th u disequilibrium dating of middle and late pleistocene wood and shells from banks and victoria islands arctic canada an attempt has been made at dating middle and late pleistocene deposits from banks and victoria islands using the th u disequilibrium method geological correlations are difficult to establish because of the discontinuous nature of the terrestrial units and the relative absence of datable fossil wood and shells the th u geochemical system likely remained closed for extended periods because of the presence of permafrost which implies very limited or absent water circulation the th u age determinations obtained confirmed the previously established stratigraphic framework and provide chronological information in that samples of sangamonian age are differentiated from those that are younger wisconsinan and holocene or older middle pleistocene the th u disequilibrium method carefully used in areas where water circulation is limited can thus provide chronological control for sediments older than those that can be dated by the radiocarbon method

run out distance and erosion of debris flows in the nile deep sea fan system evidence from lithofacies and micropalaeontological analyses abstract debrites with different volumes depositional periods and setting are observed in the nile deep sea fan system here we present debrites from four late quaternary environments i within the rosetta submarine channel at 1700 m ii within small basins created by salt tectonics in the eastern province at 1300 m iii on the open slope outside of channels at 2600 m and iv in the distal part of the fan fringe beyond channel terminations at 3050 m water depth the debrites comprise mud clasts in a generally ungraded muddy sand matrix they are relatively thin 20 150 cm in fan fringe in channel and salt tectonic basin settings and somewhat thicker 8 m in the open slope setting benthic foraminiferal assemblages and variation in the planktonic benthic foraminiferal ratios are considered as a useful tool for tentatively evaluating the origin of sediment in these debris flows and estimation of their run out distance similar microfaunal contents of clast matrix combinations document that the debrites in the rosetta channel and salt tectonic basin were locally sourced the debrite from the open slope travelled farther up to 150 km three separate debrites in a fan fringe core were deposited by debris flows that originated over 200 km away in water depths shallower than 1000 m these far travelled debris flows appear to have eroded remarkably little sediment as they travelled for 200 km across low 0 8 1 2 gradient seafloor this lack of erosion is attributed to hydroplaning or increasing pore fluid pressure at the base of debris flows this study confirms that remarkably mobile debris flows can travel long distances to produce thin debrites in the distal fringes of submarine fans

criteria for differentiating microbial caddisfly bioherms from those of marine polychaetes in a lacustrine setting paleocene second member funing formation subei basin east china abstract paleogene carbonate successions from lacustrine basins across east china yield microbial bioherms containing abundant agglutinated fossil tubes historically these have been interpreted as the dwellings of polychaetes introduced by marine incursions this is problematic because these tubes are associated with abundant non marine fauna and algae diverse and exceptionally well preserved fossil tubes from carbonate microbial bioherms were studied in outcrops and drill cores of the second member of the paleocene funing formation subei basin east china detailed macro meso and micro structural investigations of these tube dominated bioherms reveal that the putative marine polychaete tubes better fit the morphologies and construction styles of lacustrine caddisfly insecta trichoptera larval cases eliminating the need for a marine influence on this lacustrine setting individual caddisfly cases are commonly 10 25 mm in length and 1 2 5 mm in diameter and form slightly curved tapered tubes or coiled spirals twelve tubular case construction styles were further identified based on caddisfly case wall structures and dominant grain types caddisfly cases are commonly single walled with rarer nested instar cases comprising double or triple walls of different sizes most cases are fully agglutinated with neatly arranged similarly sized and closely connected particles of clastic grains carbonate particles plant fragments and phosphatic fish bones sparsely agglutinated cases are also present but much rarer and conform to the silk larval cases of caddisflies the caddisfly cases appear as gregarious aggregations within carbonate microbial bioherms the microbial caddisfly bioherms in this study are commonly developed on carbonate hardground or mudstone within the wave washed high energy shallow lacustrine littoral zone and can be traced along the western paleolake margin of the jinhu depression of the subei basin for up to 60 km the bioherms are composed of repeated microbial caddisfly couplets made up of randomly arranged and similarly sized caddisfly cases encrusted by domed planar columnar or cauliflower like microbial layers cyclic microbial caddisfly couplets fit well with caddisfly biology but are not known to occur with marine polychaetes this is the first report of microbial caddisfly bioherms in cenozoic strata of china and is the oldest cenozoic record of caddisflies in china

redistribution of energy available for ocean mixing by long range propagation of internal waves ocean mixing which affects pollutant dispersal marine productivity and global climate1 largely results from the breaking of internal gravity waves disturbances propagating along the ocean s internal stratification a global map of internal wave dissipation would be useful in improving climate models but would require knowledge of the sources of internal gravity waves and their propagation towards this goal i present here computations of horizontal internal wave propagation from 60 historical moorings and relate them to the source terms of internal waves as computed previously2 3 analysis of the two most energetic frequency ranges near inertial frequencies and semidiurnal tidal frequencies reveals that the fluxes in both frequency bands are of the order of 1 kw m 1 that is 15 50 of the energy input and are directed away from their respective source regions however the energy flux due to near inertial waves is stronger in winter whereas the tidal fluxes are uniform throughout the year both varieties of internal waves can thus significantly affect the space time distribution of energy available for global mixing

a pleistocene diatomite from the western piedmont of jebel marra volcano darfur western sudan and its hydrological and climatic significance abstract diatoms collected at 10 cm intervals from a 6 3 m pleistocene lacustrine diatomite section in the now arid piedmont west of jebel marra volcanic caldera darfur western sudan were analyzed the lake probably formed as a result of a rhyolite lava flow damming a former river the lake was initially shallow but in the upper 5 3 m diatom composition and a salinity reconstruction indicate persistent deep freshwater conditions the diatomite is overlain by an acheulian stone tool assemblage with a probable age range of 0 8 0 3 ma the lake may be coeval with the oil palm fossils discovered 90 km to the southwest which are underlain by developed oldowan early acheulian artifacts some in primary context and so of lower middle pleistocene age both the diatomite and the oil palm fossils indicate a much wetter climate than today

new altimetric estimates of mode 1 m2 internal tides in the central north pacific ocean abstract new estimates of mode 1 m2 internal tide energy flux are computed from an extended ocean topography experiment topex poseidon t p altimeter dataset that includes both the original and tandem tracks improving spatial resolution over previous estimates from o 500 km to o 250 km additionally a new technique is developed that allows separate resolution of northward and southward components half wavelength features previously seen in unseparated estimates are shown to be due to the superposition of northward and southward wave trains the new technique and higher spatial resolution afford a new view of mode 1 m2 internal tides in the central north pacific ocean as with all altimetric estimates only the coherent or phase locked signals are detectable owing to the long repeat period of the tracks emanating from specific generation sites consistent with predictions from numerical models internal tidal beams 1 are as narrow as 200 km and 2 propagate a longer distance than previously observ

climate driven variations in geothermal activity in the northern kenya rift valley high temperature continental geothermal systems are primarily associated with volcanic activity at plate margins1 it has long been known that the activity of these geothermal systems is intermittent and this is generally attributed to the effect of magmatic intrusions or fault motions recently however it has also been recognized2 4 that surface water hydrology and climate variations can influence the evolution of individual silicic caldera hosted geothermal systems although it is difficult to judge the general importance of such effects in controlling the long term behaviour of continental geothermal systems here we report uranium series ages for the hydrothermal deposits of past geothermal activity from several quaternary volcanic centres in the northern kenya rift valley we find that the ages correspond well to periods of high lake level within the rift suggesting that the elevated water table and increased availability of meteoric water associated with more humid climates can promote greater transfer of heat and mass from deep long lived heat sources to the surface

northward intrusions of low and mid latitude storms across the saharo arabian belt during past interglacials the rain regime of the levant during the late quaternary was controlled primarily by mediterranean cyclonic systems associated with north atlantic climate shifts lake levels in the dead sea basin have been robust recorders of the regional hydrology and generally indicate highstand wet conditions throughout glacial intervals and lowstands dry during interglacials however sporadic deposition of travertines and speleothems occurred in the negev desert and arava valley during past interglacials suggesting intrusions of humidity from southern sources probably in association with enhanced activity of mid latitude red sea synoptic troughs and or low latitude tropical plumes the southerly incursions of wetness were superimposed on the long term interglacial levantine arid conditions as refl ected by the current prevailing hyperaridity and could have had an important impact on human migration through the red sea dead sea corridor

mega lake in the kalahari a late pleistocene record of the palaeolake makgadikgadi system abstract a distinct series of beach ridges marking the former shorelines of large inter connected lacustrine basins in the kalahari can be clearly identified from landsat imagery and shuttle radar topography mission srtm data these basins which form the terminal sump of the okavango system in northern botswana are now almost completely dry during the quaternary they were intermittently occupied by large stable lake bodies and are thought to have periodically filled to a point of coalescence inundating an area that at its largest extent encompassed 66 000 km 2 poor chronological control has previously limited the utility of this important palaeo archive as part of a region wide lake palaeo shoreline research programme a systematic optically stimulated luminescence osl dating programme has utilised a lightweight hydraulic auger to take samples at depth from relict shoreline features twenty drill sites have generated 140 samples for dating establishing a firm chronology for multiple lake full phases in all three component basins ngami mababe and makgadikgadi of this mega lake this paper presents the final set of ages in the programme derived from four cores from the western and north eastern shorelines of makgadikgadi and uses these ages to establish a chronology of mega lake high stands during the last 300 ka providing a rare directly dated long terrestrial record of positive hydrological excursions within the southern hemisphere

coals of canada distribution and compositional characteristics abstract canada s coal resources occur in 16 sedimentary basins or groups of basins and range in age from devonian to tertiary the western canada sedimentary basin wcsb which contains the vast majority about 90 of the nation s coal resources of immediate interest underlies a large area in the provinces of british columbia alberta saskatchewan and manitoba extending northward to about the 62nd parallel in yukon and northwest territories coal deposits in the wcsb range in age from early carboniferous mississippian to paleocene rank ranges from lignite to semianthracite about 36 of the total estimated 71 000 megatonnes of resources of immediate interest in the wcsb is bituminous coal including a high proportion in the medium to low volatile range their low sulphur contents and acceptable ash levels make these medium and low volatile bituminous coals attractive 3s coking feedstocks and large quantities are mined for that purpose the lower rank western canadian coals are used mainly for electricity generation significant resources of bituminous coal occur in the coalfields of atlantic canada where they have been mined since 1720 most of these coals are classed as high volatile a bituminous and most are used for power generation large resources of coal lignite to anthracite also occur in more remote regions of canada such as the bowser basin in northwestern british columbia and sverdrup basin franklinian geosyncline in the arctic islands information on distribution and compositional attributes of these frontier region coals is commonly scarce

wet phases in the sahara sahel region and human migration patterns in north africa the carbon isotopic composition of individual plant leaf waxes a proxy for c3 vs c4 vegetation in a marine sediment core collected from beneath the plume of sahara derived dust in northwest africa reveals three periods during the past 192 000 years when the central sahara sahel contained c3 plants likely trees indicating substantially wetter conditions than at present our data suggest that variability in the strength of atlantic meridional overturning circulation amoc is a main control on vegetation distribution in central north africa and we note expansions of c3 vegetation during the african humid period early holocene and within marine isotope stage mis 3 50 45 ka and mis 5 120 110 ka the wet periods within mis 3 and 5 coincide with major human migration events out of sub saharan africa our results thus suggest that changes in amoc influenced north african climate and at times contributed to amenable conditions in the central sahara sahel allowing humans to cross this otherwise inhospitable region

ages of quaternary pluvial episodes determined by uranium series and radiocarbon dating of lacustrine deposits of eastern sahara abstract as documented by radiocarbon dating and geoarchaeological investigations the now hyperarid northwestern sudan and southwestern egypt experienced a period of greater effective moisture during early and middle holocene time about 10 5 ka we have used the uranium series technique to date lacustrine carbonates from bir tarfawi bir sahara east wadi hussein oyo depression and the great selima sand sheet localities results indicate five paleolakeforming episodes occurred at about 320 250 240 190 155 120 90 65 and 10 5 ka four of these five pluvial episodes may be correlated with major interglacial stages 9 7 5e and 1 the 90 65 ka episode may be correlated with substage 5c or 5a our results support the contention that past pluvial episodes in north africa corresponded to the interglacial periods farther north ages of lacustrine carbonates from existing oases and from the sand sheet fail to indicate pluvial conditions between about 60 and 30 ka age results and field relationships suggest that the oldest lake and ground water deposited carbonates were much more extensive than those of the younger period and that carbonate of the latest wet periods were geographically localized within depressions and buried channels

mediterranean climate and oceanography and the periodic development of anoxic events sapropels mediterranean sapropels are layers with elevated organic carbon concentrations that contrast with surrounding sediments which are organic poor sapropels occur quasi periodically in sedimentary sequences of the last 13 5 million years and exist both in the eastern and western mediterranean sub basins they have been the subject of extensive study based on records from both short conventional and long ocean drilling program sediment cores and from a wide variety of uplifted marine sediment sequences on the basin margins and islands previous syntheses in the 1990s and 2000s have discussed how the formation of sapropels is commonly ascribed to deep sea anoxia enhanced export productivity or a combination of these effects however a wealth of new evidence and insights has emerged during the past 1 2 decades based on traditional and novel proxy data as well as modelling which has revealed intriguing new aspects and nuances to the reconstructed conditions hence it is timely to present a new synthesis of current understanding of the processes behind the formation of sapropels which have over the past decade also become a matter of commercial interest in sub salt hydrocarbon exploration in this review we present a context of modern mediterranean climate and oceanography followed by an integrated assessment of the growing understanding of climatological and ocean circulation changes that were associated with sapropel deposition we find that sapropels predominantly formed during astronomically timed episodes when climatic and oceanographic conditions and ecological responses broadly preconditioned the basin for sapropel deposition there is strong correspondence with times of monsoon intensification fuelling runoff from north africa into the mediterranean sea while preconditioning due to sea level rise and regional precipitation and runoff may have contributed as well within these broad episodes of surface buoyancy gain and resultant decline in deep water ventilation specific deposition under dysoxic anoxic or even euxinic conditions occurred within a clearly dynamic system that was characterised by complex spatial and depth dependent patterns gradients with distinct temporal variability on at least decadal to centennial millennial timescales in the final section we evaluate the implications of different modes of deep water removal from silled basins to investigate why sapropels are more frequently and often more intensely developed in the eastern mediterranean than in the western mediterranean

constraints on pleistocene pluvial climates through stable isotope analysis of fossil spring tufas and associated gastropods kharga oasis egypt abstract stable isotope analyses of fossil spring carbonates tufas and freshwater gastropods melanoides tuberculata from the currently hyperarid western desert of egypt indicate that this region received enough precipitation to support a small perennial lake during the height of the oxygen isotope stage 6 5e pluvial event and a substantial volume of spring discharge during prior pluvial phases tufa and gastropod oxygen isotope ratios are generally low relative to modern precipitation in the region confirming that the majority of pleistocene pluvial precipitation came from an atlantic source as determined by previous workers mean tufa 13c 2 pdb suggests a significant proportion of c4 vegetation in the recharge area of the tufa depositing springs while the paucity of tufas with 13c greater than 0 suggests that equilibrium between spring water and atmospheric co2 represented an endpoint for the evolution of spring water carbon though spring carbonate deposition requires a substantial increase in humidity in central egypt relative to modern conditions high variation in tufa 18o up to 4 within individual stratigraphic units as well as positive covariance between tufa carbon and oxygen isotope ratios reflect significant effects of evaporation on spring water during tufa formation similarity in 18o and 13c values of tufas from different stratigraphic units indicates that climatic conditions did not vary significantly among pluvial phases recorded by spring carbonate deposition the lack of substantial carbonate formation at tufa localities during the holocene pluvial phase however suggests that tufa deposition represents substantially wetter conditions than have existed during holocene time

monsoons and the dynamics of deserts the existence of subtropical deserts such as the sahara has often been attributed to the annual mean zonal mean hadley circulation which shows strong descent in the subtropics however the zonal mean hadley circulation shows considerable evolution over the course of the year with very strong subtropical descent during winter but practically no zonal mean subtropical descent during summer when rainfall over the eastern sahara and the mediterranean is least charney 1975 proposed a biosphere albedo feedback mechanism whereby local anthropogenic effects related to over grazing could affect the radiative balance enhancing summertime diabatic descent and leading to desertification of the subtropics in general the present study which uses an idealized model suggests a monsoon desert mechanism for desertification whereby remote diabatic heating in the asian monsoon region can induce a rossby wave pattern to the west integral with the rossby wave solution is a warm thermal structure that interacts with air on the southern flank of the mid latitude westerlies causing it to descend this adiabatic descent is localized over the eastern sahara and mediterranean and over the kyzylkum desert to the south east of the aral sea by the mountains of north africa and south west asia trajectories indicate that the monsoon desert mechanism does not represent a simple walker type overturning cell instead the descending air is seen to be mainly of mid latitude origin it is speculated that the monsoon forced adiabatic descent may result in clear air and therefore a local diabatic enhancement which effectively doubles the strength of descent with this mechanism desertification can be forced by remote changes in monsoon strength rather than by local effects this conclusion is supported by the observed dramatic strengthening of descent over the mediterranean and east sahara during the onset of the asian monsoon and on the longer timescale by relating prehistoric lake levels to milankovitch monsoon forcing the latter may help to explain the perceived discrepancies between the palaeoclimate of the eastern sahara and the strength of a tropic wide monsoon the monsoon desert mechanism may not be confined to the asian monsoon alone and the existence of other monsoon climate regions over the globe may in a similar way explain the observed summertime strengthening of the oceanic sub tropical anticyclones and the existence of western continental deserts and mediterranean type climate regions

desert pavement development and landscape stability on the eastern libyan plateau egypt abstract desert pavement surfaces of the eastern libyan plateau in central egypt represent a stable landscape preserving middle and upper paleolithic artifacts detailed measurements of pavement clasts indicate significant variability in clast size density lithology and orientation between pavements but no spatial relationship among any of these pavement variables over the study area pavement characteristics are unrelated to local geomorphic features including slope gradient and aspect suggesting a desert pavement surface that has developed without significant influence from transporting mechanisms such as overland flow and slope failure meridional vertical cracks in surface clasts implicate thermal stresses due to diurnal solar variation as a mechanical weathering process whereas the presence of a clast free silty layer within all soil profiles indicates that these are accretionary pavement surfaces that have grown upward over time the desert pavement in this region has likely developed in situ through mechanical breakdown of surface clasts and desert pedogenesis indicating long term stability for this region and minimal taphonomic effects on artifacts 2 cm in diameter deposited on this surface over the last ca 100 ka

organic maturity in the western canada sedimentary basin abstract organic maturity of strata in the western canada sedimentary basin has been investigated in numerous studies using petrological chemical and physical methods compilation of these data has facilitated a regional analysis of the maturity patterns and maturation history variation in maturity patterns can be divided into three levels basin wide 1st order regional 2nd order and local 3rd order first order variations are manifested by an overall increase in maturity of strata of the same age from east to west from the plains to the rocky mountain foothills and front ranges in response to progressively deeper burial and higher paleo geothermal gradients superimposed on this first order basinal scale variation in maturity are second and third order variations which reflect local differences in depth of burial conductive and advective heat transport or effects of thrust faulting maturity gradients in the axis of the basin are low averaging 0 10 log r omax km whereas in the front ranges and foothills the gradients are substantially greater average 0 25 log r omax km variations in maturity gradients reflect lower paleo geothermal gradients resulting from rapid sediment loading and subsequent unloading in the plains and higher conductive heat transport in the deformed belt the main component of maturity of most phanerozoic strata occurred during deep burial under upper cretaceous and or paleogene molasse in foredeeps developed in response to crustal loading during the easterly migration of the foreland fold and thrust belt as a result of the west to east propagation of deformation during the laramide orogeny deep burial maturation hydrocarbon generation and uplift occurred earlier in the foreland belt late cretaceous than in the plains to the east where the main component of maturity and hydrocarbon generation occurred as late as eocene in the plains because of the low maturity gradients a thick succession of strata currently is within the oil window in the deformed belt however higher maturity gradients have resulted in a thinner succession of strata within the oil window in response to higher paleo geothermal gradients strata in the deformed belt matured more quickly leading to more rapid hydrocarbon generation and migration than areas to the east in the southern part of the cordillera a significant component of maturation post dates structural deformation of the strata whereas in northeastern british columbia and adjacent parts of alberta maturity primarily pre dates structural deformation data from some deep bore holes and surface samples in the southern part of the cordillera provide evidence for maturity post dating or accompanying emplacement of major overthrust sheets tectonic burial in most areas however there is no clear evidence for the timing of maturity relative to faulting or evidence of frictional heating the maturity gradients and levels of maturity of surface and near surface strata in the western plains and rocky mountain foothills indicate that a succession several kilometres of paleogene strata were deposited in these regions foredeep and subsequently eroded in response to post laramide uplift

reduced mixing from the breaking of internal waves in equatorial waters in the oceans heat salt and nutrients are redistributed much more easily within water masses of uniform density than across surfaces separating waters of different densities but the magnitude and distribution of mixing across density surfaces are also important for the earth s climate as well as the concentrations of organisms most of this mixing occurs where internal waves break overturning the density stratification of the ocean and creating patches of turbulence predictions of the rate at which internal waves dissipate were confirmed earlier at mid latitudes here we present observations of temperature and velocity fluctuations in the pacific and atlantic oceans between 42 degrees n and 2 degrees s to extend that result to equatorial regions we find a strong latitude dependence of dissipation in accordance with the predictions in our observations dissipation rates and accompanying mixing across density surfaces near the equator are less than 10 of those at mid latitudes for a similar background of internal waves reduced mixing close to the equator will have to be taken into account in numerical simulations of ocean dynamics for example in climate change experiments

eolian sand in peridesert northwestern libya and implications for late pleistocene and holocene sahara expansions abstract jebel gharbi is a semi arid mountain range on the northern boundary of the sahara desert in northwestern libya between the mediterranean coastal plain the jefara and the tripolitanian plateau at jebel gharbi quaternary strata dated by means of radiocarbon techniques and archaeological artifacts consist of two upper pleistocene oxygen isotope stages 4 and 2 eolian sand beds that interfinger with loess calcrete alluvium and colluvium the deposition of the eolian sand was a consequence of the expansions of the sahara desert into the jebel gharbi area that took place during arid cold spells during the latest pleistocene and late holocene the northern margin of the sahara desert reached its modern position and eolian activity was widespread on the jefara coastal plain and in the valleys of jebel gharbi the late pleistocene desert expansions were contemporaneous with the periods of highest sedimentation rate of non volcanic dust in polar ice cores the comparison between data on eolian activity in some different places outside of the present boundary of the sahara desert suggest that there are differences in the timing of eolian sediment mobilization and the rate of eolian sediment deposition inside and outside the hadley cell atmospheric circulation zone the present work confirms the influence of the size of the hadley cells on atmosphere dust circulation patterns

palhydaf palaeohydrology in africa program objectives methods major results abstract the palhydaf program aims to reconstruct the late quaternary 130 ka b p to present environmental and hydrological changes of northwest africa and to interpret them in terms of palaeoclimate for testing coupled atmospheric ocean circulation models lacustrine sequences were obtained from closed basins which lie along two meridional transects southern tunisia southern niger and algeria mali particular attention is given to groundwater storage and periods of recharge of the aquifers the chronological framework is based on 14c dates conventional and ams methods and or the th u disequilibrium method in favourable cases e g mali correlations can be made between the chronology of surface palaeohydrology and that of aquifer recharge various environmental markers mineralogy elemental chemistry stable isotopes biological remains are used for palaeoenvironmental purposes quantitative estimates of palaeoclimatic variables e g air humidity may be attempted through the combination of geochemical and palaeobiological data the major results already obtained are summarized for the following individual sectors 1 southern tunisia 1 northern algeria 2 southern algeria 3 northern niger 3 mali 4 southern niger at the present stage of investigation the following new inferences can be drown one or several humid episode s occurred in the northern sahara between 150 and 75 ka b p several factors suggest that these humid conditions also affected lower latitudes although their southern extent cannot be given precisely at present the previously postulated existence of large lakes at 20 35 ka b p is not confirmed this time interval seems to coincide with raised water tables but not to a major humid phase in all cases the 14c chronologies obtained for the late pleistocene have to be checked by comparison with other dating methods e g th u disequilibrium after the arid period of the last glacial maximum the return of humid conditions occurred in two major steps at 12 and 9 5 9 ka b p for both the southern and the northern sahara

humidity changes in southern tunisia during the late pleistocene inferred from u th dating of mollusc shells abstract calcareous deposits mainly consisting of mollusc shell accumulations which have been dated by the u th disequilibrium method mark the shorelines of paleolake highstands in the great chotts area of southern tunisia the 5 sites studied consist of discontinuous accumulations of fossils of marine like organisms e g cerastoderma glaucum melania tuberculata melanopsis praemorsa cerithium rupestre u th isochron plots and age frequency histograms for 39 shell samples are reported here limited variations for u content and 234u 238u activity ratios ar of shells support the hypothesis of closure of the geochemical system with respect to this element it is remarkable that 234u 238u ar of shells collected in chott fejej or chott jerid are clustered around different values reflecting probably different groundwater recharge from the continental intercalaire ci or complexe terminal ct aquifers furthermore waters collected near wadi el akarit show 234u 238u ar values comparable to those observed for shells 14c determinations made on aliquots of some of these samples suggested an age distribution between 18 and 34 ka bp the u th data of these 39 shell samples imply that 4 distinct flood episodes of these lakes occurred at about 30 95 100 130 150 and 180 200 ka for the episode centred around 30 ka the frequency histogram of ages shows a multimodal age group that could represent the existence of several humid pulses rather than a unique event moreover the comparison of 13c and 18o with those of older humid pleistocene phases when very large palaeolakes have been recorded suggests that these young carbonate shells are not related to a true highstand lake it is suggested that they represent a period of less humid climatic conditions with carbonate accumulation in minor water ponds in which intensive biological activity could have taken place it should be noted that this period was less arid than the present

megalakes in the sahara a review the sahara was wetter and greener during multiple interglacial periods of the quaternary when some have suggested it featured very large mega lakes ranging in surface area from 30 000 to 350 000 km2 in this paper we review the physical and biological evidence for these large lakes especially during the african humid period ahp 11 5 ka megalake systems from around the world provide a checklist of diagnostic features such as multiple well defined shoreline benches wave rounded beach gravels where coarse material is present landscape smoothing by lacustrine sediment large scale deltaic deposits and in places tufas encrusting shorelines our survey reveals no clear evidence of these features in the sahara except in the chad basin hydrologic modeling of the proposed megalakes requires mean annual rainfall 1 2 m yr and a northward displacement of tropical rainfall belts by 1000 km such a profound displacement is not supported by other paleo climate proxies and comprehensive climate models challenging the existence of megalakes in the sahara rather than megalakes isolated wetlands and small lakes are more consistent with the sahelo sudanian paleoenvironment that prevailed in the sahara during the ahp a pale green and discontinuously wet sahara is the likelier context for human migrations out of africa during the late quaternary

isotopic hydrology and paleohydrology of the madagi kenya natron tanzania basin during the late quaternary abstract three generations of stromatolites mark the recent extension of paleolakes in the magadi natron basin on the border between tanzania and kenya the last two generations are observed some 50 m above the modern lakes level they are respectively attributed to the beginning of the last interglacial and to the pleistocene holocene transition analysis of the morphology and texture of the stromatolites reveals an ecological zonation between 0 and 10 14 m below the shoreline their relatively high 18o and 13c values reflect a long residence time for the paleolake water which would have been favorable to the establishment of an isotopic equilibrium between atmospheric co2 and the dissolved inorganic carbon despite deep carbon inputs in relation to carbonititic volcanism a study of the 18o 2h and 13c contents in the hydrothermal springs perennial rivers and lacustrine brines indicates that during the last period of significant renewal of the deep saline groundwater precipitation was depleted in heavy isotopes when compared to modern precipitation this difference corresponds to a drop of about 600 1000 m in altitudinal hydroclimatic zones finally a comparison between the modern and paleohydrological data leads us to believe that the stabilization of the paleolake shorelines at the same altitude can be attributed to control by the water table level in the aquifer of the basaltic plateaus east of the basin

chronology and paleohydrology of late quaternary high lake levels in the manyara basin tanzania from isotopic data 18o 13c 14c on fossil stromatolites abstract superimposed phases of stromatolite buildup are observed on the eastern margin of lake manyara and depict a paleoshoreline at about 20 m above modern lake level radiocarbon and th u measurements permitted the dating of the last two phases of stromatolite formation at ca 90 000 yr and between 27 000 and 23 000 yr b p respectively the th u chronology is based on the decay of a strong 230th excess over 234u inherited with the detrital particles cemented into the stromatolites the various generations of stromatolites show comparable stable carbon and oxygen isotope contents and are located at the same paleolake stabilization levels this indicates that stringent hydrological conditions are necessary for the development of the encrusting benthic microbial communities responsible for stromatolite formation a comparison with similar stromatolitic units from the nearby lake natron lake magadi basin shows that such conditions occurred during only a few of the late quaternary humid episodes known in eastern africa and that they are different in each basin stromatolites do not necessarily represent all high lake levels that lake manyara experienced during the late pleistocene and holocene

lake level chronology on the southern bolivian altiplano 18 23 s during late glacial time and the early holocene stratigraphic analyses of outcrops shorelines and diatoms from the southern bolivian altiplano uyuni coipasa basin reveal two major lacustrine phases during the late glacial period and the early holocene based on a chronology established by radiocarbon and u th control a comparison of14c and230th 234u ages shows that during times of high lake level radiocarbon ages are valid however during low water periods 14c ages must be corrected for a reservoir effect the lacustrine tauca phase started a little before 16 00014c yr b p and the lake level reached its maximum between 13 000 and 12 00014c yr b p a dry event ticana occurred after ca 12 000 and before 950014c yr b p a moderate lacustrine oscillation coipasa event occurred between ca 9500 and 850014c yr b p using a reservoir corrected conventional14c chronology comparisons between the lake level chronology in the uyuni coipasa basin and data from other southern tropical areas of south america suggest that the lacustrine evolution may reflect large scale climatic changes

the sahara east mediterranean dust and climate connection revealed by strontium and uranium isotopes in a jerusalem speleothem this paper explores the potential of sr and u isotope systems in speleothems as tracers of eolian dust transport and hydrological conditions the study focuses on a speleothem from jerusalem spanning the past 220 kyr this speleothem provides a precisely dated record of dust flux from the sahara to the east mediterranean enhanced dust flux and terra rossa soil development are reflected by elevated 87sr 86sr ratios in the speleothem 0 7082 0 7086 while lower 87sr 86sr ratios 0 7078 indicate higher contribution of the local bedrock due to low dust flux and low soil accumulation the strontium isotope system in the speleothem is a robust monitor of the sahara monsoon modulated climate since dust uptake is related to development or reduction in vegetation cover of sahara soil the 234u 238u activity ratios in the speleothem range between 1 12 and 1 0 the high activity values may indicate selective removal of 234u from the soil while the low values converge to the bedrock the migration of 234u to the cave reflects mainly the regional hydrological conditions that are modulated by the north atlantic mediterranean climate system thus the speleothem provides a combined record of the monsoon north atlantic climatic systems long term stability in glacial 87sr 86sr ratios 0 7083 0 0001 over the past 220 kyr suggests an overall similarity in eolian dust sources and uniformity in the synoptic conditions that dominate the dust storm tracks during glacial periods

plio pleistocene marine continental correlation using habitat indicators from olduvai gorge tanzania abstract reconstructions of plio pleistocene habitats and lake levels from a brief interval 200 000 yr of hominid bearing sediments from bed i and lower bed ii olduvai gorge tanzania are for the first time integrated with climatic data from the deep sea in order to examine the effect of largescale climatic events in east africa the period of densely vegetated habitats and wet conditions at olduval at 1 82 myr shows a best fit with a period of northern hemisphere warming as evidenced by oxygen isotopes from deep sea core v28 239 from the western pacific this environmental response is similar to that witnessed in east africa during the period of warming climates associated with deglaciation at 10 000 8 000 yr

the zambezi river tectonism climatic change and drainage evolution abstract the longitudinal profile of the zambezi river forms two concave upwards sections with their boundary at the victoria falls this form has been ascribed to the process of pediplantation and the victoria falls identified as one of several knick points that have traversed the river since the breakup of gondwanaland an alternative model explains the river s long profile by suggesting that the upper and middle zambezi evolved as entirely separate river systems which only joined together in comparatively recent times the alluvial sequence of the middle zambezi is described and interpreted in terms of the latter hypothesis the river capture event caused a change in grade and is marked by a deposit believed to record a cataclysmic flood capture is inferred to have resulted from overtopping of the lake that formed the end sink of the proto upper zambezi rather than from headward erosion of the proto middle zambezi this event is dated from archaeological evidence to the peak of the last interglacial at the end of the middle pleistocene subsequent drainage diversions between the middle zambezi and the kalahari are interpreted as the product of rifting of the chobe graben and aggradation of the chobe swamps river capture by overtopping implies high rainfall over central southern africa at the peak of the last interglacial which is contrary to predictions that africa s rain belts then lay north of their modern mean positions this anomaly is resolved by postulating a southern polar warming episode at that time and agrees with suggestions that the last interglacial was marked by large scale ablation of the west antarctic ice sheet

u series evidence for two high last interglacial sea levels in southeastern tunisia abstract pleistocene raised marine deposits in southeastern tunisia consist of a siliciclastic unit that culminates at 3 m asl overlain by a carbonate rich unit with strombus bubonius that culminates at 5 m asl 234 u 238 u ratios on fossil ostraea shells from both units are compatible with a marine origin from the uranium incorporated into the shells and show narrowly clustered 230 th ages respectively between 147 and 110 ka and 141 and 100 ka the two units were therefore developed during marine isotopic substage 5e miss 5e last interglacial their heights are comparable to those of contemporaneous marine deposits found in many tectonically stable areas of the world such as in the bahamas and in bermuda and can therefore be used as indicators of eustatic changes during the last interglacial it is argued that on the basis of this evidence the last interglacial was characterised by two eustatic maxima

reconstructing palaeoclimate and hydrological fluctuations in the fezzan basin southern libya since 130 ka a catchment based approach abstract we propose a novel method to evaluate regional palaeoclimate that can be used to alleviate the problems caused by the discontinuous nature of palaeoenvironmental data found in deserts the technique involves processing satellite imagery and dem s to map past rivers catchments and evaluate the areas and volumes of palaeolakes this information is used to determine the new lake evaluation index lei that allows a qualitative estimate of the amount of sediment received by lakes and how long lived those lakes are lakes with considerable longevity and large sediment stores are selected for study validation is performed using image interpretation of remote sensing data ultragpr surveys and fieldwork these techniques are also used to identify and study spring deposits and fluvial landforms that provide valuable palaeoclimate information the method is applied to the fezzan basin in southern libya focusing on the wadi ash shati and wadi el agial catchments results indicate that the palaeohydrology is accurately mapped except within dune fields we analysed the sedimentology of the key deposits identified by this methodology developing a chronology using optically stimulated luminescence osl and radiocarbon dating we find evidence for relatively humid conditions during mis 5c d and e as well as during the early to middle holocene larger lakes and more extensive river systems were present during mis 5 than are found during the holocene suggestive of greater humidity the holocene humid period started at 11 ka and continued until 5 ka being interrupted by abrupt periods of aridity at 8 2 ka and 6 ka that coincide with north atlantic cooling after each of these arid events the climate was less humid than previously suggesting that they were superimposed upon an overall drying trend the termination of the holocene humid period in the sahara has received much scrutiny in recent years and sediments of palaeolake shati provide a continuous record of this we do not find evidence to support the hypothesis of either sudden or gradual aridification of the sahara at 5 ka instead we find that that aridity started to develop at 6 5 ka whereupon the lake levels oscillated until finally drying up by 5 3 ka most of the other lakes in the fezzan also dried up at 5ka we suggest that thousands of years of aridification prior to 5 ka shrunk these lakes so that additional aridity at this time led to their final desiccation because lakes are prodigious dust sources this mechanism potentially explains the rapid rise in dust flux to the atlantic at 5 ka with this final drying being the culmination of longer term aridity albeit overprinted with considerable climate variability

the last interglacial ocean the final effort of the climap project was a study of the last interglaciation a time of minimum ice volume some 122 000 yr ago coincident with the substage 5e oxygen isotopic minimum based on detailed oxygen isotope analyses and biotic census counts in 52 cores across the world ocean last interglacial sea surface temperatures sst were compared with those today there are small sst departures in the mid latitude north atlantic warmer and the gulf of mexico cooler the eastern boundary currents of the south atlantic and pacific oceans are marked by large sst anomalies in individual cores but their interpretations are precluded by no analog problems and by discordancies among estimates from different biotic groups in general the last interglacial ocean was not significantly different from the modern ocean the relative sequencing of ice decay versus oceanic warming on the stage 6 5 oxygen isotopic transition and of ice growth versus oceanic cooling on the stage 5e 5d transition was also studied in most of the southern hemisphere the oceanic response marked by the biotic census counts preceded led the global ice volume response marked by the oxygen isotope signal by several thousand years the reverse pattern is evident in the north atlantic ocean and the gulf of mexico where the oceanic response lagged that of global ice volume by several thousand years as a result the very warm temperatures associated with the last interglaciation were regionally diachronous by several thousand years these regional lead lag relationships agree with those observed on other transitions and in long term phase relationships they cannot be explained simply as artifacts of bioturbational translations of the original signals

african monsoons an immediate climate response to orbital insolation the croll milankovitch astronomical theory of climate1 3 has received strong support from the evidence of a linear climatic forcing by obliquity and precession although nonlinearity had to be assumed for eccentricity4 5 moreover interglacials have appeared to be controlled by the orbital insolation6 although a phase shift of 6 000 5 000 yr is seen between an astronomical climate index and terrestrial climate indicators dominated by the northern ice sheet dynamics climate proxy data of low latitudes are less directly dependent on the ice sheets here it is shown that during the past 464 000 yr african monsoons signalled by the east mediterranean sapropels were heaviest always and only when a northern summer monsoon index computed from the orbital variation of insolation reached maximum values this occurred during all the interglacials but also twice during glacial periods thus tropical aridity is not the single climatic pattern of glacial periods an immediate terrestrial climate response to orbital variations of insolation is established

nile floods recorded in deep mediterranean sediments clastic mud beds rich in continental organic matter are observed recurrently in the nile deep sea turbidite system they formed during flooding periods of the river similar to those that induce sapropel formation and occurred during periods of increased density stratification of the eastern mediterranean the very fine grained flood deposits are intercalated within pelagic sediments sapropels and bouma type turbidites these flood deposits form by the successive reconcentrations of surface hypopycnal plumes by convective sedimentation which in turn generate a fine grained low energy hyperpycnal flow sea level high stands seem also to favor hypopycnal plume formation and increase clastic mud bed formation consequently these muddy clastic beds provide a direct link between deep marine sedimentary records and continental climatic change through flood frequency and magnitude

three million years of monsoon variability over the northern sahara we present a 3 million year record of aeolian dust supply into the eastern mediterranean sea based on hematite contents derived from magnetic properties of sediments from ocean drilling program site 967 our record has an average temporal resolution of 400 years geochemical data validate this record of hematite content as a proxy for the supply of aeolian dust from the sahara we deduce that the aeolian hematite in eastern mediterranean sediments derives from the eastern algerian libyan and western egyptian lowlands located north of the central saharan watershed 21 n in corroboration of earlier work we relate dust flux minima to penetration of the african summer monsoon front to the north of the central saharan watershed this would have enhanced soil humidity and vegetation cover in the source regions in agreement with results from green sahara climate models our results indicate that this northward monsoon penetration recurred during insolation maxima throughout the last 3 million years as would be expected this orbital precession scale mechanism is modulated on both short 100 kyr and long 400 kyr eccentricity time scales we also observe a strong expression of the 41 kyr obliquity cycle which we discuss in terms of high and low latitude mechanisms that involve southern hemisphere meridional temperature contrasts and shifts in the latitudes of the tropics respectively we also observe a marked increase in sub milankovitch variability around the mid pleistocene transition 0 95 ma which suggests a link between millennial scale climate variability including monsoon dynamics and the size of northern hemisphere ice sheets

a 1 2 myr record of isotopic changes in a late pliocene rift lake ethiopia the hadar fluvio lacustrine formation ethiopia contains an original hominid fossil australopithecus afarensis and an abundant vertebrate faunal assemblage1 2 the formation3 4 consists of a 150 300 m thick sequence of fine to coarse detrital sediments originating from the ethiopian plateau escarpment with some interbedded pyroclastic layers the three lower members of the formation were formed in a lacustrine environment which was occasionally interrupted by fluvial drainage the upper member is quite distinct and formed as an alluvial fan delta this sedimentary sequence was deposited at the end of the pliocene between 3 9 and 2 7 myr as indicated by magnetostratigraphy5 the palaeomagnetic record spans the gilbert gauss transition period k ar dating6 7 and biostratigraphy8 we consider here the oxygen and carbon isotope variations observed throughout the formation and their palaeo hydrological implications

14 c and th u dating of pleistocene and holocene stromatolites from east african paleolakes during recent humid episodes stromatolites were built along paleolake margins some 60 m above the modern water level of lakes natron and magadi southern gregory rift valley three generations of stromatolites are observed the more recent ones frequently covering pebbles and boulders eroded from the older ones the youngest one yielded 14c ages ranging from approximately 12 000 to 10 000 yr b p their 13c values 2 6 suggest isotopic equilibrium between the paleolake total inorganic dissolved carbon and the atmospheric co2 thereby lending credence to the reliability of the 14c an initial 230th232th ratio in the detrital component was determined by thu measurements on the 14c dated stromatolites using this value a 230th234u chronology for the older stromatolites was calculated ages of 240 000 and 135 000 10 000 yr were obtained for the first and second generations respectively a humid episode apparently characterized eastern africa during each glacial interglacial transition 18o and 13c measurements on stromatolites when compared to values on modern waters and carbonates provide paleohydrological information long residence time of the paleolake waters and less seasonally contrasted regimes are inferred

evidence of widespread wildfires in coal seams from the middle jurassic of northwest china and its impact on paleoclimate abstract today wildfire is an important disturbance in many continental ecosystems and it is assumed that with ongoing climate changes the frequencies and impacts of wildfires will increase in many regions one way to obtain information about the potential long term influences of wildfires on ecosystems and the climate system itself is to study palaeo wildfires a total of 140 jurassic samples including 122 coal samples and 18 rock samples from roof and floor of coal seams were collected from 4 coalfields in northwestern china the samples were analyzed by macropetrography microscopy scanning electron microscopy gas chromatography and gas chromatography mass spectrometry to study the evidences of wildfires and their impact on the paleoclimate high contents of pyrogenic inertinites charcoal natural char and natural coke particles observed in the samples which indicate that widespread wildfire events occurred in the early middle jurassic in all over northwestern china additionally high molecular weight polycyclic aromatic hydrocarbons were detected in the coal samples these aromatic compounds were formed under high temperature and provide further evidence of wildfires according to a carbon emission model for modern forest fires the total carbon and gas emissions from wildfires in peat swamps of the middle jurassic in northwest china were calculated by the data from 21 575 coal samples of number of 1567 drill core samples the total carbon released from peat swamp forest wildfires was at least 1207 gt corresponding to emissions of co2 co and ch4 of 1926 gt 163 gt and 8 8 gt respectively the huge amount of co2 released by forest fires in the peat swamp systems of the middle jurassic in northwestern china alone was equivalent to 64 20 of the total co2 in the atmosphere at present 3000gt the large amounts of greenhouse gases released by wildfires must have led to long term changes in the atmospheric composition and have caused global warming which could have the reason of affected the paleoclimate

latitudinal distribution of mixing rate caused by the m2 internal tide abstract ten years of ocean topography experiment topex poseidon tidal data and an energy balance relation are used to estimate the mixing rate caused by m2 internal tides in the upper ocean the results indicate that latitudinal distribution of the mixing rate has a generally symmetrical structure with respect to the equator the maxima are distributed around 28 9 n and 28 9 s and can be as high as 3 8 10 5 m2 s 1 in the pacific 5 10 5 m2 s 1 in the atlantic and 3 7 10 5 m2 s 1 in the indian oceans the minimum which is only 10 of those at 28 9 is located near the equator the data imply that midlatitudes are the key regions for internal tide mixing

uranium series dating of quaternary deposits abstract this is a review of u series dating methods as applicable to terrestrial sedimentary materials alpha counting techniques allow the dating of carbonates and phosphates with a precision of 5 to 10 over the time range from 1 ka to 350 ka atom counting methods mass spectrometry and ams substantially increase precision and may increase the range problems of contamination and open system behavior are discussed correction for detrital contaminant is demonstrated in the dating of speleothem from a cave in italy

data for enhanced deposition of coal fragments at the chukchi margin western arctic ocean implications for deglacial drainage history from the laurentide ice sheet abstract distribution and composition of coarse particles 250 m were investigated in 13 sediment cores from the chukchi margin western arctic ocean petrographic clast identification is supplemented by xrf core scanning eds analysis of coal fragments and ams 14c dating of planktic foraminifera for age control coal debris distribution is also investigated in surface sediments of the study region for provenance interpretation the sediment core study focuses on the interval between the last glacial maximum and the holocene for constraining provenance and timing of the deglacial discharge we find that this interval is characterized by a distinct enrichment in sedimentary rock fragments including coal comparison with longer stratigraphic records indicates that this composition is unique for the time interval since at least maine isotope stage 5 based on the timing interpreted provenance and geographic distribution of the coal enrichment we conclude that the most likely primary source was the deglacial discharge from the mackenzie drainage basin of the laurentide ice sheet lis the identified coal bearing layer also expressed in the xrf sulfur record can be thus used as a stratigraphic marker for the lis discharge to the arctic ocean during the last deglaciation

east african climate change and orbital forcing during the last 175 kyr bp abstract variations in the temporal and spatial distribution of solar radiation caused by orbital changes provide a partial explanation for the observed long term fluctuations in african lake levels the understanding of such relationships is essential for designing climate prediction models for the tropics our assessment of the nature and timing of east african climate change is based on lake level fluctuations of lake naivasha in the central kenya rift 0 55 s 36 20 e inferred from sediment characteristics diatoms authigenic mineral assemblages and 17 single crystal 40ar 39ar age determinations assuming that these fluctuations reflect climate changes the lake naivasha record demonstrates that periods of increased humidity in east africa mainly followed maximum equatorial solar radiation in march or september interestingly the most dramatic change in the naivasha basin occurred as early as 146 kyr bp and the highest lake level was recorded at about 139 133 kyr bp this is consistent with other well dated low latitude climate records but does not correspond to peaks in northern hemisphere summer insolation as the trigger for the ice age cycles the naivasha record therefore provides evidence for low latitude forcing of the ice age climate cycles

marine deposits 35 000 25 000 years old in the chott el djerid southern tunisia the chott el djerid is a large salt covered depression in southern tunisia fig 1 the late nineteenth century idea that the basin had been invaded by the sea1 2 has been superseded by the view that any quaternary flooding of the chott must have resulted from climatic changes as the fossils found in its sediments are not exclusively marine and there is no indication of the fluctuations in sea level or of the crustal movements that would be required to fill and then empty the basin2 3 we report here evidence for a marine connection 35 000 25 000 years ago and show that it implies subsequent uplift by some 80 metres in an area long regarded as either stable2 or prone to subsidence throughout the quaternary4

significant dissipation of tidal energy in the deep ocean inferred from satellite altimeter data how and where the ocean tides dissipate their energy are long standing questions1 that have consequences ranging from the history of the moon2 to the mixing of the oceans3 historically the principal sink of tidal energy has been thought to be bottom friction in shallow seas4 5 there has long been suggestive evidence6 7 however that tidal dissipation also occurs in the open ocean through the scattering by ocean bottom topography of surface tides into internal waves but estimates of the magnitude of this possible sink have varied widely3 8 9 10 11 here we use satellite altimeter data from topex poseidon to map empirically the tidal energy dissipation we show that approximately 1012 watts that is 1 tw representing 25 30 of the total dissipation occurs in the deep ocean generally near areas of rough topography of the estimated 2 tw of mixing energy required to maintain the large scale thermohaline circulation of the ocean12 one half could therefore be provided by the tides with the other half coming from action13 on the surface of the ocean

paleolandscape and paleoenvironmental interpretation of spring deposited sediments in dakhleh oasis western desert of egypt abstract iron rich sediments in dakhleh oasis western desert of egypt have been recognized as spring mounds and as archaeological sites where paleolithic materials have been recovered detailed sedimentologic investigation and mapping of these features reveal that spring mound sediments were deposited in a shallow vegetated wetland formed by the discharge of iron rich nubian aquifer waters along the southern margin of the oasis controlled largely by localized faulting and the variable paleotopography of the basin floor the spring sediments represent peri lacustrine or lake independent features and can be differentiated from fully lacustrine deposits on the basis of their sedimentary characteristics as well as the presence of goethite and jarosite in a region where authigenic deposition during pleistocene pluvial activity principally resulted in tufas and lacustrine marls spring mound formation incorporated sediment through the ponding of surface water aeolian entrapment by local vegetation and the formation of iron precipitates in a low energy oxidative and acidic environment that was not dependent upon surface water inputs into the dakhleh basin the potentially interpluvial nature of this water resource makes it an important sedimentary archive for archaeological investigations in the basin the dakhleh spring mounds record a unique groundwater controlled paleoenvironment providing the first evidence of a bog iron in egypt and one of the few occurrences of iron rich wetland remnants in the modern sahara

the middle pleistocene al mahruqah formation in the murzuq basin northern sahara libya evidence for orbitally forced humid episodes during the last 500 000 years abstract geological and geomorphological work in the endorheic murzuq basin fezzan in the northern sahara desert sw libya has revealed that the middle pleistocene al mahruqah formation consists of at least four limestone beds at four distinct elevations the corresponding limestone beds are named from oldest to youngest the antalkhata member the brak member the bi r az zallaf member and the aqar member the four members are interpreted as lacustrine deposits and they are well distinguished by their spatial extent and lithology twenty five radiometric and thermal ionisation mass spectrometric tims 230th u ages ranging from about 490 130 ka have been determined on these lacustrine limestones according to these radiometric 230th u ages the accumulation of the four limestone members of the al mahruqah formation followed the orbitally forced milankovitch eccentricity cycle of 100 ka during at least the last 400 ka the four members formed in humid episodes of several tens of thousands of years duration when large lakes might have existed the 13c and 18o values from samples of the four members of the al mahruqah formation provide independent genetic evidence on the lacustrine origin of the deposits in the murzuq basin the spatial extent of the limestone deposits and thus the intensity of the humid episodes decreases at least from the brak member to the aqar member from about 52 000 km2 to 1400 km2 respectively in spite of the limited precision and reliability of the new 230th u ages it seems possible that short humid episodes interrupted the intervening arid episodes other long term palaeohydrological records from northern and southern africa as well as the mediterranean region support the palaeoclimatic results from the murzuq basin

a pleistocene lacustrine episode in southeastern libya fossiliferous lacustrine deposits of middle pleistocene age have been found in central fezzan libya 1 on each side of the east west depressions of the wadi shati south of the precambrian qarqaf anticline1 8 they represent the shoreline of a former brackish water lake fig 1 which some 130 000 yr bp covered an area about 100km long and 10 20 km wide at present the area may be classified as hyperarid since 1978 a collaborative study of the fossil assemblages has been undertaken it included macro and micropalaeontological determination th u dating stable isotope and chemical analyses in parallel with sedimentological and archaeological studies of the sites here we consider the th u dating and the isotopic palaeohydrology of the lacustrine episode

pleistocene and holocene herbivore diets and palaeoenvironments in the gebel akhdar libya implications for past human populations the gebel akhdar massif in cyrenaica northeast libya has yielded a long record of human occupation going back at least 100 000 years to date there is only a limited understanding of how the landscape of the region varied in response to the climatic fluctuations of the last glacial interglacial cycle and the implications of these changes for local human populations remain largely unexplored this study provides an isotope based interpretation of past environments directly linked to the archaeological record tooth enamel stable carbon isotope ratios 13c from herbivore species hunted by past human populations are used to infer the isotopic characteristics of past diet and vegetation and in turn the likely environmental conditions that prevailed during periods when humans were active within the landscape to provide a baseline from which to interpret the archaeological 13c data modern samples are considered in relation to their diet and environmental origin archaeological samples come from 2 cave sites haua fteah and hagfet ed dabba and span a period from oxygen isotope stage 4 to the mid holocene whilst results indicate a more arid environment in the pleistocene and an increase in humidity at the onset of the holocene the overall picture is one of relative environmental stability the biggest landscape change observed in the data occurs during the mid holocene neolithic when c4 plant species become evident in the herbivore diet for the first time there is little evidence to suggest that this occurred at a time of any large scale climate variation and thus the contribution of anthropogenic influences to vegetation change is considered likely

radiocarbon chronology of holocene hydrologic changes in northeastern mali abstract a detailed chronology of hydrological changes that occurred in northeastern mali during the holocene is based on approximately 30 14 c dates of molluscs which were collected from the lacustrine and paludal deposits of the ine sakane sand sea the presence of cemented paleodunes indicates that the end of the pleistocene was and arid period this was followed by an early holocene lacustrine episode ca 9500 6500 yr b p during which numerous lakes occupied depressions formed in the cretaceous limestones and between the sand ridges these lakes reflect a significant rise in the water table between ca 6500 and 5500 yr b p the lakes dried up although the water table remained close to the ground surface in several areas calcareous concretions formed at the water table fringe during this time thereby fossilizing some of the middle holocene dunes a second lacustrine episode is dated between ca 5500 and 4500 yr b p it corresponds to the establishment of numerous neolithic settlements in the area arid conditions have developed since about 4500 yr b p to attain the conditions of the present day the isotopic composition 18 o and 13 c of the fauna reflects highly variable hydrological conditions groundwaters were recharged by precipitation which was occasionally very depleted in heavy oxygen 18 o 13 evaporation induced an enrichment in 18 o high concentrations characterize some lakes the 13 c content of fossil shells reflect 1 species ecological requirements 2 isotopic composition of the total inorganic dissolved carbon tidc in groundwaters and 3 the lake metabolism

the last 140 ka in the afro asian arid semi arid transitional zone abstract during the last ten years a great number of geological or other proxy records provided radiocarbon u th or tl dated information on the past climatic oscillations in the arid semi arid zones extending between the atlantic and the pacific comparisons and a synthetic view of these results can now be attempted for the last 140 ka and compared with global changes as registered in oceanic or ice cores and with palaeomonsson models for this purpose wide spatial and time scales have to be used on the whole arid humid alternations roughly fit global changes cold phases corresponding to an extension of the arid areas to the south warm phases corresponding to the shrinking of the same to the north the last interglacial is associated with an increase of precipitation throughout the area considered isotopic stage 4 brings no evidence whatsoever of humid conditions two wetter episodes are registrated during stage 3 a major rainfall decrease is everywhere associated with the last glacial maximum 21 15 ka in most regions the arid or semi arid zones extending several hundred kilometers southwards relative to the present day pattern the two abrupt deglaciation steps and the younger dryas are recorded in all of the most sensitive regions at the margins of the present day monsoonal range during the holocene the precipitation increases everywhere by 100 400 mn relative to the present day values the optimum being at 8 5 6 5 ka a climatic deterioration follows with an irregular pattern of dry wet episodes according to the different geographic conditions the humid phase terminates at 3 5 3 ka in the whole transitional zone the major southward shift of the monsoonal precipitation range since its optimum some 8000 years ago fits the astronomical neoglacial trend and may possibly be correlated with past stage 5d although its rapidity and spatial importance may just reflect one of the sharp successive cold warm variations registered by grip during the whole stage 5

freshwater diatom influx in intertropical atlantic relationships with continental records from africa diatom assemblages from modern west african rivers and from lacustrine sediments subjected to deflation represent the present day sources of continental diatoms to the sea diatom productivity in large rivers is high especially for the genus melosira windblown diatoms derive mainly from the central and northern sahara summer dust plume where saline chloride water assemblages are widespread or from the southern edge of the sahara winter dust plume where melosira rich assemblages of dilute water predominate freshwater diatom peaks in atlantic cores may reflect 1 phases of increased river influx correlated with humid episodes on the continent or 2 phases of enhanced deflation and wind transport during arid episodes the single hypothesis for fine sediments from mid ocean sites genus melosira dominates the freshwater assemblages of many modern and fossil marine samples whatever the transport agent is therefore it is not an accurate paleoclimate indicator by itself but associated taxa may provide information on the environmental and geographical origin of the displaced diatoms this tentative approach shows that freshwater diatoms in atlantic cores may be a good tool for reconstructing paleoclimates and for establishing continent ocean correlations if species analyses are made and if the continental distribution of the taxa encountered is considered

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

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

were springline carbonates in the kurkur dungul area southern egypt deposited during glacial periods the tufa deposits in the kurkur dungul area southern egypt date from marine isotope stage mis 11 to mis 1 springs across the region were active during glacial periods with sea level below 50 m reflecting changed atmospheric circulation over the indian ocean as well as peak interglacial periods during times of low sea level reduced indonesian throughflow promoted formation of an indian ocean warm pool and anomalous rainfall on its western margin we suggest that egypt lies at the intersection of westerly maghrebian and easterly mashriqian rainfall provinces which show different timing with relation to orbital forcing and different source water regions tufa growth periods are therefore not mechanistically linked to humid periods or sapropel events identified elsewhere stable isotope and t 47 data are also inconsistent with these spring systems being part of a larger system spanning northern africa and lack a clear interaction between northern hemisphere heating and mid latitude rainfall we also follow previous researchers in concluding that formation of springline deposit formation was probably delayed compared with rainfall owing to aquifer flow distances this delay is unlikely to be sufficient to explain why rainfall is out of phase with movements of the monsoon belts but may complicate interpretation of these records supplementary material a lithofacies description and supplementary figures and tables are available at https doi org 10 6084 m9 figshare c 5246661

mediterranean pluvial periods and sapropel formation over the last 200 000 years abstract three mediterranean deep sea cores have been studied to reconstruct the hydrological conditions at the time of deposition of interglacial and glacial sapropels during the time interval 200 60 kyr bp the isotopic and sea surface temperature records of planktonic foraminifera were used to estimate the oxygen isotopic composition of surface water w in the tyrrhenian sea and levantine basin our results show a strong w salinity decrease in both basins associated with each sapropel these w decreases reflect a drastic change in the local freshwater budget and were comparable to that observed during the last sapropel s 1 centred at ca 8 kyr bp a strong precipitation increase transformed the whole mediterranean sea into a non concentration basin the low surface salinity was responsible for the water column stratification while nutrients brought in by the runoff enhanced productivity it resulted in a significant reduction in the rate of deep water oxygenation and sapropel formation in the eastern basin in addition the mediterranean sea 18 o record is very similar to that of the bay of bengal as the latter is very sensitive to the summer monsoon rain this similarity suggests that both glacial and interglacial sapropels result from enhanced monsoon like precipitation due to an increase in summer insolation driven by precession variations

internal tide radiation from mendocino escarpment abstract strong semidiurnal internal tides are observed near mendocino escarpment in full depth profile time series of velocity temperature and salinity velocity and density profiles are combined to estimate the internal tide energy flux divergence of this flux demonstrates that its source is the barotropic tide interacting with the escarpment a baroclinic energy flux of 7 kw m 1 radiates from the escarpment corresponding to 3 of the 220 kw m 1 fluxing poleward in the surface tide energy and energy flux are concentrated in packets that emanate from the flanks of the ridge surmounting the escarpment and one site 90 km north of the escarpment coherent beamlike structure along semidiurnal ray paths remains identifiable until the first surface reflection beyond the first surface reflection north of the escarpment the energy flux drops by 2 kw m 1 and beams are no longer discernible turbulence as inferred from finescale parameterizations is elevated by over two orders of magnitude relative to th