mineralogy and geochemistry of the clay fraction of sediments from the namibian continental margin and the adjacent hinterland the continental shelf off namibia can be divided into two sectors at latitude 18 40 s based on the distinctive morphologies of the two regions and on differences in composition of their surficial sediments the kunene shelf is narrow shallow and covered mainly by terrigenous sediment whereas the walvis shelf is wide deep and supports predominantly biogenic sediment the walvis shelf also possesses a number of distinguishing features its outer region has a concave profile which is attributed to upbuilding of the inner shelf during periods of lowered sea level its inner region is covered by an extensive belt of diatomaceous mud parallel to the coast the seaward and landward flanks of this deposit differ in lithology due mainly to the advanced state of dissolution of opal in deep water and to the influx of aeolian terrigenous detritus especially illite close to shore long belts of terrigenous mud extend southwards from both the kunene and orange rivers the currents responsible for transporting this sediment are poleward directed tongues of the angola benguela front in the north and mixed poleward moving south atlantic central water in the south the kunene river carries more smectite to the sea than the much larger orange the distribution of clay minerals on the continental shelf follows that of the adjacent hinterland with smectite and to a lesser extent kaolinite being significant in the north and illite predominating in the south along the upper slope the concentrations of smectite and kaolinite increase due to their importation from the kunene river by poleward directed antarctic intermediate water the geochemical character of riverine clays reflects the rock types exposed in each of the drainage basins compared with marine sediments these clays are enriched in eight of the fourteen elements investigated namely al fe mg co mn pb rb and zn in contrast the offshore element distribution patterns show little resemblance to the patterns of individual drainage basins clays in the marine sediments are enriched relative to the river sediments in ca k cd ni and sr five lithofacies recognized in the unconsolidated marine sediments are based on the relative proportions of opal calcium carbonate phosphorite glauconite and terrigenous components average element concentrations of individual lithofacies are enriched relative to the river clays in cd diatomaceous facies ca cd ni and sr calcareous and facies k and ni slightly glauconitic facies cd and sr terrigenous facies regionally the clay fractions of unconsolidated marine sediments contain high concentrations of al fe mg k mn and rb on the kunene margin due to aluminosilicates being imported by the kunene river ca and sr reach high values on the walvis outer margin in response to upwelling along the coast maximum concentrations of cd are associated with diatomaceous mud on the walvis inner shelf cu ni and zn reach high levels at a few spot locations where organic matter morg has suffered little dilution from other components especially opal and pb is concentrated with glauconitic sediment on the kunene shelf semi consolidated sediments are exposed intermittently on the outer margin they provide evidence that the palaeoclimate became much drier in post miocene times but in the north this picture is complicated by the introduction of sediment from the kunene river during the late tertiary palaeo oceanographic evidence suggests that the northern part of the study area experienced a concomitant decrease in productivity while in the southern part productivity increased

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

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

age dating and the orbital theory of the ice ages development of a high resolution 0 to 300 000 year chronostratigraphy using the concept of orbital tuning a continuous high resolution deep sea chronostratigraphy has been developed spanning the last 300 000 yr the chronology is developed using a stacked oxygen isotope stratigraphy and four different orbital tuning approaches each of which is based upon a different assumption concerning the response of the orbital signal recorded in the data each approach yields a separate chronology the error measured by the standard deviation about the average of these four results which represents the best chronology has an average magnitude of only 2500 yr this small value indicates that the chronology produced is insensitive to the specific orbital tuning technique used excellent convergence between chronologies developed using each of five different paleoclimatological indicators from a single core is also obtained the resultant chronology is also insensitive to the specific indicator used the error associated with each tuning approach is estimated independently and propagated through to the average result the resulting error estimate is independent of that associated with the degree of convergence and has an average magnitude of 3500 yr in excellent agreement with the 2500 yr estimate transfer of the final chronology to the stacked record leads to an estimated error of 1500 yr thus the final chronology has an average error of 5000 yr

quantitative analysis of detrital modes statistically rigorous confidence regions in ternary diagrams and their use in sedimentary petrology abstract detrital modes are the basic type of quantitative data used in sedimentary petrology they represent estimates of the petrographic framework composition of sand or sandstone obtained by point counting of thin sections at present geological inferences from analysis of these numerical data are semi quantitative only because many of the data processing methods employed in sedimentary petrology lack a firm theoretical basis a review of existing methods reveals a number of fundamental statistical problems associated with the use of univariate statistics and the construction of so called hexagonal fields of compositional variation it is shown that these problems can be overcome by using multivariate methods that honour the non negativity and unit sum constraints on compositional data and incorporate the covariance structure spread in compositional data can be modelled on two levels 1 by regarding the grain as unit of observation and the total point count as a sample from a multinomial distribution 2 by regarding the total count as unit of observation and a set of counts as a sample from an additive logistic normal distribution numerical methods are presented for constructing statistically rigorous confidence regions in ternary diagrams by inversion of significance tests for these two distributions statistical analyses of the framework composition and heavy mineral assemblages of modern beach and river sands illustrate their use the case histories are followed by a brief overview of popular multivariate methods for reconstructing sediment provenance

the erbutu ni cu deposit in the central asian orogenic belt a permian magmatic sulfide deposit related to boninitic magmatism in an arc setting the erbutu ni cu sulfide deposit is hosted in a small ultramafic intrusion located at the southern margin of the central asian orogenic belt inner mongolia this deposit is the first case of a magmatic sulfide deposit related to boninitic magmatism in an arc setting the host intrusion is composed of orthopyroxenite and olivine orthopyroxenite more than three quarters of the intrusion contains economic disseminated sulfide mineralization shrimp u pb dating of zircons from the intrusion gives a crystallization age of 294 2 2 7 ma similar to the age of the permian tarim plume in northwest china olivine orthopyroxenite is characterized by olivine crystals with relatively high fo contents 85 88 mol and extremely low ca contents 250 ppm coexisting orthopyroxene crystals have clinoenstatite composition with enstatite from 86 to 88 mol whole rock data show characteristic enrichments of light ree relative to heavy ree pronounced negative nb ta anomalies and moderately positive hf anomaly the 34s values of sulfide separates are 4 to 5 significantly higher than the typical mantle value 0 2 the hf values of zircons are from 4 to 9 these data indicate that the parental magma was a boninite crustal contamination was involved during magma evolution and ore formation

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

the timing of linear dune activity in the strzelecki and tirari deserts australia abstract linear dunes occupy more than one third of the australian continent but the timing of their formation is poorly understood in this study we collected 82 samples from 26 sites across the strzelecki and tirari deserts in the driest part of central australia to provide an optically stimulated luminescence chronology for these dunefields the dunes preserve up to four stratigraphic horizons bounded by palaeosols which represent evidence for multiple periods of reactivation punctuated by episodes of increased environmental stability dune activity took place in episodes around 73 66 35 32 22 18 and 14 10 ka intermittent partial mobilisation persisted at other times throughout the last 75 ka and dune activity appears to have intensified during the late holocene dune construction occurred when sediment was available for aeolian transport in the strzelecki and tirari deserts this coincided with cold arid conditions during marine isotope stage mis 4 late mis 3 and mis 2 and the warm dry climates of the late pleistocene holocene transition period and late holocene localised influxes of sediment on active floodplains and lake floors during the relatively more humid periods of mis 5 also resulted in dune formation the timing of widespread dune reactivation coincided with glaciation in southeastern australia along with cooler temperatures in the adjacent oceans and antarctica

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

orbital forcing of climate over south africa a 200 000 year rainfall record from the pretoria saltpan late pleistocene variations in rainfall in subtropical southern african are estimated from sediments preserved in the pretoria saltpan a 200000 year old closed basin crater lake on the interior plateau of south africa we show that south african summer rainfall covaried with changes in southern hemisphere summer insolation resulting from orbital precession as predicted by orbital precession geometry berger 1978 this south african record is out of phase with north african palaeomonsoon indices street and grove 1979 rossignol strick 1983 mcintyre et al 1989 the amplitude of the rainfall response to insolation forcing agrees with climate model estimates prell and kutzbach 1987 these results document the importance of direct orbital insolation forcing on both subtropical north and south african climate as well as the predicted antiphase sensitivity to precessional insolation forcing

controls on terrigenous sediment supply to the arabian sea during the late quaternary the makran continental slope the input of terrigenous sediment along the tectonically active makran continental margin off south western pakistan gulf of oman northern arabian sea is studied on the basis of sediment cores distributed along a transect from the upper slope to the abyssal plain spatial and temporal variations in sediment composition sedimentation rate and turbidite frequency in late pleistocene holocene time last 20 14c ka will be discussed and related to changes in sea level and climate and tectonic activity end member modelling of the grain size distributions of the hemipelagic and turbiditic sediments indicate that the sediments are adequately described as mixtures of three end members which represent turbidite sand turbidite silt or eolian dust and fluvial mud the geochemical and mineralogical compositions of the hemipelagic sediments indicate that the eolian dust was dominantly supplied from the northern arabian peninsula and the persian gulf region and that the fluvial input is from the makran margin the ratio of contributions of eolian and fluvial sediment in the hemipelagic intervals is used as an indicator of continental aridity i e summer monsoon intensity highest holocene turbidite frequencies and sedimentation rates are recorded at the deepest coring sites they are related to the proximity of the deformation front of the accretionary prism turbidite sedimentation on the upper continental slope was most frequent during the last glacial period of sea level lowstand and continued during the entire deglaciation and sea level rise infrequent turbidite sedimentation occurred during the holocene highstand of sea level turbidite sedimentation during the period of late sea level rise and the holocene sea level highstand is inferred to be due to the strong impact of episodic events i e flash floods and earthquakes and because of the narrow shelf of the active makran continental margin

dust response to seasonal atmospheric forcing proxy evaluation and calibration eolian grain size and flux were measured on samples from 11 arabian sea sediment traps deployed 200 1250 km offshore the timing of increased grain size is coincident with the onset of strong summer monsoon winds and dust storm activity over the arabian peninsula and middle east data spanning a full annual cycle show that eolian grain size is highly correlated with barometric pressure r 0 91 and wind speed r 0 84 enabling calibration of the downcore record in terms of these primary meteorological variables eolian flux is highly correlated with organic carbon flux r 0 80 both increase 6 8 weeks after the grain size increase and summer monsoon onset this lag and the low correlation between eolian grain size and eolian flux r 0 36 likely result from the differential sinking rates of large and small dust particles in the surface waters as well as biological scavenging associated with monsoon induced productivity marily to aridity and vegetation cover at the source and 3 that grain size is primarily a function of atmospheric transport capacity these findings are based on statistical relation ships linking dust flux to glacial interglacial climate change 5 o and grain size to biological indicators of wind driven upwelling such relationships are well documented and have provided a reasonably robust framework for interpreting the downcore dust record rea 1994 provides an excellent overview however direct evaluation of links between at mospheric dynamics dust aerosols and deep sea sediments in the modem environment are needed to better understand

dune activity as a record of late quaternary aridity in the northern kalahari new evidence from northern namibia interpreted in the context of regional arid and humid chronologies abstract sediments from presently vegetated linear dune ridges in the caprivi strip of northern namibia are optically dated to assess the timing of dune building phases and aridity in the late quaternary the dunes have been constructed from sediments likely to have been initially transported into the area by fluvial processes dune sediments do not preserve obvious evidence of hiatuses in deposition in the dated period 48 121 ka but continuous deposition and therefore continuous aridity is considered unlikely given differences in dated phases compared with chronologies from western zimbabwe to the southeast and western zambia to the north that include ages derived from the same equivalent sampling depths laboratory experiments provide evidence that dune sediments have not been bioturbated since deposition such that sample palaeodoses determined from different sampling depths are a true reflection of the duration of sediment burial independent chronologies from cave sediments provide evidence that humid episodes have occurred in the region within the dated range of dune construction it is concluded that dune construction in the late quaternary has been punctuated that the preserved record reflects the attributes of the aeolian processes that affect linear dune development and that the late quaternary record of dune building in the middle and northern kalahari was spatially complex

relocating the historical 1951 hualien earthquake in eastern taiwan based on tide gauge record summary a sequence of large earthquakes happened along the longitudinal valley in eastern taiwan in 1951 thefirstthree andlargest earthquakestookplaceneartheharbourcityofhualienatthe northerntipofthevalleyduringthefirstday thesecondofwhich eventii severelydamaged the city the epicentre of event ii had previously been allocated at some 15 km offshore which would result in a vertical subsidence of the hualien city according to model calculations for the supposed seismic source mechanism however the hualien tide gauge record speaks the opposite the site experienced a co seismic uplift by 255 mm due to event ii we conduct a series of numerical grid search simulations and determine a most likely scenario for the earthquake sequence we assert that event ii s hypocentre was actually on land rather than offshore right along the recognized active milun fault below the city of hualien at a very shallow depth this result advances our understanding of the spatio temporal seismotectonic behaviour associated with these historical events and is important towards the assessment of the potential seismic and tsunami hazards of the region

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

synthetic reconstructions of southern african environments during the last glacial maximum 21 18kyr and the holocene altithermal 8 6kyr abstract the pash database has been assembling palaeoclimatic data from the southern hemisphere for a number of years the southern african sector now contains more than 100 records spanning the last 200 000 years a number of these relate to the last glacial maximum and holocene altithermal which represent the two most recent extremes in global temperatures syntheses of these records have permitted the preparation of preliminary palaeoenvironmental maps for the two intervals the results provide insights into probable changes in atmospheric circulation as temperatures across the subcontinent rose by 6 7 c these changes are in agreement with a present day model which has been developed to explain short term variations in southern african climate the results presented here should assist in calibrating models developed to predict future climate change

late quaternary sedimentation on the eastern walvis ridge se atlantic hpc 532 and four piston cores five sediment cores from the eastern walvis ridge have been investigated by means of a quantitative coarse fraction analysis 40 m numerous biogenous terrigenous and authigenic particles have been distinguished the variations in composition suggest the following conclusions core walda ks 12 from the northern flank of the walvis ridge consists entirely of a slump which contains mainly glacconite sands and biogenous opal foraminiferal skeletons and terrigenous matter the sediments in the other four cores comprise the late quaternary oxygen isotope stages 1 6 there are several levels at the transition from warm to cold intervals where benthos plankton foraminiferal ratios increase from 10 up to 50 90 here imperforate benthonic foraminifera decrease strongly dissolution of planktonic foraminifera can only partly explain this finding lateral supply of benthos or the influence of reduced oxygen content in the bottom water might have played an additional role calcium carbonate dissolution as revealed by the fragmentation of planktonic foraminifera shows intermediate values in coldest and warmest periods dissolution maxima occur at warm cold stage boundaries dissolution minima have been found at the transition from cold to warm periods upwellinginfluence is reflected in varying amounts of biogenous opal in interglacial periods the influence of upwelling is strong in all cores with an increase in intensity to the east core v19 259 it is explained by eddies of upwelled water which drifted westward under the influence of the westward turning benguela current in glacial periods the influence of upwelling at the core sites was very reduced the benguela current problably was close to the continent and turned westward only at 17 s thus it did not affect the core sites climate in the near coastal area has been arid in the latitude of 20 s throughout the investigated time slice accumulation rates reflect the position of the cores hpc 532 and circe gb 211 are in the area of the sediment filled trough which forms the walvis ridge terrace here accumulation rates are 2 to 3 times higher in stages 2 4 and 5 than in v19 259 and walda ks 09 which are from the steeper flank of the terrace outside the trough mean values of calcium carbonate dissolution are similar in all cores lower caco3 content in v19 259 and lower sand fraction percentages are explained by lateral supply of fine grained terrigenous matter in the area of core v19 259 which probably received insoluble material from the kuene shelf area by the n s flowing countercurrent

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

size variation in the cape dune molerat bathyergus suillus and late quaternary climatic change in the southwestern cape province south africa abstract within their historic range at the southwestern tip of africa cape dune molerats bathyergus suillus tend to be significantly larger in areas of higher rainfall they also tend to be large in late quaternary fossil samples associated with independent evidence for relatively moist climate and small in samples associated with evidence for relatively dry conditions together with sedimentologic geomorphic and other faunal observations fluctuations in fossil dune molerat size imply that the regional climate was very moist during an early late quaternary interval corresponding to marine isotope stage 4 74 000 to 59 000 yr b p or perhaps to substage 5b or 5d centered on 110 000 and 90 000 yr b p respectively it was comparably moist again from roughly 14 000 to 8 000 yr b p somewhat drier than at present from about 8000 until 4000 yr b p and mainly near the modern average after 4000 yr b p together the modern and fossil data suggest that dune molerat size is a reliable index of past precipitation but it may not be useful for revealing extremely arid conditions because these seem to be associated with depositional occupational gaps in most local paleontological archeological sites

seasonal impact of mineral dust on deep ocean particle flux in the eastern subtropical atlantic ocean abstract seasonal lithogenic particle and al fluxes were obtained from a deep ocean sediment trap deployment during 1992 and 1993 off nw africa and were compared concurrently with atmospheric al concentrations and two dimensional backward trajectories of windfields from two barometric levels in the lower and mid troposphere marine al fluxes lithogenic particle fluxes and grain size distributions in the area were found to be directly linked to airmass pathways and surface mineral aerosol concentrations at 1000 m water depth highest al fluxes 10 77 mg m 2 day 1 lithogenic particle fluxes 99 25 mg m 2 day 1 and smallest mean grain sizes 11 9 m occurred during the winter and spring season concurrent with highest atmospheric dust load and al concentrations 15 300 ng m 3 in the lower troposphere a strong seasonal change of the main atmospheric dust transport from low altitude winds during winter spring to higher altitudes during summer is clearly reflected at depth by a significant coarsening of mean grain sizes 18 6 m and lowest al 0 81 mg m 2 day 1 and lithogenic particle fluxes 11 3 mg m 2 day 1 found in the sediment traps the comparison of marine atmospheric and model derived data used within this study highlights the close temporal coupling between atmospheric dust transport and the deep ocean particle stock

punctuated aridity in southern africa during the last glacial cycle the chronology of linear dune construction in the northeastern kalahari abstract the mega kalahari of central southern africa is one of the most extensive quaternary desert basins on a regional scale present day aeolian activity is restricted to episodic dune crest reactivation in the most arid southwestern desert core there is however abundant evidence of former periods of both more arid and more humid conditions many of which have little or no chronological control we have employed optical dating of quartz sand grains to develop a chronology of arid intervals as recorded by phases of linear dune construction in the northeastern sector of the mega kalahari we identify repeated phases of aeolian deposition during the last interglacial glacial cycle at ca 95 115 41 46 20 26 and post 20 ka which are separated by depositional hiatuses that we infer to correspond to more humid periods these aeolian depositional events correlate with and are inferred to relate to millennial scale cold sea surface temperature events in the southeast atlantic which have been linked to sub milankovitch climate changes recognised in northern hemisphere oceanic and cryospheric environmental archives covering the same time period while the present landscape is the product of either post 20 ka hwange national park dune field or 20 30 ka victoria falls dune field aeolian activity and subsequent erosion and reworking much of the vertical expression of the larger dune forms corresponds to the earlier periods of activity the linear ridges of the area are rich archives of late quaternary terrestrial climate change

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

climatic conditions in southern africa since the last glacial maximum inferred from pollen analysis abstract a review of late quaternary palaeoclimatic data derived from several pollen sequences between ca 22 and 34 s in southern africa shows a degree of similarity in temperature and moisture variations between the various site pollen data from sites such as wonderkrater and rietvlei transvaal tate vondo venda elim orange free state equus cave southern kalahari and boomplaas cape province suggest relative dryness during the last glacial maximum wet conditions during the late glacial and dry conditions during the early holocene ca 8000 yr b p followed by progressively moister conditions peaking soon after the development of a temperature optimum ca 6500 yr b p problems with radiocarbon dating of polleniferous spring deposits however prevent precise correlations of especially late pleistocene sequences different seasonal patterns in the late glacial and early holocene may explain palaeobotanical data and are partly compatible with simulations of past climates of kutzbach and guetter 1986

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

holocene climatic optimum in southwest africa evidence from the marine clay mineral record abstract sediment composition grain size and clay mineral record of a high resolution sediment core from the continental slope off namibia was investigated to gain information on the deposition of terrigenous matter in this part of the southwest african continental margin during the last 18 k y the depositional processes involved are fluvial input by the kunene river and eolian input from the namib and kalahari deserts each supplying characteristic mineral suites during low sea level erosion of the exposed shelf yields additional material the amount of eolian or fluvial matter depends on the strength of the transport process which is a function of aridity or humidity of the source area thus allowing paleoclimatic interpretations arid conditions prevailed during a low sea level from 18 to 15 ka and unconsolidated shelf sediment was mobilized and supplied to the slope by short distance transport by southerly winds a dramatic increase in the accumulation of terrigenous sediment is recorded from 15 to 10 ka without major changes in sediment composition which is attributed to increased runoff of the kunene river and fluvial erosion of shelf sediments this period coincides with a strengthening of the monsoonal system during a precessional minimum which is observed in numerous sites in central africa and indicates an intensified influence of the monsoon at the kunene headwaters a distinct shift in clay mineralogy towards river derived material marks a second period of increased river runoff during high sea level from 9 to 5 ka indicating maximum humidity in the source area from 6 to 5 ka this corresponds to the holocene climatic optimum observed in the arid belts of northern africa the present balance between fluvial and eolian input was reached approximately at 4 ka

deep sea sediments of the arabian sea a paleoclimatic record of the southwest asian summer monsoon modern summer sea surface temperatures sst of the arabian sea show a correlation to holocene accumulation rates of biogenic opal this relation is used to infer mean summer sst for the holocene 24 28 c and the last glaciation reduced accumulation rates of biogenic opal suggest higher sst during glacial times which can be interpretated as a record of reduced upwelling intensity