hydrological response of an east saharan palaeolake nw sudan to early holocene climate we present a palaeolimnological record in western nubia nw sudan eastern sahara isolated from large scale artesian systems and thus reflecting hydrological changes resulting from local rainfall past changes in lake water chemistry and water depth are interpreted by combining results from biological indicators diatoms ostracodes and charophytes with geochemical and stable oxygen and carbon isotope data palaeolake gureinat existed from 10 9 cal ka bp until at least 7 1 cal ka bp and is devided into five lake phases for the first two millennia the lake was shallow and fluctuating salinity conditions prevailed 500 to 3000 s cm after 8 7 cal ka bp the lake level rose waters became fresh to oligosaline and the lake had a minimum depth of at least 8 m both transgressive and stable lake phases were punctuated by at least three drops in lake level with complete desiccation at around 10 9 and 8 9 cal ka bp the third of these regressions may be coeval with the 8 2 ka climatic event sediments

holocene pollen spectra from oyo northwestern sudan problems of interpretation in a hyperarid environment the oyo site is in the hyperarid core of the eastern sahara a 3 4 m core of lake sediment yielded a continuous pollen record for the period 9 to 4 5 ka bp the lower regularly laminated carbonate sediments 9 6 ka show a pollen assemblage with the tropical taxa dominated by plants of sudanian and sahelian affinity and the local taxa dominated by grass and typha the upper sediments 6 4 5 ka grading from laminated carbonates to coarsely bedded and massive layers with aeolian sands have reduced frequencies of sudanian pollen taxa and increases in saharan types and sedge dominated local pollen spectra a possible vegetational reconstruction is a mosaic of savanna woodlands associated with groundwater supported lakes and marshes between 9 and 6 ka bp and sparser cover of steppe and semidesert communities between 6 and 4 5 ka bp as water tables dropped and hyperarid conditions were restored

the 8 2 ka event from greenland ice cores we present a collection of high resolution chemistry and stable isotope records from the plateau of the greenland ice cap during the cold event 8200 yr ago using a composite of four records the cold event is observed as a 160 5 yr period during which decadal mean isotopic values were below average within which there is a central event of 69 yr during which values were consistently more than one standard deviation below the average for the preceding period four cores in north south and central greenland show differences at decadal and shorter timescales it is not yet clear if this represents significant spatial differences in response the results show clear evidence for colder temperatures and a decrease in snow accumulation rate however the changes in chemical concentrations for the ions looked at here are small suggesting only minor changes in atmospheric circulation for this event apart from the decrease in methane concentration greenland ice cores give only weak evidence for effects outside the north atlantic region

stable isotope records of late quaternary climate and hydrology from mediterranean lakes the isomed synthesis lake isotope records can be used to assess the spatial coherency of late quaternary climate change across the circum mediterranean region we place modern and palaeo data within a simple conceptual lake response model to show that the isotope hydrology of most mediterranean lakes has been influenced strongly by water balance even in those systems that are chemically dilute i e freshwater 18o data on biogenic and endogenic carbonates from 24 lake basins are used to reconstruct multi millennial scale trends since the lgm while it is difficult to make direct comparisons between lake records in terms of single climatic parameters coherent regional isotopic trends can be identified during glacial times mediterranean lakes deposited carbonates isotopically heavier in 18o compared to the holocene partly due to source area effects isotopic enrichment was most marked during intervals corresponding to the h1 and younger dryas events confirming that late pleistocene cold stages in the north atlantic region were marked by aridity around much of the mediterranean almost all mediterranean lake records shifted to more depleted isotopic values during the last glacial interglacial transition lgit this shift is the reverse of the trend which characterised the same transition in lakes from northern and central europe and suggests that temperature changes were not an important direct driver of mediterranean lake isotopic records over glacial interglacial timescales in the early holocene many lakes in the eastern part of the region were more depleted isotopically than in recent millennia this corresponds with marine sapropel formation both chronologically and geographically and implies that increases in local rainfall contributed significantly to the creation of a freshwater lid and anoxia in the east mediterranean sea in contrast no such pattern is currently apparent from lake isotope records from the west mediterranean suggesting a possible nw se contrast in climate history during the holocene

paleoclimatic implications of late quaternary lacustrine sediments in western nubia sudan abstract owing to the hypercontinental location of western nubia secular fluctuations of climate have been filtered and wet phases can be considered as representative of conditions throughout the southeastern sahara the study area is crossed by the 20 mm isohyet between 9300 and about 4000 yr b p however there were widespread lake and swamp environments with freshwater molluscs ostracods and diatoms and a species rich savanna mammal fauna the center of the west nubian basin approx 18 n an area of about 20 000 km 2 was occupied by a semiaquatic landscape which was situated at the same latitude as paleolake chad from extensive lake carbonates up to about 4 m thick a long term rise of the groudwater table is inferred environments developed that now exist at about latitude 13 n radiocarbon dates from lake sediment sequences cluster between 30 000 and 21 000 yr b p indicating a pleistocene wet phase a gap in radiocarbon dates between 21 000 and 11 000 yr b p signals a phase of hyperaridity similar to the present hyperarid phase with eolian deflation and deposits of sand being the dominant forms of erosion and accumulation

insolation values for the climate of the last 10 million years new values for the astronomical parameters of the earth s orbit and rotation eccentricity obliquity and precession are proposed for paleoclimatic research related to the late miocene the pliocene and the quaternary they have been obtained from a numerical solution of the lagrangian system of the planetary point masses and from an analytical solution of the poisson equations of the earth moon system the analytical expansion allows the direct determination of the main frequencies with their phase and amplitude numerical and analytical comparisons with the former astronomical solution ber78 are performed so that the accuracy and the interval of time over which the new solution is valid can be estimated the corresponding insolation values have also been computed this analysis leads to the conclusion that the new values are expected to be reliable over the last 5 ma in the time domain and at least over the last 10 ma in the frequency domain

holocene vegetation and climate of the alashan plateau nw china reconstructed from pollen data a set of 55 recent pollen spectra from the alashan plateau and the qilian mountains qilianshan northwestern china has been analyzed the established relationships between the pollen spectra and modern vegetation and precipitation patterns of the studied area have been applied to 65 fossil pollen spectra from a 825 cm long sediment record collected in the topographic depression formerly occupied by eastern juyan palaeolake 41 89 n 101 85 e 892 m a s l several qualitative approaches e g indicator plant species artemisia chenopodiaceae and ephedra fragilis type s l ephedra distachya type pollen ratios and quantitative methods of vegetation and climate reconstruction have been tested with the surface pollen and then applied to the eastern juyan pollen record fossil pollen data reveal pronounced environmental changes in the studied area of the alashan plateau between ca 10 700 and 1700 cal yr b p desert taxa as chenopodiaceae nitraria calligonum reaumuria were abundant in the vegetation cover around the study site through the whole record pollen spectra dated to ca 10 700 5400 cal yr b p are characterised by highest values of chenopodiaceae e fragilis type and other desert indicating taxa suggesting rather dry climate most favourable conditions are reconstructed between 5400 and 3900 cal year b p on the basis of relative increase in abundance of artemisia pollen a return to dry conditions occurred at about 3900 cal yr b p the lake finally desiccated after 1700 cal yr b p reconstructed dry climate oscillations between 5900 and 5400 and 3100 cal yr b p correlate well with similar events found in the published records from northern and western china and central mongolia however eastern juyan record does not show the early holocene humidity maximum as suggested by the sedimentary data from monsoon influenced areas of china

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

re entering the central sahara at the onset of the holocene a territorial approach to early acacus hunter gatherers sw libya abstract the central sahara was intensely occupied at the very onset of the holocene when moister conditions re established after the younger dryas in the study area sw libya a solid 14c chronology dates the occupation of early acacus h g between 9800 and 8900 bp it is likely that the colonization by these small groups of specialized hunter gatherers was rapid entering unfamiliar areas characterized by the presence of lakes and ponds whether or not the very first re entry was a uniquely holocene phenomenon is still debated we favour an arrival from the north considering current palaeoenvironmental archaeological and possibly genetic evidences the archaeological record consists of around ninety sites surface occurrences and a few excavated sequences standing as regional and supra regional references sites are located on different geomorphological contexts and are task specifically organized subsistence relied on specialized hunting of barbary sheep and on some vegetal species the provenance and circulation of lithic raw materials reveals a high mobility at a regional but also at a wider scale tool kits are light bladelet based curated and dominated by backed armatures early acacus occupation reveals strong differences compared to the subsequent late acacus phase 8900 7400 bp when a profound reorganization in the subsistence settlement system featuring an increase in the delayed use of resources i e corralling of barbary sheep and storage of wild cereals occurred in this work we present an overview of the early acacus archaeological data with an emphasis on the territorial analysis of lithic assemblages

the arid humid transition in the sahara and the sahel during the last deglaciation at the time of the last glacial maximum the sahara and sahel regions of north africa were extremely dry new records of rainfall show that during the subsequent deglaciation the transition from arid to humid conditions in these regions occurred synchronously in two main steps comparison with other records of palaeoclimate in europe and the north atlantic ocean shows that certain common factors controlled changes in ocean and atmosphere dynamics during the deglaciation

a comparative study of different pga attenuation and error models case of 1999 chi chi earthquake abstract in order to evaluate the horizontal peak ground acceleration hpga during earthquakes the author studies the respective efficiency of two existing attenuation models mebarki a 2003a risques sismiques aleas vulnerabilite et aide a la decision par cartes sig proceedings of international conference on risks vulnerability and reliability in construction towards a reduction of disasters isbn 9961 891 01 5 pp 82 97 algiers october 11 12 mebarki a 2003b proposal of a parametric attenuation model and comparison with some worldwide earthquakes viio congreso venezolano de sismologia y ingenieria sismica barquisimeto venezuela november 12 13 cd rom mebarki a 2004 modele d attenuation sismique prediction probabiliste des pics d acceleration rfgc revue francaise de genie civil hermes ed 8 9 10 1071 1086 a comparative study of their performances is done in the case of 1999 chi chi earthquake taiwan the reported pga peak ground accelerations values correspond to hypocentral distances ranging from 15 up to 180 km with observed acceleration peaks ranging from 0 04 g up to 1 16 g the author considers two kinds of probabilistic distributions for the error model in order to describe the uncertainty and the variability that affect the values of the pga a gamma distribution and a log normal distribution the adopted error models assume that the variability of the pga is such that its coefficient of variation is equal to 55 mebarki a 2003a risques sismiques aleas vulnerabilite et aide a la decision par cartes sig proceedings of international conference on risks vulnerability and reliability in construction towards a reduction of disasters isbn 9961 891 01 5 pp 82 97 algiers october 11 12 mebarki a 2003b proposal of a parametric attenuation model and comparison with some worldwide earthquakes viio congreso venezolano de sismologia y ingenieria sismica barquisimeto venezuela november 12 13 cd rom mebarki a 2004 modele d attenuation sismique prediction probabiliste des pics d acceleration rfgc revue francaise de genie civil hermes ed 8 9 10 1071 1086 the obtained results show that both attenuation models provide adequate values of the pga when a gamma distribution is adopted for the error model the attenuation models efficiency decreases slightly when a log normal distribution is adopted for the error model

friction of drill bits under martian pressure 1 frictional behavior was investigated for two materials that are good candidates for mars drill bits diamond impregnated segments and polycrystalline diamond compacts pdc the bits were sliding against dry sandstone and basalt rocks under both earth and mars atmospheric pressures and also at temperatures ranging from subzero to over 400 c it was found that the friction coefficient dropped from approximately 0 16 to 0 1 as the pressure was lowered from the earth s pressure to mars pressure at room temperature this is thought to be a result of the loss of weakly bound water on the sliding surfaces holding the pressure at 5 torr and increasing the temperature to approximately 200 c caused a sudden increase in the friction coefficient by approximately 50 this is attributed to the loss of surface oxides if no indication of the bit temperature is available an increase in drilling torque could be misinterpreted as being caused by an increase in auger torque due to accumulation of cuttings rather than being the result of a loss of oxide layers due to elevated bit temperatures an increase in rotational speed to allow for clearing of cuttings would then cause greater frictional heating and would increase the drilling torque further therefore it would be advisable to monitor the bit temperature or if that is not possible to include pauses in drilling to allow the heat to dissipate higher friction would also accelerate the wear of the drill bit and in turn reduce the depth of the hole

model data comparison for the 8 2 ka bp event confirmation of a forcing mechanism by catastrophic drainage of laurentide lakes abstract to improve our understanding of the mechanism behind the 8 2 ka bp cooling event we compare proxy evidence with climate model simulations in which the thermohaline ocean circulation is perturbed by a freshwater pulse into the labrador sea both the proxy data and model results show a cooling that is mainly concentrated in the north atlantic region ranging from more than 5 c cooling in the nordic seas to about 0 5 1 c over europe and less than 0 5 c over the subtropical north atlantic data and model also indicate a weakening of the summer monsoon and generally a drier circum north atlantic over the south atlantic ocean the model simulates a slight warming mostly less than 0 5 c which falls within the uncertainty of proxy data and thus could not be confirmed to examine in detail the structure of the 8 2 ka bp event we also compare the modeled climatological evolution at two locations with high quality records revealing a generally consistent picture the good model data agreement confirms the hypothesis that the 8 2 ka bp event was forced by a freshwater induced weakening of the thermohaline circulation other forcings are unlikely since they would result in an alternative geographical distribution and expression of the climate response

holocene rock varnish on the messak plateau libyan sahara chronology of weathering processes abstract rock varnish coats subaerially exposed rock surfaces on the messak settafet plateau se libya central sahara holocene surfaces host three microlayers formed by different accretion processes that record a climatic transition from a wet environment to progressively more arid conditions sem eds and optical investigations shed light on the processes leading to the formation of microlaminae while direct ams 14c dating on microlayers and relative radiocarbon dating on dark varnished pastoral neolithic stone monuments suggest a reliable chronology for varnish development the innermost microlayer includes illuvial clay and calcite incorporated during the early to middle holocene 9200 to 5500 uncal ybp under wet environmental conditions the second microlayer is more typical mn rich rock varnish biomineralized during a phase of growing aridity in the middle holocene 6000 to 4000 uncal ybp the upper microlayer is composed of mostly aeolian dust and it dates back to the last hyperarid millennia a positive correlation between varnish chronological development and climate changes in the sahara reflects also broad regional control on microlamination deposition

abrupt post glacial climate events in west asia and north africa monsoon domains regions beyond the present or past penetration of the indian and african monsoons have experienced several large and abrupt climatic fluctuations over the past 13 14c kyr pollen and lake records from west asia western tibet and rajasthan east africa ethiopia and west africa western sahara sahel and subequatorial africa were selected on the basis of chronological control sensitivity of both site and environmental indicators to climate change the continuity of the record and interdisciplinary control of the palaeoclimatic interpretation conditions wetter than those of today prevailed during the early mid holocene period but major dry spells are recorded at all sites during the intervals 11 0 9 5 kyr bp 8 7 kyr bp and 3 4 kyr bp several records also suggest dry events of minor amplitude around 6 kyr bp potential boundary forcings of insolation and sea surface and tropical land surface conditions are discussed the solar radiation accounts for the general envelop of the post glacial monsoon fluctuations but explains neither the timing nor the amplitude of the short term changes in spite of apparent covariation between fluctuations in sea surface conditions in the north atlantic and the monsoon record no direct mechanism could be found relating the intensity of the oceanic thermohaline conveyor belt to the monsoon strength changes in tropical land surface conditions soil moisture negative feedback and changes in ch4 production from wetlands provide a more satisfactory hypothesis for explaining abrupt reversal events

abrupt onset and termination of the african humid period rapid climate responses to gradual insolation forcing a detailed ca 100 yr resolution and well dated 18 ams 14 c dates to 23 cal ka bp record of latest pleistocene holocene variations in terrigenous eolian sediment deposition at odp site 658c off cap blanc mauritania documents very abrupt large scale changes in subtropical north african climate the terrigenous record exhibits a well defined period of low influx between 14 8 and 5 5 cal ka bp associated with the african humid period when the sahara was nearly completely vegetated and supported numerous perennial lakes an arid interval corresponding to the younger dryas chronozone punctuates this humid period the african humid period has been attributed to a strengthening of the african monsoon due to gradual orbital increases in summer season insolation however the onset and termination of this humid period were very abrupt occurring within decades to centuries both transitions occurred when summer season insolation crossed a nearly identical threshold value which was 4 2 greater than present these abrupt climate responses to gradual insolation forcing require strongly non linear feedback processes and current coupled climate model studies invoke vegetation and ocean temperature feedbacks as candidate mechanisms for the non linear climate sensitivity the african monsoon climate system is thus a low latitude corollary to the bi stable behavior of high latitude deep ocean thermohaline circulation which is similarly capable of rapid and large amplitude climate transitions

holocene palaeoclimate in the northern sahara margin jefara plain northwestern libya the palaeoclimatic evolution of northwestern libya has been deduced from the study of the palustrine and aeolian sediments the holocene environmental variations have been recognized through geological and stratigraphic surveys and by pollen analysis and malacofauna content of the sediments some organic rich horizons have been dated using the radiocarbon method the data show that in general the presence of marshes is linked with the period of higher humidity which began around 9 4 ka bp and ended around 5 0 ka bp from c 5 0 ka bp up to about 4 4 ka bp there was a simultaneous presence of marshes and vegetation of arid climate as a consequence of rainfall in the mountains and or a decrease in temperatures the holocene wet period was interrupted by two arid spells dated about 8 2 and 5 5 5 4 ka bp the end of the wet phase must have been gradual the climatic events that occurred in the jefara plain seem to be well correlated with those identified in saharan africa even if the mediterranean is only

holocene climatic instability a prominent widespread event 8200 yr ago the most prominent holocene climatic event in greenland ice core proxies with approximately half the amplitude of the younger dryas occurred 8000 to 8400 yr ago this holocene event affected regions well beyond the north atlantic basin as shown by synchronous increases in windblown chemical indicators together with a significant decrease in methane widespread proxy records from the tropics to the north polar regions show a short lived cool dry or windy event of similar age the spatial pattern of terrestrial and marine changes is similar to that of the younger dryas event suggesting a role for north atlantic thermohaline circulation possible forcings identified thus far for this holocene event are small consistent with recent model results indicating high sensitivity and strong linkages in the climatic system

transition from arid to hyper arid environment in the southern levant deserts as recorded by early pleistocene cummulic aridisols abstract the time at which deserts established their current arid or hyper arid conditions remains a fundamental question regarding the history of earth cosmogenic isotope exposure ages of desert pavement and welded calcic gypsic salic reg soils that developed on relatively flat alluvial surfaces 2 ma ago in the negev desert indicate long geomorphic stability under extremely dry conditions over a short interval during their initial stage of development between 1 2 ma these cumulative soils are characterized by calcic soils reaching maximum stage iii of carbonate morphology this interval is the only period when calcic soil horizons formed on stable abandoned alluvial surfaces in the southern negev desert since 1 ma pedogenesis changed toward more arid soil environment and the formation of gypsic salic soil horizons that were later followed by dust accumulation the dichotomy of only moderately developed calcic soil stages ii iii during a relatively long time interval 105 106 years indicates an arid environment that does not support continuous development but only occasional calcic soil formation the very low 18o and relatively high 13c values of these early pedogenic carbonates support soil formation under arid climatic conditions such an environment was probably characterized by rare and relatively longer duration rainstorms which occasionally allowed deeper infiltration of rainwater and longer retention of soil moisture this in turn enabled the growth of sparse vegetation that enhanced deposition of pedogenic carbonate at 1 ma these rare events of slightly wetter conditions ceased and less atmospheric moisture reached the southern negev desert leading to deposition of soluble salts and dust deposited in the soils the combination of long term hyperaridity scarcity of vegetation and lack of bioturbation salts cementation dust accumulation and tight desert pavement cover has protected the surfaces from erosion forming one of the most remarkably stable landscapes on earth a landscape that essentially has not eroded but accumulated salt and dust for more than 106 yr

micromorphological approach to polycyclic pedogenesis on the messak settafet plateau central sahara formative processes and palaeoenvironmental significance the messak settafet is a wide plateau located in the libyan central sahara sw fezzan it is cut into the nubian sandstone and interpreted as relic of a tertiary peneplain heavily affected by aeolian deflation notwithstanding degradation phenomena discontinuous thin soil bodies are locally preserved below the hamada several profiles located in correspondence of different geomorphological units the hamada the endorheic depressions and the southern escarpment were described and sampled for micropedological analyses micromorphology outlined that the main characteristics of the residual b horizons from each geomorphological units are i high concentration of ferruginous nodules ii multiple generations of clay illuviation iii fragmentation and deformation of textural pedofeatures and iv calcite mobilization and redeposition according to horizon macro and micromorphological characteristics these pedologic bodies do not seem to be in equilibrium with the present day climatic conditions and should be regarded as paleosols generated by processes regulated by large water availability and warm temperature such as rubification clay translocation and calcite redistribution the development of horizons acted mainly since the early middle pleistocene during subsequent pluvial periods and was interrupted by several erosional and aeolian input phases arid periods pedological processes ended at the onset of the severe dry environmental conditions at the mid late holocene transition the development of this kind of paleosols in the central sahara is also discussed in comparison with pedogenetic processes occurring in temperate regions during interglacial and interstadial intervals

climate change versus land management in the po plain northern italy during the bronze age new insights from the vp vg sequence of the terramara santa rosa di poviglio abstract the sedimentary infilling of the moat surrounding the villaggio piccolo of the terramara santa rosa di poviglio was analysed in order to obtain palaeoenvironmental inferences from sediments and pollen assemblage the high resolution stratigraphic sequence preserves evidence of the environmental changes that occurred in the po plain in northern italy during the late holocene our interdisciplinary approach permitted to study climatic and anthropic contributions to the environmental changes in this region the relationships between these changes and land use changes were investigated focussing on adaptive strategies of the terramare people during the middle and recent bronze ages 1550 1170 yr bc the terramare are archaeological remains of banked and moated villages located in the central alluvial plain of the po river the terramara of santa rosa consists of two adjoining settlements villaggio grande and villaggio piccolo the moat that separates the two parts of the site is c 23 m large and reaches a maximum depth of 4 m from the extant ground level the stratigraphic sequence vp vg exposed by archaeological excavation inside the moat was sampled for pedosedimentary thin section and pollen analyses chronology is based on archaeological evidence stratigraphic correlations and radiocarbon dating pedosedimentary features and biological records pollen of aquatics and algal remains demonstrate that shallow water probably subjected to seasonal water level oscillations has always been present in the moat in the lower units of the sequence the laminations indicate standing water while occurrence of reworked pollen testified the supply of sediments to the plain from catchment zones located in the apennine open vegetation was widespread economy was based on wood management fruit collection on the wild or from cultivated woody plants crop fields with a fairly diversified set of cereals especially increasing in variety during dryness or phases of water crisis probably grapevines were cultivated near the moat where the wet habitat was favourable to the growing of wild plants the extraordinary high resolution of this sequence makes visible the management of woods including coppicing at the middle bronze and early recent bronze ages the economy of santa rosa di poviglio should have been probably less based on animal breeding than it was in the other terramare villages already studied for pollen this research also confirms the chronological correspondence between an environment stressed by dry conditions and the collapse of the terramare civilization

mid to late holocene landscape change and anthropogenic transformations on mo orea society islands a multi proxy approach archaeology s ability to generate long term datasets of natural and human landscape change positions the discipline as an inter disciplinary bridge between the social and natural sciences using a

high resolution record of northern hemisphere climate extending into the last interglacial period high resolution record of northern hemisphere climate extending into the last interglacial period

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

depositional environments and sequence stratigraphy of paralic glacial paraglacial and postglacial upper ordovician siliciclastic deposits in the murzuq basin sw libya abstract the application of sequence stratigraphy methods to glaciogenic deposits is more problematic than in normal paralic deposits because changes in the relative sea level are strongly influenced by an interplay between glacial advance and retreat and concomitant isostatic loading and rebound of the continental shelf the study of outcrop and subsurface sections of the upper ordovician glaciogenic melaz shuqran and the mamuniyat formations murzuq basin sw libya yielded important information on understanding changes in the relative sea level related to glacier movements three depositional sequences were recognized depositional sequence one ds 1 corresponds to the entire melaz shuqran formation that was bounded below by subglacial erosion surface i e sequence boundary sb formed during glacier advance into shallow water areas this surface correlates with a transgressive surface ts in deep water areas and records an initial rise in the relative sea level owing to glacier advance and loading of the continental shelf transgression occurred due to the slower rate of eustatic sea level fall than the isostatic loading the transgressive systems tract tst comprises shoreface sandstones and offshore mudstones diamictites with ice rafted debris further rise in relative sea level was associated with glacial retreat which resulted in sediment starvation in deep water areas and the formation of a condensed section these deposits represent glacial depositional systems the highstand systems tract hst comprises prograding deltaic deposits which was formed when the rate of sediment supply exceeds the rate of relative sea level rise the hst deposits represent paraglacial depositional systems depositional sequence two ds 2 corresponds to the lower and middle part of the mamuniyat formation and is bounded below by deep erosional surface sb that cuts into ds 1 as an incised valley which was formed during glacial advance across the continental shelf incision occurred due to the faster rate of eustatic sea level fall than the rate of isostatic loading glacier retreat and the concomitant rise in relative sea level resulted in the deposition of braided fluvial sediments in a lowstand systems tract lst and tide dominated estuarine sediments in a tst these deposits represent glacial depositional systems the hst sediments represent prograding foreshore to shoreface deposits which were formed when the rate of sediment supply exceeded the rate of relative sea level rise these deposits represent paraglacial depositional systems depositional sequence three ds 3 corresponds to the upper part of the mamuniyat formation and is bounded below by an erosional surface sb that was formed as a result of isostatic rebound associated with relative sea level fall ds 3 lst comprises gilbert type deltaic conglomeratic sandstones grading upwards into sandstones subsequent relative sea level rise during the late ordovician resulted in deposition of shoreface tst sandstone further rise in the relative sea level resulted in sediment starvation in deep water areas and the formation of thin hardground layer i e condensed section that marks the upper boundary of late ordovician deposits this study provides predictive model for the spatial and temporal distribution of ancient glaciogenic depositional facies that may have important implications in hydrocarbon explorations

recent environmental change and prehistoric human activity in egypt and northern sudan abstract this paper reviews the various late quaternary records that are available from western egypt and northern sudan which includes more than 500 published radiocarbon dates and various sedimentary archives from local landscape components including palaeolakes soils drainages wadis and archaeological sites this palaeoenvironmental compilation frames the spatial and temporal context of local cultural activities when the region was most hospitable 9000 6000 bp at this time monsoonal weather influenced the portion of the african continental interior creating enough convective rainfall for occasional surface water storage in this part of the modern sahara rapid hydroclimatic changes play a key role in geomorphic evolution and resource availability as watering holes formed and dried up in the early to middle holocene neolithic people developed various subsistence strategies including opportunistic hunting of small animals e g gazelle and hare and food related e g wild sorghum millet and legumes activities gathering plant cultivation and livestock rearing during its wettest phases during the monsoonal maximum the area was drought prone sustaining a meager steppe shrub desert flora further desertification and aeolian deflation during the middle and late holocene fostered technological innovation migration and settlement as well as the further development of agrarian communities and complex culture

cupressus dupreziana a dendroclimatic record for the middle late holocene in the central sahara dendroclimatology of cupressus dupreziana the tassili cypress has been attempted on samples obtained from the door beams of the old cities of ghat and barkat located at the foot of the tassili where the cypress still lives the tree rings of 20 samples were measured and dated by 24 ams 14c dates a mean ring width chronology has thus been obtained spanning though discontinuously 5220 to 100 14c bp 5990 65 cal bp as the tree ring width in dry lands depends mainly on water availability the mean ring width chronology represents a detailed record of changes in rainfall on a decade scale for the middle and late holocene of the central sahara it indicates main drought spells at 5200 5000 14c bp 5900 5760 cal bp and at 4350 bp 5120 cal bp followed by phases of enhanced precipitation and by the onset of extreme arid conditions at 1550 14c bp 1500 cal bp

mid to late holocene climate evolution of the lake telmen basin north central mongolia based on palynological data abstract palynological and sedimentological data from lake telmen in north central mongolia permit qualitative reconstruction of relative changes in moisture balance throughout the mid to late holocene the climate of the atlantic period 7500 4500 yr ago was relatively arid indicating that lake telmen lay beyond the region of enhanced precipitation delivered by the expanded asian monsoon maximum humidity is recorded between 4500 and 1600 cal yr b p during the subboreal 4500 2500 yr ago and early subatlantic 2500 yr present periods additional humid intervals during the medieval warm epoch 1000 1300 a d or 950 650 ago and the little ice age 1500 1900 a d or 450 50 yr b p demonstrate the lack of long term correlation between temperature and moisture availability in this region a brief aridification centered around 1410 cal yr b p encompasses a decade of cold temperatures and summer frost between a d 536 and 545 1414 1405 yr b p inferred from records of mongolian tree ring widths these data suggest that steppe vegetation of the lake telmen region is sensitive to centennial and decadal scale climatic perturbations

holocene climate variability although the dramatic climate disruptions of the last glacial period have received considerable attention relatively little has been directed toward climate variability in the holocene 11 500 cal yr b p to the present examination of similar to50 globally distributed paleoclimate records reveals as many as six periods of significant rapid climate change during the time periods 9000 8000 6000 5000 4200 3800 3500 2500 1200 1000 and 600 150 cal yr b p most of the climate change events in these globally distributed records are characterized by polar cooling tropical aridity and major atmospheric circulation changes although in the most recent interval 600 150 cal yr b p polar cooling was accompanied by increased moisture in some parts of the tropics several intervals coincide with major disruptions of civilization illustrating the human significance of holocene climate variability c 2004 university of washington all rights reserved

evaluating the life expectancy of a desert pavement abstract this paper integrates prior scholarship on desert pavements with a case study of pavements on stream terraces in the sonoran desert to analyze the processes and site conditions that facilitate the survival of ancient desert pavements this synthesis identifies vital factors key factors and site specific factors promoting pavement stability hyperaridity is the vital factor in pavements surviving for 106 years or more aided by minimal bioturbation and clast size reduction three key factors aid in pavements surviving for 104 to 105 years accumulation of allochthonous dust underneath pavement cobbles a flat topography and a lack of headward retreating swales or gullies a unified explanation for pavement longevity however did not emerge from a literature review because a variety of site specific factors can also promote pavement antiquity including resistant bedrock beneath the pavement disk shaped cobbles to promote dust accumulation and microclimatological and ecological reasons for minimal bioturbation both key and site specific explanations for pavement longevity apply well to a case study of pavements on stream terraces in the sonoran desert central arizona the buildup of cosmogenic 10be and in situ 14c optically stimulated luminescence and varnish microlamination ages reveal stable pavements range in age between 30 and 332 ka with conditions for longevity including flat surface topography pavements underlain by consolidated granitic bedrock a lack of headward retreating gullies and swales 87sr 86sr analyses indicating the infiltration of allochthonous dust floating disk shaped pavement cobbles and a quartzite lithology resistant to disintegration however 10be ages also indicate evidence for the instability of desert pavements on stream terraces underlain by unconsolidated playa clays and unconsolidated fanglomerate these weaker materials allowed the growth of headward retreating swales that in turn promoted exposure of newer gravels by surface erosion

the holocene climatic optimum and pollen records of sapropel 1 in the eastern mediterranean 9000 6000 bp abstract the most recent sapropel in the deep eastern mediterranean sea has been deposited between 9 and 6 ka bp climate conditions as revealed by the pollen records of this sapropel in marine cores were most favorable for temperate deciduous trees which is in agreement with the inferences from records of peripheral land pollen sites the abundance of deciduous oak pollen is much higher than that of artemisia sage brush indicating that annual precipitation in the mid elevation borderlands was at least 550 mm without summer drought but more probably in the range 800 1300 mm the pollen of pistacia which formed a savanna at low elevations is also at its highest abundance and signals the absence of frost in winter while being capable of withstanding summer drought the early holocene therefore appears as the post glacial climatic optimum with the highest moisture and mildest winters in southwest asia this is also the time of the neolithic population explosion with incipient domestication of cereals possibly following natural selection of the tough rachis mutation in wheat and barley by the extreme aridity of the preceding younger dryas

holocene lakes and prehistoric settlements of the western faiyum egypt abstract holocene lake stages in the faiyum depression commenced with a high lake stand during the 10th millennium bp followed by an early holocene lake from 8500 to 7000 bp a pronounced recession and the development of a palaeosol preceded another rise to a mid holocene high lake level from 6500 to 5100 bp a major drop in level coincided with the late neolithic and early dynastic the moeris lake witnessed by herodotus is also documented the drop in lake level during early ptolemaic times marked the end of the freshwater lake and was apparently in part a result of declining nile floods terminal palaeolithic sites are associated with the early holocene lake and neolithic sites with the mid holocene phase prehistoric settlements were placed near lake margin marshes and ponds the richness of the lake margin in aquatic resources and its susceptibility to short and long term fluctuations influenced both subsistence and settlements and is believed to have encouraged a para agricultural economy

paleoclimatic variations in west africa from a record of late pleistocene and holocene lake level stands of lake bosumtwi ghana abstract a detailed investigation of geomorphological evidence of paleoshorelines and exposed stratigraphic sections of lake deposits combined with a chronology based on radiocarbon dated charcoal and in situ 14c dating of wave polished bedrock provide important new constraints on lake level changes of lake bosumtwi ghana thick sequences of laminated silts alternating with transgressive sands and deltaic gravels attest to a long history of climatically controlled lake level variations the post glacial rise in lake level began sometime after 16 3 ka reached stable levels first at 14 5 0 6 ka and then rose again after ca 14 3 ka a significant lake level regression spanned the interval from 12 6 0 3 to 11 6 0 5 ka synchronous with the younger dryas deep lake conditions were reestablished after ca 11 ka at which time the lake overtopped the crater overflow continued until 8 8 0 5 ka when another significant but short lived regression occurred deep but probably not overflowing conditions were again reestablished by 7 2 0 3 ka and continued until around 3 2 0 1 ka when lake level dropped precipitously multicentury late holocene highstands occurred at 2 2 0 1 and 1 7 0 2 ka although these were significantly lower than those registered in the late glacial and early holocene the timing of late glacial events is similar to those recorded elsewhere in africa and the higher latitudes and likely reflects the dominant control of high latitude northern hemisphere conditions on the african tropics during the times of large northern hemisphere ice sheets mid to late holocene variations appear to be less coupled with changes across africa and elsewhere suggesting that regional forcing may be more important during warmer periods

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

meridional shifts of the atlantic intertropical convergence zone since the last glacial maximum the position of the intertropical convergence zone is thought to be linked to changes in the earth s climate state analyses of marine sediment show that over the past 25 000 years the intertropical convergence zone over the atlantic ocean migrated in response to climate changes

pollen inferred climate changes and vertical shifts of alpine vegetation belts on the northern slope of the nyainqentanglha mountains central tibetan plateau since 8 4 kyr bp fossil pollen from nam co and modern pollen from altitudinal vegetation belts around the lake are investigated to reveal alpine vegetation succession in response to climate changes during the holoc

environmental change and archaeology lake evolution and human occupation in the eastern sahara during the holocene abstract the west nubian palaeolake is the most large scale hydrographic evidence in the eastern sahara of the early to mid holocene wet phase that affected northern africa it is the result of a significant increase in local rainfall due to the northward shift of the tropical rainfall belt a series of fieldwork based differential gps dgps measurements along several profiles across the west nubian palaeolake basin provides the first precise topographic data from this up to a 5330 km2 large palaeolake feature in combination with sedimentological geochemical and archaeological results an almost complete picture of significant palaeoclimatic changes and human occupation during the early to mid holocene for this region is presented different stages of palaeolake evolution ranging from non existence of the lake through stable freshwater conditions to its extinction were identified in the period from 9400 to 3800 14c yr bp these lake stages coincide with phases of intensive human inhabitation between ca 6300 and 3500 14c yr bp and include at least four settlement phases distinguishable by style of pottery these are known from adjacent areas of the palaeolake region emphasizing strong prehistoric cultural connections in the eastern sahara during the highstands of the palaeolake in the early to mid holocene the dotted wavy line pottery relates to the early khartoum type culture with its supra regional distribution from the nile valley to the chad and possibly with slightly different forms even to the atlantic coast later in the holocene western nubia with its large palaeolakes and migration paths along palaeowadis such as wadi howar acted as an important natural and cultural link between the nile valley and the chad basin until the region was deserted during the fourth millennium bp

the calcareous tufa in the tadrart acacus mt sw fezzan libya an early holocene palaeoclimate archive in the central sahara article i nfo the palaeoclimatic signi cance of calcareous tufa deposits found in the presently hyperarid tadrart acacus massif central sahara sw libya is discussed on the basis of u th datings stable isotope analyses and the regional hydrogeological context these deposits formed mostly between c 9600 and 8100 yr bp the isotopic signature 18 o and 13 c of this tufa indicates that the water forming these carbonates originated as isotopically depleted rainwater as a result of the northward expansion of the sw african monsoon system during the early holocene carbonate sedimentation in the tadrart acacus decreased shortly before c 8000 yr bp attributed to a drop in precipitation in the course of the well known 8 2 kyr event a correlation between tufa development and climate change in n and e africa and in the mediterranean basin during the early holocene is discussed the effect of early holocene climate change on both the landscape and the dwellers living in the tadrart acacus region is also evaluated

cosmogenic 3he surface exposure dating of stone pavements implications for landscape evolution in deserts the formation of stone pavements a ubiquitous gravel armor mantling landforms in arid regions of the world has been previously attributed to erosion by wind and water or alternating shrinking and swelling of soil horizons implying that gravel is concentrated at the land surface in a time transgressive manner a newly proposed model for pavement evolution differs from these models in that pavement clasts are continuously maintained atthelandsurfaceinresponsetodepositionandpedogenicmodificationofwindblowndust in situ cosmogenic 3 he surface exposure ages on volcanic and alluvial landforms in the mojave desert of california are used to understand pavement evolution over geologic time scales and to test this new model these exposure ages are stratigraphically consistent show internal consistency at each site and for stone pavements adjacent to pristine continuously exposed volcanic bedrock are indistinguishable at the 1 level we conclude that stone pavements are born at the surface and that pavements may provide one of the longest term records of geologic hydrologic and climatic processes operating on desert surfaces

palaeoclimate and the formation of sapropel s1 inferences from late quaternary lacustrine and marine sequences in the central mediterranean region abstract synchronous responses to climatic changes during the late pleistocene holocene transition are inferred from marine and lacustrine stratigraphic records in the central mediterranean region new stratigraphic data are presented from well dated sequences in the meso adriatic depression mad two crater lakes in the lazio region and in the tyrrhenian sea the sequences all span the last termination and the holocene but we focus here on the evidence in each record for the time period during which sapropel s1 formed in the mediterranean ca 9 0 to 6 8 cal kyr b p the new records provide evidence of palaeoenvironmental changes on land and sea that can be reconstructed at a high temporal resolution and which throw some light on the processes which led to the formation of s1 the collective evidence indicates that i organic rich sediments occurred in both the marine and the crater lake sites during the time of formation of the s1 sapropel ii there is evidence of increased stratification and anoxia in the sea water column during the period of s1 formation iii the s1 period in the study area is divisible into two sub phases s1a and s1b which reflect short term variations in oceanographic conditions stratification and anoxia iv changes in stratification in the marine column were contemporaneous with regional climate variations that are inferred from the terrestrial records we conclude that the key factor that initiated the formation of s1 was increased discharge of freshwater into the mediterranean following a change post 9 0 cal kyr b p to a warmer and wetter climate furthermore the period of s1 formation was interrupted by a short lived episode ca 500 years of comparatively cooler and drier conditions during the early middle holocene transition emht

the oxygen and carbon isotopic records in holocene freshwater mollusc shells from the faiyum paleolakes egypt their paloenvironmental and paleoclimatic implications abstract stable oxygen and carbon isotopes of freshwater mollusc shells from a sequence of dated palaeolake deposits in the faiyum depression egypt provide an outline record of lake development during the holocene it is argued that the carbonate o 18 record provides a proxy for climate change and that the isotope changes are driven by variations in monsoon precipitation in nile headwaters and in local evaporation processes shells from sediments dated to ca 10 000 to 7700 cal bp had low values for oxygen and carbon isotopes indicating higher precipitation in the local area and intensified monsoonal activity leading to higher nile flood discharge and flooding of the entire faiyum depression this was followed by a transitional period characterized by marked variability but with precipitation on average less than before from ca 6000 to 4300 cal bp precipitation was decreased until a dramatic reduction in nile flood discharge occurred late in the fifth millennium bp this reduction signalled the onset of arid conditions in the faiyum partly as a consequence of changes in nile headwaters that was concomitant with declining lake levels in east africa lake level decline in the faiyum was further aggravated after 2700 bp as the nile inflow further weaken and regulation increased

climate change and the collapse of the akkadian empire evidence from the deep sea the akkadian empire ruled mesopotamia from the headwaters of the tigris euphrates rivers to the persian gulf during the late third millennium b c archeological evidence has shown that this highly developed civilization collapsed abruptly near 4170 150 calendar yr b p perhaps related to a shift to more arid conditions detailed paleoclimate records to test this assertion from mesopotamia are rare but changes in regional aridity are preserved in adjacent ocean basins we document holocene changes in regional aridity using mineralogic and geochemical analyses of a marine sediment core from the gulf of oman which is directly downwind of mesopotamian dust source areas and archeological sites our results document a very abrupt increase in eolian dust and mesopotamian aridity accelerator mass spectrometer radiocarbon dated to 4025 125 calendar yr b p which persisted for 300 yr radiogenic nd and sr isotope analyses confirm that the observed increase in mineral dust was derived from mesopotamian source areas geochemical correlation of volcanic ash shards between the archeological site and marine sediment record establishes a direct temporal link between mesopotamian aridification and social collapse implicating a sudden shift to more arid conditions as a key factor contributing to the collapse of the akkadian empire

holocene climate phases from buried soils in tigray northern ethiopia comparison with lake level fluctuations in the main ethiopian rift abstract stratigraphic analysis of alluvial colluvial sequences and 14c dating have been used as proxies for holocene climate changes in the highlands of tigray northern ethiopia the studied records show alternations of buried soils and peaty clayey sediments pointing to wet stabilization phases and organic free colluvium layers resulting from the abrupt occurrence of dry climate episodes the 14c dates mostly unpublished cluster in the 11 090 9915 9465 9135 8450 7330 6720 3635 2710 2345 and 1265 790 cal yr b p time spans evidence of subsequent pedogenesis is lacking in the area apart from a buried humified horizon dated at 300 60 14c yr b p 460 295 cal yr b p both the timing and the pattern of tigray paleoclimatic events fit the corresponding framework based on lake level changes previously implemented for the main rift valley these findings give further support for arguing that the forcing mechanisms of the wet dry fluctuations during the holocene were effective over a large scale

sahara and sahel vulnerability to climate changes lessons from holocene hydrological data abstract the examination of more than 1500 paleohydrological dated records collected between 10 and 28 n during the last 50 years have been used to improve our knowledge and understanding of the sahara and sahel vulnerability to the atlantic monsoon changes in the long term we have analyzed the distribution of water bodies mainly lakes and wetlands over time and space the central saharan massifs played a major role in favoring water supply to the lowlands throughout the whole african humid period in addition distinct east west dynamics is recorded with humidity starting and stopping several millennia earlier to the east than to the west of the sahara a series of time lags are discussed 1 between the maximum of deep fresh water lake formation during the early holocene and the maximum of water body extensions during the mid holocene which highlight the primary role of aquifer water level in lake response to climate change 2 between the hydrological history of the sahara and the sahel and the forcings mainly insolation changes during the early and mid holocene which involves complex interactions between remnant ice sheet in the northern hemisphere open water bodies in the sahara and sahel and the atlantic monsoon system