our sst record indicates continuous warming throughout the deglaciation phase in the benguela current while its northern counterpart the canary current experienced prominent cooling during heinrich event 1 h1 on the other hand for the time period corresponding to the younger dryas yd cooling event the benguela sst record exhibits a cold temperature interval that corresponds to that observed in the eastern north atlantic sst records this observation suggests that interhemispheric climate response in atlantic eastern boundary current systems was different with respect to the two abrupt climate events associated with termination i for the h1 the eastern south atlantic sst record strongly supports the hypothesis that an anti phase thermal behavior in south atlantic surface waters was forced by the slowdown of the north atlantic deep water formation during cold spells in the north atlantic in contrast the abrupt cooling in the eastern south atlantic coincident with the yd period was probably induced by more vigorous global atmospheric circulation enhancing the upwelling intensity in both eastern boundary current systems this atmospheric control may have overridden any effect caused by changes in thermohaline circulation on the south atlantic ssts during the yd which leads to the assumption that the thermohaline circulation was already much closer to its interglacial mode during the yd than during the h1

oversampling of sedimentary series collected by giant piston corer evidence and corrections based on 3 5 khz chirp profiles 1 the depth scale accuracy of marine sedimentary series collected by coring is of key importance for the precise calculation of sedimentation rates and fluxes for three giant piston cores collected during the interpole md99 114 international marine past global changes study images v cruise md99 2227 md99 2246 and md99 2251 the 3 5 khz chirp profiles recorded on board are compared to synthetic seismograms computed from physical property logs measured on cores in each case the perfect match of main deep reflectors requires a significant upward shift of the water sediment w s interface in the synthetic seismograms with respect to the 3 5 khz profiles since no drastic perturbation of the physical property logs is observed this upward shift is interpreted as resulting from a significant sediment oversampling in the upper part of the cores the affected depth intervals are consistent with the thickness of the perturbed zones observed in penetrometry and anisotropy of magnetic susceptibility records 10 15 m to retrieve the true in situ sediment thickness a linear depth correction is applied between consecutive acoustic reflectors to achieve a perfect match between the synthetic seismogram and the corresponding 3 5 khz profiles depth correction laws amount of material excess as a function of initial depth are deduced from this resynchronization procedure first estimations of upper core oversampling rates range from 30 core md99 2227 to 37 core md99 2246 moreover we observe that some undersampling may also exist in the lower part of the sediment cores

mid holocene climate change in lake bosumtwi ghana lake bosumtwi is one of the most widely studied palaeoclimate archives in west africa results from numerous ams 14c dates of samples from four piston cores from lake bosumtwi show that an abrupt sedimentary transition from a mid holocene sapropel to calcareous laminated muds occurred at about 3200 cal yr b p high resolution analyses of the nitrogen isotopic composition of organic matter across this transition confirm its abrupt nature and suggest that the change may signal a step toward increased aridity and intensified surface winds that affected western equatorial africa from ghana to the congo basin northern and eastern africa experienced a similar abrupt shift toward aridity during the late holocene but at about 5000 cal yr b p a difference in timing that illustrates the regional nature of climate changes during the holocene and the importance of feedback mechanisms in regulating holocene climate variability furthermore an abrupt change at about 3000 cal yr b p occurs at several sites adjacent to the tropical and subtropical atlantic which may hint at major changes in the surface temperatures of the tropical atlantic and or pacific at this time

mangrove palynology in recent marine sediments off cameroon abstract based on a palynological analysis of marine sediment surface samples we obtained the modern distribution of pollen and spores on the continental shelf off cameroon to examine their dispersal relative to vegetation sources on the adjacent continent special attention is paid to the mangrove pollen assemblage whose dominant type is rhizophora which is the most abundant pollen in the overall modern assemblages this mangrove grows mainly in the river deltas west and east of mount cameroon in the northern part of the study area but patches also are found along smaller estuaries further south concentrations and percentages of rhizophora are highest proximal to mangrove vegetated coastline and decrease continuously southward due to 1 the littoral position of the mangrove ecosystem 2 the prolific pollen production of rhizophora and 3 their efficient dispersal by water the other mangrove types e g avicennia pandanus and nypa along the cameroon caostline are very poorly represented in the offshore pollen assemblage the spores group was the second most common assemblage component with the highest concentrations and percentages of spores in the axis of the main estuaries common features in the distribution patterns of the two dominant taxa rhizophora and spores which are very different in terms of production zones size and morphology suggest that marine currents are crucial in distributing the palynomorphs in the open sea the variation of mangrove pollen abundance in cores on the continental shelf can be indicative of palaeogeographical modifications of the environment

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

a high resolution palaeoclimatic record for the last 27 500 years in tropical west africa from the carbon and nitrogen isotopic composition of lacustrine organic matter variations in the carbon and nitrogen isotopic composition of bulk organic matter in the sediments of lake bosumtwi ghana reflect climatically induced changes to the lake and the catchment flora cores spanning the last 27 5 kyr of sedimentation in the lake show large oscillations in 13corg pdb and 15norg air the late pleistocene record is particularly detailed showing changes of ca 20 in carbon and ca 10 in nitrogen isotopic compositions these variations are of complex origin although different in magnitude major isotopic excursions in the two records are generally in phase and reveal the occurrence of two major dry intervals at and immediately following the last glacial maximum the allerod younger dryas period also seems to have been marked by generally dry conditions in this part of tropical west africa nitrogen isotopic evidence suggests that during the period 9 2 3 2 kyr the lake had an extremely stable water column probably due to the absence of a windy or cool dry season or both regular circulation of the water column recommenced during the late holocene and has persisted until the present day

geographical distributions of organic walled dinoflagellate cysts in surficial sediments of the benguela upwelling region and their relationship to upper ocean conditions the organic walled cyst content of 41 surface sediment samples from the south eastern south atlantic ocean have been studied to create a dataset that can be used for palaeoceanographic reconstructions in order to obtain insight into which environmental factors influence the distribution of individual cyst species the cyst associations have been compared with oceanographic characteristics of the overlying water masses i e temperature salinity density and stratification gradients the associations and relationships have been established by visual examination of the dataset and the multivariate ordination techniques detrended correspondence analysis and canonical correspondence analysis special attention has been given to the factors of transport and preservation of the cysts five associations have been recognised as being characteristic of 1 areas influenced by coastal upwelling and or river outflow 2 open ocean 3 agulhas current and southern benguela current 4 benguela current and 5 walvis bay shelf break area the factors dominant in influencing either directly or indirectly the cyst distributions appear to be the stratification in the upper 50 m of the water column nutrient concentration and seasonality variations in sea surface temperatures and salinities have only minor effect on cyst distribution

climatic instability in west equatorial africa during the mid and late holocene abstract millennial scale climatic variations have punctuated the holocene characterised by abrupt changes from warm to cool or wetter to drier conditions amongst these climatic events there is increased evidence for an abrupt multicentennial shift of climatic conditions around 3 8 3 7 kyr bp 4 1 cal kyr bp in mid to low latitude regions which had a profound impact on landscape and population migration in the mediterranean region subtropical tropical and equatorial africa a number of continental proxies lake levels pollen sequences stable isotopes record this abrupt change towards drier conditions however regionalism in climatic conditions is reflected in the vegetation records possibly in relation to orographic conditions and the influence of sea surface conditions hitherto there have been very few marine sequences that record this particular climatic shift at high resolution we present here new data from the congo deep sea fan containing integrated marine and terrestrial proxies around 5 4 cal kyr bp shifts in surface conditions off the congo river mouth are observed with possible establishment of seasonal coastal upwelling and lower sea surface temperatures in parallel pollen data indicate fluctuations of herbaceous afromontane taxa and charred grass cuticles suggesting more open vegetation in the lowland regions and an increase in cloud forest and or afromontane vegetation at higher altitudes within the congolese region

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