biological remains geochemistry and stable isotopes for the reconstruction of environmental and hydrological changes in the holocene lakes from north sahara abstract a detailed reconstruction of the holocene palaeoenvironments and palaeohydrology is presented for wwamp and lacustrine calcareous sediments which lie in closed depressions along the northern margin of the great western erg algeria it is based on a multidisciplinary approach using numerous techniques the chronological framework is established on the basis of 23 14 c dates on inorganic calcite and mollusc shells ranging from ca 9300 to 3000 yr b p and on an isolated 14 c age of ca 10 900 yr b p the significance of various environmental markers is discussed biological remains are abundant and diversified about 150 taxa including diatoms chrysophytes charophytes ostracods molluscs and foraminifera they provide information on the water chemistry total salinity ionic ratio ph and partial pressure of carbon dioxide and on the habitats water depth development of aquatic plants ephemerality etc emphasis is given to the total salinity individual organisms and palaeobiological assemblages are ordered as a function of their salt requirements and tolerances calibration based on modern diatom samples allows a tentative estimate of the palaeosalinities which range from fresh to marine like and even hypersaline waters attention is drawn to the possible colonization of marine organisms in continental basins without any connection with the sea stable isotope analyses have been carried out on both inorganic and biogenic mollusc and ostracod shells charophyte gyronites carbonates the 18 o contents demonstrate wide changes in the hydrological balance fluctuations in 13 c contents inform on water mixing residence time and on the importance of biological effects on total dissolved inorganic carbon geochemical analyses were performed on inorganic calcite and on ostracod shells high contents in fe and mn indicate reducing environments during stages of high salinity sr ca and mg ca are used as salinity indicators zn mg ratios suggest that the major water supply was the aquifer of the great western erg today down to about 50 m rather than surface waters from the atlas mountains salinities resulted from the balance between inputs from the water table evaporation rate leaching of pre existing salt crusts and seepage through the lake bottom a reconstruction of the evolution of the palaeosystems through time and space has been attempted with special attention to the instability of the water and ionic balances for different time scales short term fluctuations occurring during a single hydrological cycle annual or seasonal are deduced from the apparent discrepancies between the various indicators found in a given sample long term 10 2 10 3 years changes confirm the existence of a climatic phase being wetter than today from at least ca 9300 and may be from ca 10 900 yr b p to 3000 yr b p for the first time the salinity deduced from diatoms and the 18 o content of the water deduced from that of the carbonate at 25 c are used simultaneously to model the hydrological history of the basins and to estimate the relative humidity of the atmosphere during the holocene major environmental trends recorded from ca 9300 to 3000 yr b p do not show a clear synchronims between the different profiles studied therefore they cannot be interpreted in terms of palaeoclimate but instead demonstrate the importance of local hydrology in the evolution of closed basins associated with groundwater a common situation in arid and semi arid zones

sahel megadrought during heinrich stadial 1 evidence for a three phase evolution of the low and mid level west african wind system abstract millennial scale dry events in the northern hemisphere monsoon regions during the last glacial period are commonly attributed to southward shifts of the intertropical convergence zone itcz associated with an intensification of the northeasterly ne trade wind system during intervals of reduced atlantic meridional overturning circulation amoc through the use of high resolution last deglaciation pollen records from the continental slope off senegal our data show that one of the longest and most extreme droughts in the western sahel history which occurred during the north atlantic heinrich stadial 1 hs1 displayed a succession of three major phases these phases progressed from an interval of maximum pollen representation of saharan elements between 19 and 17 4 kyr bp indicating the onset of aridity and intensified ne trade winds followed by a millennial interlude of reduced input of saharan pollen and increased input of sahelian pollen to a final phase between 16 2 and 15 kyr bp that was characterized by a second maximum of saharan pollen abundances this change in the pollen assemblage indicates a mid hs1 interlude of ne trade wind relaxation occurring between two distinct trade wind maxima along with an intensified mid tropospheric african easterly jet aej indicating a substantial change in west african atmospheric processes the pollen data thus suggest that although the ne trades have weakened the sahel drought remained severe during this time interval therefore a simple strengthening of trade winds and a southward shift of the west african monsoon trough alone cannot fully explain millennial scale sahel droughts during periods of amoc weakening instead we suggest that an intensification of the aej is needed to explain the persistence of the drought during hs1 simulations with the community climate system model indicate that an intensified aej during periods of reduced amoc affected the north african climate by enhancing moisture divergence over the west african realm thereby extending the sahel drought for about 4000 years

coal characteristics elemental composition and modes of occurrence of some elements in the isaalan coal balikesir nw turkey abstract this study uses coal petrography micro ftir mineralogical and geochemical analyses to determine the coal characteristics and controlling factors of elemental enrichments in the isaalan coal nw turkey the obtained data show that this subbituminous coal has a relatively low ash yield avg 19 high hydrogen content avg 5 4 and total s content avg 2 4 on an air dry basis the maceral composition along with micro ftir data on ulminite suggests that lignin and resin rich vegetation was predominant the coal facies data imply the precursor peat was accumulating under mesotrophic anoxic conditions where water level was high and fluctuating the mineralogical composition of the bulk coal samples is composed mainly of quartz pyrite and clay minerals and lesser amounts of feldspars and carbonates the statistical analysis of the bulk compositional data shows the major elements including al fe k na mg p and the vast majority of trace elements have inorganic affinity whereas b presumably has organic affinity the trace element concentrations are variable and especially as and elements including b ba cr cs ni sb and v are enriched compared to most world coals the enrichments and distributions of cr cs ni and v were controlled by clastic inputs into a palaeomire whereas as enrichments were controlled mainly by epigenetic and to a lesser extent syngenetic factors introduced so42 and as rich surface and groundwater from the basin margin area along with anoxic conditions in the palaeomire resulted in syngenetic formation of as bearing pyrite the circulating intra seam solutions later liberated as from syngenetically formed pyrite which resulted in as and ni rich overgrowths around framboidal and massive pyrite crystals these solutions could also easily mobilize b that was absorbed by organic matter therefore b enrichment appears to be related to epigenetic factors tectonic activities during coalification allowed hydrothermal solutions from neighbouring epithermal mineralisation to penetrate and circulate along with underground waters that leached overlying volcanoclastic volcanic strata and supplied a number of elements to the studied coal thus infilling as bearing pyrite and silica were formed mainly during coalification all these results suggest that as enrichment in the study area is more likely a combination of epigenetic factors and syngenetic detrital rather than volcanogenic contributions as suggested earlier for some other turkish coal deposits

mid late holocene vegetation response to climatic drivers and biotic disturbances in the banni grasslands of western india abstract tropical grasslands and savannas are globally extensive and are of significant environmental economic and ecological importance these ecosystems are anticipated to be particularly sensitive to future changes in climate and understanding how these systems have responded to climatic changes in the past can provide us with insights into their potential responses to future global change in this study the temporal dynamics of c 3 c 4 vegetation changes in response to changes in moisture availability local fire events and changing levels of herbivory in a summer rainfall region of western india are reconstructed for the past 4600 cal yr bp paleodata such as stable carbon isotope of bulk organic matter 13 c org oxygen isotope from carbonate shells 18 o shell macro charcoal and herbivore dung fungal spores are reported from the retrieved cores of two wetland sites located in the banni grasslands of western india results show that vegetation in the banni was composed mostly of c 3 vegetation from 4600 to 2500 cal yr bp after which there was a decline in c 3 vegetation from the late holocene to the present there was a mix of both c 3 and c 4 vegetation with c 4 grasses being more abundant in the ecosystem these shifts were coincident with rainfall changes from more mesic conditions during 4600 to 2500 cal yr bp to more arid conditions towards the present as indicated by 18 o shell isotope data the period of increase in c 4 vegetation also coincides with a period of increased biotic disturbances in the ecosystem particularly fire given the current scenarios of global warming recurrent drought events and increased anthropogenic use of similar ecosystems such studies can provide us insights into potential future trajectories of these ecosystems

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

milankovitch forcing of fluctuations in the level of tropical lakes from 18 to 0 kyr bp an atmospheric general circulation model has been used to simulate the climates of january and july at 3 000 yr intervals from 15 kyr bp onwards the model simulates a strengthened monsoon circulation and increased precipitation in the northern hemisphere tropics culminating at 9 6 kyr bp the computed hydrological budgets were used to estimate the area of closed lakes between 8 9 and 26 6 n giving with minor exceptions results in good agreement with geological evidence

the influence of changing orbital parameters and surface boundary conditions on climate simulations for the past 18 000 years abstract general circulation model experiments at 3000 year intervals for the past 18 000 years were made to estimate the magnitude timing and pattern of the climatic response to prescribed changes of orbital parameters date of perihelion axial tilt eccentricity and glacial age lower boundary conditions ice sheets land albedo sea ice and sea surface temperature the experiments used the community climate model ccm of the national center for atmospheric research ncar the response of monsoon circulations and tropical precipitation to the orbitally produced solar radiation changes was much larger than the response to changes of glacial age boundary conditions the continental interior of eurasia was 2 4 k warmer in summer and summer monsoon precipitation of north africa south asia was increased by 10 20 between 12 000 and 6000 yr bp before present when perihelion occurred during northern summer rather than in winter as now and the earth s axial tilt was larger than now southern hemisphe

late holocene environmental change and the anthropization of the highlands of santo antao island cabo verde abstract cabo verde was the first tropical archipelago colonized by europeans historians have suggested that the first colonizers initiated archipelago wide ecosystem degradation loss of vegetation cover and erosion however the human environment interactions that led to the archipelago s current environmental status remain poorly understood here we report the first palaeoecological study of past vegetation change and disturbance regimes for cabo verde we present a 2130 yr old sediment sequence from a volcanic caldera cova de paul located at 1200 m asl on santo antao island for which we analyzed fossil pollen non pollen palynomorphs npps charcoal silica bodies and grain size distribution our analyses do not show evidence of the presence of temperate tropical or subtropical forests growing on the summits of santo antao in pre human times the pollen record shows that scrubland and grasslands dominated the highlands and underwent compositional changes ca 1850 and 1300 cal yr bp these shifts overlap with erosion phases and are linked to intensified seasonality steady rates of sedimentation marked the period 1230 to ca 350 cal yr bp but an increase in charcoal concentrations indicate a drying phase increases in regional and local fire peaks of coprophilous fungi and the presence of new world crop pollen zea mays are interpreted as the onset of portuguese settlement of the highlands after 450 bp sustained erosion between ca 350 and 100 cal yr bp indicate soil degradation and the pollen record shows the increase in introduced herbaceous taxa rosaceae centaurea verbenaceae and exotic tree taxa e g pinus while shifting abundances of coprophilous fungi indicate changes in land use the record shows stabilization of soils in the last century due to recent afforestation of the highlands further palaeoecological studies have the potential to provide further detail of the long term dynamics of cabo verde ecosystems and to inform conservation initiatives

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

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

comparison of elements of environmental concern in regular and reclaimed soils near abandoned coal mines ptolemais amynteon northern greece impact on wheat crops abstract a comparative study related to the concentrations of trace elements especially those of environmental concern occurring in regular and reclaimed soils on abandoned coal mines as well as on their respective wheat crops has been undertaken in the vicinity of ptolemais amynteon greece the aim was to assess the impact of land reclamation on the crops the results of elemental analysis show that the concentration of as is 1 5 to 1 7 times higher in the regular soils than in the reclaimed ones similarly co is 2 4 to 2 6 times higher cr is 1 7 to 2 8 times higher ni is 1 9 to3 1 times higher pb is 0 8 to 4 6 times higher and hg is equal to or 1 7 times higher cu cd and se have the same concentration in both kind of soils and mo and u are 1 7 to 2 0 times lower in the regular soils as compared to the reclaimed soils nevertheless all concentrations are within the reported range for regular soils moreover the concentrations of the studied elements in wheat grains harvested from both types of soils are approximately the same the reason for the uniform uptake of elements by the plants is the very low solubility of the elements in the soil solutions this is attributed to the high soil ph and the nature of the occurrence or bonding of the elements in soil minerals

accurate and efficient regularized inversion approach for the interpretation of isolated gravity anomalies a very fast and efficient approach for gravity data inversion based on the regularized conjugate gradient method has been developed this approach simultaneously inverts for the depth z and the amplitude coefficient a of a buried anomalous body from the gravity data measured along a profile the developed algorithm fits the observed data by a class of some geometrically simple anomalous bodies including the semi infinite vertical cylinder infinitely long horizontal cylinder and sphere models using the logarithms of the model parameters log z and log a rather than the parameters themselves in its iterative minimization scheme the presented numerical experiments have shown that the original non logarithmed minimization scheme which uses the parameters themselves z and a instead of their logarithms encountered a variety of convergence problems the aforementioned transformation of the objective functional subjected to minimization into the space of logarithms of z and a overcomes these convergence problems the reliability and the applicability of the developed algorithm have been demonstrated on several synthetic data sets with and without noise it is then successfully and carefully applied to seven real data examples with bodies buried in different complex geologic settings and at various depths inside the earth the method is shown to be highly applicable for mineral exploration and for both shallow and deep earth imaging and is of particular value in cases where the observed gravity data is due to an isolated body embedded in the subsurface

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

geoelectromagnetic measurements across the southern senegal basin west africa abstract magnetotelluric and differential geomagnetic sounding surveys consisting of nine soundings were performed in 1984 along a 200 km profile across the southern senegal basin they were intended to obtain information concerning the resistivity structure of the crust and upper mantle and the distribution of the induced electric currents magnetotelluric data indicate that two dimensional resistivity models are appropriate for the region the zone above the basement is highly inhomogeneous in geoelectrical structure very conductive sediments 0 6 3 ohm m appear in the mesozoic cenozoic senegal basin these sediments lie at depths of up to 4500 m on the west end of the profile below this a modest resistivity material 10 30 ohm m extends to a maximum depth of about 3000 m the material at depth on the cast part of the traverse line is thought to be palaeozoic sediments of the bove basin the depth of the magnetotelluric basement lies between about 250 m in the east and 4800 m in the west the crust is characterised by a drop in electrical resistivity at a depth of 15 km below the east part of the profile considering the total section we observe a general trend towards lower resistivities at depths in excess of 100 km the transition from 2000 ohm m to about 2 ohm m occurs in the depth range 100 to 175 km an analysis of the geomagnetic variation field has identified a concentration of telluric current flow beneath the deep basin it appears that the additional currents flowing in the striking direction of the senegal basin are largely controlled by sedimentary rocks of high conductivity lying at depths less than 5 km model studies show that the local conductivity distribution is able to explain the currents circulating in the thick well conducting sediments

the petrology of greek brown coals abstract results are given in the petrography of greek coals collected from most of the major coal bearing basins in greece rank was determined by measuring reflectances on the maceral varieties eu ulminite a and eu ulminite b and on the maceral textinite reflectances obtained from these components indicate a coalification stage of brown coal for all samples within this group of samples there is however a fairly wide scatter of reflectance values indicating for some of them the transition zone from peat to brown coal and for others a coalification stage close to the transition into bituminous coals reflectances obtained from eu ulminite a and eu ulminite b were found to correlate well with chemical rank parameters such as volatile matter and calorific value composition was determined by maceral analysis the coals are in general characterized by low amounts of macerals of the inertinite group low to intermediate amounts of macerals of the liptinite group and high amounts of macerals of the huminite group within the latter group densinite attrinite eu ulminite and textinite make up the bulk of the samples typical macerals observed in the coals are illustrated by two black and white and three colour plates cluster analysis based on maceral distribution mineral matter and reflectance indicates that the samples studied can be divided into three major groups the first one is dominated by eu ulminite and densinite with relatively high reflectances the second is dominated by attrinite textinite and texto ulminite with somewhat lower reflectances the third is represented by a single sample in which textinite and resinite are the most abundant macerals this sample also has the lowest reflectance

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

multidecadal variations in the early holocene outflow of red sea water into the arabian sea we present holocene stable oxygen isotope data from the deep arabian sea off somalia at a decadal time resolution as a proxy for the history of intermediate upper deep water these data show an overall 18o reduction by 0 5 between 10 and 6 5 kyr b p superimposed upon short term 18o variations at a decadal centennial timescale the amplitude of the decadal variations is 0 3 prior and up to 0 6 subsequent to 8 1 kyr b p we conclude from modeling experiments that the short term 18o variations between 10 and 6 5 kyr b p most likely document changes in the evaporation precipitation balance in the central red sea changes in water temperature and salinity cause the outflowing red sea water to settle roughly 800 m deeper than today

holocene climatic evolution at 22 23 n from two palaeolakes in the taoudenni area northern mali abstract in the endorheic hydrologically isolated taoudenni depression nowadays receiving 5 mm mean annual rainfall continuous sections in fossiliferous holocene deposits from two palaeolakes with different geological evolutions have been compared the following climatic sequence is determined 9000 8300 yr b p the beginning of a humid phase 8300 6700 yr b p climatic optimum from 6700 yr b p onwards a change in regime 4500 4000 yr b p end of humid phase aridification begins the data imply a 600 km northern shift of the monsoonal belt which confirms models described in the literature

geochemical controls on leaching of lignite fired combustion by products from greece abstract the mobility of inorganic pollutants is of key concern for a range of industrial and engineering applications of fly ash produced during the combustion of lignite in power generation this paper investigates the role that the geochemical features of lignite the ash composition and the partitioning of elements during combustion play in determining leaching properties of lignite fired by products the work is based on surveys on three lignite fired power plants in greece calcium rich ashes show a high abatement potential for so 2 and other gaseous pollutants for most elements the concentrations in the parent lignite and the ashes follow the same trend relative enrichments in cd co cr cu mo ni pb u v w zn fingerprint the regional and local geological settings of the lignite basins the total and leachable concentrations of highly volatile elements are strongly influenced by the interaction with ubiquitous free lime a broad array of elements is highly insoluble in alkaline ash while a few oxyanionic forming elements display substantial mobility their mode of occurrence in the parent lignite plays a primary role in the leaching of combustion ashes the outcomes of this study may assist in addressing the impact of co firing high ash or high ca alternative fuels on the leaching properties of combustion by products

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

drought and dust deposition in the west african sahel a 5500 year record from kajemarum oasis northeastern nigeria a high resolution multiproxy palaeolimnological record from the manga grasslands northeastern nigeria spanning the last 5500 calendar years reveals the episodic deterioration in sahelian climate as significant biogeophysical thresholds were crossed desert dust deposition began to increase 4700 cal bp rainfall during the summer monsoon season declined permanently after 4100 cal bp a further significant change in atmospheric circulation giving rise to multidecadal to centennial scale droughts and enhanced dust deposition occurred 1500 cal bp hence the post 1968 sahel drought is not unique the prolonged arid episode that occurred around 1200 1000 cal bp in ethiopia the sahel and tropical mexico may have been linked to an abrupt cooling event in the north atlantic and to a cluster of intense el nino southern oscillation events in the eastern equatorial pacific

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

high resolution pollen record from core kw31 gulf of guinea documents the history of the lowland forests of west equatorial africa since 40 000 yr ago abstract pollen data from core kw31 recovered off the mouth of the niger river 3 31 1n 05 34 1e 1181 m water depth provide an exceptional record of vegetation changes in the west african lowlands between 40 000 and 3500 cal yr b p the highly diverse microflora testify for the permanency of rain and secondary forests in the niger river catchment at least as gallery formations along rivers during the last glacial period when dry conditions occurred in relation to enhanced trade wind circulation the direct consequence of the post glacial warming and the correlative increase in monsoon fluxes over west africa was the increase in forest diversity and the expansion of rain and secondary forests on the nearby continent comparison between kw31 pollen record and continental pollen data from 5 s to 25 n allows the evaluation of migration rates of tropical forest populations throughout north west africa at the beginning of the holocene and the vegetation response to the shift toward aridity recorded widely at the end of the african humid period around 4000 cal yr b p

a reassessment of lake and wetland feedbacks on the north african holocene climate 1 large parts of the sahara were vegetated during the early to mid holocene several positive feedbacks most notably related to vegetation have been shown to have favored the northward migration of the desert boundary during this period numerous lakes and wetlands existed in the sahara region and might have acted as a local moisture source however earlier model studies of the effects of open water surfaces on the mid holocene north african climate suggested that these were weak and did not contribute significantly to this northward migration of the north african climate zones using a state of the art climate model we suggest that the effect of open water surfaces on the mid holocene north african climate might have been much stronger than previously estimated regionally more than doubling the simulated precipitation rates it is thus possible that this effect combined to other known positive feedbacks favored the appearance of the green sahara

dust clouds rain types and climatic variations in tropical north africa abstract dust and processes of raindrop formation in the clouds play a very important role in the climatic evolution of tropical north africa sedimentologic stratigraphic pedologic geomorphologic and palynologic data converge to show that a major environmental change occurred in tropical africa about 7000 yr b p in the sudanian and sudano guinean zones wet tropical zone from 15 000 to 7000 yr b p rivers deposited mostly clay while from 7000 to 4000 yr b p they deposited mostly sand during the first period pedogenesis was vertisolic montmorillonite dominant associated with pollen belonging mostly to vegetation typical of hydromorphic soils while during the second period pedogenesis was of ferruginous type kaolinite dominant with pollen belonging mostly to vegetation typical of well drained soils the great change near 7000 yr b p is linked chiefly to a major hydrological change that appears related to a change in the size of raindrops from fine rains associated with considerable atmospheric dust raindrop diameter essentially less than 2 mm to the second period associated with thunderstorm rains raindrop diameter mostly greater than 2 mm the size of raindrops is related particularly to cloud thickness and dust concentration in the troposphere thunderstorm activity is influenced also by fluctuations of the atmospheric electricity modulated by the sun

pollen analyses off senegal evolution of the coastal palaeoenvironment during the last deglaciation pollen and dinoflagellate cysts from marine sediments in core a180 48 15 19 n 18 06 w 2450 m water depth 530 cm length are used to reconstruct palaeoenvironmental conditions of nearshore tropical west africa during the last deglaciation high concentrations and influxes of pollen and dinoflagellate cysts between 11 000 and 10 000 yr bp are interpreted as reflecting an increase in continental trade wind circulation and related coastal upwelling at 15 n latitude the sea surface temperature difference between glacial and interglacial times was not as strong as previously suggested together with local fresh ground water input this smaller temperature difference may explain the persistence of the rhizophora mangrove and guinean gallery forests near the shore until their massive extension during the early holocene humid maximum around 9500 yr bp pollen data from the core are compared with data from rosilda n110 z from the continental shelf at the same latitude

a review of deep structure and ocean continent transition in the senegal basin west africa abstract in the mauritania senegal basin gravity and magnetic data reveal long n s lineations reflecting probable extensional structures which are possibly related to opening of the atlantic along the coast where the basin is considerably thicker a large gradient in the gravity field is closely associated with an important linear magnetic high this gravity feature which dominates the whole of the area concerned reflects a crustal thinning to the west together with the existence of elongate dense intrusive bodies suggesting that it is associated with a crustal discontinuity and a change towards a crust of oceanic type the long coastal magnetic high is presumed to be an onshore segment of the magnetic lineations identified on the mauritania senegal continental margin as the landward boundary of the magnetic quiet zone a comparison of conjugate magnetic quiet zones of west africa and eastern north america shows that the west africa coastal magnetic anomaly may be closely related to the east coast magnetic anomaly which probably marks the boundary between continental and oceanic basement such correlation implies that the cape vert peninsula senegal may rest on a basement of oceanic type this appears to be consistent with geological and geophysical interpretations which suggest seaward rapid thinning of the continental crust together with major subsidence and seaward progradation

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

palaeoenvironment of seam i in the marathousa lignite mine megalopolis basin southern greece the present study aims to elucidate the depositional environment of seam i in the megalopolis basin southern greece using existing geological data supported by maceral and mineralogical analyses as well as by natural radioactivity determinations very high pyrite and uranium contents and a very low tissue preservation index usually suggest a marine influence during deposition in seam i however these features are most probably the result of a karstic aquifer supplying the palaeomire with sulphate rich neutral to weakly alkaline water the pyrite formation and the u precipitation are attributed to the anaerobic conditions which prevailed during peat accumulation the petrographic mineralogical and geochemical data of the studied profile point to several changes in the surface runoff and the supply of clastic material during deposition

late quaternary vadose carbonate diagenesis in coastal and desert dune and beach sands is there a palaeoclimatic signal a study of the variation in cement amounts in aeolianite and sandy beach calcarenites that range in age from holocene to last interglacial is used to assess whether a palaeoclimatic control on vadose diagenesis can be identi ed examples are taken from modern day arid to subhumid settings and represent a geographical distribution ranging from libya and oman to the mediterranean the caribbean and mexico the results indicate that a palaeoclimatic signal can be identi ed in some deposits however in other sediments there is substantial variability in mean cement abundance within deposits as well as between these sand bodies moreover many of the better cemented sediments are located in areas where there is accelerated diagenesis such as in the sea spray zone close to the groundwater table or near to a palaeosurface rather than in climatically wet regions the inference is that palaeoclimatic interpretations are substantially complicated by other factors that affect diagenetic processes and change therefore caution is needed when studying the role of climate in vadose diagenesis in the light of the effects of other intrinsic and extrinsic controls this paper does not aim to provide a de nitive comparison of sites from different climatic zones the approach taken here is a to see if there is an overall palaeoclimatic signal in the samples studied b to use examples to illustrate how explanations controls other than those relating to climate can account for the variabilities observed and c if there is a climatic effect to see if it is the main over riding control on vadose diagenesis the conclusions drawn from this research highlight that it is possible to misinterpret evidence if the study is approached with preconceived notions of simplistic relationships between diagenesis and climate copyright 2004 john wiley sons ltd

interaction of an antecedent fluvial system with early normal fault growth implications for syn rift stratigraphy western corinth rift greece continental overfilled conditions during rift initiation are conventionally explained as due to low creation of accommodation compared with sediment supply alternatively sediment supply can be relatively high from the onset of rifting due to an antecedent drainage system the alluvial lower group of the western plio pleistocene corinth rift is used to investigate the interac tion of fluvial sedimentation with early rifting this rift was obliquely super imposed on the hellenide mountain belt from which it inherited a significant palaeorelief detailed sedimentary logging and mapping of the well exposed syn rift succession document the facies distributions palaeocurrents and stratigraphic architecture magnetostratigraphy and bios tratigraphy are used to date and correlate the alluvial succession across and between fault blocks from 3 2 to 1 8 ma a transverse low sinuosity braided river system flowed north north east to east across east west striking active fault blocks 4 to 7 km in width deposits evolved downstream from coarse alluvial conglomerates to fine grained lacustrine deposits over 15 to 30 km the length scale of facies belts is much greater than and thus not directly controlled by the width of the fault blocks at its termination the distribu tive river system built small stacked deltas into a shallow lake margin the presence of a major antecedent drainage system is supported by i a single major sediment entry point ii persistence of a main channel belt axis iii downstream fining at the scale of the rift basin the zones of maximum sub sidence on individual faults are aligned with the persistent fluvial axis sug gesting that sediment supply influenced normal fault growth instead of low accommodation rate during the early rift phase this study proposes that facies progradation can be controlled by continuous and high sediment sup ply from antecedent rivers

stratigraphic and geophysical integrated methodologies for the interpretation of sulphur water formational environment in salento italy abstract the salento coal deposits south eastern italy are unutilized because these deposits are thin and nearly uneconomic but they have a high scientific and economic value due to their high organic sulphur content the studied area is located in the western salento peninsula where wells used by a fish farm ittica ugento have shown high concentrations of hydrogen sulphide data from surface and boreholes stratigraphic surveys integrated with electrical resistivity tomography ert allow us to define the structure depths and geometry of the aquifer and its relationship with saltwater intrusion induced polarization ip with pole dipole array survey has been carried out near the coastline the value measured was over 50 msec a direct relationship is shown to exist between ip values and the aquifer containing sulphur water the high resolution of the data obtained with the applied methods not only shows the validity of the methodology but is the key to evaluating the groundwater resources of the area the proposed mechanism is that of entrapment of sulphur water in a graben structure when sulphate enriched waters of marine origin come into contact with organic substances and lignite deposits galatone formation oligocene they are deprived not only of free oxygen but also generate hydrogen sulphide as a result of the reduction of sulphates

enrichment of u re v cr se and rare earth elements in the late permian coals of the moxinpo coalfield chongqing china genetic implications from geochemical and mineralogical data abstract rare metals in coal deposits have attracted much attention in recent years because of their potential economic significance this paper reports the abundance and enrichment origin of rare metals in the late permian coals k1 and k2 coals of the moxinpo coalfield chongqing southwestern china the k1 coal is characterized by highly elevated concentrations of u re v cr se and nb ta zr hf ree assemblages the latter assemblage is also enriched in the k2 coal the high temperature ash 815 c of the k1 coal is enriched in v cr se re u and ree the ash of the k2 coal and also the floor strata of each seam are enriched in ree potentially making all of the units economically viable sources for these elements the minerals in the k1 coal are mainly represented by kaolinite illite and mixed layer illite smectite and pyrite while the minerals in the k2 coal consist mainly of kaolinite and tobelite nh 4 k al 2 alsi 3 o 10 oh 2 authigenic roscoelite k v 3 al 2 alsi 3 o 10 oh 2 is commonly observed in the k1 coal under the sem and was probably formed by interaction of kaolinite with v derived from permeating u re v cr se rich solutions during early diagenesis the tobelite enriched in the k2 coal was formed by reaction between kaolinite already present in the coal and nh 4 derived from decomposition of the organic matter during hydrothermal alteration at a relatively high temperature the mafic tuffs directly underlying the k1 coal and containing limestone residual breccias not only served as the substrate for coal accumulation but also as the source of sediment from the uplifted areas around the coal basin the latter is indicated by low al 2 o 3 tio 2 ratios from 10 09 to 14 24 positive eu anomalies enrichment of medium rare earths relative to upper continental crust and detrital calcite of terrigenous origin the highly elevated concentrations of u re v cr se assemblages in the coal were derived from exfiltrational hydrothermal solutions and were then deposited in a euxinic environment the terrigenous materials in the k2 coal however were derived from felsic intermediate rocks at the top of the kangdian upland although the elevated concentrations of nb ta zr hf ree assemblages are attributed to the input of hydrothermal solutions

holocene landscape evolution of the manga grasslands ne nigeria evidence from palaeolimnology and dune chronology this paper presents a synthesis of results from investigations into palaeolimnology and dune chronology in the manga grasslands and adjacent areas of ne nigeria in order to reconstruct the evolution of this semi arid landscape since the late glacial a marked wet phase gave way to a fall in lake levels during the late glacial dune emplacement was active at this time to the south of the manga grasslands but seems to have been absent from the grasslands themselves wet conditions prevailed during much of the early to mid holocene accompanied by periods of significant barchanoid dune reactivation a marked deterioration in climate and vegetation commenced around 4 1 ka bp leading to the formation of the present day semi arid landscape the data suggest that there have been major changes in humidity in this part of subsaharan west africa during the late glacial to holocene and that the relationship between changes in precipitation and aeolian activity is complex

fluvial system dynamics of sudano sahelian zone during the late holocene the yame river dogon country mali abstract in west africa most palaeoenvironmental studies have focused on the role of climatic change on the environment because of the richness of its deposits the yame river mali a tributary of the niger river offers an opportunity to reconstruct the evolution of this fluvial system in comparison with both climatic and anthropogenic changes during the late holocene to investigate more closely the spatio temporal response of the fluvial system sedimentary analyses were conducted on seven reaches distributed along the 137 km length of the yame valley this approach testifies of the possibility for each time slice and river reach 1 to reconstruct the fluvial style and processes and 2 to estimate the sediment storage volume reflecting sediment distribution patterns results reveal a wide variability of the sedimentary cascade suggesting contrasting responses to external and local controls humid 4200 2900 cal bp 450 24 cal bp or arid 2350 1700 cal bp phases have been recorded in fluvial archives while an intensification in erosion and sediment supplies even during the arid period 2900 2350 cal bp and the 20th century may be associated with an increase in human pressure two other periods are related to both climate and anthropogenic factors during 1700 1400 cal bp phase huge sediment supplies can be explained mainly by the reactivation of both hydrological processes and human occupation after an intense and long arid event dispersed sediment sinks recorded during the 450 24 cal bp period originate from the combination of intensification of colluvial processes and a wet phase context originating from global climatic change thus this study provides evidence that climate is the strongest driver for the fluvial response of these semi arid and tropical rivers while human disturbance appear as a secondary factor due to the high sensitivity of the environment to climate variability in such areas

takarkori rock shelter sw libya an archive of holocene climate and environmental changes in the central sahara abstract rock shelters in the central saharan massifs preserve anthropogenic stratigraphic sequences that represent both a precious archive for the prehistory of the region and a powerful proxy data for holocene palaeoenvironments the geoarchaeological micromorphology and archaeobotanical pollen analysis approaches were integrated to investigate the anthropogenic sedimentary sequence preserved within the takarkori rock shelter a holocene archaeological site located in the libyan central sahara southern tadrart acacus massif the site was occupied throughout the early and middle holocene african humid period by groups of hunter gatherers before and by pastoral communities later the investigation on the inner part of the sequence allows to recognize the anthropogenic contribution to sedimentation process and to reconstruct the major changes in the holocene climate at the bottom of the stratigraphic sequence evidence for the earliest frequentation of the site by hunters and gatherers has been recognized it is dated to c 10 170 cal yr bp and is characterized by high availability of water freshwater habitats and sparsely wooded savannah vegetation a second early holocene occupation ended at c 8180 cal yr bp this phase is marked by increased aridity sediments progressively richer in organics testifying to a more intense occupation of the site and pollen spectra indicating a decrease of grassland and the spreading of cattails which followed a general lowering of lake level or widening of shallow water marginal habitats near the site after this period a new occupational phase is dated between c 8180 and 5610 cal yr bp this period saw the beginning of the frequentation of pastoral groups and is marked by an important change in the forming processes of the sequence sediments and pollen spectra confirm a new increase in water availability which led to a change in the landscape surrounding the takarkori rock shelter with the spreading of water bodies the upper part of the sequence dating between c 5700 and 4650 cal yr bp records a significant environmental instability towards dryer climatic conditions consistent with the end of the african humid period though some freshwater habitats were still present increasing aridity pushed the expansion of the dry savannah the final transition to arid conditions is indicated by the preservation of ovicaprines dung layers at the top of the sequence together with sandstone blocks collapsed from the shelter s vault on the contrary the outer part of the sequence preserves a significantly different palaeoenvironmental signal in fact the surface was exposed to rainfall and a complex pedogenetic evolution of the sequence occurred encompassing the formation of an argillic laminar horizon at the topsoil the evolution of a desert pavement and the deposition of mn rich rock varnish on stones these processes are an effect of the general environmental instability that occurred in the central sahara since the middle holocene transition finally the local palaeoclimatic significance of the sequence fits well with holocene regional and continental environmental changes recorded by many palaeohydrological records from north africa this highlights the potential of geoarchaeological and archaeobotanical investigations in interpreting the palaeoenvironmental significance of anthropogenic cave sediments in arid lands

holocene vegetation zonation in the eastern sahara recent discoveries of fossil bearing holocene lake sediments from the eastern sahara have brought further confirmation and detail to earlier indirect evidence that a major pluvial episode occurred between 9 500 and 4 500 yr bp1 3 the botanical results mainly palynological show that savanna and desert grassland occupied regions that today are plantless hyperarid deserts we present here the first evidence from the entire saharan region of a latitudinal zonation of holocene vegetation extending across a 500 km wide belt in north west sudan we have completed detailed pollen analyses of three of the several sites discovered so far and the salient features from two the northernmost and the southernmost are presented here to demonstrate a steep gradient of past vegetative cover

the geology and coal petrology of a miocene lignite profile at meliadi mine katerini greece abstract the present study describes vertical variations in petrographic composition of a miocene lignitebearing succession from the moschopotamus basin greece the lignite beds are described megascopically in terms of lithotypes and microscopically by maceral contents examination of lithotype variations within the seams and the distribution of facies critical macerals such as humotelinite and humodetrinite and the relatively low inertinite contents suggest a formation of the miocene lignites as reed marsh to wet forest type swamps in a lower delta plain depositional setting reflectances measured on the maceral type eu ulminite b range from 0 36 to 0 42 r random placing the meliadi coals into a transition zone from lignite to subbituminous coal

petrology mineralogy and geochemistry of lignites from crete greece abstract coal from the deposit of plakia island of crete greece was examined petrologically for the determination of rank maceral composition and trace element geochemistry the coal is of lignite to subbituminous b rank r o ran 0 36 0 44 some samples are rich in resinite this was subdivided into two groups based on its morphological and fluorescence properties mineral matter in the coals consists predominantly of silicate and sulphur bearing minerals the low temperature ash lta is dominated by quartz illite and gypsum whereas the mineralogy of the high temperature ash hta is comprised of silicates fe bearing minerals and anhydrite the coals are enriched in the elements as cr co cu mn mo rb se u v ce and nd when compared to world coals thorium zn and most of the rees are present in concentrations within the mean range for most coals bromine cl and se are volatilized at 1000 c while u th v cr co cu mo and zn are enriched in the coal ash by a factor of 2 8 also there are positive correlations among as mo u and s in the coal as well as between the amount of sulphur bearing minerals in coal ash and total concentration of as mo and u in the coal sulphur content in the coals ranges from 3 5 to 5 7 approximately 4 8 33 6 of sulphur is volatilized at 120 c and a large percentage of total sulphur is volatilized at 1000 c this has an important implication for the use of the plakia coals for power generation because of the emission of so x compounds to the atmosphere fluidized bed combustion and flue gas desulphurization processes are recommended to achieve reduction in trace element and sulphur emissions the hta is relatively acidic and the high content of sio 2 in the ash may create erosion problems in the boilers finally potential problems of slagging and fouling are anticipated based on the indices e g base acid ratio of the lignitic ash

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

reconnaissance study of mineral matter and trace elements in greek lignites abstract mineral matter in low temperature ashes major oxides and trace elements were determined for twenty eight selected lignite samples and their respective 1000 c ashes from various locations in greece using x ray diffraction x ray fluorescence spectroscopy and inaa lignites ashed at lower temperature 150 c reveal the presence of quartz feldspars layer silicates pyrite gypsum bassanite and anhydrite in all samples in northwestern greece epirus kozani florina and ptolemais calcite siderite magnesite epsomite hexahydrite jarosite and meta aluminite coexist with the common occurring minerals large variation in the concentration of major constituents in the 1000 c ashes is observed sio 2 varies from 11 98 to 87 72 al 2 o 3 from 4 53 to 23 36 fe 2 o 3 from 3 01 to 39 72 cao from 0 55 to 43 05 mgo from 0 18 to 9 17 and so 3 from 0 02 to 32 72 smaller variations are exhibited by tio 2 na 2 o k 2 o and p 2 o 5 these variations are attributed to the depositional environment high concentrations of pb 548 ppm zn 2991 ppm ba 1520 ppm as 1131 and 1675 ppm mo 565 and 712 ppm w 191 420 ppm and sb 69 ppm are reported in the 1000 c ashes of the selected lignites from serres and drama northern greece samples from another location in northern greece serres yield 5000 ppm u while in drama dipotama u concentration reaches as high as 17 600 ppm the u enrichment of lignites in these locations is attributed to the leaching of this element from the superimposed u bearing volcanic rhyodacitic rocks of oligocene age in southern greece peloponnesus lignite samples from megalopolis xidias drossato and vounargo show high f ranging from 154 to 218 ppm present results indicate that not only the wide variety and combination of elements that can be used to fingerprint lignitic basins but also the possibility of extracting such valuable elements from lignites and their ashes whenever the concentration and economics warrants such an undertaking

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

simulated climate and biomes of africa during the late quaternary comparison with pollen and lake status data new compilations of african pollen and lake data are compared with climate ccm1 ncar boulder and vegetation biome 1 2 gsg lund simulations for the last glacial maximum lgm and early to mid holocene emh the simulated lgm climate was ca 4 c colder and drier than present with maximum reduction in precipitation in semi arid regions biome simulations show lowering of montane vegetation belts and expansion of southern xerophytic associations but no change in the distribution of deserts and tropical rain forests the lakes show lgm conditions similar or drier than present throughout northern and tropical africa pollen data indicate lowering of montane vegetation belts the stability of the sahara and a reduction of rain forest the paleoenvironmental data are consistent with the simulated changes in temperature and moisture budgets although they suggest the climate model underestimates equatorial aridity emh simulations show temperatures slightly less than present and increased monsoonal precipitation in the eastern sahara and east africa biome simulations show an upward shift of montane vegetation belts fragmentation of xerophytic vegetation in southern africa and a major northward shift of the southern margin of the eastern sahara the lakes indicate conditions wetter than present across northern africa pollen data show an upward shift of the montane forests the northward shift of the southern margin of the sahara and a major extension of tropical rain forest the lake and pollen data confirm monsoon expansion in eastern africa but the climate model fails to simulate the wet conditions in western africa

can mima like mounds be vertisol relics far north region of cameroon chad basin abstract non anthropogenic earth mounds defined as mima like mounds in this study have recently been observed in non carbonate watersheds along the sudano sahelian belt in the chad basin in the diamare piedmont northern cameroon they are particularly well developed within stream networks in less eroded areas they occur as whaleback flattened morphologies or even as buried features all these shapes are composed of clay rich sediment associated with high proportions of secondary carbonate nodules and fe mn micro nodules their soil structure is prismatic to massive and vertical cracks are observed locally grain size distributions emphasize the clay rich nature of the sediment with average clay contents of 32 12 8 n 186 which is significantly higher than the clay content in the adjacent sediments in the landscape mean 10 4 n 21 moreover high proportions of smectite characterize the soil with average contents of 34 7 n 25 at the micro scale the groundmass has a cross striated b fabric with embedded smooth subangular quartz and feldspar grains of the silt size fraction all the characteristics point to altered vertic properties in the clay rich sediment composing the mima like mounds mima like mounds are thus interpreted as degraded vertisols compared to present day vertisols occurring in the piedmont mima like mounds are located upstream it is thus proposed that the vertisol areas were more extensive during a former and wetter period than the present day subsequent changing climatic conditions increased erosion revealing the gilgai micro relief by preferential erosion in micro lows rather than in micro highs mima like mounds of the chad basin might thus result from pedogenesis combined with later erosion these local processes can be inherited from regional climatic variations during the late pleistocene holocene and likely be related to the african humid period

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

radiocarbon dating and paleoclimatic significance of subfossil limicolaria in northwestern sudan the discovery identification and radiocarbon dating of the large achatinid land snail limicolaria kambeul chudeaui at six places in northwestern sudan north of the 100 mm isohyet provide convincing evidence that at least 300 mm annual rainfall existed there 6000 yr ago since then the rainfall north of ca 20 n lat in the eastern sahara has certainly been 300 mm and probably 200 mm accelerator mass spectrometry has allowed the organic fraction presumably conchiolin to be accurately radiocarbon dated whereas carbonate fractions are ca 600 yr too old comparative studies show the subfossil form l k chudeaui to be the most primitive in its species complex and to occupy a narrow east west belt across africa north of 15 n and west of 35 e the northern limit of living forms lies within the southern part of the sahelian zone in forest or forest savanna they cannot live in open grassland therefore the distribution of l k chudeaui marks the former northern position of this zone during the middle holocene indicating a latitudinal shift of at least 5 500 km

coals of greece a review of properties uses and future perspectives abstract greece relies heavily on lignites to satisfy its domestic needs for power generation and produces some of the cheapest and most cost effective kwh within the european community currently it mines 70 tons year of soft brown coal to cover over 70 of the country s domestic power needs brown coal in greece is found in 68 coal basins of various sizes generally it is of poor quality that is of low calorific values and relatively high ash content henceforth a detailed knowledge of maceral and ash composition is essential to appropriately construct boilers to avoid abrasion of mechanical parts fouling and corrosion of boilers and to take appropriate environmental measures though the composition of the maceral groups is relatively the same in almost all greek coal basins the composition of maceral subgroups especially that of huminite varies substantially this variation affects boiler s performance since the maximum heat release takes place at various heights inside the boiler humotelinitic brown coals rich in resinite release their thermal energy at lower temperatures than humocollinitic coals which have low resinite contents furthermore coal ash rich in gypsum anhydrite and other minerals with hardness less than 3 do not affect the mechanical parts of the boiler whereas coal ash rich in quartz feldspars and pyrites do finally greek brown coals have variable trace elemental composition coals from basins surrounded by carbonate rocks western greece do not exhibit high concentrations of heavy elements whereas coals from basins surrounded by mafic rocks north central greece are rich in cr ni cu and zn while coal basins surrounded by igneous rocks north eastern greece are rich in pb zn co hg as and u

late quaternary climatic reconstruction for the maghreb north africa abstract the paleoclimatic evolution of the maghreb since 40 000 yr b p is not yet known in great detail for that purpose data from more than 35 sites have been collected although only about 11014c dates and a few prehistoric remains can be used to correlate these data however the sites are well distributed over the maghreb and the convergent results from paleosoils paleolakes alluvian or eolian deposits pollen analysis etc induce one to draw up a certain pattern in climatic evolution for the period between 40 000 and 20 000 yr b p the data are still fragmentary but are concordant in indicating wetter environments generalized as far as the northern sahara the effect of dropping temperatures on the increasing water budget and soil moisture is not yet clear but rainfall could have been more regular and more abundant as a consequence of the shifting of middle latitude cyclone paths towards the south in both summer and winter the environmental indicators show that between about 20 and 18 000 yr b p rainfall progressively became more irregular however proofs of aridity in the south maghreb and the northern sahara is evident only for the period between 12 10 000 yr b p at that time the saharan high pressure belt was reinforced and well established at higher latitudes than at present this dry climate lasted more or less until the thermal optimum after 6000 yr b p there is some evidence that the climate became wetter again but the environmental conditions were rather different from those of before 20 000 yr b p for instance the middle holocene dark humic sois are the result of a pedogenesis different from that of the late pleistocene reddish paleosoils for the last two millennia it is not easy to distinguish between a natural trend towards aridification and desertification through man s intervention

late quaternary eolian deposition in central italy abstract records of eolian quartz from two continuous sediment sequences drilled in lagaccione and lago di vico volcanic lakes in central italy contribute to the knowledge of eolian deposition in the central mediterranean during the last 100 000 years the chronology is based on 14 c and 40 ar 39 ar dating and tephra analysis pollen data provide the paleoenvironmental framework and enable correlation between the cores eolian inputs were high during the steppe phases corresponding to oxygen isotope stages 4 and 2 low inputs correspond to the forest phases of the last interglacial and the middle holocene eolian inputs have increased in the late holocene patterns of eolian deposition in central italy resemble the antarctic dust record from the vostok ice core the italian patterns may also correspond with hydrological changes registered in north africa the main source of dust loading over the mediterranean now north africa may have played an important role in dust supply throughout the last climatic cycle

paleoenvironmental events during the last 13 000 years in the central red sea as recorded by pteropoda a high resolution record was obtained by investigating a sedimentary sequence from two cores taken in the central red sea r v meteor cruise 5 leg 2 the numerical variations between nonmigratory and migratory pteropods and their stable isotopic record were studied together with variations in the carbonate and total organic carbon content in order to estimate changes in the structure of the water column during the last 13 kyr the results indicate the existence of a highly stratified water column during deglaciation and early holocene 13 8 5 ka during this period the depth of the mixed layer varied between less than 50 100 m at most and productivity was markedly reduced in comparison to the recent situation the intermediate and deep water were constantly highly depleted in oxygen a pronounced younger dryas event was recognized at 10 4 ka which coincides with an arid phase on the border land the oxygenation of the intermediate water improved remarkably after 7 5 ka and peaked during the period between 4 6 and 2 0 ka during this period the mixed layer reached its present depth and the productivity which was maximal during middle holocene attained present level during the last 4 6 kyr

petrological and organic geochemical characteristics of coal samples from florina lava moschopotamos and kalavryta coal fields greece abstract coal samples from florina vevi lava moschopotamos and kalavryta basins greece have been subjected to petrographic and organic geochemical analysis the reflectance of the samples ranged from 0 26 to 0 42 which ranks them as soft brown coals and sub bituminous c coals lower reflectance values were encountered in the florina and kalavryta coals whereas higher values were obtained for lava and moschopotamos samples maceral composition of the samples from the florina area shows a high concentration of textinite a and texto ulminite a the samples from the lava and moschopotamos area are comparatively rich in humodetrinite samples from florina are rich in resinite whereas samples from lava are rich in liptodetrinite rock eval pyrolysis data show that the samples from florina and kalavryta have generally high hi si and s2 and low t max values the coal samples from moschopotamos and lava show the opposite having low hi s1 and s2 values these distinct differences are related to the maceral composition tissue rich in resinitic material and rank of the coals gas chromatography of the saturate fraction indicates that the samples from the lava and moschopotamos are rich in n alkanes from n c 23 to n c 33 in contrast the samples from florina are rich in diterpanes whereas those from kalavryta are rich in sesquiterpenoids gas chromatography mass spectrometry of the saturate fraction reveals that the diterpenoids present in the lignites from kalavryta are derivatives of abietanic acid and pimaric acid which are found in pinaceae the lignites from florina mainly contain the diterpane phyllocladane which implies an input from cupressaceae and or taxodiacaea the difference in the composition of these xylitic lignites may be attributed to differences in gymnosperm input as a response to climatic change as the kalavryta coals are of pliocene age whereas the florina coals are of miocene age the study suggests that the petrology and organic geochemistry of these coals are related to the different peat forming plant communities that were present in the florina kalavryta lava and moschopotamos basins a mainly non woody angiosperm dominated vegetation made up the peat that went to form the coal deposits of lava and moschopotamos while the florina and kalavryta lignites were formed by peats derived mainly from gymnosperms

phytolith taphonomy in the middle to late holocene fluvial sediments of ounjougou mali west africa in semi arid ecosystems where lacustrine sediments are rare bio proxies preserved in fluvial deposits are needed to understand environmental changes in this study we evaluate the potential of phytoliths as a bio proxy in the yame river s deposits at ounjougou mali west africa covering the middle to late holocene 7790 4000 cal bp in soils phytolith assemblages result mainly from decomposition of local vegetation but in alluvial deposits the taphonomy of phytoliths is more complex depending on the type of transport and deposition in order to define the spatial origin of the phytolith assemblages either from local valley floor or regional vegetation catchment area we took the sedimentary characteristics of the deposits into account using a combination of general and index approaches phytoliths of 14 modern and 29 fossil samples from the yame valley were analyzed the predominant source area of represented phytoliths varies with the fluvial energy of transport channel deposits carried during periods of strong fluvial activity contain higher numbers of savanna grass phytoliths and display a lower ratio of dicotyledon versus poaceae phytoliths d p than those deposited in the floodplain where phytoliths from the local gallery forest are more strongly represented from the 5th millennium bp onwards higher percentages of grass short cell phytoliths gscp and lower d p values point to gradual vegetation opening due to increasing aridity high amounts of burned phytoliths show regular fire incidence in the gallery forest and attest for human impact on the landscapes of ounjougou from the 7th millennium bp onwards after 4500 cal bp there is evidence for pearl millet cultivation

geologic sections across the onshore senegal mauritania basin derived from geoelectric studies on the basis of magnetotelluric two dimensional models along three traverses the regional electrical structure within the senegal mauritania basin sediments has been studied correlation of the computed models with the wells drilled in the region shows that the main gross lithologic units were detected the models can be generalized into three resistivity layers i post turonian ii pre senonian and iii basement in the southern part of the basin there are relatively conductive formations below the mesozoic that are interpreted as sediments of the paleozoic bove basin sequence by combining the information that has been provided by the geoelectric cross sections and the lithologic and electric log data a schematic model for the generalized sedimentary structure of senegal has been generated the interpretation that emerges shows that the basin is a westward sloping open homocline in which the structure is controlled by north south trending basement faulting that portrays a staircase structural

monsoon climate of the early holocene climate experiment with the earth s orbital parameters for 9000 years ago values for the precession and obliquity of the earth 9000 years ago indicate that the global average solar radiation for july 9000 years ago was 7 percent greater than at present when the estimated solar radiation values are used in a low resulation climate model the model simulates an intensified continent scale monsoon circulation this result agrees with paleoclimatic evidence from africa arabia and india that monsoon rains were stronger between 10 000 and 5000 years ago than they are today

enrichment of arsenic lead and antimony in balingian coal from sarawak malaysia modes of occurrence origin and partitioning behaviour during coal combustion abstract the concentrations of mineral forming elements and trace elements in coal and coal ashes from balingian coalfield sarawak malaysia were assayed and their modes of occurrence enrichment origin and partitioning behaviour during coal combustion were studied as well balingian coal is high in as cu pb sb th and zn of particular concern are as as high as 181 ppm pb 325 ppm and sb 96 ppm all of which are highly enriched in balingian coal relative to their respective coal clarke values arsenic pb and sb in the coal are mostly organic and to a lesser extent they are bound to discrete minerals in the form of aluminosilicates during coal combustion in the power plant a substantial portion of organically associated as pb and sb in the coal vaporize and are released into the atmosphere whereas ba and mn are highly enriched in the coal ashes as compared to the raw coal the as pb and sb anomalies observed in balingian coal may be related to a large sb as and probably pb mineralization in the vicinity of the coalfield with some localised epigenetic infiltration of these elements from the surrounding rocks by circulating groundwaters

across an early holocene humid phase in western sahara pollen and isotope stratigraphy paleohydrology and paleoecology of the region around the chemchane sabkha mauritania are reconstructed from the comparative analyses of mineralogy stable isotope ratios of bulk carbonates and palynology from sediment cores the age of the section is from ca 13 5 ka to the middle holocene maximum lake expansion between about 8 3 and 6 5 ka is recorded in a girdle of stromatolite carbonates these document a lower salinity lake concomitant with a general establishment of sahel sudan vegetation the chemchane area is the northernmost lat 21 n occurrence of humid phase elements that are related to maximum intensity of monsoon activity during the early holocene

glacialanterglacial record from intertropical africa high resolution pollen and carbon data at rusaka burundi abstract in northern intertropical africa many geological data have documented several abrupt climatic fluctuations taking place during the time of the last deglaciation one of them is an aridity event generally placed in the interval 11 000 10 000 bp and synchronous with the younger dryas cold episode of the temperate regions that has been explained by the influence of the atlantic ocean circulation on global climate in this paper we present high resolution 50 to 150 years studies of organic matter and pollen undertaken on a new core from the rusaka peat bog 3 26 south 29 37 east 2070 m altitude dated by the conventional l2 dates and the ams methods 18 dates obtained on bulk sediment and macroremains although the sediment at the bottom of the core was deposited during the last glacial period prior to 21 000 bp the dating provides a reliable chronology for the last 12 000 bp only several vegetational changes are documented in the interval 12 000 9000 bp but their chronological placement can only be achieved with a few years of uncertainties a great dispersion of ams dates is attributed to contamination by charcoal dated between 11 700 and 11 500 bp and to a possible age plateau at ca 10 000 bp therefore the results are preferably presented versus depth to keep synchronism in the stratigraphic placement of the successive events at rusaka increase in carbon storage c n ratio for terrestrial plants and tree cover began at ca 12 000 bp soon after the swamp had been established this date is in good agreement with a major increased hydrological balance well documented throughout the intertropical region the forest installation is progressive rather complex and seen as a two step process the first forest phase 11 700 bp to 10 600 bp is dominated by hagenia a montane taxon in good correspondance with previous records from other authors in east african mountains the second forest regeneration took place post 10 000 bp after a clearly reversed trend of tree cover from 10 600 to 10 000 bp ending with a short episode of minimum trees lasting no more than about 200 years but there is no indication of cooling whatsoever and the opening of the forest is better explained by increased aridity rather than normal pattern of forest succession this will remain a controversal interpretation among palynologists until high resolution pollen data can be obtained for the 13 000 bp and earlier period nevertheless the timing of the arid episode at rusaka is in good agreement with other evidence mentioned from highland and lowland pollen data a detailed comparison with 25 available pollen diagrams is limited by poor or inconsistent dating or low resolution of the pollen data nevertheless pollen from four of them document an arid event in good agreement with its timing at rusaka and high resolution isotopic results from two sites in equatorial lowlands and from the sahel this work shows that the arid younger dryas occurred at the same time both in lowlands and highlands and therefore was most likely a precipitation signal rather than a response to temperature decrease

palynological evidence of cultural and environmental connections in sudanese nubia during the early and middle holocene abstract pollen and non pollen palynomorphs have been studied from three archaeological sites 8 b 10c 8 b 76 and 8 b 81 on sai island in the river nile and one 2 r 66 from the amara west district in northern upper nubia of the present sudan the research aimed at obtaining information on the environmental conditions and changes occurring in the area during the early and middle holocene archaeologically this is a crucial period as it saw one of the most relevant economic transitions from hunting fishing gathering to animal herding the archaeopalynological analyses are useful to reconstruct the environment and plant landscapes that supported human plant selection in this part of north africa between about 8700 and 4300 cal bc however pollen was not present everywhere because the sediments were very poor in organic content or damaged by the repeated hydration dehydration cycles determined by the nile river floodings the data obtained the most consistent from any archaeological site in this area are coherent with the regional and interregional palaeoenvironmental data the interdisciplinary studies that allowed the correct interpretation of the pollen records presented in this paper included the archaeological features and the faunal mainly gastropod and algal remains from the same sites the state of preservation of most pollen showing thinned exine and the remarkable records of the terrestrial alga fritschiella outline the seasonality of the area that has been evident in the past as in the present pollen samples included prevailing amounts of grasses and sedges that with some hygro hydrophilous taxa largely represent the riverine and wetland vegetation of the nile valley they suggest that the land use was not intensive and was not able to substantially modify the natural cover and cyclic renewal of the soils and the vegetation the interdisciplinary evidence from the sai island showing that site 8 b 76 has been continuatively occupied during the 8 2 ka bp dry phase proves the key role played by the great river on attracting humans and supplying resources even and especially during the arid oscillations of the holocene

geoelectrical structure of the northern part of the senegal basin from joint interpretation of magnetotelluric and geomagnetic data magnetotelluric mt and differential geomagnetic sounding dgs surveys were made in the senegal basin west africa to investigate the conductivity structure of the crust and upper mantle magnetic and electric variations were recorded at 11 sites along a east west profile 220 km in length the period range of the measurements was 10 10 000 s for the western stations the mt response displays low skew with moderate anisotropy the dgs data indicate the presence of an anomalous geomagnetic variation field in the deep basin and on the eastern margin of our study area a telluric current concentration in the thick conductive sediments of the deep basin might cause this anomaly both mt and dgs data are then compared with two dimensional model results to explain the observed features along the profile results reveal a strong anomalous crustal structure in the central part of the profile involving a conductive zone 2 ohm m at depths in the range 4 23 km within the deep basin the results of the two dimensional modeling are not conclusive acceptable fits between observed and calculated data have been obtained with alternate models a continuous conductive zone 0 7 2 ohm m may exist beneath the uppermost layers at a depth of approximatively 1 km but the observed data can be explained equally well by poorly conducting material intruded into the highly conductive sedimentary cover this ambiguity is inherent in the two dimensional models themselves both models show that the sediments increase westward to reach a thickness greater than 4000 m when approaching the coast it is suggested that the flow of current observed in this study is largely controlled by strong lateral variation in the resistivity of the basin and or by variation of the sedimentary thickness the models also indicate the existence of two decreased resistivity layers in the eastern part of the profile one at the base of the crust and the second at 80 km with a resistivity of 20 ohm m no such layers were detected at these depths below the deep basin at some depth greater than 150 km there is a general trend toward lower resistivities from 2000 to about 2 ohm m across the entire profile this may indicate the presence of a small degree of partial melt

sediment and pollen evidence for an early to mid holocene humid period in the eastern sahara a major problem in the study of holocene palaeoenvironments of the arid and wind deflated sahara is the low preservation potential of sediments from which a record of past climatic change can be established several indirect and imprecisely dated pieces of evidence have suggested humid episodes in the holocene that may have supported more productive ecosystems and denser human populations than today searches for reliable proxy data on past climates however have failed to yield satisfactory results we report here the discovery of buried lake muds in north west sudan in the hyperarid core of the eastern sahara which yield sedimentological and palynological data clearly interpretable as recording an early to mid holocene humid episode that supported a relatively deep stratified lake surrounded by tropical savanna woodland vegetation

concentration of elements in lacustrine coals from zone a hat creek deposit no 1 british columbia canada abstract coal samples from the 170 m thick a zone of the hat creek coal deposit no 1 in central british columbia were examined for major minor and trace element distribution the coal is lignite subbituminous in rank and of eocene age this study shows 1 as b br cl and s are the only elements organically associated in these samples 2 concentration of certain elements such as fe mn and co can be related to the presence or absence of carbonate and or sulphide minerals 3 distribution of certain elements i e al b br cl cr fe th u and zn follow similar patterns of concentration or depletion across the vertical thickness of zone a and suggest three depositional cycles 4 concentration of boron versus chromium sodium sulphur and uranium for these coals clearly placed them in a freshwater environment 5 rates of sedimentation using the na k ratio also suggest three depositional cycles 6 in resinite samples hand picked from these coals the concentrations of all elements except as and se were lower than their concentrations in the whole coals

holocene palaeoecology of the eastern sahara selima oasis buried lake sediments and tufa strandlines of holocene age provide evidence of a former lake in the selima oasis depression northwest sudan near the hyperarid core of the eastern sahara desert a 1 6 m section through laminated lake muds and precipitates has been analysed in detail and 30 radiocarbon dates geochemical sedimentological diatom and pollen analyses indicate that the unit dated from approximately 8 4 ka bp to 6 ka bp and was deposited in an environment that passed through one arid to humid to arid cycle the arid episodes from prior to 8 4 ka bp and subsequent to 7 ka bp are characterized by high caco3 deposition abundant diatom indicators of relatively high salinity and well developed periphytic communities and pollen spectra dominated by desert grassland indicators a humid period centered on 8 ka bp is indicated by low caco3 deposition high organic rich mud and al and si oxide deposition diatoms indicating a fresh deep 20 m lake and a pollen assemblage with maximum values for sahelo sudanian tree taxa suggesting a thorn tree and shrub savanna vegetational mosaic this reconstruction concordant with higher groundwater table and precipitation between 9 3 and 7 ka bp agrees with palaeoecological inferences from other sites in north africa

a holocene vegetation history from lowland guatemala a 5 45 m core from lake peten itza lowland guatemala contains a near complete record of holo cene sedimentation the age depth relationship for the core is based on ams 14c dating of terrestrial wood fragments and provides a reliable chronology in this karst region where hard water lake error has typically confounded sediment geochronology in the basal part of the sequence pollen of the moraceae urticaceae group dominate indicating the presence of widespread tropical forest during the early holocene c 8600 5600 yr bp relative abundance of pollen of high forest taxa declined beginning as early as 5600 yr bp indicating climatic drying or perhaps initial land clearance deforestation by prehistoric mayan inhabitants is documented clearly in the pollen record beginning about 2000 yr bp 106 cal bc 122 cal ad 95 4 2 sigmas by the appearance of disturbance taxa e g ambrosia and poaceae and presence of zea forest regrowth occurred following the classic maya collapse c ad 900 as reflected by

mediterranean quaternary sapropels an immediate response of the african monsoon to variation of insolation abstract in the subsurface of the east mediterranean sea eleven discrete basin wide organic rich black sapropels were deposited during the last 465 000 years they are dated by oxygen isotopic chronostratigraphy the most recent sapropel has been shown to be the consequence of heavy nile river summer floods due to increased monsoonal summer precipitation over ethiopia and not of scandinavian ice sheet meltwater influx into the east mediterranean sea as was previously believed while most sapropels were deposited during interglacials two thick sapropels formed during glacial periods as shown by their position in the oxygen isotopic stratigraphy and by their pollen record in core rc9 181 the new evidence presented here is that the sapropel sequence in this core including the two glacial sapropels correlates on a one to one basis with the highest values of an index of insolation on top of the atmosphere the concept of this index derives from an analysis of the african monsoon process the index variation is computed for the last 465 000 years from the caloric northern summer insolation in the tropical latitudes controlled by variations of the earth s orbital parameters it is conjectured that this index might be linked through the climate system to the monsoonal precipitation which in africa produces the nile river summer flood

interpretation of satellite magnetic anomalies over the west african craton abstract satellite magnetic anomaly maps of west africa display persistent anomalies that are spatially related to the geological structure of the west african craton the reguibat and man shields to the north and south of the taoudeni basin have positive magnetization contrasts whereas the basin which was disrupted by rifting and volcanism in the late proterozoic and mesozoic has a negative magnetization contrast a forward model of the craton is developed based on the surface geology and including aspects of the evolution of deep seated thermal petrological and magnetization contrasts anomalies calculated from the model correspond fairly well to the most reliable observed anomalies the anomaly associated with the basin is due partly to the anomalies arising from the shields and partly to intra basin deep seated demagnetization resulting from mesozoic thermal and metasomatic demagnetization another similar demagnetized zone probably contributes to the reguibat shield anomaly the man shield anomaly results primarily from the archean segment of this shield the largest magnetization contrast required for a 70 km thick shield slab is 0 3 a m 1