reconstruction of megalake chad using shuttle radar topographic mission data in the 2 500 000 km2 lake chad basin in central africa the 2000 shuttle radar topographic mission srtm data have been used to supplement the existing topographic data srtm data produce much sharper images of the region s topography and provide new insights into debates about the nature and extent of late quaternary lake chad this paper shows that the accuracy of srtm30 the recently released 30 arc seconds topographic data from srtm largely surpasses that of previous global digital elevation models dems available in the region using a gis we identified from srtm30 elevation data key features in the landscape topography providing further evidence for the existence of a megalake chad the srtm30 data corroborate the presence of two ancient shorelines associated with stillstands of the paleolake at the elevation of the mayo kebbi and bahr el ghazal spillovers we found a general flattening of the topography in the region covered by megalake chad which is most likely the result of wave cut action the srtm30 data show that the remains of the highest paleoshoreline have a constant elevation of 325 5 m amsl at its maximum extent megalake chad had an area of about 340 000 km2 only 8 less than the present day world s largest lake the caspian sea the srtm30 data also revealed ancient drainage networks in the sahara that lead to megalake chad we compiled available 14c dates to constrain holocene megalake chad events the results presented in this paper have significant consequences for improving our knowledge of regional paleohydrology and continental climate change this study is also the first step for a gis based reconstruction of late quaternary paleohydrology in tropical africa

abrupt return of the summer monsoon 15 000 years ago new supporting evidence from the lower white nile valley and lake albert abstract the last glacial period ended with an abrupt return to warmer and wetter conditions at a number of sites in intertropical africa asia australia and south america 15 000 years 15 ka ago similar abrupt warming is also evident at this time in high latitudes and in air trapped in greenland ice we here report new supporting evidence of this event from a climatically sensitive region in africa the white nile valley of the central sudan and lake albert in the uganda headwaters during the last glacial maximum the white nile valley was even more arid than it is today with desert dunes active as far south as latitude 12 s the sudden overflow of lake victoria in the ugandan headwaters of the white nile 14 5 ka confirmed here by sr isotope statigraphy caused extensive flooding in the lower white nile valley and severe flooding in egypt the volume of fresh water flowing into the eastern mediterranean at this time ultimately resulted in accumulation of highly organic sediments sapropel 1 on the floor of the mediterranean sea flooding in the central sudan attained an elevation of 382 m along a north south distance of 400 km the recessional shoreline of the 382 m white nile is indicated by concentrations of now buried freshwater gastropod shells and beach sands dated respectively by calibrated ams radiocarbon and osl to 15 16 ka this mega flood event marks the abrupt return of the summer monsoon and may reflect a globally synchronous event

high resolution fluvial records of holocene environmental changes in the sahel the yame river at ounjougou mali west africa abstract the yame river in the bandiagara plateau dogon country mali is characterised by extensive alluvial sedimentary records particularly in the 1 km long ounjougou reach where holocene floodplain pockets are inset in the pleistocene formations these alluvial records have been investigated via geomorphologic fieldwork and sedimentologic and micromorphologic analyses and are supported by 79 radiocarbon dates the alluvial deposits of the valley floor correspond to a vertical accretion of 3 10 m the reconstruction of fluvial style changes provides evidence of four main aggradation periods from 11 500 to 8760 cal bp the alluvial architecture and grain size parameters indicate a wandering river this period included phases of pulsed high energy floods and avulsion related to a northward shift of the summer monsoon to around 14 n after 11 500 cal bp from 7800 to 5300 cal bp a swampy floodplain environment with standing water pools within a sudanian savanna woodland mosaic corresponds to the culmination of the holocene humid period from 3800 cal bp onwards rhythmic sedimentation attests to an increase in the duration and or intensity of the dry season giving a precise date for the local termination of the holocene optimum period during the last two millennia and for the first time during the holocene the alluvial formations are progressively restricted whereas the colluvial deposits increase indicating strong soil erosion and redeposition within the watershed related to an increase in human impact four major periods are characterised by incision i1 ante 11 500 i2 8760 7800 i3 6790 6500 cal bp i4 2400 1700 cal bp pointing to dramatic changes in fluvial style they result from high energy flood flows during dry spells and confirm the capacity of the floodplain pocket in the upstream reach of the sahelian belt to record rapid holocene climatic change

late quaternary vegetation and climate of the sahel abstract pollen and phytogeographic evidence provides a vegetational history of the sahel for the period 0 18 000 yr b p the zonal vegetation fluctuated latitudinally and its most extreme positions occurred at 18 000 and 8500 yr b p the first involved a southward shift of the sahelian wooded grassland to 10 n under the arid conditions of the last glacial maximum the second change shows a rapid northward migration of humid vegetation guinean elements reach 16 n and sahelo sudanian elements extend to the southern margin of the modern sahara 21 n when the atlantic monsoon flux increased in the middle holocene the extensive spread of sudanian elements into the modern sahelian zone suggests the appearance of a markedly dry season the modern sahelian semiarid conditions appeared abruptly at 2000 yr b p

late quaternary floods and droughts in the nile valley sudan new evidence from optically stimulated luminescence and ams radiocarbon dating our results show that the late pleistocene nile in northern sudan was shifting position and actively aggrading at 145 20 kyr 83 24 kyr 32 8 kyr and 20 7 0 2 kyr and indicate for the first time a phase of high energy flow in the white nile at 27 8 3 2 kyr with still high but somewhat reduced flow in that river at 13 3 kyr 10 kyr and 4 8e4 0 kyr beach ridges associated with a 386 m strandline of the white nile have osl ages of 27 5 2 7 kyr and 14 5 1 6 kyr the holocene terraces and former channels of the main nile have ages of 11 kyr 6 5e5 0 kyr and 4 8e4 0 kyr after which there was a general decline in flood discharge the now arid main nile valley in northern sudan was significantly wetter during the early to middle holocene with a lake up to 450 km 2 in area fed by an overflow channel from the early holocene nile between 9 5 kyr and 7 5 kyr previously stable late pleistocene dunes were reactivated at intervals during the holocene with five samples from the white nile valley indicating brief phases of holocene dune activity at 9 9 2 0 kyr 9 0 2 8 kyr 6 6 0 9 kyr 4 8 0 9 kyr and 2 9 0 5 kyr the earliest of which occurred within periods of generally wetter climate and higher nile flow the youngest freshwater shells on the khor abu habl alluvial fan west of the white nile correspond to a time of regionally wetter climate between 1 7 and 1 0 kyr our results suggest that millennial scale climatic instability may have been characteristic of holocene climates in this region 2010 elsevier ltd all rights reserved

late quaternary climate and vegetation of the sudanian zone of northeast nigeria the lake tilla crater lake in northeastern nigeria 10 23 n 12 08 e provides a ca 17 000 14c yr multiproxy record of the environmental history of a sudanian savanna in west africa evaluation of pollen diatoms and sedimentary geochemistry from cores suggests that dry climatic conditions prevailed throughout the late pleistocene before the onset of the holocene the slow rise in lake levels was interrupted by a distinct dry event between ca 10 900 and 10 500 14c yr b p which may coincide with the younger dryas episode the onset of the holocene is marked by an abrupt increase in lake levels and a subsequent spread of guinean and sudanian tree taxa into the open grass savanna that predominated throughout the late pleistocene the dominance of the mountain olive olea hochstetteri suggests cool climatic conditions prior to ca 8600 14c yr b p the early to mid holocene humid period culminated between ca 8500 and 7000 14c yr b p with the establishment of a dense guinean savanna during high lake levels frequent fires were important in promoting the open character of the vegetation the palynological and palaeolimnological data demonstrate that the humid period terminated after ca 7000 14c yr b p in a gradual decline of the precipitation evaporation ratio and was not interrupted by abrupt climatic events the aridification trend intensified after ca 3800 14c yr b p and continued until the present

mid and late holocene fluvial chronology of tunisia abstract regarding fluvial response to recent climate change it is important to identify and understand previous changes of fluvial dynamics related to past climatic shifts in this paper periods of increased flood activity in central tunisian river systems are presented changes in holocene flood dynamics are reconstructed through a comprehensive analysis of 195 radiocarbon dates from alluvial archives holocene floodplain deposits in central tunisia normally consists of cohesive overbank fines which exhibit excellent preservation conditions against lateral erosion and therefore permit chronostratigraphic records of the entire holocene at most based on published radiocarbon dates two 14 c cumulative probability density plots are created for central tunisia the former represents activity dates indicating phases of increased fluvial dynamics the latter documents stability dates showing periods of soil formation and enhanced landscape stability within the floodplain and the catchment due to geomorphological considerations activity dates and stability dates should be mutually exclusive there is a good match for the mid holocene with increased periods of flooding from 6 2 to 6 0 4 8 to 4 5 4 1 to 3 7 and 3 3 to 3 0 ka these flood periods coincide with low soil formation signals on the other hand periods of increased alluvial soil formation from 6 8 to 6 6 5 9 to 4 8 4 5 to 4 1 3 7 to 3 3 3 0 to 2 8 and 1 8 to 1 6 ka coincide with periods of low flood plain aggradation the data reveal a coupling with north atlantic cooling short term periods of enhanced fluvial dynamics in central tunisia match well with north atlantic cooling the findings indicate a strong climatic link from 4 7 ka until today compared to the holocene floodplain dynamics in tunisian drylands the temperate mid latitudes show mid holocene switches in fluvial dynamics with opposite signs especially after 5 ka north atlantic driven flooding in the temperate central european river systems contrast sharply to flood dynamics in arid to semiarid mediterranean tunisia while under temperate climate conditions floodplain sedimentation decreases it increases under more arid conditions this may indicate that fluvial archives of dissimilar climatic zones reveal different threshold values and sedimentation patterns and therefore different responses to climatic shifts

the early holocene palaeoenvironment of ounjougou mali phytoliths in a multiproxy context abstract the site complex ounjougou on the dogon plateau mali comprises sediments up to 100 000 years old with numerous pleistocene and holocene sequences the site ravin de la mouche 11 4 10 2 ka is of special archaeological significance because in its early holocene deposits pottery sherds have been found which are among the oldest in africa for a better understanding of the environmental conditions which might have contributed to the innovation of pottery making a multi proxy approach was applied to the sediments of ravin de la mouche including phytoliths pollen palynofacies micromorphology and charcoal the multi proxy approach also allows reconstructing the complex taphonomy of the site in our phytolith study we applied a combination of the general and the indices approaches we recorded a maximum of morphotypes and used the summarized data for a calculation of the indices d p ic and iph in comparison with modern surface samples and data from other african phytolith studies with the general approach a number of morphotypes could be detected which are useful in describing the late pleistocene and the early holocene vegetation phytoliths were extracted from the pleistocene base and the early holocene layers ha1 ha2 and ha3 the pleistocene sediment samples with an age of ca 30 40 ka bp have no grass short cell phytoliths gscp and their composition is difficult to interpret ha1 is a coarse fluvial deposit with mainly redeposited phytoliths of pleistocene origin the palaeosoil in ha2 contains phytolith assemblages developed in situ from a terrestrial plant cover the vegetation was an open tropical grassland and a gallery forest with palms and marantaceae annuals from the grass subfamilies chloridoideae and panicoideae probably with a low biomass production dominated the grassland this might explain the insignificant role of fire as indicated by the very low number of micro charcoals ha3 results from a rhythmic deposition of alluvial sediments pointing to pronounced seasonality of rainfall and discharge it contains pollen charcoal and phytolith assemblages with a similar composition as in ha2 the early holocene annual grassland on the dogon plateau probably harboured a high number of species from the grass subfamily panicoideae with edible grains we suggest that the massive expansion of useful panicoid grasses during the early holocene triggered the development of important cultural innovations mainly pottery production cooking wild cereal grains in a ceramic container would have enabled a very effective exploitation of the vast sahelo sudanian grasslands which remained to be successful until modern times

late quaternary river development in the southwest chad basin osl dating of sediment from the komadugu palaeofloodplain northeast nigeria the komadugu river system is the major nigerian tributary to lake chad its large alluvial complex represents an important archive of the late quaternary environmental history of the chad basin modern floodplains are incised into an older fluvial terrace that extends over 5000 km2 evidence from satellite images suggests that the ancient river system was dammed up by the bama beach ridge during high stands of mega lake chad this caused the formation of extensive wetlands with a chaotic network of relic channels the first set of luminescence ages presented here fits well into the previously established environmental history of the chad basin both the early and mid holocene pluvial periods as well as a final relatively humid period during the late holocene are reflected by point bar to overbank deposits or channel fills the onset of the holocene was characterised by erosion of older dune fields as indicated by grain size distributions of the alluvium satellite images and sedimentological observations show that the floodplain was dominated by meandering channels shifted by frequent avulsion reflecting a high variability in precipitation and discharge patterns at the beginning and end of holocene humid periods copyright 2007 john wiley sons ltd

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

late quaternary sediment fluxes from tropical watersheds abstract inherited saprolite stores and continued weathering in quaternary time juxtapose abundant clay and fresh rock in tropical landscapes this influences sediment fluxes and affects the interpretation of sediment sequences derived from tropical watersheds detrital kaolinites derive from inherited saprolite sources as well as from soil clays and appear in delta and ocean sediments these sediments appear to correspond with sub milankovitch millennial scale cycles of climate change but may also record century scale episodes of rapid warming dansgaard oeschger events destabilisation of sediment sources and increased sediment fluxes in the late quaternary followed millennia of climatic deterioration cooling aridity and vegetation change and led to altered patterns of sedimentation during the last glacial maximum lgm sediment yield from slopes increased 10 around the lgm when rainfall was reduced by 30 60 and led to fan building and braided channels rainfall increased 40 80 from the lgm to the early holocene maximum and this led to channel cutting and major sediment fluxes to delta and ocean sinks vegetation recovery lagged the rapid warming by several millennia and was interrupted by younger dryas yd aridity influencing slope and stream behaviour holocene sedimentation has been by both vertical and lateral accretion increasing floodplain sediment stores

geoarchaeology the four dimensional 4d fluvial matrix and climatic causality abstract geoarchaeology is the application of geological and geomorphological techniques to archaeology and the study of the interactions of hominins with the natural environment at a variety of temporal and spatial scales geoarchaeology in the uk over the last twenty years has flourished largely because it has gone beyond technological and scientific applications over the same period our ability to reconstruct the 3 dimensional stratigraphy of fluvial deposits and the matrix of fluvial sites has increased dramatically because of a number of technological advances these have included the use of lidar laser imaging and radar to produce high resolution digital surface models the use of geophysics particularly ground penetrating radar and electrical resistivity to produce sediment depth models and the use of gis and data visualisation techniques to manipulate and display the data these techniques along with more systematic and detailed sedimentological recording of exposed sections have allowed the construction of more precise 3 dimensional volumetric models of the matrix of artefacts within fluvial deposits additionally a revolution in dating techniques particularly direct sediment dating by luminescence methods has enabled the creation of 4 dimensional models of the creation and preservation of these sites these 4 dimensional models have the ability to provide far more information about the processes of site creation preservation and even destruction and also allow the integration of these processes with independent data sources concerning cultural evolution and climatic change all improvements in the precision of dating fluvial deposits have archaeological importance in our need to translate events from a sequential or geological timeframe to human timescales this allows geoarchaeology to make a more direct contribution to cultural history through the recognition of agency at the individual or group level this data can then form a component of biocomplexity or agent based modelling which is becoming increasingly used in the natural sciences particularly ecology and geomorphology and which can be used to test scenarios including the impact on and response of hominins to abrupt or catastrophic environmental change whilst catastrophic events clearly represent the atypical they can be illuminating in revealing cognitive processes resulting in abandonment coping mitigation and innovation these points are exemplified using two in depth case studies one from the holocene geoarchaeological record of the river trent in central england and the other from the palaeolithic record from rivers in south west britain in the former the interaction between climate change and human activity is illustrated at the year to century timescale whilst in the other the timescale is millennial these case studies have deliberately been chosen to be as different as possible in temporal and spatial scale with the aim of examining the applicability of methodological and theoretical aspects of geoarchaeology lastly the paper considers the problem of scale in geoarchaeology and concludes it is process dependency which ultimately affects the questions we can ask and that questions of human response to climate change are fundamentally a product of materiality and cognitive processes this demands an in depth contextual approach to such questions rather than database driven assertions of causality

geometric characterization of igneous intrusions 3 d seismic insights from the campos basin se brazil abstract igneous intrusions are important for the exploration and appraisal phases of an oilfield due to their impact on petroleum systems their presence can be associated with the generation migration accumulation and entrapment of hydrocarbons in particular 1 sills with low permeability might form barriers to fluid flow and migration 2 sills may re use fracture networks providing insights on fluid migration and interaction s 3 sills may influence source rock maturation 4 sill induced uplifting of overburden and faulting may generate traps and 5 their geometric complexities enables the understanding of the interconnectedness of sill complexes which is the focus of this study therefore we provide a systematic characterization of sill geometries and their morphometric parameters we used three dimensional seismic data calibrated with well logs to interpret sills in the southern campos basin brazil throughout the late cretaceous until the paleogene sills intruded into the cretaceous to the eocene post salt sedimentary sequence including shallow platform carbonates transgressive marls and shales a morphometric analysis assisted by seismic attributes allowed the geometric characterization of sills namely 1 saucer shaped 2 slightly saucer shaped 3 climbing saucer shaped 4 transgressive and 5 rough layer parallel the documented sills comprise a wide range of sizes 0 43 14 35 km2 that increase with depth up to 3 18 km extend vertically from 0 06 to 0 23 km and vary from low 1 82 3 31 to high dip 6 05 15 74 the sill complex area extends over 67 35 km2 this study suggests that a thorough characterization of sill intrusions may have a substantial influence on the prediction of other igneous occurrences the mapped sills might have implications not just on the acoustically similar shallow platform albian carbonates of the macae group but may also provide insightful geometric constraints on the brazilian pre salt igneous intrusions

scale linkage and contingency effects of field scale and hillslope scale controls of long term soil erosion anthropogeomorphic sediment flux in agricultural loess watersheds of southern germany abstract agricultural landscapes with a millennial scale history of cultivation are common in many loess areas of central europe over time patterns of erosion and sedimentation have been continually modified via the variable imposition of anthropogenic discontinuities and linkages on fragmented hillslope sediment cascades which eventually caused the complicated soilscape pattern these field records challenge topographically oriented models of hillslope erosion and simple predictions of longer term change of spatial soilscape by cultivation activities a thorough understanding how soilscape patterns form in the long term however is essential to develop spatial concepts of the sediment budget particularly for the spatial modeling of anthropogenic hillslope sediment flux using gis in this study i used extensive datasets of anthropogenic soil truncation and burial in a typical undulating loess watershed in southern germany 10 km2 wetterau basin n of frankfurt a m spatial soilscape properties and historic sediment flux as caused by cultivation over seven millennia were evaluated by these data the soilscape pattern on the low gradient hillslopes of the study area was found to be marked by a statistical near random pattern of varying depth thickness of truncation and overthickened burial moreover it was shown that truncation and burial had developed independently from each other and did not correlate with either hillslope gradient or downslope curvature hence in the field any combination of few nearly preserved severely truncated or completely removed soil profiles with either no some or a thick sediment cover is present thereby lacking an obvious spatial pattern here i suggest putting long term change of the soilscape into a contextual anthropogeomorphic systems perspective that accommodates components of human induced soil erosion operating at different spatial scales to interpret the longer term spatial consequences at the hillslope system level in the study area system scale linkages are marked by the spatial intersection of a finer scaled managed field system with a broader hillslope scale framework of natural erosion controls in the low gradient study area field borders exert control over the spatial reference of soil erosion and sedimentation sites over time this brought about a growing historical and spatial contingency change to the soilscape because of arbitrary spatial changes of the field system which are inherent in its socio agricultural maintenance thus the very low gradient and low erosivity setting of the study area have singled out the agency of human induced spatial and connectivity controls and contingency for long term spatial hillslope sediment flux although these findings may be less true for different settings they allow for deriving a generic conceptual model of the linkages between natural and anthropogenic subsystems to interpret the effects of long term human induced sediment flux accordingly the resulting balance between on hillslope net storage and net delivery to streams is scaling with basic physiographic properties of erosivity and sedimentation as well as the degree of anthropogenic hillslope fragmentation for loess areas in europe variable fields are fundamental anthropogeomorphic units that determine appropriate system scaling for historic sediment flux analysis and constrain retrodiction and prediction of changing fluxes at a point and a time at watershed scales methodical implications address adequate sampling strategies to record soilscape change as a result of which a critical review of the applicability of the catena concept to long cultivated hillslopes in central europe was included finally the suggested refined generic model of long term human controlled sediment flux involves a number of research opportunities particularly for linking modeling approaches to long term field records of cultivation related change in the soilscape

fluvio sedimentary response of the monsoon fed indian rivers to late pleistocene holocene changes in monsoon strength reconstruction based on existing 14c dates abstract the primary objective of the present study is to identify major phases of alluviation in the indian region since the abrupt deglacial intensification of the monsoon 15 cal ka bp on the basis of analysis of 68 radiocarbon dates from two major hydro geomorphic regions of india the central ganga basin cgb and the deccan peninsula dp the recognition of main phases of alluviation and incision has been achieved by evaluating the temporal distribution and clustering of the radiocarbon dates from alluvial sequences the clusters were detected on the basis of the interpretation of the summed probability distribution plots derived by using oxcal version 4 0 1 and calpal version may 2006 software packages the summed probability plots reveal that periods of alluviation in the cgb represented by three clusters 13 9 12 3 11 9 11 2 and 9 8 9 0 cal ka bp occur roughly before the onset of early holocene monsoon optimum phase two other clusters occur in the intervals 3 6 2 8 and 1 1 0 9 cal ka bp the peak monsoon period generally lacks clusters of radiocarbon dates implying fluvial erosion and channel incision this period also shows clustering of radiocarbon dates of the abandoned channels in comparison 14c dates from dp alluvial units form clusters at 16 4 14 12 8 11 2 10 8 8 9 8 1 6 7 and 5 1 3 9 cal ka bp indicating an association with the deglacial early holocene humid phase alluviation in the dp appears to have continued more or less uninterrupted till the middle of the holocene epoch the beginning and end of the discernible gap in the radiocarbon dates of cgb 9 0 3 6 cal ka broadly corresponds with the two well established short term events of the holocene 8 2 and 4 2 ka cal bp in comparison the prominent gap of dp radiocarbon dates 3 9 2 1 ka cal bp approximately begins with the 4 2 ka cal bp short term event onset of aridity and ends with the 2 0 ka cal bp enhanced monsoon event notwithstanding the inter regional differences in the fluvio sedimentary response in the india region the clusters of radiocarbon dates indicate that the century to millennium scale variations in fluvial activity in the indian subcontinent were intimately linked to long term fluctuations in the monsoon strength during the late quaternary

diatom inferred salinity and carbonate oxygen isotopes in holocene waterbodies of the western sahara and sahel africa abstract thirteen holocene palaeolakes in the western sahara and sahel have provided diatom records with carbonate oxygen isotope profiles available from eight of them most of these palaeolakes were groundwater fed lake water chemistry is reconstructed using diatom transfer functions lake water salinity and 18 o records are assembled with some isotopic and chemical groundwater data to better understand the response of the hydrological systems to climate changes over the past 15 000 yr data are in general agreement with climate simulations using coupled atmosphere ocean vegetation models which show a mid holocene wetting over the whole of northwest africa and a rapid drying by 6 4 ka the lake record also shows that at many sites the major lake infilling lags the end of the younger dryas by 1 2 ka regional differences also appear in the timing of the lake hydrological optimum ca 10 5 8 5 and 7 5 4 5 ka in the northern sahara 10 8 5 ka in the air tenere 10 5 7 or 4 5 in the sahel and 7 5 ka in lake chad the whole of the holocene is punctuated by short term drying events changes in water isotopic composition through time are partly explained by changes in rainfall amount and air humidity during the wet holocene period however the very low values in the southern sahara also imply changes in the moisture transport pattern or rainfall mechanisms data suggest an apparent decrease in 18 o content of precipitation along the monsoon flow in contrast with modern patterns changes in water availability and quality have driven population migrations in and out of the sahara sahel but relationships between climate and cultures are complex short term dry events might have driven inventive adaptations in the sahara drying at 5 4 5 ka coincides with both the collapse of the classical neolithic civilization and the settlement of new cultures

holocene environmental change in the west african sahel sedimentological and mineral magnetic analyses of lake sediments from jikariya lake northeastern nigeria previous studies have demonstrated long term changes in effective moisture in sub saharan africa here we reconstruct holocene environments using a similar to 7 m lake sediment sequence recovered from the northeastern nigerian sahel and attempt to distinguish basin specific changes from regional climatic variations the sequence was analysed for sedimentological properties mineral magnetism and pollen and dated by cs 137 pb 210 excess and c 14 extremely arid conditions of the terminal pleistocene ended ca 11 500 cal bp calendar years when climate ameliorated and a lake developed until the occurrence of an arid event leading to lake desiccation at similar to 11 200 cal bp following this climate ameliorated and a water body re emerged very wet conditions predominated 11 200 5600 cal bp followed by drought between 5600 and 5500 cal bp and a return to moderate humidity from 5500 to 4000 cal bp after 4000 cal bp a marked deterioration occurred culminating in lake desiccation at ca 800 cal bp after this time the climate remained generally dry and the re emerging lake was shallow comparison of these results with other well dated sequences in the region demonstrates the importance of basin specific influences on the palaeolimnological records in addition to regional climatic controls disentangling these different controls as well as the reconstruction of holocene climate therefore requires a multiple basin approach copyright c 2008 john wiley sons ltd

palynological evidence for climatic and oceanic variability off nw africa during the late holocene pollen and organic walled dinoflagellate cyst assemblages from core geob 9503 5 retrieved from the mud belt 50 m water depth off the senegal river mouth have been analyzed to reconstruct short term palaeoceanographic and palaeoenvironmental changes in subtropical nw africa during the time interval from ca 4200 to 1200 cal yr bp our study emphasizes significant coeval changes in continental and oceanic environments in and off senegal and shows that initial dry conditions were followed by a strong and rapid increase in humidity between ca 2900 and 2500 cal yr bp after ca 2500 cal yr bp the environment slowly became drier again as indicated by slight increases in sahelian savannah and desert elements in the pollen record around ca 2200 cal yr bp this relatively dry period ended with periodic pulses of high terrigenous contributions and strong fluctuations in fern spore and river plume dinoflagellate cyst percentages as well as in the fluxes of pollen dinoflagellate cysts fresh water algae and plant cuticles suggesting episodic flash flood events of the senegal river the driest phase developed after about 2100 cal yr bp

tracing the indian lithospheric mantle beneath central tibetan plateau using teleseismic tomography abstract we determined a detailed three dimensional p wave velocity structure of the crust and upper mantle down to 400 km depth beneath central tibet by applying teleseismic tomography to 28 146 high quality p wave arrival times the data were collected very carefully from the original seismograms of 1625 teleseismic events recorded by 131 broadband stations of five portable seismic networks deployed in central tibet our results show that the indian lithospheric mantle has subducted beneath central tibet and its frontier has passed through the bangong nujiang suture and extended northward beneath the qiangtang terrane at 34 north latitude a prominent low velocity anomaly is revealed within the tethyan himalayan sequences close to the yarlung zangbo suture which is the boundary between the tethyan himalayan sequences and the lhasa terrane the subduction of the indian lithospheric mantle has caused the east west extension in central tibet the indian slab beneath the region has not sutured with the asian lithospheric mantle in the north the obtained tomographic images can explain many geological and geophysical features such as strong seismic anisotropy in the upper mantle and k rich and ultra potassic lavas in northern tibet

holocene alluvial morphopedosedimentary record and environmental changes in the bardenas reales natural park ne spain abstract a holocene alluvial morphostratigraphy based on geomorphological mapping sedimentological analysis soil development and radiocarbon chronology is presented for the semiarid blanca basin in the bardenas reales natural park ne spain four morphopedosedimentary alluvial units mu1 mu2 mu3 and mu4 were differentiated which may correspond to two palaeogeographical stages the first one includes the morphopedosedimentary unit mu1 and is the most extensive in the area it is made up of a complex aggradation fill up sequence comprising a braided channel system a sand mud flat 9200 7900 cal bp and a playa lake environment related to a nearly closed depression an intensive fluvial entrenchment period is recognized leading the opening of the blanca basin before the second stage that includes the units mu2 mu3 and mu4 this set of units forms a sequence of cut and fill and nested fill terraces recording the alluvial activity in the blanca basin during the last 6 ka the mu2 unit 5900 3300 cal bp in age represents a period of high geomorphic instability with rapid alluvial sedimentation channel incision oscillating changes on the other hand the mu3 unit has an age of 1050 500 cal bp and appears as a thin extensive alluvial cover overlying previous units finally the mu4 unit comprises superposition of numerous flood depositional sequences centimetric in scale dated from 450 10 cal bp the established soil chronosequence supports the numerical chronology of the four units these accumulation periods mu1 mu2 mu3 mu4 are associated with dry phases and or high hydrological variability floods and droughts at a regional scale changes on the north atlantic atmospheric circulation mode over southern europe may provide the key to explain the nature of alluvial activity during holocene in bardenas reales

late quaternary upwelling off tropical nw africa new micropalaeontological evidence from odp hole 658c planktonic foraminifera and radiolaria have been analysed in a late quaternary 40 0 ka sediment sequence from ocean drilling program odp hole 658c located under a coastal upwelling system near cap blanc offshore northwest africa in order to document the palaeoceanographic history of the area temporal variations in species abundance and faunal assemblage analysis reveal a tripartite phased sequence of palaeoceanographic change through the late quaternary phase 1 spans 40 14 5 ka and is characterised by moderate upwelling but heinrich event 2 is distinguished as a brief episode of strengthened upwelling phase 2 begins with a change in a number of variables at ca 14 5 ka and extends to ca 5 5 ka this phase is characterised by a general strengthening of upwelling but may be subdivided into three minor phases including a the recognition of the younger dryas marked by a temporary reduction in upwelling strength followed by b an intensification of upwelling and c upwelling and high productivity between 8 and 5 5 ka this phase of upwelling corresponds with maximum holocene cooling possibly triggered by the collapse of the laurentide ice sheet phase 3 extends from 5 5 to 0 ka and is characterised by weak upwelling and significant calcite dissolution these phases are related to climatic events particularly the african humid period ahp which is coincident with phase 2 the ahp is characterised by increased precipitation linked to an intensification of the african monsoon that enhances north east trade wind driven coastal upwelling and is associated with the expansion of continental vegetation across north africa copyright 2006 john wiley sons ltd

past hydrological events reflected in the holocene fluvial record of europe abstract a comprehensive database of radiocarbon dated fluvial units in great britain poland and spain has been compiled to investigate the relationship between environmental change flooding and holocene river dynamics following the methodology recently developed by macklin and lewin macklin m g lewin j 2003 river sediments great floods and centennial scale holocene climate change journal of quaternary science 18 101 105 radiocarbon dates in fluvial sequences that coincide with a modification in sedimentation rate or style have been highlighted allowing geomorphologically significant changes in holocene river activity to be identified data analysis has been undertaken at both national and sub national scales and on catchments of different size type and land use history multiple phases of higher flood frequency characterized by accelerated erosion and sediment deposition on floodplains are recognized and compared with a range of climate proxies the relative and varying roles of climate and land use on river dynamics are considered and the value of the database for reconstructing past hydrological events as well as for predicting river response to future environmental change is assessed

geomorphic response to rapid climatic and hydrologic change during the late pleistocene and early holocene in the humid and sub humid tropics abstract the data base concerning late quaternary environmental change in the humid tropics is dependent on records from scattered sites the intensity and duration of wet dry oscillations remain speculative and the responses of hillslopes and river systems are based on over simplified models however available evidence indicates that prolonged aridity affected all but a few favoured core areas of equatorial climate after 20 000 bp lasting 5 7 ka dry conditions at the lgm were marked by semi arid landforms and reduced steam activity large palaeofloods occurred after 13 000 be but dry conditions returned after 11 000 bp before the early holocene pluvial led to abundant sedimentation lasting nearly 2 ka after 9500 bp in africa amazonia and australasia erosion channel cutting and flood deposition occurred at the pleistocene holocene transition followed by multiple shifts from lateral to vertical accretion fill cut episodes during the early holocene while thick overbank deposits formed during the later holocene re establishment of the lowland rainforests was delayed until after 9000 bp in africa australia and brazil and several wet dry oscillations followed in the mid holocene period hillslope activity at the lgm was marked by local sediment transfers and fan formation at the termination sediment fluxes increased rapidly but mass movement probably peaked after 10 000 bp sediment delivery to stream channels was not immediate and large sediment stores remain on the landscape today many in areas of potential sensitivity of erosion

understanding the impacts of late quaternary climate change in tropical and sub tropical regions abstract the understanding of climate impacts on tropical rivers and catchments has developed in part from models developed for temperate landscapes and a common rhythm is often apparent when millennial scale studies are compared at this level issues arising from the complex response of rivers to internal and external factors appear less important than the climate driven signal understanding the fluvial record however depends on the availability and interpretation of other sources of proxy data such as pollen spectra lake levels and ocean cores in many rainforest areas few pollen records exist and many of these reveal a hiatus at the time of the last glacial maximum examples of fluvial archives from africa indonesia and tropical queensland australia are discussed in relation to independent records of vegetation change and inferences are made concerning critical episodes in fluvial history focusing on the transitions from oxygen isotope stage ois 3 towards glacial conditions in ois2 and the pleistocene holocene transition to modern climates links between climate vegetation change and fluvial activity are presented as descriptive models

seasonality change and the third millennium bp rainforest crisis in southern cameroon central africa the third millennium bp crisis of the central african rainforest is not sufficiently understood the low resolution of most pollen profiles and a large plateau of the calibration curve aggravate the exact dating of the event and its causal climatic parameters are debated we present a high resolution pollen profile from the swamp site nyabessan in the southern cameroonian rainforest covering the period 3100 2300 cal yr bp between 3100 and 2500 cal yr bp the climate was favourable for a regional evergreen forest with caesalpiniaceae and lophira and a local raphia swamp forest around 2500 2400 cal yr bp a significant decrease of mature forest and swamp forest taxa and an increase of pioneers indicate that the rain forest was seriously disturbed and replaced by secondary formations the dominance of trema orientalis a pioneer well adapted to seasonal desiccation points to a much more accentuated seasonality after 2500 cal yr bp which seems to be linked to a southwards shift of the itcz during the northern hemisphere winter months we propose that the rain forest crisis between 2500 and 2200 cal bp created favourable conditions for farming and paved the way for a major expansion of bantu speaking populations

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