isotopic reconstruction of the african humid period and congo air boundary migration at lake tana ethiopia abstract the african humid period of the early to mid holocene 12 000 5000 years ago had dramatic ecological and societal consequences including the expansion of vegetation and civilization into the green sahara while the humid period itself is well documented throughout northern and equatorial africa mechanisms behind observed regional variability in the timing and magnitude of the humid period remain disputed this paper presents a new hydrogen isotope record from leaf waxes d wax in a 15 000 year sediment core from lake tana ethiopia 12 n 37 e to provide insight into the timing duration and intensity of the african humid period over northeastern africa d wax at lake tana ranges between 80 and 170 with an abrupt transition from d enriched to d depleted waxes between 13 000 11 500 years before present 13 11 5 ka a similarly abrupt transition from d depleted to d enriched waxes occurs ca 8 5 8 ka and is followed by generally d enriched waxes throughout the late holocene trends in d wax covary with changes in northern hemisphere summer insolation and reflect increased precipitation at lake tana during the ahp however the transition from d depleted to d enriched waxes occurs earlier at lake tana ca 8 ka vs 5 ka than in many other regional records and the amplitude of d depletion during the ahp is larger at lake tana as well we attribute this early enrichment to a reduction of moisture derived from westerly sources the congo basin and atlantic ocean which we suggest are d depleted relative to moisture sourced from the east indian ocean and the north red sea and mediterranean sea our new record highlights the importance of both the northward migration of the tropical rain belt as well as east west migration of the congo air boundary to precipitation source and amount during the african humid period

paleosol carbonates from the omo group isotopic records of local and regional environmental change in east africa abstract pliocene and pleistocene sedimentary rocks from the omo turkana basin in east africa are well known for fossil and archeological evidence of human evolution and provide a unique opportunity to study four million years of environmental change in a rift basin this study uses carbon and oxygen isotope ratios of pedogenic carbonates to examine environmental variability across 5000 km 2 within the omo turkana basin an expanded isotopic dataset including the first isotopic data on pedogenic carbonates from the shungura formation and new data from the nachukui and koobi fora formations is compared to published isotopic records from both the omo turkana basin and the lower awash basin ethiopia regionally the carbon isotope record indicates a steady increase of c 4 vegetation in floodplain environments for the past 4 million years the oxygen isotopic record indicates that the isotopic composition of rainfall was depleted in 18 o relative to today s waters and that both basins likely received more rainfall in the pliocene than today a shift to higher 18 o values in paleosol carbonate after 2 ma in the omo turkana basin but not in the lower awash basin suggests that the ecology and hydrology in these two rift basins were influenced by different climatic regimes in addition to regional trends pedogenic carbonates sampled from different parts of the basin show that the distribution of c 4 vegetation and soil water 18 o values varied with proximity to the axial river system and specifically that c 3 vegetation was more dominant in soils of the shungura formation compared with coeval sediments downstream in the nachukui and koobi fora formations this large isotopic dataset from pedogenic carbonates provides the opportunity to examine how terrestrial systems responded to global climate change during the last 4 million years from both local and regional perspectives the isotopic data indicate that local basin and climate dynamics strongly influenced the impact of large scale environmental change in east african rift basins

precessional forcing of lacustrine sedimentation in the late cenozoic chemeron basin central kenya rift and calibration of the gauss matuyama boundary the fluviolacustrine sedimentary sequence of the chemeron formation exposed in the barsemoi river drainage tugen hills kenya contains a package of five successive diatomite fluvial cycles that record the periodic development of freshwater lakes within the axial portion of the central kenya rift the overwhelming abundance in the diatomite of planktonic species of the genera aulacoseira and stephanodiscus and the virtual absence of benthic littoral diatoms and detrital material indicate areally extensive deep lake systems a paleomagnetic reversal stratigraphy has been determined and chronostratigraphic tie points established by 40 ar 39 ar dating of intercalated tuffs the sequence spans the interval 3 1 2 35 ma and bears a detailed record of the gauss matuyama paleomagnetic transition the 40 ar 39 ar age for this boundary of 2 589 0 003 ma can be adjusted to concordance with the astronomical polarity time scale apts on the basis of an independent calibration to 2 610 ma 29 kyr older than the previous apts age the diatomites recur at an orbital precessional interval of 23 kyr and are centered on a 400 kyr eccentricity maximum it is concluded that these diatomite fluvial cycles reflect a narrow interval of orbitally forced wet dry climatic conditions that may be expressed regionally across east africa the timing of the lacustrine pulses relative to predicted insolation models favors origination of moisture from the northern africa monsoon rather than local circulation driven by direct equatorial insolation this moisture event at 2 7 2 55 ma and later east african episodes at 1 9 1 7 and 1 1 0 9 ma are approximately coincident with major global climatic and oceanographic events

experimental study of phase relations involving osumilite in the system k2o feo mgo al2o3 sio2 h2o at high pressure and temperature phase relations and mineral chemistry for garnet grt orthopyroxene opx sapphirine spr water undersaturated cordierite crd osumilite osu sillimanite sil k feldspar kfs quartz qtz and a water undersaturated liquid liq have been determined experimentally in the system kfmash k2o feo mgo al2o3 sio2 h2o under low ph2o and fo2 conditions four compositions have been studied with 100 mg mg fe ranging from 65 6 to 89 7 based on our experimental data a p t grid is derived for the kfmash system in the presence of quartz orthopyroxene and liquid osumilite has been found in various mineral assemblages from 950 to 1100 c and 7 5 to 11 kbar in the temperature range 1000 1100 c the pair os grt is stable over a pressure range of about 3kbar the divariant reaction os opx grt kfs qtz runs to the right with increasing pressure because osumilite is the most magnesian phase it is restricted to mg rich compositions at high pressure the reaction defining the upper pressure stability limit of os grt is located around 11 kbar with a nearly flat dp dt slope over the temperature range 950 100 c over the entire temperature range investigated osumilite is not stable beyond 12 kbar the data imply a restricted pressure range between 11 and 12 kbar for the stability of the assemblage os opx sil kfs qtz at 1050 c and above osumilite occurs in various mineral assemblages together with the high t pair spr qtz when coexisting with garnet orthopyroxene or sapphirine osumilite is always the most magnesian phase at 1050 and 1100 c liquid is invariably the most fe rich phase in the run product our data support a theoretical p t grid for the kfmas system in which osumilite is stable outside the field of the high t assemblage spr qtz moreover our grid indicates that os opx sil kfs qtz has a more restricted pressure and compositional stability domain than os grt in agreement with natural occurrences osumilite is stable over a large pressure range such that in mg rich rocks and at high temperature it can occur at any depth in normal thickness continental crust

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

greatly expanded tropical warm pool and weakened hadley circulation in the early pliocene the pliocene warm interval has been difficult to explain we reconstructed the latitudinal distribution of sea surface temperature around 4 million years ago during the early pliocene our reconstruction shows that the meridional temperature gradient between the equator and subtropics was greatly reduced implying a vast poleward expansion of the ocean tropical warm pool corroborating evidence indicates that the pacific temperature contrast between the equator and 32 n has evolved from 2 c 4 million years ago to 8 c today the meridional warm pool expansion evidently had enormous impacts on the pliocene climate including a slowdown of the atmospheric hadley circulation and el nino like conditions in the equatorial region ultimately sustaining a climate state with weak tropical sea surface temperature gradients may require additional mechanisms of ocean heat uptake such as enhanced ocean vertical mixing

earth s precession cycle and quaternary climatic change in tropical africa high abundances of the freshwater diatom melosira spp reflecting eolian input from dry lake beds in tropical africa occur in deep sea sediments of the equatorial atlantic at discrete times during the late quaternary1 2 we report here an approximately 260 kyr record of melosira accumulation in an equatorial atlantic sediment core and demonstrate significant influence of the earth s precessional cycle on the climate of tropical africa

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

isotopic evidence for plio pleistocene environmental change at gona ethiopia abstract a 4 5 ma record of fluvial and lacustrine deposits is well exposed at gona in the afar depression of ethiopia we use isotopic values of pedogenic carbonate and fossil teeth to reconstruct plio pleistocene environmental change at gona an increase in 13c values of pedogenic carbonates since 4 5 ma points to a shift from woodlands to grassy woodlands in the early pliocene 10 4 to 3 9 vpdb to more open but still mixed environments in the late pleistocene 3 0 to 1 4 vpdb this pattern is also seen in isotopic records elsewhere in east africa however at 1 5 ma the higher proportion of c4 grasses at gona is largely a result of a local facies shift to more water limited environments the wide range of 13c values of pedogenic carbonate within single stratigraphic levels indicates a mosaic of vegetation for all time intervals at gona that depends on depositional environment elements of this mosaic are reflected in 13c values of both modern plants and soil organic matter and plio pleistocene soil carbonate indicating higher amounts of c4 grasses with greater distance from a river channel in both the modern and ancient awash river systems 18o values of pedogenic carbonates increase up section from 11 9 in the early pliocene to 6 4 vpdb in the late pleistocene the wide range of 18o values in paleovertisol carbonates from all stratigraphic levels probably reflects short term climate changes and periods of strong evaporation throughout the record based on the comparison between 18o values of plio pleistocene pedogenic carbonates and modern waters we estimate that there has been a 6 5 increase in mean annual 18o values of meteoric water since 4 5 ma 18o values of pedogenic carbonate from other east african records indicate a similar shift increasing aridity and fluctuations in the timing and source of rainfall are likely responsible for the changes in 18o values of east african pedogenic carbonates through the plio pleistocene

human evolution in a variable environment the amplifier lakes of eastern africa abstract the development of the cenozoic east african rift system ears profoundly re shaped the landscape and significantly increased the amplitude of short term environmental response to climate variation in particular the development of amplifier lakes in rift basins after three million years ago significantly contributed to the exceptional sensitivity of east africa to climate change compared to elsewhere on the african continent amplifier lakes are characterized by tectonically formed graben morphologies in combination with an extreme contrast between high precipitation in the elevated parts of the catchment and high evaporation in the lake area such amplifier lakes respond rapidly to moderate precessional forced climate shifts and as they do so apply dramatic environmental pressure to the biosphere rift basins when either extremely dry or lake filled form important barriers for migration mixing and competition of different populations of animals and hominins amplifier lakes link long term high amplitude tectonic processes and short term environmental fluctuations east africa may have become the place where early humans evolved as a consequence of this strong link between different time scales

aquatic biomarkers record pleistocene environmental changes at paleolake olduvai tanzania abstract several hypotheses invoke climatic variability as a driving force for hominin evolution thus high resolution records of climate and environmental variability from anthropologically significant locations can help test these hypotheses sedimentary sequences recovered by the olduvai gorge coring project ogcp help evaluate climatic and environmental changes at olduvai gorge tanzania through the analyses of various biogeochemical proxies the stratigraphic sequence of ogcp core 2a can be correlated with horizons associated with hominins and is chronologically constrained by distinctive dated horizons such as the bed i basalt and tuff ib the lacustrine interval from 76 6 to 86 9 m depth is ideally suited for high resolution analyses of biogeochemical proxies as it is rich in organic carbon 1 toc the hydrogen isotopic composition of nc31 in this interval of ogcp core 2a records the effects of precession driven wet dry cycles on the terrestrial environment that led to alternations between woodland and grassland ecosystems comparable to those documented by previous investigations of outcrop analogues at olduvai here we examine stratigraphic variations in the abundance of biomarkers c28 steradienes c17 and c23 n alkanes c27 and c28 a norsteranes fern 8 ene and chromans derived from aquatic organisms algae cyanobacteria sponges macrophytes etc to determine the response of paleolake olduvai to climate variability in general these aquatic biomarkers reflect the productivity of the lake environment and exhibit the same precession driven wet dry cycles recorded by terrestrial biogeochemical signatures however they also provide evidence of abrupt

permanent el nino like conditions during the pliocene warm period during the warm early pliocene 4 5 to 3 0 million years ago the most recent interval with a climate warmer than today the eastern pacific thermocline was deep and the average west to east sea surface temperature difference across the equatorial pacific was only 1 5 0 9 c much like it is during a modern el nino event thus the modern strong sea surface temperature gradient across the equatorial pacific is not a stable and permanent feature sustained el nino like conditions including relatively weak zonal atmospheric walker circulation could be a consequence of and play an important role in determining global warmth

neogene biomarker record of vegetation change in eastern africa the evolution of c4 grassland ecosystems in eastern africa has been intensely studied because of the potential influence of vegetation on mammalian evolution including that of our own lineage hominins although a handful of sparse vegetation records exists from middle and early miocene terrestrial fossil sites there is no comprehensive record of vegetation through the neogene here we present a vegetation record spanning the neogene and quaternary periods that documents the appearance and subsequent expansion of c4 grasslands in eastern africa carbon isotope ratios from terrestrial plant wax biomarkers deposited in marine sediments indicate constant c3 vegetation from 24 ma to 10 ma when c4 grasses first appeared from this time forward c4 vegetation increases monotonically to present with a coherent signal between marine core sites located in the somali basin and the red sea the response of mammalian herbivores to the appearance of c4 grasses at 10 ma is immediate as evidenced from existing records of mammalian diets from isotopic analyses of tooth enamel the expansion of c4 vegetation in eastern africa is broadly mirrored by increasing proportions of c4 based foods in hominin diets beginning at 3 8 ma in australopithecus and slightly later kenyanthropus this continues into the late pleistocene in paranthropus whereas homo maintains a flexible diet the biomarker vegetation record suggests the increase in open c4 grassland ecosystems over the last 10 ma may have operated as a selection pressure for traits and behaviors in homo such as bipedalism flexible diets and complex social structure

hydrogeochemistry of lake turkana kenya mass balance and mineral reactions in an alkaline lake abstract lake turkana in northwestern kenya is a closed basin alkaline ph 9 2 lake of moderate salinity tds 2500 ppm principal ions are na hco 3 and ci the lake is essentially polymictic in the northern basin and little compositional variation occurs in surface waters the omo river is the principal influent providing some 80 90 of water input to the lake chloride has an apparent accumulation time of about 2500 years after accounting for burial of interstitial water the bottom sediments are predominantly detrital and fine grained yet mineral water reactions are very important for the geochcmical budget ca 2 is precipitated as calcite na is removed as an exchangeable cation on smectite mg 2 is probably incorporated into a mg silicate phase most likely poorlycrystalline smectite as it enters the lake water k may be used in illite regradation cation exchange is a very important process in the mass balance of this lake over 40 of incoming na is removed as an exchangeable cation after cation exchange and interstitial water burial na has a response time of 2650 years which compares favorably with that of chloride these processes seem to occur rapidly within the water mass of the lake other reactions may be important in regulating interstitial water compositions several changes occur in the upper 3m of sediment interstitial water ph drops to 8 3 and alkalinity increases slightly with depth so 2 4 decreases slightly and amorphous silica saturation is approached these changes are a response to organic matter oxidation and the dissolution of unstable silicates rather than a reversal of reactions occurring in the lake water high rates of sedimentation up to 1 cm per year may minimize the effects of diffusion between the interstitial waters and the lake water although burial of interstitial water assumes considerable importance

isotope characteristics of meteoric water and groundwater in the sahelo sudanese zone the stable isotopic concentration 18o and 2h of water in phreatic aquifers in the sahelo sudanese zone of africa is representative of the original mean rainfall contributing to ground water recharge the isotopic decrease of 18o in the rainwater and groundwater 0 084 100 km 1 showed an east west gradient this suggests a supply of water vapor transported by the zonal flows east african jet tropical easterly jet and easterly waves thus increasing the contribution from the evaporating surfaces of the indian ocean in relation to that of the guinean monsoon the gradient also indicates the importance of atmospheric recycling of continental water through evapotranspiration a model of conservation of the isotopic masses is presented which takes into account the return of continental vapor toward the rain giving clouds the remaining fraction of water vapor during the east west transit between djibouti and dakar is thus determined by evapotranspiration

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

an astronomically tuned climate framework for hominins in the turkana basin understanding the influence of orbital climate cycles on hominin evolution remains a key challenge in paleoanthropology the two major unresolved issues are the absence of a climate proxy yielding highresolution b20 kyr terrestrial climate records and the lack of age control on hominin fossil occurrences at sufficiently high resolution here we present a novel climate proxy strontium isotope ratios 87sr 86sr of lacustrine fish fossils from the turkana basin that solves these issues by recording orbitally forced variation in summer monsoon intensity over the ethiopian highlands we successfully applied the climate proxy to a 150 kyr time interval of 2 million year old paleolake deposits containing hominin fossils existing age control of the studied interval was improved by a new magnetostratigraphic record precisely locating the base of the olduvai chron c2n near the bottom of the sequence spectral analysis demonstrates that 87sr 86sr variability is primarily determined by precession which enables us to place hominin fossils in an astronomically tuned climate framework the sr climate proxy is potentially applicable to all homininbearing lake deposits in the turkana basin ranging in age from 4 2 to 0 8 million years ago ma our results demonstrate that between 2 and 1 85 ma the turkana basin remained well watered and inhabited by hominins even during periods of precession maxima when summer monsoon intensity was lowest this is in contrast to other basins in the east african rift system ears that were impacted heavily by precessionforced droughts we hypothesize that during lake phases the turkana basin was an aridity refugium for permanent water dependent fauna including hominins over the precessional climate cycles

the late quaternary tectonic biogeochemical and environmental evolution of ferruginous lake towuti indonesia abstract there is a paucity of long and continuous continental records from south east asia suitable to inform on past changes and underlying causes of the region s climate and associated diverse ecosystem evolution during the late quaternary in 2015 the towuti drilling project tdp collected a series of sedimentary drill cores from the tectonic ferruginous and highly biodiverse lake towuti sulawesi one of indonesia s oldest lakes the drill cores contain 1 myr of uninterrupted lacustrine sedimentation to document long term environmental and climatic change in the tropical western pacific the impacts of geological and environmental changes on the biological evolution of aquatic taxa and the geomicrobiology and biogeochemistry of metal rich ultramafic hosted lake sediment here we use lithostratigraphic mineralogical geochemical and geochronological datasets to elucidate lake towuti s tectonic emergence and its biogeochemical responses to climatic and volcanic forcings since lake formation our data document that lake towuti emerged during a phase of accelerated tectonic subsidence from a landscape characterized by active river channels shallow lakes and swamps into a permanent lake at 1 ma the lacustrine sediments feature quasi rhythmic alternations of green organic rich and red sideritic clay beds reflecting changes in lake mixing and biogeochemistry as a response to temperature and hydrological changes driven by orbital scale changes in insolation and continental ice volume through the mid to late pleistocene clay deposition is interrupted by two beds of diatomaceous oozes composed primarily of planktonic diatoms that reflect phases of substantially increased primary productivity the occurrence of these diatomaceous oozes in close association with multiple tephra beds suggests a trophic state change driven by the addition of volcanically sourced p possibly in combination with a lake mixing state that supports recycling of p data on lake age and ontogeny are also in agreement with molecular clock estimates of 0 7 ma 0 18 1 37 ma for the divergence of lake towuti s telmatherinid fishes from a riverine ancestor our data therefore are compatible with an evolutionary model in which lake towuti s endemic fauna is a result of geographic speciation in the malili lakes a set of large lakes in southeast sulawesi driven by physical or chemical dispersal limits imposed by the regional rivers and lakes more detailed chronological constraints and refined climate and environmental proxy datasets are currently in preparation and will help to paint a more detailed history of the region s climate and environmental history in future studies

an isotopic study of paleosol carbonates from olduvai gorge the isotopic composition of paleosol carbonates from olduvai gorge can be used to give estimates of the paleoclimatic history of east africa during the plio pleistocene oxygen isotopes are indicative of major changes in the isotopic composition of meteoric water while carbon isotopes are indicative of major changes in the fraction of biomass using the hatch slack c4 photosynthetic pathway three major periods of climatic change in this terrestrial setting occurred at about 1 67 1 3 and 0 6 myr these involved major changes in flora as well as major changes in the isotopic composition of meteoric water overall the climate has become warmer and more arid over the past 2 2 myr

biomarker records of late neogene changes in northeast african vegetation open savannah grasslands dominated by c4 plants became a significant component of northeast african vegetation during the late neogene we present molecule specific carbon isotopic mea surements of terrestrial plant biomarkers preserved in marine sed iments off northeast africa that allow reconstruction of orbital scale vegetation changes in short time windows over the past 9 4 m y the biomarker data show large amplitude vegetation vari ability as early as 3 8 ma with the greatest c4 expansion occurring after 3 4 ma we sampled orbital scale oscillations of up to 5 almost as large as the observed late neogene range of 7 sug gesting that large and repeated oscillations between more open and more closed landscapes were an important aspect of northeast af rican vegetation change during the past 4 m y

atmospheric circulation patterns during late pleistocene climate changes at lake malawi africa abstract the climate of tropical africa transitioned from an interval of pronounced orbitally paced megadroughts to more humid and stable conditions approximately 70 000 years ago scholz et al 2007 the regional atmospheric circulation patterns that accompanied these climatic changes however are unclear due to a paucity of continental paleoclimate records from tropical africa extending into the last interglacial we present a new 140 kyr record of the deuterium hydrogen isotopic ratio of terrestrial leaf waxes d wax from drill cores from lake malawi southeast africa that spans this important climatic transition d wax shifts from highly variable and relatively d depleted to more stable and d enriched around 56 ka contemporary with the onset of more humid conditions in the region moisture source and transport history dominate the d wax signal at lake malawi with local rainfall amount playing a secondary role for much of the paleorecord analysis of modern moisture sources for lake malawi suggests that d depletion of waxes during the megadroughts may have been caused by an enhanced contribution of the drier d depleted air mass currently located in central southern africa to the lake malawi catchment this d depleted air mass is associated with the descending limb of the hadley cell which implies significant changes in the hadley circulation during the megadroughts and related changes in the position of the intertropical convergence zone over africa these findings demonstrate the ability of d wax to serve as an atmospheric tracer when used in conjunction with additional proxy records for moisture balance and elucidate potential mechanisms for pronounced hydrological change in southeast africa during the late pleistocene

hydrogen isotope ratios of lacustrine sedimentary n alkanes as proxies of tropical african hydrology insights from a calibration transect across cameroon abstract hydrogen isotope values d of sedimentary aquatic and terrestrial lipid biomarkers originating from algae bacteria and leaf wax have been used to record isotopic properties of ancient source water i e precipitation and or lake water in several mid and high latitude lacustrine environments in the tropics however where both processes associated with isotope fractionation in the hydrologic system and vegetation strongly differ from those at higher latitudes calibration studies for this proxy are not yet available to close this gap of knowledge we sampled surface sediments from 11 lakes in cameroon to identify those hydro climatological processes and physiological factors that determine the hydrogen isotopic composition of aquatic and terrestrial lipid biomarkers here we present a robust framework for the application of compound specific hydrogen isotopes in tropical africa our results show that the d values of the aquatic lipid biomarker n c17 alkane were not correlated with the d values of lake water carbon isotope measurements indicate that the n c17 alkane was derived from multiple source organisms that used different hydrogen pools for biosynthesis we demonstrate that the d values of the n c29 alkane were correlated with the d values of surface water i e river water and groundwater which on large spatial scales reflect the isotopic composition of mean annual precipitation such a relationship has been observed at higher latitudes supporting the robustness of the leaf wax lipid d proxy on a hemispheric spatial scale in contrast the d values of the n c31 alkane did not show such a relationship but instead were correlated with the evaporative lake water d values this result suggests distinct water sources for both leaf wax lipids most likely originating from two different groups of plants these new findings have important implications for the interpretation of long chain n alkane d records from ancient lake sediments in particular a robust interpretation of palaeohydrological data requires knowledge of the vegetation in the catchment area as different plants may utilise different water sources our results also suggest that the combination of carbon and hydrogen isotopes does help to differentiate between the metabolic pathway and or growth form of organisms and therefore the source of hydrogen used during lipid biosynthesis

isotopic records of climate seasonality in equid teeth abstract we investigate how oxygen isotopes in equid teeth can be used as a record of seasonality first we use in situ laser ablation and conventional microsampling techniques to understand time averaging of environmental signals in intra tooth isotope profiles in modern feral horse teeth n 5 from mongolia where there is a large seasonal gradient in the oxygen isotopic composition of precipitation we demonstrate that laser ablation can be used to sample inner middle and outer enamel layers in large mammalian herbivore teeth the inner enamel layer records less attenuated isotopic signals than other layers as predicted by the mineralization patterns but intra tooth signal amplitude is similar for laser and conventional sampling methods second we use modern zebra teeth n 21 collected in eastern africa to evaluate how intra tooth oxygen isotope variation relates to rainfall patterns in the tropics we show that the intra tooth isotopic range increases with intra annual precipitation isotopic range which in turn relates to aridity in the equatorial bimodal rainfall region but is influenced by other hydroclimate processes in the region as a whole finally we address isotopic seasonality during the early pleistocene in eastern africa using oxygen isotopes in fossil equid teeth from southwestern kenya n 11 and northern tanzania n 5 we find variable isotopic seasonality in the past similar to present day eastern africa consistent with the notion that hominins and other mammals were able to accommodate environmental variability on intra annual scales in addition to well documented orbital cycles

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

stable isotope proxy records in tropical terrestrial environments abstract the tropics refers to an area that is loosely equatorial characterized by relatively high temperatures at low altitude comparatively low seasonal contrast in temperatures and generally high or highly seasonal rainfall the stable isotopes of carbon oxygen and hydrogen are increasingly important tools of relevance to understanding trajectories and mechanisms of past environmental change in the tropics in the case of carbon this is because at the broadest level the carbon isotope composition of organic and carbon containing inorganic proxy materials formed in the tropics can be read as being derived from the balance between c3 tree and c4 grass photosynthesis hence providing a relative measure of vegetation type and indirectly climate likewise changes in the oxygen and hydrogen isotope composition of many proxy materials again at the broadest level can be read as indicative of changes in hydrological conditions and or altitude in the past this review provides an entry point into the literature relating to the use of stable isotopes as terrestrial environmental proxies for past environmental change with a focus on the measurement and interpretation of stable isotope data from the wide range of proxy materials preserved in tropical environments

stable isotopes in east african precipitation record indian ocean zonal mode observational stable isotope data and model simulations show that the 18 o composition of precipitation in east africa is closely related to the coupled ocean atmosphere system over the indian ocean during the rainy seasons 18 o is a very sensitive recorder of the indian ocean zonal mode iozm the link between 18 o and the iozm is established through adjustments in the large scale overturning circulation over the indian ocean and associated changes in convective activity and precipitation over east africa the iozm is recorded as significant departures in 18 o not only over east africa but as a dipole mode with opposite sign to the east and west of the indian ocean

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

stable isotopic composition of pedogenic carbonates from the eastern margin of the thar desert rajasthan india pedogenic carbonates in a 19 m thick dune section 16r at didwana and several shorter profiles from the eastern margin of the thar desert rajasthan were studied for their stable isotope composition carbon isotope results show the c4 biomass grasses has dominated local vegetation for most of the 250 000 years represented by the 16r section our results display a decrease in 18o and 13c values during the last interglacial and early glacial periods the higher 13c values in our dune records correlate to episodes in the strong upwelling and decreased sedimentation rates in indian ocean cores probably the result of a strong asian monsoon the high 13c values may therefore denote the expansion of c4 vegetation in response to warm wetter monsoon circulation conversely low 18o and 13c values mark the expansion of c3 vegetation probably caused by enhanced winter rains and lower temperatures

glacial aridity in central indonesia coeval with intensified monsoon circulation abstract the last glacial maximum was cool and dry over the indo pacific warm pool ipwp a key region driving global oceanic atmospheric circulation both low and high latitude teleconnections with insolation ice sheets and sea level have been suggested to explain the pervasive aridity observed in paleoecological and geomorphic data however proxies tracking the h and o isotopic composition of rainfall e g speleothems sedimentary biomarkers suggest muted aridity or even wetter conditions than the present complicating interpretations of glacial ipwp climate here we use multiproxy reconstructions from lake sediments and modern rainfall isotopic measurements from central indonesia to show that contrary to the classical amount effect intensified australian indonesian monsoon circulation drove lighter h and o isotopic composition of ipwp rainfall during the lgm while at the same time dry conditions prevailed precipitation isotopes are particularly sensitive to the apparent increase in monsoon circulation and perhaps also decreased moisture residence time implied by our data explaining contrasts among proxy records while illuminating glacial ipwp atmospheric circulation a key target for climate models

early pleistocene lake formation and hominin origins in the turkana omo rift abstract prior research has correlated the formation of plio pleistocene lakes in east africa to global regional climate changes and interpreted the lacustrine basins as significant settings of hominin evolution paleo lake lorenyang from the turkana omo rift is important to these issues as its marginal deposits contain some of if not the earliest currently known specimens of acheulian stone tools and african homo erectus magnetostratigraphic and sedimentological evidence indicates that the oldest preserved paleo lake lorenyang deposits are dated at 2 148 2 128 ma and derive from the nw turkana basin predating those from the omo basin by 100 kyr and the ne turkana basin by 190 kyr apparently the lake expanded asynchronously in the rift potentially due to a volcano tectonic influence on the location of drainage networks depositional slopes or topographic elevation differences within and between the basins at the time of flooding the onset of the lake temporally coincides with the eruption of basalt lava flows dated to 2 2 2 0 ma that blocked the southeast outlet of the turkana basin this provides a plausible mechanism for hydrologic closure and lacustrine basin formation through volcano tectonic impounding it also points to a non climatic cause for the initial formation of paleo lake lorenyang at 2 14 ma first appearances for african h erectus 1 87 ma and acheulian tools 1 76 ma in the turkana omo rift postdate the lake s initial formation by about 270 kyr and 380 kyr respectively such timing differences contrast with studies that correlate all three to the 400 kyr eccentricity maximum at 1 8 ma although the turkana omo rift is just one example it does provide alternative insights to views that link climate hominin evolution and lake formation in east africa

coherent changes of southeastern equatorial and northern african rainfall during the last deglaciation during the last deglaciation wetter conditions developed abruptly 14 700 years ago in southeastern equatorial and northern africa and continued into the holocene explaining the abrupt onset and hemispheric coherence of this early african humid period is challenging due to opposing seasonal insolation patterns in this work we use a transient simulation with a climate model that provides a mechanistic understanding of deglacial tropical african precipitation changes our results show that meltwater induced reduction in the atlantic meridional overturning circulation amoc during the early deglaciation suppressed precipitation in both regions once the amoc reestablished wetter conditions developed north of the equator in response to high summer insolation and increasing greenhouse gas ghg concentrations whereas wetter conditions south of the equator were a response primarily to the ghg increase

a leaf wax biomarker record of early pleistocene hydroclimate from west turkana kenya abstract climate is thought to play a critical role in human evolution however this hypothesis is difficult to test due to a lack of long high quality paleoclimate records from key hominin fossil locales to address this issue we analyzed organic geochemical indicators of climate in a drill core from west turkana kenya that spans 1 9 1 4 ma an interval that includes several important hominin evolutionary transitions we analyzed the hydrogen isotopic composition of terrestrial plant waxes dwax to reconstruct orbital timescale changes in regional hydrology and their relationship with global climate forcings and the hominin fossil record our data indicate little change in the long term mean hydroclimate during this interval in contrast to inferred changes in the level of lake turkana suggesting that lake level may be responding dominantly to deltaic progradation or tectonically driven changes in basin configuration as opposed to hydroclimate time series spectral analyses of the isotopic data reveal strong precession band 21 kyr periodicity indicating that regional hydroclimate was strongly affected by changes in insolation we observe an interval of particularly high amplitude hydrologic variation at 1 7 ma which occurs during a time of high orbital eccentricity hence large changes in processionally driven insolation amplitude this interval overlaps with multiple hominin species turnovers the appearance of new stone tool technology and hominin dispersal out of africa supporting the notion that climate variability played an important role in hominin evolution

lake level changes and hominin occupations in the arid turkana basin during volcanic closure of the omo river outflows to the indian ocean response to comments by schuster and nutz quaternary research 92 2 pp 598 600

impact of sea ice processes on the carbonate system and ocean acidification at the ice water interface of the amundsen gulf arctic ocean 1 from sea ice formation in november 2007 to onset of ice melt in may 2008 we studied the carbonate system in first year arctic sea ice focusing on the impact of calciumcarbonate caco3 saturation states of aragonite xar and calcite xca at the ice water interface uiw based on total inorganic carbon ct and total alkalinity at and derived ph co2 carbonate ion co3 22 concentrations and x we investigated the major drivers such as brine rejection caco3 precipitation bacterial respiration primary production and co2 gas flux in sea ice brine frost flowers and uiw we estimated large variability in seaice ct at the top mid and bottom ice changes due to caco3 and co2 gas flux had large impact on ct in the whole ice core from march to may bacterial respiration was important at the bottom ice during all months and primary production in may it was evident that the sea ice processes had large impact on uiw resulting in a five times larger seasonal amplitude of the carbonate system relative to the upper 20 m during ice formation co2 increased by 30 mmol kg 21 co 3 22 decreased by 50 mmol kg 21 and the xar decreased by 0 8 in the uiw due to co2 enriched brine from solid caco3 conversely during ice melt co3 22 increased by 90 mmol kg 21 in the uiw and x increased by 1 4 between march and may likely due to caco3 dissolution and primary production we estimated that increased ice melt would lead to enhanced oceanic uptake of inorganic carbon to the surface layer

vegetation change in the baringo basin east africa across the onset of northern hemisphere glaciation 3 3 2 6 ma abstract vegetation in east africa is generally thought to have shifted from forests to more open grasslands and savannas as global climate cooled and high latitude ice sheets expanded during the plio pleistocene such a shift would have greatly influenced landscape resources and potentially hominin evolution as well existing records of african vegetation spanning these time scales are generally derived from offshore marine records that record continental scale changes or paleosol carbonate records that record very local vegetation changes during the short time intervals of soil carbonate formation here we present a new record of basin scale vegetation change from the late pliocene 3 3 2 6 ma derived from a drill core from the chemeron formation located in the baringo basin tugen hills region of the kenya rift valley specifically we present a new record of the relative abundance of c4 grasses and c3 vegetation based on the carbon isotopic composition of leaf wax biomarkers 13cwax which captures a signal of regional vegetation change these data demonstrate that vegetation in the baringo basin varied greatly between c3 forests and c4 grasslands and that vegetation exhibits both long term secular trends and orbital scale variations the contribution of c3 plants was lower than estimates based on low resolution carbon isotope data from paleosol carbonates and organic matter in the basin c3 plants averaged 53 of the vegetation during the late pliocene from 3 3 to 3 04 ma after which time 13cwax indicates more open vegetation and 41 c3 plants this transition may have been driven by changes in basin geomorphology but also possibly occurred as part of larger scale drying and expansion of c4 vegetation in east africa in addition to this secular change we observe high amplitude variability in the 13cwax record including oscillations between 80 and 0 c3 plants these vegetation changes are correlated with changes in precipitation inferred from 2hwax and lake level oscillations inferred from sedimentary facies implying that high amplitude orbital scale variations in precipitation drove significant changes in vegetation resources during the late pliocene in the baringo basin these variations have important implications for changes in terrestrial resources in light of the evolutionary innovations in the hominin fossil record related to changes in foraging strategies

stratigraphic classification of ancient lakes balancing tectonic and climatic controls lakes and lake deposits present two fundamental paradoxes 1 modern lakes are vastly complicated but the rock records of lakes are relatively simple extensive observations reveal three distinct facies associations of common and widespread occurrence these are referred to here as fluvial lacustrine fluctuating profundal and evaporative facies associations 2 most explanations of modern and ancient lakes attribute their nature to climate but neither modern lake parameters lake size depth and salinity nor the character of ancient lake strata thickness extent lithology correlate with measured or inferred climatic humidity we propose that it is the relative balance of rates of potential accommodation mostly tectonic with sediment water fill mostly a function of climate that controls lake occurrence distribution and character lake basins may be termed overfilled balanced fill or underfilled depending on the balance between these rates we conclude that climate and tectonics exert coequal influence on lake deposits at both mesoscales 1 m to hundreds of meters and macroscales hundreds to thousands of meters

tropical ocean temperatures over the past 3 5 million years determining the timing and amplitude of tropical sea surface temperature sst change is an important part of solving the puzzle of the plio pleistocene ice ages alkenone based tropical sst records from the major ocean basins show coherent glacial interglacial temperature changes of 1 to 3 c that align with but slightly lead global changes in ice volume and deep ocean temperature over the past 3 5 million years tropical temperatures became tightly coupled with benthic 18o and orbital forcing after 2 7 million years we interpret the similarity of tropical sst changes in dynamically dissimilar regions to reflect top down forcing through the atmosphere the inception of a strong carbon dioxide greenhouse gas feedback and amplification of orbital forcing at 2 7 million years ago connected the fate of northern hemisphere ice sheets with global ocean temperatures since that time

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

african climate change and faunal evolution during the pliocene pleistocene abstract environmental theories of african faunal evolution state that important evolutionary changes during the pliocene pleistocene interval the last ca 5 3 million years were mediated by changes in african climate or shifts in climate variability marine sediment sequences demonstrate that subtropical african climate periodically oscillated between markedly wetter and drier conditions paced by earth orbital variations with evidence for step like 0 2 ma increases in african climate variability and aridity near 2 8 ma 1 7 ma and 1 0 ma coincident with the onset and intensification of high latitude glacial cycles analysis of the best dated and most complete african mammal fossil databases indicates african faunal assemblage and perhaps speciation changes during the pliocene pleistocene suggesting more varied and open habitats at 2 9 2 4 ma and after 1 8 ma these intervals correspond to key junctures in early hominid evolution including the emergence of our genus homo pliocene pleistocene shifts in african climate vegetation and faunal assemblages thus appear to be roughly contemporary although detailed comparisons are hampered by sampling gaps dating uncertainties and preservational biases in the fossil record further study of possible relations between african faunal and climatic change will benefit from the accelerating pace of important new fossil discoveries emerging molecular biomarker methods for reconstructing african paleovegetation changes tephra correlations between terrestrial and marine sequences as well as continuing collaborations between the paleoclimatic and paleoanthropological communities