coarse grained deep water sedimentation along a border fault margin of lake malawi africa seismic stratigraphic analysis lake malawi africa serves as an important modern analog for understanding depositional processes in an active rift setting this paper explores the depositional processes and stacking patterns of coarse grained facies across a large fan delta and associated sub lacustrine fan system offshore of the south rukuru river using single channel seismic data coupled with deep water vibracores and gravity cores a companion paper wells et al 1999 discusses the details of the depositional processes of the system inferred from core data using 2900 km of single channel seismic data eight genetic facies were defined based on seismic reflection character external geometry accompanying core data and location within the modern geomorphic system five sand dominated facies include 1 mouth of canyon fan facies 2 progradational fan delta facies 3 channel fill facies 4 canyon fill facies and 5 basin plain fill facies these facies occur as surficial deposits in water depths of more than 600 m suggesting that coarse grained sedimentation characterizes certain localities in even the deepest parts of lake malawi climatically driven lake level changes influenced the internal character of these facies their thickness and to some extent their location within the south rukuru study area however climate was generally subordinate to tectonism in controlling the temporal and spatial distribution of the facies tectonic controls can be categorized as 1 orientation of the first order border fault and intrabasinal faults 2 syndepositional footwall uplift and rotation and 3 temporal changes in the rate and location of maximum subsidence these controls affect the location and stacking of the mouth of canyon fans and the presence and orientation of sub lacustrine canyons and the zone of maximum sediment accumulation and depositional gradients

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

relative geomagnetic intensity of the field during the last 140 ka relative magnetic field palaeointensities have been obtained from three marine cores in the somali basin western indian ocean the correlation between the records based on changes in the low field susceptibility and the inclination is extremely detailed a time versus depth correlation has been established from the 18o record the remanence intensity was normalized with respect to the anhysteretic remanent magnetization with special attention to changes in grain sizes coercivities and mineralogy the synthetic record of relative palaeointensity has been extended to cover the last 140 ka the quasi cyclic pattern observed during the last 80 ka confirms the results previously obtained in the mediterranean sea and thereby establishes its dominantly dipolar character between 140 and 80 ka the field intensity exhibits a much longer cycle spectral analysis of the intensity records shows the presence of two dominant periodicities but the length of the signal is too short to document the presence of a stationary process

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

renewed investigations into the middle stone age of northern malawi j desmond clark and his colleagues were first to investigate the rich middle stone age msa from ca 285 to 30 000 years ago deposits in the karonga district of northern malawi in the 1960s this work demonstrated the enormous potential of the area to inform about middle to late pleistocene hominin lifeways but further studies were hindered by difficulties in dating the sites and understanding their fine scale depositional and paleoenvironmental contexts with the advances that have been made in the fields of geoarchaeology lithic analysis palaeoenvironments and geochronology over the last fifty years the time is ideal to renew these investigations recent survey and excavation in karonga malawi show that msa lithic artifacts are preserved in a variety of stratified sedimentary contexts and that they exhibit limited weathering or other indications of post depositional transport these deposits are in close proximity to the high resolution paleoenvironmental records derived from sediments at the bottom of nearby lake malawi which provide context for the human behavior recorded by the artifacts results of excavations at one of these stratified sites the airport site are detailed here this provides a renewed example of the site formation and behavioral data available in the region

biogenic silica deposition in lake malawi east africa over the past 150 000years abstract we report a 150 000 year record of the abundance of biogenic silica bsi in drill cores from the northern and central basins of lake malawi the periods of highest diatom burial at both sites occurred around 65 69 and 51 60 thousand years ago ka after the termination of prolonged intense drought in the region these peaks are believed to reflect elevated delivery of dissolved silica to the lake due to acceleration of chemical weathering in the drainage basin the droughts that preceded these bsi depositional episodes occurred at precessional frequency corresponding to times of minimum austral spring or summer insolation in the region prior to 60 ka subsequent arid spells have not been as severe due to the reduced amplitude of precessional forcing as a result of the superimposed effect of eccentricity however biogenic silica records do not show significant sensitivity to precessional forcing the bsi burial flux in the north basin displays strong millennial scale variability since 50 ka with peak values occurring during cold times in the northern hemisphere as reflected in the greenland ice core records we observe no relationship between bsi and deposition of volcanic tephras in this system the central basin core also displays millennial scale variability in bsi abundance during the past 50 ky of comparable magnitude to that in the north basin but without systematic correlation to the north basin bsi or greenland ice core records in general we conclude that bsi profiles from the north and probably south basins are more readily interpreted in terms of regional climate dynamics than are bsi profiles from more central locations in long narrow tropical rift lakes diatom burial rates are suspected to be too influenced by silica limitations imposed by diatom production and burial in the upwelling systems at either end of these low latitude lacustrine systems

δ13c records from fish fossils as paleo indicators of ecosystem responses to lake levels in the plio pleistocene lakes of tugen hills kenya abstract the carbon isotopic ratios of organic matter in fish fossils from diatomites and other lake beds in the hspdp drill core from tugen hills kenya 2 56 3 29 ma reflect trophic resource uses and can indicate the dietary habitats of fish in the paleolake this information offers insight into how fish communities responded to lake level fluctuations during the plio pleistocene in the east african rift valley we have compared this record with fish fossil isotopes from both a previously published study of a lake malawi drill core 139 ka present and core top modern ca 1978 samples collected at the water sediment boundary from lake turkana kenya of known environmental provenance both the lake malawi drill core fossils 7 2 to 27 5 vpdb and modern lake turkana samples 16 to 24 6 vpdb have 13c values indicating a mix of near shore and deep water pelagic species in contrast the 13c values for the tugen hills core fossils vary only between 20 and 27 vpdb the absence of 13c values greater than 19 suggests none of these fossils are derived from near shore benthic habitats the lack of shallow water benthic lacustrine fish fossils through the tugen hills lake cycles may indicate that the rate of change from low lake stands to deeper lake phases was very rapid and shallow water communities were not established for long enough to leave a fish fossil record at the core site these results strongly suggest that lake level responses to climate variability in the baringo basin of the east african rift were very abrupt during the plio pleistocene transition

interhemispheric synchrony of the last deglaciation inferred from alkenone palaeothermometry the relative timings of the last deglacial warming in the southern and northern hemispheres are not well constrained but are a crucial component in understanding the mechanisms of deglaciation1 a clearer picture of the degree of interhemispheric synchrony has been obscured by a dearth of high resolution temperature records that can be tied to the absolute calendar timescale moreover the quantification of tropical temperatures during the last glacial cycle is controversial2 8 here we apply the alkenone method of sea surface temperature reconstruction9 10 to several high resolution sediment cores recovered from the tropical indian ocean between 20 n and 20 s the inferred initial sea surface temperature warming 15 000 calendar years ago at 20 s is in phase with northern hemisphere sea this study and air11 temperature changes but lags antarctic warming12 14 by several millennia this finding along with the results of recent modelling studies15 16 provides strong support for the idea that changes in the ocean s global thermohaline circulation were not the only cause of interhemispheric climate teleconnection during the last deglaciation

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

lake malawi s response to megadrought terminations sedimentary records of flooding weathering and erosion abstract sediment records from the northern basin of lake malawi provide a means of evaluating the lake basin s response to climate change over the past 75 ky notably to increased precipitation at the terminations of droughts transitions from drier to wetter conditions provide an opportunity to evaluate the system s response to climate shifts upon termination of drought episodes at 62 and 72 ka enhanced precipitation and an associated increase in streampower led to enhanced physical erosion and landscapes were flooded by rising lake waters these processes appear to have left their mark in the sedimentary record bringing about a spike of deposition of organic matter probably of terrestrial origin at times of increased rainfall this was immediately followed by a period of deposition of chemically weathered material that had been retained on the landscape during arid times and mobilized in response to increased precipitation after this altered material was removed perhaps a thousand years after the transition to wetter conditions fresher material richer in soluble elements including nutrients was exposed to chemical weathering leading to substantial diatom blooms the lag between the onset of wetter conditions and the diatom blooms is inconsistent with significant storage of bioavailable silica in soils in this system however biological cycling of silica including formation and dissolution of phytoliths may have played a role in mobilization of the silica necessary for the diatom productivity

a 1 3 million year record of synchronous faulting in the hangingwall and border fault of a half graben in the malawi nyasa rift abstract this paper analyzes throw depth t z profiles from a high resolution 2d reflection seismic grid in the central basin of lake malawi to investigate whether evidence exists 1 for migration of faulting away from the border fault of the half graben and 2 that faults in the hangingwall lengthened over the last 1 3 million years we use the high precision age model from a 2005 scientific drilling project in our study area to constrain the ages of our seismic horizons and examine a fault array and two individual faults within the hangingwall of the central basin border fault we account for climatic and sedimentological controls on stratal growth with a lake level curve that accompanies the age model a comparison of our hangingwall t z profiles with published throw distance t x profiles for the border fault shows synchronous faulting over the last 1 3 m y rather than basinward migration of faulting furthermore we find no evidence for significant propagation of the tips of the hangingwall faults in the last 1 3 m y and conclude that the lack of basinward migration of faulting is a consequence of strain localization on faults established at an early stage in basin development

reconstructing paleoenvironments and palaeoclimates in drylands what can landform analysis contribute quaternary period palaeoenvironmental and palaeoclimatic reconstructions are based on a wide and diverse array of proxy data sets some of which are geomorphological in nature in drylands where organic proxies may be limited the use of landforms is particularly important but challenging the capacity to establish the age of depositional forms particularly through the use of luminescence dating has advanced the use of landforms in dryland palaeo research though interpretation of these geoproxy records can be complex especially at the nexus of palaeoclimate and palaeoenvironmental interpretations of past conditions in this paper the use of aeolian and lacustrine forms in quaternary research is considered focusing on the relationships between dynamics form and climate and on the essential linkage between process research and palaeoenvironmental research it is concluded that landform analysis is a critical part of dryland palaeoenvironmental climate reconstruction contributing a different set of data compared to other data sources in terms of the elements of past conditions that are revealed five principles are identified to improve the use of geoproxy records in quaternary research 1 greater use of geomorphic process studies by quaternary scientists to better inform palaeolandform interpretation 2 further development of the use of chronometric data especially in terms of interpreting large data 3 interpret landform records in location specific contexts not in general terms 4 capitalise of the complexity of spatially extensive landform records which may offer better representations of real quaternary environmental complexity than at a point proxies 5 establish ways of integrating spatially extensive geoproxy records with other palaeoenvironmental records these challenges are major but not insurmountable and should represent goals for geomorphologists chronologists and quaternary scientists alike copyright 2011 john wiley sons ltd

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

scientific drilling in the great rift valley the 2005 lake malawi scientific drilling project an overview of the past 145 000years of climate variability in southern hemisphere east africa abstract the recovery of detailed and continuous paleoclimate records from the interior of the african continent has long been of interest for understanding climate dynamics of the tropics and also for constraining the environmental backdrop to the evolution and spread of early homo sapiens in 2005 an international team of scientists collected a series of scientific drill cores from lake malawi the first long and continuous high fidelity records of tropical climate change from interior east africa the paleoclimate records which include lithostratigraphic geochemical geophysical and paleobiological observations documented in this special issue of palaeo3 indicate an interval of high amplitude climate variability between 145 000 and 60 000 years ago when several severe arid intervals reduced lake malawi s volume by more than 95 these intervals of pronounced tropical african aridity in the early late pleistocene around lake malawi were much more severe than the last glacial maximum lgm a well documented period of drought in equatorial and northern hemisphere tropical east africa after 70 000 years ago climate shifted to more humid conditions and lake levels rose during this latter interval however wind patterns shifted rapidly and perhaps synchronously with high latitude shifts and changes in thermohaline circulation this transition to wetter more stable conditions coincided with diminished orbital eccentricity and a reduction in precession dominated climatic extremes the observed climate mode switch to decreased environmental variability is consistent with terrestrial and marine records from in and around tropical africa

fish fossils as paleo indicators of ichthyofauna composition and climatic change in lake malawi africa abstract numerous biological and chemical paleorecords have been used to infer paleoclimate lake level fluctuation and faunal composition from the drill cores obtained from lake malawi africa however fish fossils have never been used to examine changes in african great lake vertebrate aquatic communities nor as indicators of changing paleolimnological conditions here we present results of analyses of a lake malawi core dating back 144 ka that describe and quantify the composition and abundance of fish fossils and report on stable carbon isotopic data 13c from fish scale bone and tooth fossils we compared the fossil 13c values to 13c values from extant fish communities to determine whether carbon isotope ratios can be used as indicators of inshore versus offshore pelagic fish assemblages fossil buccal teeth pharyngeal teeth and mills vertebra and scales from the fish families cichlidae and cyprinidae occur in variable abundance throughout the core carbon isotopic ratios from numerous fish fossils throughout the core range between 7 2 and 27 5 similar to those found in contemporary lake malawi benthic and pelagic fish faunas these results are the first paleo record of fish fossils from a lake malawi sediment core and the first reported 13c values from lake malawi fish fossils this approach provides a new methodology and framework for interpreting pelagic versus inshore fish faunas lake level fluctuations and the evolution of the lake malawi fish assemblages

evidence from high resolution topography for multiple earthquakes on high slip to length fault scarps the bilila mtakataka fault malawi geomorphological features such as fault scarps and stream knickpoints are indicators of recent fault activity determining whether these features formed during a single earthquake or over multiple earthquakes cycles has important implications for the interpretation of the size and frequency of past events here we focus on the bilila mtakataka fault malawi where the 20 m high fault scarps exceed the height expected from a single earthquake rupture we use a high resolution digital elevation model 1 m to identify complexity in the fault scarp and knickpoints in river profiles of 39 selected scarp profiles 20 showed evidence of either multi scarps or composite scarps and of the seven selected river and stream profiles five showed evidence for multiple knickpoints a near uniform distribution of vertical offsets on the sub scarps suggests they were formed by separate earthquakes these independent methods agree that at least two earthquakes have occurred with an average vertical offset per event of 10 and 12 m this contrasts earlier studies which proposed that this scarp formed during a single event and demonstrates the importance of high resolution topographic data for understanding tectonic geomorphology we use a one dimensional diffusion model of scarp degradation to demonstrate how fault splays form multi scarps and estimate the diffusion age t of the bilila mtakataka fault scarp to be 48 m25 m2 corresponding to 6400 4000 years since formation we calculate that a continuous rupture would equate to a mw 7 8 0 3 earthquake greater than the largest seismic event previously recorded in east africa

seasonal signals in lake malawi sediments the sedimentary record of lake malawi one of the large rift valley lakes of east africa contains thick sequences of biologically undisturbed finely laminated muds and silts laminated sequences were recovered in cores from the extensive diatom ooze and silty diatom clay facies of the lake basin that is permanently anoxic below 250 m the sequences consist of light dark laminae couplets with average thickness of 1 mm the light laminae are completely dominated by the diatom genus melosira with a relatively minor contribution from the genus stephanodiscus the dark laminae consist of 50 diatoms equal proportions of melosira and stephanodiscus few nitzschia and occasional littoral diatom genera and 50 terrestrial plant debris mineral grains and fine clay and organic material based on a 210pb and 14c calculated sedimentation rate of 1 mm yr 1 for lake malawi basin sediments and the well defined seasonal variations in fluvial input and diatom productivity levels our data suggest that the lamination couplets are annual varves representing biannual sedimentation sequences the light laminae result from diatom blooms occurring during the dry windy season april october when the upper 250 m of the lake is well mixed localized upwelling occurs and the epilimnion becomes enriched with nutrients the dark laminae represent sedimentation in the rainy low wind velocity season november march when runoff and allochthonous terrestrial input are at a maximum and primary productivity is at a minimum

lacustrine paleochemical interpretations based on eastern and southern african ostracodes abstract ostracode assemblages in modern african lakes reflect water chemistry variations and so are potentially useful as paleochemical indicators the water chemistry ranges of 33 modern ostracode taxa have been evaluated for five parameters total conductivity k20 na ca2 cl and alkalinity co32 hco3 strong correlations exist for distribution patterns of ostracodes across all of these parameters except ca2 four somewhat arbitrary ostracode range assemblages can be defined based upon increasing alkalinity and salinity and named for characteristic taxa these are range i the sternocypris assemblage k20 4000 mho an application of the ostracode typology to the fossil record using a core from lake nakuru kenya as an example suggests a fluctuating alkaline lake prior to about 10 000 yr b p followed by fresh water conditions until about 8000 yr b p and finally a return to higher alkalinity throughout the remainder of the holocene since the core has also been studied for paleochemical interpretation based upon diatoms an independent verification of the technique is possible the two interpretations agree through 80 of the core suggesting a good reliability for the method

water level fluctuations of lake tanganyika in phase with oceanic changes during the last glaciation and deglaciation there has been considerable controversy about the magnitude of fluctuations of the levels of lake tanganyika the earth s second deepest lake 1 470 m following the discovery of submerged valleys extending down to 550 m below present lake levels1 these fluctuations register changes in the precipitation evaporation evapotranspiration ratio in a large equatorial tropical area of catchment south of the equator here we report new palaeohydrological data back to 40 kyr bp from carbon dating of the total organic matter in two diatomaceous cores the results constrain the vertical lake level fluctuations more than has hitherto been possible2 5 and show that the fluctuations from 26 kyr bp are correlated with changes in global sea level and ice volume surpris ingly fluctuations seem to be in phase with those of the african lakes north of the equator which are clearly linked to the milankovitch mechanisms

deltas of the lake malawi rift east africa seismic expression and exploration implications high resolution air gun sourced seismic reflection surveys over the offshore regions of five river deltas in lake malawi in the east african rift system reveal considerable variability in acoustic facies and stratigraphic architecture this variability can largely be attributed to the influences of different structural settings and to a lesser degree to high amplitude 100 400 m and high frequency 1000 to 100 000 yr fluctuations in lake level deltas on flexural and axial margins in the rift lake show well developed progradational geometries in contrast a delta on a steep accommodation zone margin distributes coarse sediments over a broad depositional apron rather than concentrating sediment in discrete progradational lobes as on the other deltas a large border ault margin river delta displays the most complex tectonic and stratigraphic architecture of all the deltas studied it contains several delta associated facies including prograding clinoform packages fan deltas stacked against a boundary fault and extensive subaqueous fans flexural margin lowstand deltas may be the most prospective for hydrocarbon exploration due to their large internally well organized progradational lobes and their close proximity to deep water high total organic carbon lacustrine source facies

seasonal and interannual particle export in an african rift valley lake a 5 yr record from lake malawi southern east africa a time sequencing sediment trap was deployed at 300 350 m in lake malawi east africa from 1987 to 1992 to measure particulate export to the deep anoxic hypolimnion in the northern and central lake regions the monthly settling particulate samples provide a data set of seasonal and interannual export fluxes of organic carbon biogenic opal calcium carbonate and lithogenic material maximum total particle fluxes 50 400 mg m 2 d 1 occurred primarily during the dry windy season april through october when algal productivity is high because of wind induced upwelling of nutrient enriched metalimnion and hypolimnion waters peak flux particulates contained an abundance of aulacoseira and stephanodiscus diatom valves and chains the total particle mass flux during the wet austral summer months november through march was consistently one to three orders of magnitude less than that measured during the dry months and consisted of mineral shards terrestrial plant debris and scattered diatom tests the 5 yr trap data provide support for the claim that the light dark lamination couplets abundant in northern and central lake cores reflect seasonal delivery to the sediments of diatom rich particulates during the windy months and diatom poor material during the wet season however interannual and spatial variability in upwelling and productivity patterns as well as el nino southern oscillation enso related rainfall and drought cycles appears to exert a strong influence over the magnitude and geochemical composition of particle export to the deep hypolimnion of lake malawi

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

controls of sedimentation in the malawi rift valley central africa abstract the malawi rift is the southermost classic rift valley trough in the cenozoic african rift valley system much of its floor is flooded by lake malawi which is approximately 570 km long 60 km across and up to 500 m deep this rift basin is a potentially useful analogue of many economically important ancient sedimentary troughs rift valley structures are extremely important in controlling both the distribution of sediments in the basin as a whole and also details of sedimentation on a local scale on the basin wide scale an important association between the type of rift boundary structure and amount of rift floor subsidence is recognised this association appears to apply to several section of the african rift system pre rift topography is major control on the position of major rivers and their deltas or turbidite fans along the basin margin the area of pre rift drainage network captured in the course of rift movements is a significant factor in the hydrologic budget of the basin climate is however the dominant control on the hydrologic budget and at present the basin is flooded to overflow level prevailing winds are important in beach ridge and back beach aeolian dune development climatic conditions produce thermal stratification of the lake with an oxic epilimnion down to 250 m depth overlying an anoxic hypolimnion the oxic bottom waters in the shallower parts of the lake appear to promote iron precipitation in the sediments

stratigraphic placement and age of modern humans from kibish ethiopia in 1967 the kibish formation in southern ethiopia yielded hominid cranial remains identified as early anatomically modern humans assigned to homo sapiens however the provenance and age of the fossils have been much debated here we confirm that the omo i and omo ii hominid fossils are from similar stratigraphic levels in member i of the kibish formation despite the view that omo i is more modern in appearance than omo ii 40ar 39ar ages on feldspar crystals from pumice clasts within a tuff in member i below the hominid levels place an older limit of 198 14 kyr weighted mean age 196 2 kyr on the hominids a younger age limit of 104 7 kyr is provided by feldspars from pumice clasts in a member iii tuff geological evidence indicates rapid deposition of each member of the kibish formation isotopic ages on the kibish formation correspond to ages of mediterranean sapropels which reflect increased flow of the nile river and necessarily increased flow of the omo river thus the 40ar 39ar age measurements together with the sapropel correlations indicate that the hominid fossils have an age close to the older limit our preferred estimate of the age of the kibish hominids is 195 5 kyr making them the earliest well dated anatomically modern humans yet described

organic geochemical records from lake malawi east africa of the last 700years part ii biomarker evidence for recent changes in primary productivity abstract relatively few well dated and high resolution paleoclimate records of the past few centuries presently exist from tropical east africa here we examine the bulk and molecular geochemical records of two varved sediment cores from lake malawi which together provide a continuous record of environmental variability in east africa of the last 730 years we observe a number of changes in the aquatic ecosystem of lake malawi which are likely attributed to both natural climatic forcing and anthropogenic activities biomarkers of dinoflagellates dinosterol and bacterivorous ciliates tetrahymanol display increased accumulation rates from 1900 ad to the present while a simultaneous decrease in accumulation rates of diatom biomarkers isololiolide loliolide is observed increased accumulation rates of retene a compound derived from conifers are also noted since 1930 ad and likely reflect increased soil erosion due to deforestation of the lake malawi watershed spectral analysis of the high resolution toc record indicates a periodicity of 204 years similar to the 206 year cycle noted in 14c and 10be records suggesting a link between east african climate and solar forcing

facies architecture of flexural margin lowstand delta deposits in lake edward east african rift constraints from seismic reflection imaging abstract lake edward is single half graben rift basin unique in the east african rift system as a result of its connection to lake george via the kazinga channel a paleo river valley which is incised into the flexural margin of the lake using recently acquired normal incidence seismic reflection data and thirty three lake bottom sediment samples we examine the consequences of this hydrologic connection for understanding depositional processes and facies development along the flexural margin of lake edward three depositional sequences were defined in the study area based on top and bottom discordances of seismic reflections climatically induced changes in lake level appear to have forced sequence development sequence i deposits indicate that highstand conditions prevailed earlier in the pleistocene evidence for high frequency lake level change comes from lowstand delta clinoforms in two separate stratigraphic intervals on the flexural margin an older lowstand event occurred during sequence ii time and reduced lake level by at least 37 m a younger lowstand event occurred during the deposition of sequence iii and reduced lake level by 18 m clastic sedimentation on the flexural margin of lake edward is strongly modified by inflow from the kazinga channel and lake george during periods of lowered lake level we suggest that the presence of an incised valley at the margin of the rift controls the stratigraphic development of clinoforms diagnostic of lowstand deltaic sedimentation the morphology and orientation of these clinoforms suggest that discharge from lake george during arid periods utilized the kazinga channel to transport sediment toward the exposed flexural margin of lake edward dipping reflections in sigmoid oblique clinoforms suggest that these flows were capable of transporting sediment loads that included coarse grain sizes deposition by this process occurred several times during the late quaternary history of lake edward and represents an important modern analogue for linked incised valley lacustrine delta sequences in the stratigraphic record

a revised 30 000 year paleoclimatic and paleohydrologic history of lake albert east africa abstract new geochemical and palynological data and a sedimentological reinterpretation of two cores from lake albert allow a revision of the paleoclimatic and paleohydrologic history of northwest uganda over the last 30 000 14 c yr bp our findings document one and possibly three periods of significantly reduced inflow to the nile river system from the white nile based on sediment fabric and mineralogy shifts in percentages of total organic carbon toc and total organic nitrogen ton hydrogen index hi and 13 c and 15 n as well as poor preservation of sporopollenin and siliceous microfossils we recognize two mature paleosols that formed between 18 000 and 12 500 14 c yr bp at the surface of exposed lacustrine muds a thin fossiliferous zone between the two soils documents a brief period of flooding lake albert was below the level of the coring site and thus a closed basin for several thousand years during the late pleistocene around 12 500 14 c yr bp the younger vertisol was flooded by a major rise in lake level relatively deep water conditions were established soon after this transgression but an apparent change in sedimentation rate sometime between 8 000 and 3 400 14 c yr bp may reflect another briefer period of low water which resulted in interruption of sedimentation at the coring site palynological data document a rapid and abrupt shift to drier conditions around lake albert from 11 400 to 9 900 14 c yr bp coincident with the cool younger dryas event of the northern hemisphere geochemical evidence suggests that throughout the terminal pleistocene and early holocene and again in the late holocene the water column was relatively well mixed with little tendency to persistent anoxia

lacustrine fan delta deposition alongside intrabasinal structural highs in rift basins an example from the early cretaceous jiuquan basin northwestern china development of fan deltas alongside intrabasinal structural highs has been overlooked compared to those forming on basin margins however these fan deltas may provide important clues regarding the tectonic and climatic controls on deposition during rift development this paper documents fan delta deposition alongside an intrabasinal structural high within the early cretaceous xiagou formation of the jiuquan basin china using subsurface geological and geophysical data deposits observed in drill core support fan delta deposition occurring almost exclusively through subaerial and subaqueous gravity flows subsurface mapping reveals a consistent decrease in the areal extent of fan deltas from lowstand to highstand system tracts suggesting that deposition alongside the structural high is sensitive to lake level changes the temporal and spatial distribution of the fan deltas display retrogradational stacking patterns where fan deltas exhibit a decreasing lateral extent up sequence until fan delta deposition terminated and was replaced by deposition of fine grained lacustrine deposits the retrogradational stacking patterns observed alongside the intrabasinal structural high are not observed in fan deltas along the basin margin in the lower parts of the xiagou formation subsidence profiles also show differential subsidence across the basin during the earliest stages of this formation likely resulting from border fault movements these data suggest that non uniform stacking patterns in the lower parts of the xiagou formation reflect basin scale tectonic movements as the dominant control on synrift deposition patterns however later stages of xiagou formation deposition were characterized by uniform subsidence across the basin and uniform retrogradational stacking patterns for fan deltas alongside the intrabasinal structural high and border fault these observations suggest that basin scale tectonic movements played a relatively limited role in controlling sediment deposition and imply a potential change to regional scale processes affecting fan delta deposition during later synrift stages climate change is favored here as the region scale control on the uniform retrogradational fan delta stacking patterns this assertion is supported by pollen assemblages isotope signatures and organic geochemical analyses which collectively suggest a change from a humid to semi arid environment during later synrift stages we suggest that variations in stacking patterns between different fan delta systems can provide insights into the basin and regional scale processes that control rift basin deposition

ash from the toba supereruption in lake malawi shows no volcanic winter in east africa at 75 ka the most explosive volcanic event of the quaternary was the eruption of mt toba sumatra 75 000 y ago which produced voluminous ash deposits found across much of the indian ocean indian peninsula and south china sea a major climatic downturn observed within the greenland ice cores has been attributed to the cooling effects of the ash and aerosols ejected during the eruption of the youngest toba tuff ytt these events coincided roughly with a hypothesized human genetic bottleneck when the number of our species in africa may have been reduced to near extinction some have speculated that the demise of early modern humans at that time was due in part to a dramatic climate shift triggered by the supereruption others have argued that environmental conditions would not have been so severe to have such an impact on our ancestors and furthermore that modern humans may have already expanded beyond africa by this time we report an observation of the ytt in africa recovered as a cryptotephra layer in lake malawi sediments 7 000 km west of the source volcano the ytt isochron provides an accurate and precise age estimate for the lake malawi paleoclimate record which revises the chronology of past climatic events in east africa the ytt in lake malawi is not accompanied by a major change in sediment composition or evidence for substantial temperature change implying that the eruption did not significantly impact the climate of east africa and was not the cause of a human genetic bottleneck at that time

seasonal and regional manifestation of arctic sea ice loss abstractthe arctic ocean is currently on a fast track toward seasonally ice free conditions although most attention has been on the accelerating summer sea ice decline large changes are also occu

organic geochemical records of environmental variability in lake malawi during the last 700 years part i the tex86 temperature record abstract we have applied the tex86 paleothermometer to produce a surface water temperature record for lake malawi spanning the past 700 years over much of the record temperature fluctuates from 24 27 c with a mean of 25 c however there has been a substantial increase in temperature of 2 0 c during the past 100 years the tex86 temperature record reveals a strong similarity to the instrumental record both records demonstrate warming 0 7 1 4 c over the past 50 years as well as a cooling anomaly around 1959 comparison of the tex86 temperature record with the proxy records of primary productivity suggests that wind induced upwelling and or precipitation have a strong influence on the surface temperature of lake malawi

dip analysis as a tool for estimating regional kinematics in extensional terranes abstract in basin analyses that rely almost solely on seismic reflection data to configure the underlying structure it has become common practice to use extensional fault geometries to infer kinematic histories this procedure relies on the existence of fault geometries predicted by ideal orthogonal extension and assumes the structure is well constrained in areas where data are sparse or of equivocal quality this often leads to circular analysis that is ideal orthogonal extension is assumed to have been operative then under constrained fault geometries are mapped according to model predictions fault geometries so derived are subsequently used as evidence that orthogonal extension was operative and that the direction of extension is determinable we develop an alternative tool for inferring kinematics which is independent of fault geometries the method relies on the empirical evidence that the dip of the hangingwall s is systematic and predictable in extensional systems even when fault geometries are non ideal although not without significant limitations dip analysis can provide additional constraints for structural and kinematic interpretations especially where seismic reflection data are the main source of information and dip domains define discrete crustal blocks

lithostratigraphy physical properties and organic matter variability in lake malawi drillcore sediments over the past 145 000years abstract scientific drill cores recovered from lake malawi exhibit a remarkable down core lithologic variability and are indicative of radically changing environmental conditions forced by large amplitude lake level shifts over the past 150 kyr here we present detailed lithologic and sedimentary fabric descriptions of the key sedimentary units along with down core physical properties data down core organic matter geochemistry toc c n and 13c data sets and images and descriptions from core sections and from sediment smear slide microscopy these data reveal a fundamental change in lake malawi s limnology and regional climate at ca 60 70 ka prior to this time the lake was characterized by large amplitude variations in lake level and water chemistry but after 60 ka the lake remained comparatively high and the central basin drill site accumulated mainly organic rich laminated sediments organic matter sources changed dramatically during the different lake stages during major lake high stands a mixed assemblage of algal diatom dominated woodland and aquatic macrophyte c4 pathway and grassland c3 pathway organic matter was deposited whereas during extreme low lake stages water depths

late pleistocene desiccation of lake victoria and rapid evolution of cichlid fishes lake victoria is the largest lake in africa and harbors more than 300 endemic species of haplochromine cichlid fish seismic reflection profiles and piston cores show that the lake not only was at a low stand but dried up completely during the late pleistocene before 12 400 carbon 14 years before the present these results imply that the rate of speciation of cichlid fish in this tropical lake has been extremely rapid

lacustrine environment during lower beaufort upper permian karoo deposition in northern malawi abstract this paper provides the first detailed sedimentological facies analyses of a lower beaufort upper permian gondwana sequence from mt chombe are in northern malawi approximately 300 m of undeformed non marine mudstones siltstones and sandstones are preserved in isolated grabens exposed along the mzuzu karonga highway where it crosses the lake malawi escarpment five lithofacies are distinguished a finely laminated paper shales and muddy siltstones deposited in lakes with anoxic bottom waters b massive muddy siltstones and siltstones deposited in oxic lake waters c thinly bedded horizontally stratified siltstones from episodic sheet flows into a lake d cross stratified and flaser bedded siltstones and sandstones of higher energy lake environments and e massive and cross bedded coarse grained sandstones with small channels and intraformational pebble layers interpreted as marginal lacustrine deposits new dates by pollen indicate an uppermost permian interval the whole sequence appears to have been deposited in a large shallow lake complex as none of the strata displays subaerial features such as desiccation cracks pedogenesis or rootlets all facies are reworked as is also suggested by the lack of in situ faunal or floral remains organic matter is mainly land derived although some algal dominated intervals occur clastic mineralogy is immature uniform and dominated by feldspar and quartz clay minerals include smectite kaolinite and illite carbonate contents greater than 5 and up to 40 are taken as an evidence of periods of authigenic precipitation in a freshwater lake setting the mt chombe sequence is lithologically similar but differs in significant sedimentology aspects from equivalent sequences in the adjacent luwanga and ruhuhu basins where sub aerial deposition has been reported although all three basins are part of a stable cratonic palaeoenvironment the absence of the lower permian upper coal measures from karoo sequences in northern malawi is taken as evidence of some syndepositional topographic relief in the region

a high resolution paleoclimate record spanning the past 25 000 years in southern east africa high resolution profiles of the mass accumulation rate of biogenic silica and other geochemical proxies in two piston cores from northern lake malawi provide a climate signal for this part of tropical africa spanning the past 25 000 years the biogenic silica mass accumulation rate was low during the relatively dry late pleistocene when the river flux of silica to the lake was suppressed millennial scale fluctuations due to upwelling intensity in the late pleistocene climate of the lake malawi basin appear to have been closely linked to the northern hemisphere climate until 11 thousand years ago relatively cold conditions in the northern hemisphere coincided with more frequent north winds over the malawi basin perhaps resulting from a more southward migration of the intertropical convergence zone

measurement of 10be from lake malawi africa drill core sediments and implications for geochronology abstract the cosmogenic radionuclide 10 be was measured from drill core sediments from lake malawi in order to help construct a chronology for the study of the tropical paleoclimate in east africa sediment samples were taken every 10 m from the core mal05 1c to 80 m in depth and then from that depth in core mal05 1b to 382 m sediment samples were then later taken at a higher resolution of every 2 m from mal05 1c they were then leached to remove the authigenic fraction the leachate was processed to separate out the beryllium isotopes and 10 be was measured at the tams facility at the university of arizona the 10 be 9 be profile from lake malawi sediments is similar to those derived from marine sediment cores for the late pleistocene and is consistent with the few radiocarbon and osl ir measurements made from the same core nevertheless a strong correlation between the stable isotope 9 be and the cosmogenic isotope 10 be suggests that both isotopes have been well mixed before deposition unlike in some marine sediment cores in addition the correlation of beryllium isotopes to a proxy of lake level toc total organic matter from lake malawi indicates that the concentrations of 10 be in the lake sediments result from the combined effects of global and local climates on lake level local hydrology and sediment transport in the lake malawi basin rather than as a direct response to its production in the atmosphere modulated by the intensity of the earth s dipole therefore a direct correlation of the 10 be 9 be to a chronology derived from the paleomagnetic variations measured from marine sediments was not possible nevertheless a comparison of the 10 be 9 be chronology allowing for decay at lake malawi to that of the global marine paleomagnetic record suggests that the bottom of core mal05 1b is no more than 750 ka in age

the seismic stratigraphic record of lake level fluctuations in lake challa hydrological stability and change in equatorial east africa over the last 140 kyr abstract seismic reflection data from crater lake challa mt kilimanjaro equatorial east africa reveal a 210 m thick sedimentary infill containing distinct seismic stratigraphic signatures of late quaternary lake level fluctuations extrapolation of a well constrained age model on the cored upper part of the sequence suggests that these lake level fluctuations represent a detailed and continuous record of moisture balance variation in equatorial east africa over the last 140 kyr this record indicates that the most severe aridity occurred during peak penultimate glaciation immediately before 128 kyr bp coeval with heinrich event 11 and during a last interglacial megadrought period between 114 and 97 kyr bp in comparison last glacial maximum lgm aridity was modest it was preceded by 75 000 years of relatively stable and moist climate conditions interrupted by eleven short lived dry spells five of which match the timing of heinrich events 2 to 6 climate history near the east african equator reflects variation in the precessional forcing of monsoon rainfall modulated by orbital eccentricity but precession driven moisture fluctuations were less extreme than those observed in northern and southern tropical africa the near continuous moist climate from 97 to 20 5 kyr bp recorded in the lake challa record contrasts with the trend towards greater aridity after 70 kyr bp documented in equatorial west africa this long period of moist glacial climate and a short relatively modest lgm drought can be attributed to greater independence of western indian ocean monsoon dynamics from northern high latitude glaciation than those in the tropical atlantic ocean this rather persistent moist glacial climate regime may have helped maintain high biodiversity in the tropical forest ecosystems of the eastern arc mountains in tanzania

late pleistocene temperature history of southeast africa a tex86 temperature record from lake malawi we present a tex86 derived surface water temperature record for lake malawi that provides the first continuous continental record of temperature variability in the continental tropics spanning the past similar to 74 kyr with millennial scale resolution average temperature during marine isotope stage mis 5a was 26 5 degrees c with a range from 25 7 to 27 3 degrees c comparable to holocene temperatures mis 4 was a relatively cold period with temperatures generally decreasing from 25 5 degrees c at 68 ka to a minimum of 20 degrees cat similar to 60 ka 1 5 2 degrees c colder than the last glacial maximum lgm termination of mis 4 is characterized by a rapid increase of 3 4 degrees c in only similar to 0 5 kyr temperatures were relatively stable throughout mis 3 at the resolution of this study with an average of 23 8 degrees c and a range from 25 1 to 22 9 degrees c the lack of millennial scale temperature variability during mis 3 suggests that lake malawi s documented response to the bipolar seesaw brown et al 2007 is not reflected in its thermal history our temperature estimates for the lgm and holocene are consistent with a previously published tex86 record from lake malawi with a temperature of similar to 22 6 degrees c for the lgm similar to 25 26 degrees c in the mid holocene and similar to 25 28 for the late holocene in general the present extended tex86 record indicates that temperature variability in tropical east africa during late mis 5 and mis 4 was as great as that associated with the deglaciation and holocene a decrease in southern hemisphere insolation between 70 and 60 ka may have played an important role in forcing temperatures during mis 4 but after 60 ka other factors such as the extent of the polar ice sheets or atmospheric co2 may have forced temperature in tropical africa to a greater extent than local summer insolation

tropical east african climate change and its relation to global climate a record from lake tanganyika tropical east africa over the past 90 kyr abstract forcing mechanisms of tropical climate in continental areas remain poorly understood due in large part to a lack of continuous long term high fidelity records sediment core t97 52v from lake tanganyika provides new insight into the timing and mechanisms behind east african climate change over the past 90 kyr this record is particularly important because other than a recently recovered scientific drill core from lake malawi there are no other continuous well dated records from east africa prior to 60 ka the high resolution age model presented here provides a large degree of age certainty for the past 45 kyr and our suite of proxies allows a thorough examination of lake tanganyika s dynamics from core stratigraphy and chemical analyses we present evidence of a lake level drop greater than 400 m sometime prior to 90 ka much greater than that inferred for the lgm suggesting a period of intense aridity sometime around 100 ka additionally core t97 52 v preserves evidence of worm burrows detected by x radiographic imagery as indicated by burrow shaped deposits of iron oxide indicating a shallow lake at the time of deposition of that material intermittently high lake levels between 78 ka and 72 ka developed at the same time as a weakened asian monsoon and a pluvial phase in northeast brazil suggesting a global reorganization of climate possibly forced by a reduction in orbital eccentricity over the past 60 ka this core preserves the same events recorded in a core collected 100 km away in the southern basin of lake tanganyika including an unexplained increase in biogenic silica at 37 ka suggesting that this vast lake is responding coherently across both major bathymetric basins to regional and global climate forcing

effect of aridity and rainfall seasonality on vegetation in the southern tropics of east africa during the pleistocene holocene transition fossil pollen analyses from northern lake malawi southeast africa provide a high resolution record of vegetation change during the pleistocene holocene transition 18 9 ka recent studies of local vegetation from lowland sites have reported contrasting rainfall signals during the younger dryas yd the lake malawi record tracks regional vegetation changes and allows comparison with other tropical african records identifying vegetation opening and local forest maintenance during the yd our record shows a gradual decline of afromontane vegetation at 18 ka around 14 5 ka tropical seasonal forest and zambezian miombo woodland became established at 13 ka drier more open formations gradually became prevalent although tropical seasonal forest taxa were still present in the watershed during the yd this drought intolerant forest type was likely restricted to areas of favorable edaphic conditions along permanent waterways the establishment of drought tolerant vegetation followed the reinforcement of southeasterly tradewinds resulting in a more pronounced dry winter season after 11 8 ka the onset of the driest most open vegetation type was coincident with a lake low stand at the beginning of the holocene this study demonstrates the importance of global climate forcing and local geomorphological conditions in controlling vegetation distribution

development of coarse grained facies in lacustrine rift basins examples from east africa detailed studies of high resolution and multifold seismic reflection data from the two largest east african rift lakes malawi and tanganyika reveal a complex suite of coarse grained depositional facies these facies occur within specific regions of the controlling half grabens that compose the rift lakes sand prone environments include subaqueous channels and small drowned fluvial complexes channel systems range from large erosional canyons to deep water turbidite channel levee systems lowstand and highstand deltas of axial and shoaling side rivers are volumetrically important coarse grained facies fan deltas develop along the base of major border faults during lake lowstands subaqueous talus fan deposits occur along the base of the border faults during lake highstands lowstand deltas are the best preserved progradational facies in these rift lakes in addition to simple tectonic control drastic tectonically or climatically induced lake level change significantly regulates the production of coarse grained lacustrine synrift deposits

sensitivity of the atlantic intertropical convergence zone to last glacial maximum boundary conditions 1 recent paleoproxy records suggest that the mean latitude of the atlantic intertropical convergence zone itcz varied synchronously with north atlantic climate over a range of timescales throughout the holocene and last glacial maximum we show that the present day meridional mode of atmosphere ocean variability in the tropical atlantic is a potentially useful model for understanding these paleoclimate changes the tropical atlantic in a coupled atmospheric general circulation and slab ocean model responds to last glacial maximum conditions with a southward displacement of the itcz this response arises primarily through the land ice sheet that forces increased north atlantic trades analogous to the forcing on the present day meridional mode changes to sea ice coverage and to ocean heat transport associated with a weakened atlantic thermohaline circulation also cause a meridional mode response though through different mechanisms our results highlight the potential for tropical atlantic paleoclimate to be driven from the high latitude influences in particular land ice on glacial interglacial timescales

a comparison of two sets of synthetic reflection seismograms for a complex two dimensional model of the oceanic crust mantle transition the algorithm chosen for the calculation of synthetic seismograms often represents a compromise between the realistic representation of complex geology and economy comparison of synthetic seismic reflection sections calculated using a relatively inexpensive ray theory and a relatively expensive generalized reflectivity algorithm for a complex laterally inhomogeneous model of the oceanic moho demonstrates that ray theory provides a close approximation to the full wave theory the chief difference between the two synthetic sections is the more accurate variation in amplitude with horizontal range predicted by the reflectivity synthetics due to its inclusion of the horizontal smoothing of the reflected amplitudes over the width of the first fresnel zone while this fresnel zone smoothing will become increasingly important with increasing reflector depth the results shown here suggest that inexpensive ray theory synthetics may have sufficient accuracy in many similar applications

filtering of milankovitch cycles by earth s geography abstract earth s land sea distribution modifies the temperature response to orbitally induced perturbations of the seasonal insolation we examine this modification in the frequency domain by generating 800 000 yr time series of maximum summer temperature in selected regions with a linear two dimensional seasonal energy balance climate model previous studies have demonstrated that this model has a sensitivity comparable to general circulation models for the seasonal temperature response to orbital forcing on land although the observed response in the geologic record is sometimes significantly different than modeled here differences attributable to model limitations and feedbacks involving the ocean atmosphere cryosphere system there are several results of significance 1 in mid latitude land areas the orbital signal is translated linearly into a large 10 c seasonal temperature response 2 although the modeled seasonal response to orbital forcing on antarctica is 6 c the annual mean temperature effect

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

late quaternary lacustrine deposits of the dead sea basin high resolution sequence stratigraphy from downhole logging data abstract sequence architecture and depositional sequences of the quaternary lacustrine succession deposited in the northern dead sea sub basin were examined using logging data collected during the 2010 2011 icdp campaign methods borrowed from sequence stratigraphy techniques were used to investigate the characteristics of sediments deposited in the central part of the northern lake high resolution wire logging data combined with a detailed lithological description of the icdp 5017 1 a deep borehole were used to examine depositional systems and related processes controlling their formation analysis of sedimentary stacking patterns and stratal surfaces within the late pleistocene holocene lacustrine succession revealed 10 depositional sequences it was possible to identify key stratal boundaries and to discern between three sedimentary stacking patterns interpreted here as representing lowstand systems tracts lst transgressive systems tracts tst and highstand systems tracts hst examined together they may be interpreted in terms of relative lake level changes on the basis of the stratigraphic analysis complemented with new age dating this article presents a record of the sediment accumulation pattern and a relative lake level curve reconstructed for the last ca 225 ka results show that stratigraphic units and depositional and erosional surfaces examined in the deep 5017 1 a borehole can be correlated to the proximal area of the basin this means that changes in relative lake levels were generally synchronous and uniform across the dead sea basin the creation of accommodation space in the northern dead sea was found to generally be in phase with paleoclimatic modulating lake levels and not due to tectonics

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

paleoecological response of ostracods to early late pleistocene lake level changes in lake malawi east africa abstract ostracod species from at least seven genera were recovered from drill cores recently collected at lake malawi a large 29 500 km2 and deep 706 m african rift valley lake located in the southern african tropics 9 14 s these genera include limnocythere candonopsis ilyocypris sclerocypris gomphocythere possible allocypria and an unknown cypridosine taphonomic variables such as the percentage of valve breakage adult individuals carbonate and oxidized coatings as well as associated mineralogy can be used to delineate lake level low and highstands this record of lake level fluctuations is correlated with paleoecological changes in ostracod communities throughout the core record of the past 145 ka two major assemblages found within the lake malawi cores include a limnocythere dominated shallow saline alkaline assemblage that occurred between 133 and 130 ka and between 118 and 90 ka and a deeper water cypridopsine assemblage that lived in waters 10s to 100s of meters in depth and dominated the ostracod assemblage during intervals of lake level transitions 136 133 ka 129 128 ka and 86 63 ka between the occurrence of the littoral assemblage and the appearance of indicators of bottom water anoxia changes in occurrences and abundances indicate variations in paleoecological affinities related to lake chemistry and oxygenation of bottom waters the characteristics of the different lineages influence how that lineage is likely to respond to environmental variability such as lake level fluctuations clades that include large numbers of endemic species in lake malawi tend to be specialists and thus more susceptible to environmental perturbations on the other hand cosmopolitan species within the lake all appear to be generalists suggesting that they have high tolerances to environmental variability and therefore these genera tend to be monospecific within the lake and are less likely to have radiated within the basin the distribution of these ostracods can be used to evaluate hypotheses about the environmental history at the landscape scale and their potential influence on species distribution and diversification histories

an empirical test of the relative frequency of bipolar reduction in beds vi v and iii at mumba rockshelter tanzania implications for the east african middle to late stone age transition abstract mumba rockshelter tanzania is the only east african site spanning a continuous record of more than 100 000 years including the middle stone age msa to late stone age lsa transition rather than examine the presence or absence of traditional markers of behavioral modernity we have endeavored here to respond to the calls of shea 2011 and basell 2008 by examining the proportional similarity of a trait present throughout the msa lsa sequence at mumba rockshelter the bipolar technique of lithic reduction we use a quantified experimentally derived proxy to track the relative amount of bipolar percussion among mumba s beds vi v and iii namely waste shatter our examination of mehlman s previously unanalyzed collections from mumba demonstrates that in terms of bipolar production bed v is statistically indistinguishable from the late stone age lsa bed iii but significantly different from the middle stone age msa bed vi given that bed v dates to 56 9 4 8 49 1 4 3 ka cal bp this result is consistent with other evidence that the origins of the lsa in east africa began well before 40 kya and that mumba bed v represents one of the earliest manifestations of the lsa in east africa we conclude with a discussion of factors that may have influenced the increased dependence of bipolar production at the site

abrupt change in tropical african climate linked to the bipolar seesaw over the past 55 000 years 1 the tropics play a major role in global climate dynamics and are vulnerable to future climate change we present a record of east african climate since 55 ka preserved in lake malawi sediments that indicates rapid shifts between discrete climate modes related to abrupt warming d o events observed in greenland although the timing of the malawi events cannot be determined exactly our age model implies that they occur prior to their greenland counterparts consistent with southward excursions of the intertropical convergence zone during greenland stadials the magnitude of each of the events recorded in malawi sediments corresponds to the scale of the subsequent greenland warming this suggests that a tropical component of climate sets a template for abrupt high northern latitude climate fluctuations associated with the bipolar seesaw

large temperature variability in the southern african tropics since the last glacial maximum 1 the role of the tropics in global climate change is actively debated particularly in regard to the timing and magnitude of thermal and hydrological response continuous high resolution temperature records through the last glacial maximum lgm from tropical oceans have provided much insight but surface temperature reconstructions do not exist from tropical continental environments here we used the tex86 paleotemperature proxy to reconstruct mean annual lake surface temperatures through the last glacial maximum lgm in lake malawi east africa 9 14 s we find a 3 5 c overall warming since the lgm with temperature reversals of 2 c during the younger dryas 12 5 ka bp and at 8 2 ka bp maximum holocene temperatures of 29 c were found at 5 ka bp a period preceding severe drought in africa these results suggest a substantial thermal response of southeastern tropical africa to deglaciation and to varying conditions during the holocene

the pretoria saltpan a 200 000 year southern african lacustrine sequence abstract the pretoria saltpan is a circular crater 1130 m in diameter and is situated some 40 km n of pretoria lat 25 34 30 long 28 04 59 e a recent tube sampling and core drilling programme has revealed an infilling consisting of some 90 m of fine lacustrine sediments chiefly organic muds underlain below 30 m depth by micrites which rest upon a further 61 m of coarse clastic debris granite bedrock was encountered at 151 m broad sedimentary zones correspond with major phases in the evolution of the crater lake superimposed cyclical patterns of accumulation reflect environmental changes on millenial to seasonal timescales 14c age determinations on algal debris from the upper 20 m of the core indicate a mean rate of sedimentation of about 1 m 2000 yr suggesting that the lacustrine sequence may span almost 200 000 yr over this period major environmental changes are apparent from sedimentological chemical mineralogical and isotopic analyses of the core and studies of the pollen spectra and diatom assemblages present within it this long continental sequence is therefore providing a high resolution palaeoenvironmental record for southern mid latitudes over much the same period as is covered by the vostok ice core in antarctica

approaches to middle stone age landscape archaeology in tropical africa the southern montane forest grassland mosaic ecosystem in the humid subtropics southern rift valley of africa comprised the environmental context for a large area in which modern human evolution and dispersal occurred variable climatic conditions during the late pleistocene have ranged between humid and hyperarid changing the character of the ecosystem and transforming it at different points in time into a barrier a refuge and a corridor between southern and eastern african populations alluvial fans presently blanket the areas adjacent to major river systems which were key areas of prehistoric human habitation these sets of variables have created conditions that are both challenging and advantageous to conduct archaeological research lateritic soil development has resulted in poor organic preservation and facilitated insect bioturbation which has demanded an integrated micro macro scale approach to building a reliable geochronology an integrated field and analytical methodology has also been employed to identify the nature and degree of post depositional movement in alluvial deposits which preserve a wide range of spatial integrity levels in buried stone artifact assemblages between 47 and 30 ka in karonga northern malawi this paper describes the methodological advances taken toward understanding open air middle stone age archaeology in sub tropical africa and explores the inferential potential for understanding pleistocene human ecology in the important southern rift valley region

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

late pleistocene paleohydrography and diatom paleoecology of the central basin of lake malawi africa analysis of sedimentary diatom assemblages 10 to 144 ka form the basis for a detailed reconstruction of the paleohydrography and diatom paleoecology of lake malawi lake level fluctuations on the order of hundreds of meters were in ferred from dramatic changes in the fossil and sedimentary archives many of the fossil diatom assemblages we observed have no analog in modern lake malawi cyclotelloid diatom species are a major component of fossil assemblages prior to 35 ka but are not found in significant abundances in the modern diatom communities in lake malawi salinity and alkalin ity tolerant plankton has not been reported in the modern lake system but frequently dominant fossil diatom assemblages prior to 85 ka large stephanodiscoid species that often dominate the plankton today are rarely present in the fossil record prior to 31 ka similarly prior to 31 ka common central basin aulacoseiroid species are replaced by species found in the shallow well mixed southern basin surprisingly tychoplankton and periphyton were not common throughout prolonged lowstands but tended to increase in relative abundance during periods of inferred deeper lake environments a high resolution lake level reconstruction was generated by a principle component analysis of fossil diatom and wetsieved fossil and mineralogical residue records prior to 70 ka fossil assemblages suggest that the central basin was periodically a much shallower more saline and or alkaline well mixed environment the most significant reconstructed lowstands are 600 m below the modern lake level and span thousands of years these conditions contrast starkly with the deep dilute dysaerobic environments of the modern central basin after 70 ka our reconstruction indicates sustained deeper water environments were common marked by a few brief but significant lowstands high amplitude lake level fluctuations appear related to changes in insolation seismic reflection data and additional sediment cores recovered from the northern basin of lake malawi provide evidence that supports our reconstruction

vegetation response to glacial interglacial climate variability near lake malawi in the southern african tropics abstract pollen records from lake malawi africa spanning the last 135 kyr show substantial and abrupt vegetation response to multiple episodes of extreme aridity between 135 and 75 ka peaks in both the relative abundance and total production of podocarpus pollen define the first two of these drought episodes from 135 to 127 and again from 117 to 105 kyr bp podocarpus percentages remain above 16 with peak values as high as 38 indicating a period marked by a cool climate resulting in expansion of montane forest taxa to lower elevations marine palynological records from the angola margin and congo fan show similar peak podocarpus percentages at this time oxygen isotope stage 5d indicating a similar climate across the african continent at this latitude from 105 to 90 ka continuing drought resulted in total pollen accumulation rates in lake malawi to fall to less than 300 grains cm2 yr of predominately grass pollen this episode in african history was severe enough to cause the disappearance of pteridophytes and forest taxa such as uapaca and brachystegia as well as montane taxa podocarpus olea spp and ericaceae within the pollen source area of lake malawi these taxa all remain nearly absent from the surrounding vegetation for the next 18 000 years the resultant semi desert vegetation would have been inhospitable for early humans living within or traveling through the lake malawi region increasing moisture following these arid intervals allowed expansion creation and maintenance of a more diverse landscape vegetation mosaic around lake malawi including zambezian miombo woodland humid evergreen woodland and afromontane forests the relative abundance of each fluctuated in response to either cooling i e afromontane expansion from 60 to 56 ka or moisture balance i e increasing humid evergreen woodland between 75 and 65 ka notably there was no significant change in vegetation composition during the last glacial maximum lgm 30 15 ka as compared to the previous 20 000 years