a multi model analysis of the role of the ocean on the african and indian monsoon during the mid holocene we investigate the role of the ocean feedback on the climate in response to insolation forcing during the mid holocene 6 000 year bp using results from seven coupled ocean atmosphere general circulation models we examine how the dipole in late summer sea surface temperature sst anomalies in the tropical atlantic increases the length of the african monsoon how this dipole structure is created and maintained and how the late summer sst warming in the northwest indian ocean affects the monsoon retreat in this sector similar mechanisms are found in all of the models including a strong wind evaporation feedback and changes in the mixed layer depth that enhance the insolation forcing as well as increased ekman transport in the atlantic that sharpens the atlantic dipole pattern we also consider changes in interannual variability over west africa and the indian ocean the teleconnection between variations in sst and sahelian precipitation favor a larger impact of the atlantic dipole mode in this region in the indian ocean the strengthening of the indian dipole structure in autumn has a damping effect on the indian dipole mode at the interannual time scale

geochemical composition of tanzanian shelf sediments indicates holocene climatic and sea level changes we present a high resolution geochemical and grain size record from a holocene sediment core off the pangani river mouth tanzania elemental ratios between biogenic elements and al i e ca al mg al and sr al are mainly influenced by terrigenous dilution on carbonate concentration and or limitation of carbonate production as a result of variations in the supply of fine grained terrigenous sediments of the pangani river such elemental ratios increased significantly at the end of the mid holocene between 5 and 3 5 ka demonstrating a gradual transition from the humid early and mid holocene to the arid late holocene in east africa among the elemental ratios between terrigenous elements and al si al and k al ratios correlate to grain size variation indicating a change in sedimentation regime fe al and ti al ratios show that the sediment source area has shifted from the terrestrial volcanic region of tanzania fe ti rich to the coastal and inner shelf regions fe ti poor around 7 5 ka in response to arid climate and high sea level our geochemical results correspond with a sea surface temperature record derived from the same sediment core indicating that the end of the east african humid period could have been gradual and related to the cooling water in the western indian ocean

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

solar forcing of nile discharge and sapropel s1 formation in the early to middle holocene eastern mediterranean we present high resolution records for oxygen isotopes of the planktic foraminifer globigerinoides ruber 18oruber and bulk sediment inorganic geochemistry for holocene age sediments from the southeast mediterranean our 18oruber record appears to be dominated by nile discharge rather than basin scale salinity temperature changes nile discharge was enhanced in the early to middle holocene relative to today the timing of the long term maximum in nile discharge during the early holocene corresponds to the timing of maximum intensity of the indian ocean influenced southwest indian summer monsoon sim this coincidence suggests a major influence of an indian ocean moisture source on nile discharge in the early to middle holocene while presently the atlantic ocean is the main moisture source nile discharge was highly variable on multicentennial time scale during the early to middle holocene being strongly influenced by variable solar activity this solar driven variability is also recorded in contemporaneous sim records however not observed in an atlantic ocean derived west african summer monsoon record from the holocene this supports the hypothesis that the indian ocean moisture source predominantly controlled nile discharge at that time solar driven variability in nile discharge also influenced paleoenvironmental conditions in the eastern mediterranean bulk sediment ba al and v al used as indicators for export productivity and redox conditions respectively varied both in response to solar forcing on multicentennial time scales we suggest that changes in nile discharge on these time scales have been concordant with nutrient inputs to and shallow ventilation of the eastern mediterranean

effects of aridity and vegetation on plant wax δd in modern lake sediments we analyzed the deuterium composition of individual plant waxes in lake sediments from 28 watersheds that span a range of precipitation d h vegetation types and climates the apparent isotopic fractionation ea between plant wax n alkanes and precipitation differs with watershed ecosystem type and structure and decreases with increasing regional aridity as measured by enrichment of 2 h and 18 o associated with evaporation of lake waters the most negative ea values represent signatures least affected by aridity these values were 125 5 for tropical evergreen and dry forests 130 for a temperate broadleaf

environmental changes and the rise and fall of civilizations in the northern horn of africa an approach combining δd analyses of land plant derived fatty acids with multiple proxies in soil the domains of the ancient polities d mt and aksum in the horn of africa s highlands are a superior natural system for evaluating roles of environmental change on the rise and fall of civilizations to compare environmental changes of the times of the two polities we analyzed stable hydrogen isotopic ratios d of land plant derived fatty acids n c26 30 and other proxies from soil sequences spanning the holocene from the region three results suggest that trends in d values unambiguously reflect changes in rainfall first increases in d coincide with dry periods inferred from studies of eastern african lakes second changes in d values were parallel among sections during overlapping time intervals third consideration of vegetation history did not alter directions of trends in d values over time by unambiguously recording precipitation the d values also enhanced interpretations of proxies that are affected by both climate and land clearing both d mt ca 2750 2350 cal y bp and the aksumite ca 2100 1250 cal y bp rose during wetter intervals of the drier part of the holocene after ca 6000 cal y bp analyses of charred matter indicated that fire had been a common agent of land clearing in all sites the influence of climate on fire varied however prior to the emergence of d mt d values were correlated with c4 c3 plant ratios estimated from 13c values there are no c4 trees and precipitation may have been the main influence on canopy openness after ca 4300 cal y bp there was no significant relationship between d and c4 c3 plant ratios suggesting that factors such as fire influenced canopy openness regardless of climate furthermore the impact of land clearance differed between sites and between d mt and the aksumite s times in one site the interval from 3550 cal y bp to the decline of d mt had several anomalies that suggested dramatic increases in thermal severity of fire and human impact among these were a large contribution of charred matter to a high total organic carbon that low hydrogen and oxygen indices suggest was severely altered by other factors than humification these results support hypotheses about the rise of civilizations being favored by specific climatic conditions but suggest that patterns of land clearing differed during the declines of d mt and the aksumite

a 25 000 year record of climate induced changes in lowland vegetation of eastern equatorial africa revealed by the stable carbon isotopic composition of fossil plant leaf waxes the debate of climate versus co2 in controlling the long term dynamics of tropical african vegetation has focused on events at the upper tree line since the relevant paleodata tend to be from mid elevation sites 2000 3000 m less well known is the relative importance of co2 in regulating the dynamics of tropical lowland 1500 m vegetation particularly that of the dry open woodlands bush and grasslands covering much of eastern equatorial africa here we examine the stable carbon isotopic composition of n alkanes in the sediment record of lake challa a lowland crater lake near mt kilimanjaro covering the last 25 000 years the distributions of the n alkanes with dominance of the long chain odd carbon numbered components and their isotopic composition reveal a mixed origin the c23 and c25 n alkanes are depleted in 13c with 13c values between 30 and 50 in shallow lakes these n alkanes are thought to derive from non emergent aquatic plants but this is unlikely in this steep sided crater lake as it lacks a significant littoral habitat the c27 n alkanes are predominantly derived from leaf wax lipids of terrestrial plants brought into the lake predominantly by local soil run off their 13c values in particular that of the n c31 alkane reveal a marked transition in local lowland vegetation from being dominated 70 100 by c4 plants during the glacial period until 16 5 cal kyr bp to a more mixed c3 c4 composition 30 60 c4 during the holocene the start of the late glacial trend towards a greater proportion of c3 plants coincided with the start of increasing monsoon rainfall 1500 years after the onset of the rise in atmospheric co2 and 3500 years after the onset of post glacial warming the transition was interrupted during the dry younger dryas period 13 0 11 7 cal kyr bp when c4 plants again became much more prevalent almost reaching their glacial period abundance notably the principal trend in leaf wax 13c values infers c4 dominance during both wet and dry phases of the glacial period and a mixed c3 c4 vegetation during both wet and dry phases of the holocene our results indicate that long term variation in pco2 exerted important control on the composition of drought adapted savanna vegetation in this currently semi arid lowland region of equatorial east africa

quantitative reconstructions of annual rainfall in africa 6000 years ago model data comparison precipitation are based on 85 pollen sites 14 c dated at 6000 500 years b p and distributed over the whole of africa to improve the reliability of the pollen based climate reconstruction two methods are used the modern analogues technique mat and the plant functional types pft methods we then conduct a model data comparison for five distinct regions allowing an evaluation of model outputs the sahara sahel the eastern sahara western equatorial africa east africa and madagascar the pollen inferred reconstructions are compared with 21 mid holocene simulations yielded by atmospheric general circulation models agcms and coupled ocean atmosphere vegetation models oavgcms the large scale feature of the hydrological changes is shown to be well captured by most of the models data show that during the mid holocene the sahara was considerably wetter than today 200 to 700 mm yr the results reinforce the conclusion that the agcms significantly underestimate this precipitation increase in the sahara whereas the oavgcm simulations are in accordance with the data our results show that vegetation and ocean feedbacks do not have a strong impact in the intertropical zone and that models fail to properly reproduce the climatic conditions in east africa and madagascar the model data comparison also suggests that the lengthening of the dry season during boreal winter in the west equatorial region is a robust feature although the pollen based reconstruction shows no change or only slight drying there

on the origin and timing of rapid changes in atmospheric methane during the last glacial period we present high resolution records of atmospheric methane from the gisp2 greenland ice sheet project 2 ice core for four rapid climate transitions that occurred during the past 50 ka the end of the younger dryas at 11 8 ka the beginning of the bolling allerod period at 14 8 ka the beginning of interstadial 8 at 38 2 ka and the beginning of interstadial 12 at 45 5 ka during these events atmospheric methane concentrations increased by 200 300 ppb over time periods of 100 300 years significantly more slowly than associated temperature and snow accumulation changes recorded in the ice core record we suggest that the slower rise in methane concentration may reflect the timescale of terrestrial ecosystem response to rapid climate change we find no evidence for rapid massive methane emissions that might be associated with large scale decomposition of methane hydrates in sediments with additional results from the taylor dome ice core antarctica we also reconstruct changes in the interpolar methane gradient an indicator of the geographical distribution of methane sources associated with some of the rapid changes in atmospheric methane the results indicate that the rise in methane at the beginning of the bolling allerod period and the later rise at the end of the younger dryas were driven by increases in both tropical and boreal methane sources during the younger dryas a 1 3 ka cold period during the last deglaciation the relative contribution from boreal sources was reduced relative to the early and middle holocene periods

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

the african humid period and the record of marine upwelling from excess 230th in ocean drilling program hole 658c using a high resolution 230 th normalized record of sediment flux we document the deglacial and holocene history of north african aridity and coastal upwelling at ocean drilling program hole 658c at both the end of the younger dryas and after the 8 2 ka event there are significant drops in terrigenous accumulation at our site indicating an increase in the monsoon moisture flux over africa at this time at 5 5 ka there is an abrupt end to the african humid period and a return to stronger upwelling conditions for carbonate and opal fluxes the 230 th normalization completely changes the shape of each record based on percentage variations alone this site is a clear example of how variations in one sediment component can obscure changes in the others and it demonstrates the need for radionuclide measurements more generally in paleoceanography by taking our new records and a large amount of previous data from this site we conclude that increases in african moisture are tightly coupled to decreases in coastal upwelling intensity

importance of the indian ocean for simulating rainfall anomalies over eastern and southern africa the relative contributions of the indian ocean and pacific ocean sea surface temperatures ssts to the rainfall variability over eastern central and southern africa during the austral spring summer are examined the variability of african rainfall is statistically related to both oceans but the variability in the two oceans is also related to separate the effects of the indian and pacific oceans a suite of numerical model simulations is presented goga the atmosphere is forced by observed ssts globally ioga the atmosphere is forced by observed ssts only in the indian ocean basin and poga the atmosphere is forced by observed ssts only in the tropical pacific basin while the sst variability of the tropical pacific exerts some influence over the african region it is the atmospheric response to the indian ocean variability that is essential for simulating the correct rainfall response over eastern central and southern africa analyses of the dynamical response s seen in the numerical experiments and in the observations indicate that the pacific and indian oceans have a competing influence over the indian ocean african region this competition is related to the influence of the two oceans on the walker circulation and the consequences of that variability on low level fluxes of moisture over central and southern africa finally given the high correlation found between sst variability in the indian and pacific oceans with the pacific leading by 3 months we speculate on an approach to long lead dynamical climate prediction over central east and southern africa

how far north did the african monsoon fringe expand during the african humid period insights from southwest moroccan speleothems we present new high resolution oxygen isotope delta o 18 records from three nw african speleothems located at 31 degrees n the present day rainfall patterns at 31 degrees n in nw africa are linked to negative winter north atlantic oscillation phases however on multimillennial time scales our delta o 18 records together with other hydroclimate records provide new evidence of humid conditions during the mid holocene a period that was presumably characterized by arid climate thus the apparent increase in moisture during the mid holocene is interpreted better as an increase in summer rainfall this is most likely linked to the expansion of the west african summer monsoon fringe during the african humid period which terminated in our record abruptly around 4 kyr bp the temporospatial difference with speleothem records from n morocco suggests that the high atlas mountains might have been a topographic barrier to further expansion of the west african summer monsoon fringe into higher latitudes plain language summary the holocene african humid period in north africa characterized by the expansion of vegetation into the green sahara has been linked to the intensification of the west african summer monsoon wasm however the temporospatial pattern of the african humid period especially the northernmost expansion of the wasm remain a matter of controversy largely owing to the lack of precisely dated and high resolution paleoclimatic records this study presents new highresolution paleoclimate data based on speleothem oxygen isotope records from a key site at 31 degrees n in nw africa our data suggest that the wasm expanded to 31 degrees n in nw africa during the mid holocene and terminated abruptly at 4 kyr bp

phytoliths pollen and microcharcoal from the baringo basin kenya reveal savanna dynamics during the plio pleistocene transition abstract as part of the hominin sites and paleolakes drilling project hspdp phytoliths pollen and microcharcoal were examined from the 228 m 3 29 to 2 56 ma baringo tugen hills barsemoi drill core btb13 a total of 652 samples were collected at 10 to 32 cm intervals corresponding to sub millennial to millennial scale temporal resolution microcharcoal was well preserved throughout the core and often peaked in abundance 5 kyr before and after insolation peaks phytolith preservation varied between excellent to total dissolution in alternating intervals throughout the core pollen was rarely preserved these combined datasets indicate that prior to 3 1 ma woody cover fluctuated between open savanna 80 cover at typically precessional 19 23 kyr periodicities during the mid piacenzian warm period mpwp 3 26 3 01 ma intervals with exceptionally high microcharcoal abundance suggest that regional turnover from wooded to open habitats was driven in part by fire after 3 1 ma low elevation woody cover likely never exceeded 40 with oscillations between mesic tall grass vs xeric short grass savanna at precessional periodicities mesic c4 tall grass panicoideae peaked in abundance during insolation maxima whereas xeric c4 short grass chloridoideae peaked during insolation minima the onset of northern hemisphere glaciation nhg at 2 75 ma coincided with the appearance of deep lake phases and increases in grass density and fire frequency spectral analysis and intervals with well preserved phytoliths indicate that precession and interhemispheric insolation gradients influenced vegetation via their effects on equatorial precipitation and fire this study fills a crucial gap in pliocene vegetation reconstructions from the east african rift valley and its associated hominin localities it also provides orbitally resolved regional vegetation data useful in paleodata model comparisons for the onset of the mpwp which is often used as an analog for future warming and nhg

late pleistocene precipitation δ18o interpolated across the global landmass global water cycles ecosystem assemblages and weathering rates were impacted by the 4 c of global warming that took place over the course of the last glacial termination fossil groundwaters can be useful indicators of late pleistocene precipitation isotope compositions which in turn can help to test hypotheses about the drivers and impacts of glacial interglacial climate changes here a global catalog of 126 fossil groundwater records is used to interpolate late pleistocene precipitation 18o across the global landmass the interpolated data show that extratropical late pleistocene terrestrial precipitation was near uniformly depleted in 18o relative to the late holocene by contrast tropical 18o responses to deglacial warming diverged late pleistocene 18o was higher than modern across india and south china but lower than modern throughout much of northern and southern africa groundwaters that recharged beneath large northern hemisphere ice sheets have different holocene pleistocene 18o relationships than paleowaters that recharged subaerially potentially aiding reconstructions of englacial transport in paleo ice sheets global terrestrial late pleistocene precipitation 18o maps may help to determine 3 d groundwater age distributions constrain pleistocene mammal movements and better understand glacial climate dynamics

lowland forest collapse and early human impacts at the end of the african humid period at lake edward equatorial east africa in africa the early holocene was characterized by wetter warmer conditions than today followed by rapid aridification at 5 2 ka however a lack of lowland vegetation records has prevented a detailed evaluation of forest response to holocene climate change additionally although modern vegetation communities are linked to human disturbance few studies have addressed how prehistoric human activities helped engineer the character of modern african ecosystems understanding the architecture of lowland and highland forests is important to prevent further degradation from climate land use change we present an 11 000 yr fossil pollen record from lake edward uganda we show that guineo congolian forests dominated the highlands and lowlands in equatorial east africa in the early holocene highlighting the importance of rainfall and temperature in controlling forest communities these forests remained until 5 2 ka when the climate became drier the lacustrine ecosystem response to aridification was abrupt however forest decreased gradually replaced by deciduous woodlands woodlands dominated until after an arid period at 2 ka however forest did not recover increased disturbance indicators and grasses suggest that the arrival of iron age people resulted in the modern fire tolerant vegetation although late holocene climate played a role in vegetation opening the modern ecosystem architecture in east africa is linked to early human activities

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

paleohydrology of lake turkana and its influence on the nile river system abstract the detailed paleohydrological record of lake turkana the largest lake in the eastern branch of the east african rift is necessary for determining the connectivity between adjacent watersheds in tropical africa the migration of both the intertropical convergence zone itcz and the congo air boundary cab constrains rainfall amount and duration for most of east africa lake turkana in northern kenya is the world s largest desert lake and experiences two itcz associated rainy seasons annually with cumulative rainfall of 200 mm yr evidence from new continuous high fidelity sediment core records and high resolution chirp seismic reflection data suggests that lake turkana received enough rainfall during the african humid period ahp to fill the lake to its sill 100 m above current lake level and spilled over into the white nile river system an atmospheric configuration with an eastward shifted cab over the turkana region and the northern kenya rift is invoked as an additional source of rainfall for the catchment this configuration began abruptly at 11 ka and lasted until 5 ka when lake turkana became a closed basin and was cut off from the white nile prior to the ahp lake turkana experienced at least two desiccation events following the last glacial maximum at 18 5 and 17 ka when the lake was at least 100 m lower than modern as evidenced by basin wide high amplitude reflections and 14 c dated shallow water facies in sediment cores lake level fluctuations generally follow trends in mean solar insolation however the onset of lake level extremes are much more abrupt than rates of insolation change lake turkana s location in relation to atmospheric convergence and dynamic rainfall patterns makes its susceptible to extreme climate change over relatively short timescales

isotopic composition of waters from ethiopia and kenya insights into moisture sources for eastern africa 1 oxygen and deuterium isotopic values of meteoric waters from ethiopia are unusually high when compared to waters from other high elevation settings in africa and worldwide these high values are well documented however the climatic processes responsible for the isotopic anomalies in ethiopian waters have not been thoroughly investigated we use isotopic data from waters and remote data products to demonstrate how different moisture sources affect the distribution of stable isotopes in waters from eastern africa oxygen and deuterium stable isotopic data from 349 surface and near surface groundwaters indicate isotopic distinctions between waters in ethiopia and kenya and confirm the anomalous nature of ethiopian waters remote data products from the tropical rainfall measuring mission trmm and national centers for environmental prediction ncep reanalysis project show strong westerly and southwesterly components to low level winds during precipitation events in western and central ethiopia this is in contrast to the easterly and southeasterly winds that bring rainfall to kenya and southeastern ethiopia large regions of high equivalent potential temperatures e at low levels over the sudd and the congo basin demonstrate the potential for these areas as sources of moisture and convective instability the combination of wind direction data from ethiopia and e distribution in africa indicates that transpired moisture from the sudd and the congo basin is likely responsible for the high isotopic values of rainfall in ethiopia

hydrogen isotope ratios of recent lacustrine sedimentary n alkanes record modern climate variability abstract hydrogen isotope ratios were measured on n alkanes n c12 to n c31 extracted from recent lake surface sediments along a n s european transect to test if modern climate variability is recorded in these biomarkers d values of the n alkanes are compared to d values of meteoric water from the iaea gnip database spanning a range from 119 in northern sweden to 41 in southern italy to lake water d values and to mean annual temperatures varying between 2 0 c in the north and 13 7 c in the south d values of the short chained n alkanes n c12 to n c20 excluding algal derived n c17 and n c19 are higher in the north and lower in the south the isotopic fractionation e for hydrogen between meteoric water and the short chained n alkanes is increasing from n to s by more than 100 and is significantly correlated to mean annual temperature for n c16 and n c18 this suggests that these n alkanes may originate from a different source in the northern lakes possibly due to petroleum contamination or are synthesized using a different biochemical pathway the n c17 and n c19 alkanes of algal origin the n c21 and n c23 alkanes originating from water plants and the long chain n alkanes n c25 n c27 n c29 and n c31 of terrestrial origin clearly correlate with d values of meteoric water lake water and mean annual temperature indicating that they excellently record the d value of meteoric water the mean hydrogen isotope fractionation ec17 w of 157 sd 13 between n c17 and meteoric water is fairly constant over the wide range of different climates and lake environments suggesting only minor influence of environmental factors on this biochemical fractionation this suggests that d values of n c17 are suitable to reconstruct the isotopic composition of source water the mean fractionation between the long chain n alkanes and water is 128 sd 12 the mean difference of 31 between both e values is likely due to evaporative enrichment of deuterium in the leaf water if this is the only influence on the enrichment the difference between the d values of terrestrial and aquatic compounds might be suitable to reconstruct terrestrial evapotranspiration of the lake environment

hydroclimate variability in the nile river basin during the past 28 000 years it has long been known that extreme changes in north african hydroclimate occurred during the late pleistocene yet many discrepancies exist between sites regarding the timing duration and abruptness of events such as heinrich stadial hs 1 and the african humid period ahp the hydroclimate history of the nile river is of particular interest due to its lengthy human occupation history yet there are presently few continuous archives from the nile river corridor and pre holocene studies are rare here we present new organic and inorganic geochemical records of nile basin hydroclimate from an eastern mediterranean em sea sediment core spanning the past 28 ka bp our multi proxy records reflect the fluctuating inputs of blue nile versus white nile material to the em sea in response to gradual changes in local insolation and also capture abrupt hydroclimate events driven by remote climate forcings such as hs1 we find strong evidence for extreme aridity within the nile basin evolving in two distinct phases during hs1 from 17 5 to 16 ka bp and from 16 to 14 5 ka bp whereas peak wet conditions during the ahp are observed from 9 to 7 ka bp we find that zonal movements of the congo air boundary cab and associated shifts in the dominant moisture source atlantic versus indian ocean moisture to the nile basin likely contributed to abrupt hydroclimate variability in northern east africa during hs1 and the ahp as well as to non linear behavior of hydroclimate proxies we note that different proxies show variable gradual and abrupt responses to individual hydroclimate events and thus might have different inherent sensitivities which may be a factor contributing to the controversy surrounding the abruptness of past events such as the ahp during the late pleistocene the nile basin experienced extreme hydroclimate fluctuations which presumably impacted paleolithic cultures residing along the nile corridor

african hydroclimatic variability during the last 2000 years abstract the african continent is characterised by a wide range of hydroclimate regimes ranging from humid equatorial west africa to the arid deserts in the northern and southern subtropics the livelihoods of much of its population are also vulnerable to future climate change mainly through variability in rainfall affecting water resource availability a growing number of data sources indicate that such hydroclimatic variability is an intrinsic component of africa s natural environment this paper co authored by members of the pages africa 2k working group presents an extensive assessment and discussion of proxy historical and instrumental evidence for hydroclimatic variability across the african continent spanning the last two millennia while the african palaeoenvironmental record is characterised by spatially disjunctive datasets with often less than optimal temporal resolution and chronological control the available evidence allows the assessment of prominent spatial patterns of palaeomoisture variability through time in this study we focus sequentially on data for six major time windows the first millennium ce the medieval climate anomaly 900 1250 ce the little ice age 1250 1750 ce the end of the lia 1750 1850 ce the early modern period 1850 1950 and the period of recent warming 1950 onwards this results in a continent wide synthesis of regional moisture balance trends through history allowing consideration of possible driving mechanisms and suggestions for future research

a 10 000 year high resolution diatom record from pilkington bay lake victoria east africa a new diatom record from lake victoria s pilkington bay subsampled at 21 to 25 year intervals and supported by 20 ams dates reveals a 10 000 calendar year environmental history that is supported by published diatom and pollen data from two nearby sites with their chronologies adjusted here to account for newly documented ancient carbon effects in the lake these three records provide a coherent finely resolved reconstruction of holocene climate change in equatorial east africa after an insolation induced rainfall maximum ca 8800 8300 cal yr b p precipitation became more seasonal and decreased abruptly ca 8200 and 5700 yr b p in apparent association with northern deglaciation events century scale rainfall increases occurred ca 8500 7000 5800 and 4000 yr b p conditions after 2700 yr b p were generally similar to those of today but major droughts occurred ca 1200 600 yr b p during europe s medieval warm period

sedimentation in lake malawi east africa during the past 10 000 years a continuous paleoclimatic record from the southern tropics a suite of piston cores recovered from lake malawi 9 14 s 34 35 e east africa in 1986 has been analyzed for major and minor elements organic c and n calcium carbonate and diatoms an internally consistent stratigraphy was constructed from calcium carbonate abundance and variations in the two most abundant diatom genera stephanodiscus and melosira with age control obtained primarily from 14c dating of the carbonate differences with time in fe abundance in a transect of cores from different water depths have been interpreted to reflect changes in chemocline depth the depth to the chemocline was on the order of 100 m shallower than present prior to 3500 yr b p indicating less seasonality carbonate production and preservation appears to be related to climatically induced changes in both salinity and chemical distributions in the water column the carbonate which precipitates from surface waters is most abundant during the interval from about 10 000 to 6000 yr b p this micrite most likely represents periods of low lake level when salinity increased and carbonate precipitation was enhanced sedimentary evidence suggests that lake levels were 100 150 m lower than present during this period this record is different from climatic trends in northern intertropical africa but appears to also be related to changes in insolation and monsoon circulation this is the northernmost basin in africa reported to exhibit a southern hemisphere response to the early holocene northern hemisphere summer insolation maximum the climatic hingeline north of lake malawi 9 s implied by our results is significantly south of that indicated by general circulation model simulations however the cores show evidence for periods of abrupt climate change during the interval of generally arid climate

holocene palynology and palaeoenvironments in the savanna biome at tswaing crater central south africa abstract a radiocarbon dated pollen and microscopic charred particle record from the holocene section of the tswaing crater in the savanna biome of south africa give new evidence for environmental changes during the period c 9400 1800 cal years bp pollen grains are scarce or absent in layers dating from before 9400 cal years bp but deposits rich in pollen occur in overlying layers the section dated between 9400 cal years bp and c 7200 cal years bp contains grass asteraceae and dry savanna pollen types that suggest fluctuating but generally dry moisture conditions later between c 7200 and 1800 cal years bp broad leaved savanna woodland elements and local swamp pollen indicate relatively stable vegetation and wetter mildly fluctuating climatic conditions which is consistent with previously published biomarker analysis between c 6200 and 5500 cal years bp the numbers of charred particles increased slightly this indicates burning activity which can probably be attributed to dry season ignition of denser fuel under relatively moist conditions a decrease of local swamp pollen between c 3600 and 3500 cal years bp suggests that conditions became briefly drier again as pollen of woody elements declined in favour of open grassland pollen a comparison between the tswaing pollen profile and various other sequences within the central interior of south africa suggests generally similar conditions over the central interior of the sub continent during the holocene deviating from sequences further afield along the coastal areas of southern africa between 9400 cal years bp and ca 7200 cal years bp the western and southern coasts were probably controlled by different atmospheric and oceanic circulation regimes under the influence of a strong westerly winter rain system while the north eastern area where tswaing is situated experienced weaker precession and less summer rain from the intertropical convergence zone

pollen derived rainfall and temperature estimates from lake tanganyika and their implication for late pleistocene water levels abstract palaeoclimatic estimates of mean annual temperature and rainfall in the southern tanganyika basin between 25 000 and 9000 yr b p have been established from two pollen sequences based on the best analogue method the results give evidence of a mean temperature decrease of about 4 2 c during the last glaciation a value consistent with that previously obtained in the catchment area on the burundi highlands this cooling was synchronous with a decrease of mean annual precipitation of about 180 mm yr postglacial climatic conditions were established by 12 700 yr b p with warming and wetness continuing to increase from this date onward these new palaeoclimatic data will be useful for hydrological reconstructions of lake tanganyika particularly during the last glacial age for which the magnitude of water level fall has been a controversial issue our rainfall estimates are more consistent with low values 250 to 300 m fall than with high ones 600 m previously proposed

global monsoons in the mid holocene and oceanic feedback the response of the six major summer monsoon systems the north american monsoon the northern africa monsoon the asia monsoon the northern australasian monsoon the south america monsoon and the southern africa monsoon to mid holocene orbital forcing has been investigated using a coupled ocean atmosphere general circulation model foam with the focus on the distinct roles of the direct insolation forcing and oceanic feedback the simulation result is also found to compare well with the ncar csm the direct effects of the change in insolation produce an enhancement of the northern hemisphere monsoons and a reduction of the southern hemisphere monsoons ocean feedbacks produce a further enhancement of the northern africa monsoon and the north american monsoon however ocean feedbacks appear to weaken the asia monsoon although the overall effect direct insolation forcing plus ocean feedback remains a strengthened monsoon the impact of ocean feedbacks on the south american and southern african monsoons is relatively small and therefore these regions especially the south america experienced a reduced monsoon regime compared to present however there is a strong ocean feedback on the northern australian monsoon that negates the direct effects of orbital changes and results in a strengthening of austral summer monsoon precipitation in this region a new synthesis is made for mid holocene paleoenvironmental records and is compared with the model simulations overall model simulations produce changes in regional climates that are generally consistent with paleoenvironmental observations

half precessional dynamics of monsoon rainfall near the east african equator extensive records exist with which to assess the relationship between external climate forcings such as changes in insolation and climate variability for middle and high latitudes but records from equatorial regions are relatively few especially from regions experiencing the passage of the intertropical convergence zone a continuous and well resolved climate proxy record of hydrological variability during the past 25 000 years from equatorial east africa is now presented and analysed

an 11 000 year lithostratigraphic and paleohydrologic record from equatorial africa lake edward uganda congo abstract sedimentologic and geochemical analyses of four cores from lake edward uganda congo document a complex record of moisture balance in the edward basin from 11 000 cal yr bp to present highly organic diatomaceous muds provide evidence for shifts in wind intensity and stratification within an early holocene wet phase lake level variations within this period may have shifted due to tectonic lowering of the lake s outlet level the onset of mid holocene aridity as documented by the appearance of authigenic calcite at 5200 cal yr bp initiated a period of falling lake levels that culminated in a late holocene lowstand between 4000 and 2000 cal yr bp this lowstand is documented by coarse sediments whose fabric and mineralogy depend on the core site although different cores yield different ages for this regression it appears that declining lake levels culminated in a maximum lake lowstand of 14 m lake levels then rose rapidly attaining modern positions by 1700 cal yr bp this lake level history suggests that although many paleoclimatic changes in africa are apparently synchronous throughout northern hemispheric africa other events may be spatially heterogeneous these patterns highlight the need for a well dated network of paleoclimate sites within the african continent

environmental changes at lake cheshi zambia since 40 000 years b p abstract sediment and microfossil analyses of a 7 5 m core from lake cheshi suggest that south central africa experienced late quaternary climate changes similar to those in east africa the lake formed around 34 000 yr b p after a prelacustrine phase of at least 6000 yr from climatic or tectonic causes ratios of precipitation to evaporation were probably similar to those of today until a decline about 15 000 13 000 yr b p when the lake shrank and became chemically concentrated maximal lake levels occurred between 8000 and 4000 yr b p and were followed by a low stand under presumably arid conditions about 3500 yr b p encroachment of sudd vegetation contributed to shallowing during the last 3000 yr a phase of microfossil dilution may reflect human activity in the basin or climatic or hydrological changes melosira valve morphology seems to reflect mixing regimes sponge and testate amoeba remains were most numerous relative to diatoms during low water phases

can sedimentary leaf waxes record d h ratios of continental precipitation field model and experimental assessments abstract d h ratios of leaf waxes d wax derived from terrestrial plants and preserved in lake sediments can provide important information on past continental hydrology ideally d wax can be used to reconstruct precipitation d h ratios d p which is a well established paleoclimate proxy however many other factors such as vegetation and relative humidity rh also affect d wax variation how the combination of these factors affects sedimentary d wax is unclear here we use a transect of 32 lake surface sediments across large gradients of precipitation relative humidity and vegetation composition in the southwestern united states to study the natural factors affecting sedimentary d wax d values of c 28 n alkanoic acids show significant correlation with d p values r 2 0 76 with an apparent isotopic enrichment of 99 8 indicating that sedimentary d wax values track overall d p variation along the entire transect leaf waxes produced by plants grown under controlled conditions rh 80 60 40 show a small increase in d h ratios as rh decreases consistent with prediction from the craig gordon model however the isotopic effect of rh on d wax along the natural transect is partially countered by the opposing influence of vegetation changes the correlation between d wax and d p values is significantly higher r 2 0 84 in the drier portions of the transect than in the wetter regions r 2 0 64 this study suggests that d h ratios of sedimentary leaf waxes can be used as a proxy for precipitation d variations with particularly high fidelity in dry regions although more studies in other regions will be important to further test this proxy

age and duration of weathering by 40k 40ar and 40ar 39ar analysis of potassium manganese oxides supergene cryptomelane k1 2 mn3 mn4 8o16 h2o samples from deeply weathered pegmatites in southeastern brazil subjected to 40k 40ar and 40ar 39ar analysis yielded 40k 40ar dates ranging from 10 1 0 5 to 5 6 0 2 ma million years ago laser probe 40ar 39ar step heating of the two most disparate samples yielded plateau dates of 9 94 0 05 and 5 59 0 10 ma corresponding within 2 to the 40k 40ar dates the results imply that deep weathering profiles along the eastern brazilian margin do not reflect present climatic conditions but are the result of a long term process that was already advanced by the late miocene weathering ages predate pulses of continental sedimentation along the eastern brazilian margin and suggest that there was a time lag between weathering and erosion processes and sedimentation processes

reconstructing east african monsoon variability from grain size distributions end member modeling and source attribution of diatom rich sediments from lake chala abstract grain size analysis and end member modeling of the clastic fraction of the 25 kyr sediment sequence from lake chala a meromictic crater lake on the lower east slope of mt kilimanjaro reveal crucial aspects of climate driven environmental change in equatorial east africa since the last glacial maximum the finely laminated sediments of lake chala contain only up to 40 of clastic components the rest are mainly diatom frustules and amorphous organic matter measured grain size distributions were split into six statistically meaningful end members of which four could be linked to a distinct source and transport mechanism of clastic mineral input fine aeolian dust from distal sources em1 fine catchment runoff em2 coarser aeolian dust from proximal sources em5 and coarse erosive material from the upper crater slopes em6 the two other end members em3 and em4 represented frustules of the two most common diatom taxa in lake chala afrocymbella barkeri and nitzschia fabiennejansseniana which had partly survived sample pre treatment temporal variation in normalized abundance of the two dust derived end members are valuable proxies for past changes in monsoon circulation over equatorial east africa during northern hemisphere cold periods e g last glacial maximum and younger dryas the inter tropical convergence zone shifted southward enhancing northeasterly monsoon winds in the lake chala area and increasing advection of fine dust from the dry horn of africa region simultaneously more modest continental heating reduced the prevalence of small scale atmospheric turbulence and thus the occurrence of dust devils resulting in reduced influx of coarse dust from drylands nearby conversely abrupt intensification of the southeasterly monsoon at the onset of the holocene is recorded by an abrupt increase in the amount of coarse dust delivered to lake chala temporal variation in the end members representing catchment run off em2 and erosion em6 mainly reflect changes in lacustrine sedimentation dynamics associated with major lake level fluctuation as evidenced by other paleoenvironmental proxies overall this study shows that subdivision of the clastic fraction of lacustrine sediments into statistically robust grain size end members can provide multiple independent and quantitative proxies which help constrain reconstructions of a region s multi faceted climate history

high resolution modelling closes the gap between data and model simulations for mid holocene and present day biomes of east africa abstract east africa hosts a striking diversity of terrestrial ecosystems which vary both in space and time due to complex regional topography and a dynamic climate the structure and functioning of these ecosystems under this environmental setting can be studied with dynamic vegetation models dvms in a spatially explicit way yet regional applications of dvms to east africa are rare and a comprehensive validation of such applications is missing here we simulated the present day and mid holocene vegetation of east africa with the dvm lpj guess and we conducted an exhaustive comparison of model outputs with maps of potential modern vegetation distribution and with pollen records of local change through time overall the model was able to reproduce the observed spatial patterns of east african vegetation to see whether running the model at higher spatial resolutions 10 10 contribute to resolve the vegetation distribution better and have a better comparison scale with the observational data i e pollen data we run the model with coarser spatial resolution 0 5 0 5 for the present day as well both the area and point wise comparison showed that a higher spatial resolution allows to better describe spatial vegetation changes induced by the complex topography of east africa our analysis of the difference between modelled mid holocene and modern day vegetation showed that whether a biome shifts to another is best explained by both the amount of change in precipitation it experiences and the amount of precipitation it received originally we also confirmed that tropical forest biomes were more sensitive to a decrease in precipitation compared to woodland and savanna biomes and that holocene vegetation changes in east africa were driven not only by changes in annual precipitation but also by changes in its seasonality

forward modeling of δ18o in andean ice cores tropical ice core archives are among the best dated and highest resolution from the tropics but a thorough understanding of processes that shape their isotope signature as well as the simulation of observed variability remain incomplete to address this we develop a tropical andean ice core isotope forward model from in situ hydrologic observations and satellite water vapor isotope measurements a control simulation of snow 18o captures the mean and seasonal trend but underestimates the observed intraseasonal variability the simulation of observed variability is improved by including amount effects associated with south american cold air incursions linking synoptic scale disturbances and monsoon dynamics to tropical ice core 18o the forward model was calibrated with and run under present day conditions but can also be driven with past climate forcings to reconstruct paleo monsoon variability the model is transferable and may be used to render a paleo climatic context at other ice core locations

reconstruction of a pleistocene paleocatena using micromorphology and geochemistry of lake margin paleo vertisols olduvai gorge tanzania abstract olduvai gorge tanzania contains a rich record of pleistocene paleoclimate and paleoenvironment as well as an abundance of paleontological and archaeological data the 2 2 ma of volcaniclastic infill can be divided into time slices using dated tuffs sediments were deposited in a semi arid closed rift basin containing a shallow saline alkaline lake until 1 75 ma four trenches in uppermost bed i sample a 20 ka time slice between ng eju tuff base 1 818 0 006 ma and tuff if top 1 803 0 002 ma they form a 1 km transect of the lake margin flat that contains both stacked and cumulative paleosols that are interpreted as a heterogeneous paleocatena closer to the lake paleosols are thinner vertically stacked and separated by thin tuffs or tufa further from the lake margin there is additional volcaniclastic input and paleosols are thicker and cumulative macroscale and micromorphological features identify these clay rich paleosols as paleo vertisols abundant pedogenic slickensides and a variety of ped shapes were observed in the field as well as micro ped structures and stress cutans in thin section although weakly developed these paleo vertisols also have distinct horizons defined by soil color changes differing ped shapes and bulk geochemistry and are indicative of monsoonal precipitation seasonality paleosols and parent materials were analyzed for bulk geochemistry of major rare and trace elements geochemical proxies reveal changing hydrology not definitively identifiable in the field or micromorphology molecular weathering ratios show increased weathering and mass balance calculations indicate greater translocations positive and negative through time lower soil moisture due to a drier climate likely created better drained conditions allowing for increased pedogenesis the records of femn oxides and zeolites precipitated in rhizoliths are further evidence for changing redox conditions and water chemistry redox sensitive elements such as fe and mn were mobilized during saturated soil conditions and precipitated during drier conditions as the hydrology changed due to increased aridity zeolites precipitated as rhizoliths due to capillary rise and evaporative pumping of saline alkaline water the fluctuating hydrology identified in the paleo vertisols deposited between 1 79 and 1 81 ma is indicative of a precession cycle and consistent with records from olduvai gorge and throughout east africa this 20 ka time slice of the 2 ma drying trend identified in africa coincides with the first hominin migrations out of africa at 1 8 ma and suggests that hominins were living in a water stressed environment with annual precipitation seasonality and large variations in precipitation due to precession

response of the african monsoon to orbital forcing and ocean feedbacks in the middle holocene simulations with a climate model that asynchronously couples the atmosphere and the ocean showed that the increased amplitude of the seasonal cycle of insolation in the northern hemisphere 6000 years ago could have increased tropical atlantic sea surface temperatures in late summer the simulated increase in sea surface temperature and associated changes in atmospheric circulation enhanced the summer monsoon precipitation of northern africa by more than 25 percent compared with the middle holocene simulation with prescribed modern sea surface temperatures and provided better agreement with paleorecords of enhanced monsoons

fishing in a fluctuating landscape terminal pleistocene and early holocene subsistence strategies in the lake turkana basin kenya abstract during the african humid period ahp c 15 5 5 ka the rivers and lakes of much of the continent swelled due to changes in monsoonal rainfall driven by earth s orbital precession this period witnessed the growth of diverse fisher forager communities whose members adapted their settlement patterns and created new technologies in order to take advantage of aquatic resources around lake turkana in northern kenya numerous surface sites have been documented along former shorelines dating to the ahp relatively few have been excavated and dated however and just three all from the eastern basin have published faunal analyses here we present archaeofaunal assemblages from the kalokol region of the western basin where three sites with microlithic technology bone harpoons and radiocarbon dates falling within the ahp were excavated we present a detailed taphonomic assessment of the fish assemblages and a comparison with both natural and anthropogenic and ancient and modern fish bone accumulations taxa identified at the kalokol sites are discussed in terms of the occupants possible fishing technologies and strategies drawing on ethological and ethnographic data our analysis combining our data with those published from the eastern basin enables a broader discussion of how people may have responded to fluctuating ahp environments in the turkana basin

holocene carbon cycle dynamics based on co2 trapped in ice at taylor dome antarctica a high resolution ice core record of atmospheric co2 concentration over the holocene epoch shows that the global carbon cycle has not been in steady state during the past 11 000 years analysis of the co2 concentration and carbon stable isotope records using a one dimensional carbon cycle model uggests that changes in terrestrial biomass and sea surface temperature were largely responsible for the observed millennial scale changes of atmospheric co2 concentrations

northern hemisphere controls on tropical southeast african climate during the past 60 000 years the processes that control climate in the tropics are poorly understood we applied compound specific hydrogen isotopes d and the tex86 tetraether index of 86 carbon atoms temperature proxy to sediment cores from lake tanganyika to independently reconstruct precipitation and temperature variations during the past 60 000 years tanganyika temperatures follow northern hemisphere insolation and indicate that warming in tropical southeast africa during the last glacial termination began to increase 3000 years before atmospheric carbon dioxide concentrations d data show that this region experienced abrupt changes in hydrology coeval with orbital and millennial scale events recorded in northern hemisphere monsoonal climate records this implies that precipitation in tropical southeast africa is more strongly controlled by changes in indian ocean sea surface temperatures and the winter indian monsoon than by migration of the intertropical convergence zone

model proxy and isotopic perspectives on the east african humid period abstract both north and east africa experienced more humid conditions during the early and mid holocene epoch 11 000 5000 yr bp 11 5 ka relative to today the north african humid period has been a major focus of paleoclimatic study and represents a response of the hydrological cycle to the increase in boreal summer insolation and associated ocean atmosphere and land surface feedbacks meanwhile the mechanisms that caused the coeval east african humid period are poorly understood here we use results from isotope enabled coupled climate modeling experiments to investigate the cause of the east african humid period the modeling results are interpreted alongside proxy records of both water balance and the isotopic composition of rainfall our simulations show that the orbitally induced increase in dry season precipitation and the subsequent reduction in precipitation seasonality can explain the east african humid period and this scenario agrees well with regional lake level and pollen paleoclimate data changes in zonal moisture flux from both the atlantic and indian ocean account for the simulated increase in precipitation from june through november isotopic paleoclimate data and simulated changes in moisture source demonstrate that the western east african rift valley in particular experienced more humid conditions due to the influx of atlantic moisture and enhanced convergence along the congo air boundary our study demonstrates that zonal changes in moisture advection are an important determinant of climate variability in the east african region

late quaternary behavior of the east african monsoon and the importance of the congo air boundary both atlantic and indian ocean climate dynamics exert influence over tropical african hydroclimate producing complex patterns of convergence and precipitation to isolate the indian ocean influence on african paleohydrology we analyzed the deuterium hydrogen ratio of higher plant leaf waxes delta d wax in a 25 000 year sediment record from lake challa 3 degrees s 38 degrees e in the easternmost area of the african tropics whereas both the seismic record of inferred lake level fluctuations and the branched and isoprenoidal tetraether bit index proxy record changes in hydrology within the challa basin delta d wax as a proxy for the isotopic composition of precipitation delta d p is interpreted as a tracer of large scale atmospheric circulation that integrates the history of the moisture transported to the lake challa area specifically based on modern day isotope rainfall relationships we argue that lake challa delta d wax reflects the intensity of east african monsoon circulation the three hydrological proxy records show generally similar trends for the last 25 000 years but there are important differences between them primarily during the middle holocene we interpret this deviation of delta d wax from local hydrological history as a decoupling of east african monsoon intensity which heavily influences the isotopes of precipitation in east africa today from rainfall amount in the challa basin in combination the hydrological proxy data from lake challa singularly highlight zonal gradients in tropical african climate that occur over a variety of timescales suggesting that the congo air boundary plays a fundamental role in controlling hydroclimate in the african tropics

benthic foraminiferal response to late glacial and holocene sea level rise and rainfall variability off east africa analogous to west and north africa east africa experienced more humid conditions between approximately 12 and 5 kyr bp relative to today while timing and extension of wet phases in the north and west are well constrained this is not the case for the east african humid period here we present a record of benthic foraminiferal assemblages and sediment elemental compositions of a sediment core from the east african continental slope in order to provide insight into the regional shallow indian ocean paleoceanography and east african climate history of the last 40 kyr during glacial times the dominance of a benthic foraminiferal assemblage characterized by bulimina aculeata suggests enhanced surface productivity and sustained flux of organic carbon to the sea floor during heinrich stadial 1 h1 the nuttallides rugosus assemblage indicates oligotrophic bottom water conditions and therefore implies a stronger flow of southern sourced aaiw to the study site during the east african humid period the saidovina karreriana assemblage in combination with sedimentary c n and fe ca ratios suggest higher river runoff to the indian ocean and hence more humid conditions in east africa between 8 5 and 8 1 kyr contemporaneous to the globally documented 8 2 kyr event a severe reduction in river deposits implies more arid conditions on the continent comparison of our marine data with terrestrial studies suggests that additional moisture from the atlantic ocean delivered by an eastward migration of the congo air boundary during that time period could have contributed to east african rainfall since approximately 9 kyr the gaining influence of the millettiana millettii assemblage indicates a redevelopment of the east african fringe reefs

impact of deep convection on the isotopic amount effect in tropical precipitation the empirical amount effect observed in the distribution of stable water isotope ratios in tropical precipitation is used in several studies to reconstruct past precipitation recent observations suggest the importance of large scale organized convection systems on amount effect with a series of experiments with isocam3 0 we quantify the sensitivity of amount effect to changes in modeled deep convection the magnitude of the regression slope between long term monthly precipitation amount and isotope ratios in precipitation over tropical ocean reduces by more than 20 with a reduction in mean deep convective precipitation by about 60 indicating a decline in fractionation efficiency reduced condensation in deep convective updrafts results in enrichment of lower level vapor with heavier isotope that causes enrichment in total precipitation however consequent increases in stratiform and shallow convective precipitation partially offset the reduction in the slope of amount effect the net result is a reduced slope of amount effect in tropical regions except the tropical western pacific where the effects of enhanced large scale ascent and increased stratiform precipitation prevail over the influence of reduced deep convection we also find that the isotope ratios in precipitation are improved over certain regions in the tropics with reduced deep convection showing that analyses of isotope ratios in precipitation and water vapor are powerful tools to improve precipitation processes in convective parameterization schemes in climate models further our study suggests that the precipitation types over a region can alter the fractionation efficiency of isotopes with implications for the reconstructions of past precipitation

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

climate of east africa 6000 14c yr b p as inferred from pollen data this paper presents a spatial reconstruction of climate in east africa at 6000 14c yr b p two different approaches using pollen data have been used the standard best modern analogues method and the new plant functional type method based on groups of pollen taxa both methods have been applied to 32 fossil pollen spectra dated at 6000 14c yr b p for each site we have estimated two climatic parameters annual precipitation and mean annual temperature three bioclimatic parameters ratio of actual to potential evapotranspiration and the mean temperature of the coldest and warmest months results show that the temperature lapse rate was less steep at 6000 14c yr b p than it is today the climate was wetter north of 3 s precipitation anomalies 50 mm yr and slightly drier farther south precipitation anomalies 50 mm yr the ratio of actual to potential evapotranspiration also indicates conditions similar to or wetter than today north of 3 s anomalies 10 the climate was warmer than today by 2 c north of the equator and was cooler southward 1 c although both methods indicate similar climatic patterns the plant functional type method provides more consistent results

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

southward migration of the intertropical convergence zone through the holocene titanium and iron concentration data from the anoxic cariaco basin off the venezuelan coast can be used to infer variations in the hydrological cycle over northern south america during the past 14 000 years with subdecadal resolution following a dry younger dryas a period of increased precipitation and riverine discharge occurred during the holocene thermal maximum since approximately 5400 years ago a trend toward drier conditions is evident from the data with high amplitude fluctuations and precipitation minima during the time interval 3800 to 2800 years ago and during the little ice age these regional changes in precipitation are best explained by shifts in the mean latitude of the atlantic intertropical convergence zone itcz potentially driven by pacific based climate variability the cariaco basin record exhibits strong correlations with climate records from distant regions including the high latitude northern hemisphere providing evidence for global teleconnections among regional climates

environment and climate of early human evolution evaluating the relationships between climate the environment and human traits is a key part of human origins research because changes in earth s atmosphere oceans landscapes and ecosystems over the past 10 myr shaped the selection pressures experienced by early humans in africa these relationships have been influenced by a combination of high latitude ice distributions sea surface temperatures and low latitude orbital forcing that resulted in large oscillations in vegetation and moisture availability that were modulated by local basin dynamics the importance of both climate and tectonics in shaping african landscapes means that integrated views of the ecological environmental and tectonic histories of a region are necessary in order to understand the relationships between climate and human evolution

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

a 3 million year index for north african humidity aridity and the implication of potential pan african humid periods abstract mediterranean sediments are valuable archives of both african monsoon variability and higher latitude climate processes and can also be used to provide an environmental context for early human migrations and settlements however the long history of mediterranean palaeoclimate studies largely pre dates the advent of widespread x ray fluorescence xrf core scanning so there are few continuous and high resolution geochemical records from this key region that extend beyond the last glacial cycle here we present xrf core scanning results for odp site 967 eastern mediterranean that have been fully calibrated into element concentrations spanning the last 3 million years my comparison with independent geochemical data from conventional xrf highlights disparities for certain element element ratios thus suggesting the need for caution when taking ratios of scanning xrf data principal component analysis of the calibrated xrf dataset reveals two dominant components detrital inputs pc1 and a sapropel monsoon run off signal pc2 which we use to establish a new orbitally tuned chronology we observe inverse covariation between pc2 and a previously published aeolian dust record from odp site 967 larrasoana et al 2003 and combine these records to produce a composite index of humidity and aridity for the wider north african region over the past 3 my we propose that by combining run off and dust signals in a single metric our index captures the effects of both strengthening northward migration increased run off and weakening southward retreat increased dust of the north african monsoon comparison of the index with published records of northwest and east african palaeohumidity suggests that it tracks the timing of green sahara periods throughout the plio pleistocene and that at least 30 of these intervals coincided with increased humidity across east africa we tentatively suggest that these specific episodes may be termed pan african humid periods as a means to highlight large scale climate trends and to provide an environmental framework for palaeo anthropological research

cooling and drying in northeast africa across the pliocene abstract terrestrial records suggest that northeast africa experienced drying during the pliocene however these records are often incomplete in time and space and questions about this shift in climate remain here we use marine sediments from deep sea drilling project dsdp site 231 in the gulf of aden to generate a multi proxy organic geochemical record of northeast african climate spanning 5 3 2 ma this new record provides a regional perspective on climate and serves as context for the fossil record of early hominin evolution we measured leaf wax carbon 13cwax and hydrogen dwax isotopic composition and tex86 tetraether index of 86 carbons to investigate past changes in vegetation aridity and ocean temperature respectively in the earliest pliocene we infer warm subsurface ocean temperatures from tex86 semi arid conditions on land and extensive c4 grasslands based on dwax 13cwax and previously published pollen after 5 ma ocean temperatures gradually cooled and at 4 3 ma there was a transition to arid conditions on land based on dwax and pollen grasslands yielded to a mid pliocene landscape of dry shrublands this drying appears to be an atmospheric response to cooling ocean temperatures which may reflect changes in tropical ocean circulation the intensification of indian monsoon winds or perhaps other changes associated with pliocene cooling

nature of crustal terranes and the moho in northern costa rica from receiver function analysis 1 the central american subduction zone in northern costa rica shows along strike variations in both the incoming and overriding plates by analyzing the subducting oceanic moho m1 and the upper plate moho m2 with receiver functions we investigate the variability in the hydration state of the subducting cocos plate and the nature of crustal terranes within the overriding caribbean plate we calculate high quality p and pp wave receiver functions using broadband data of the global seismology network geoscope project and the crseize pocosol and corisubmod experiments in addition we estimate the depth h and vertically averaged vp vs to moho and present a sensitivity study to explore the effects of a dipping interface on receiver functions and the h and estimates our results are consistent with a drier oceanic mantle subducting beneath the southernmost part of the nicoya peninsula as compared to a serpentinized oceanic mantle subducting beneath the northern part in the caribbean plate we describe the nature of the mesquito nicoya and chorotega terranes by integrating new and published vp vs estimates both the nicoya and chorotega terranes display high vp vs 1 80 1 92 consistent with their oceanic character in contrast the oceanic mesquito terrane mostly displays lower vp vs 1 62 1 80 more compatible with continental crust which may indicate that subduction zone magmatism is modifying the crust to display continental character our estimates show that the deepest m2 42 km is observed in the volcanic arc region whereas the shallowest m2 27 33 km is observed in parts of the fore arc and back arc regions