late pleistocene and holocene vegetation history of the bale mountains ethiopia abstract a sediment core recovered from garba guracha a glacial lake at 3950 m altitude in the bale mountains of ethiopia at the boundary of the ericaceous and afroalpine vegetation belts provides a 16 700 year pollen record of vegetation response to climatic change the earliest vegetation recorded was sparse and composed mainly of grasses amaranthaceae chenopodiaceae and artemisia indicating an arid climate at 13 400 cal bp amaranthaceae chenopodiaceae pollen declined sharply and cyperaceae increased suggesting a change to moister conditions the younger dryas interval is represented by a small increase in artemisia and reduced cyperaceae indicating aridity just after the start of the holocene 11 200 cal bp the upper altitudinal limit of the ericaceous belt rose and woody ericaceous vegetation extended across the sanetti plateau in response to increased moisture and temperature the marked change from clastic to organic lake sedimentation at this time reflects the increase in woody vegetation cover in the lake catchment accompanied by soil stabilisation and increased leaf litter and soil humus content from about 6000 cal bp and especially after 4500 cal bp mid altitude dry afromontane juniper podocarpus forests developed on the northern slopes of the mountains in response to reduced rainfall in a shortened wet season erica shrub and forest decreased in area and altitude and the afroalpine ecosystem expanded on the plateau podocarpus declined from about 2000 cal bp as juniperus increased to its present dominance at 2500 3300 m altitude human impact on the high altitude afroalpine and ericaceous vegetation has been relatively minor confirming that the endemic biodiversity of the ethiopian mountains is a legacy of natural holocene vegetation change following repeated expansion and contraction of the upland ecosystems during the quaternary

quaternary glaciation of the bale mountains ethiopia central ethiopia comprises a high plateau at 2000 3000 m formed from tertiary lava flows and bisected by the eastern african rift ten volcanic mountains rise to altitudes of just over 4000 m but on only three has quaternary glaciation been substantiated by published field observations on the bale mountains 4400 m a previous report based on limited evidence proposed an ice cap extending to 600 km2 based on aerial photographs and ground surveys this paper reports evidence of a more complex situation a wide spread of large erratic boulders on the plateau records a central ice cap of 30 km2 though ice probably extended for a further 40 km2 further north two groups of deeply incised and clearly glaciated valleys contain moraines and roches moutonnees 60 km2 on interfluves between them and on the open north slopes are moraines from an earlier stage of the same glaciation or from a distinct older event altogether about 180 km2 may have been glaciated cores dated by 14c from inside and outside the glaciated area suggest that at least the northern valley glaciers may date from the last glacial maximum estimated equilibrium line altitudes for these glaciers and the ice cap are 3750 4230 m

late quaternary pollen records from two ugandan mires evidence for environmental changes in the rukiga highlands of southwest uganda abstract high resolution pollen data from two mires in the rukiga highlands southwest uganda ahakagyezi swamp and muchoya swamp provide detailed evidence of the environmetal history of the region since ca 42 000 yr b p according to these data there a was period pre ca 42 000 yr b p when climatic conditions were similar to the present conditions considerably colder and drier than the present occurred during two periods in the past pre ca 42 000 yr b p to ca 32 000 yr b p and ca 21 000 yr b p to ca 12 000 yr b p the most recent period of climatic improvement began ca 14 000 yr b p moist lower montane forest colonised the muchoya catchment by ca 11 100 yr b p and the ahakagyezi catchment by ca 10 600 yr b p indicating the establishment there of climatic conditions broadly similar to the present pollen data from both mires reveal forest disturbances beginning ca 2200 yr b p this disturbances is likely to have been human induced

abrupt return of the summer monsoon 15 000 years ago new supporting evidence from the lower white nile valley and lake albert abstract the last glacial period ended with an abrupt return to warmer and wetter conditions at a number of sites in intertropical africa asia australia and south america 15 000 years 15 ka ago similar abrupt warming is also evident at this time in high latitudes and in air trapped in greenland ice we here report new supporting evidence of this event from a climatically sensitive region in africa the white nile valley of the central sudan and lake albert in the uganda headwaters during the last glacial maximum the white nile valley was even more arid than it is today with desert dunes active as far south as latitude 12 s the sudden overflow of lake victoria in the ugandan headwaters of the white nile 14 5 ka confirmed here by sr isotope statigraphy caused extensive flooding in the lower white nile valley and severe flooding in egypt the volume of fresh water flowing into the eastern mediterranean at this time ultimately resulted in accumulation of highly organic sediments sapropel 1 on the floor of the mediterranean sea flooding in the central sudan attained an elevation of 382 m along a north south distance of 400 km the recessional shoreline of the 382 m white nile is indicated by concentrations of now buried freshwater gastropod shells and beach sands dated respectively by calibrated ams radiocarbon and osl to 15 16 ka this mega flood event marks the abrupt return of the summer monsoon and may reflect a globally synchronous event

groundwater mediated response to holocene climatic change recorded by the diatom stratigraphy of an ethiopian crater lake abstract the diatom stratigraphy of a 23 m sediment core from lake tilo a maar lake in the ethiopian rift valley provides a 10 000 yr record of lake salinity and trophic status until 5500 14c yr b p the phytoplankton was dominated by aulacoseira granulata with only minor changes in the abundance of other diatoms the lake was over 50 m deep eutrophic and oligosaline at 5500 yr b p geothermal groundwater inflow inferred from calcite and silica deposition rates declined abruptly and the lake became more oligotrophic as indicated by a rapid rise in cyclotella stelligera about 4500 yr b p lake salinity began to increase reaching approximately its present state ca 2500 yr b p but with a temporary reversal to lower salinity at 4000 3500 yr b p the record shows no evidence of salinity increases equivalent to early holocene low stands of the larger river fed rift valley lakes probably because of high rates of geothermal groundwater influx it responded to reduced rainfall at 4500 yr b p when levels of the larger lakes also fell because geothermal groundwater flux had diminished 1000 years earlier independently of climate because geothermal groundwaters can form a significant proportion of a crater lake s hydrological budget and dominate its salinity budget these results show that variable geothermal groundwater flux can override lake hydrochemical response to climate change palaeoclimatic interpretation of palaeosalinity proxies from the sediments of volcanic crater lakes should be approached with caution

constraints on the lithospheric structure of venus from mechanical models and tectonic surface features radar images of the surface of venus show numerous structures that are interpreted as having formed due to horizontal compression and extension of the lithosphere many of these features exhibit characteristic scales widths or spacings of deformation one of 10 20 km and another of 100 300 km for a range of simple models we test the hypothesis that these length scales are controlled by dominant wavelengths arising from unstable compression or extension of the venus lithosphere results show that the existence of tectonic features that exhibit both length scales can be explained if at the time of deformation the lithosphere consisted of a crust that was relatively strong near the surface and weak at its base and an upper mantle that was stronger than or of nearly comparable strength to the upper crust the spacings of these features imply crustal thicknesses in the approximate range 5 30 km and a thermal gradient not greater than 25 k km 1 features that exhibit only the smaller scale of deformation can be explained by either a lithosphere with a thick crust that overlies a weak mantle or a lithosphere with a strong mantle but with small internal strength contrasts for a broad range of parameters the models require not only that smaller scale compressional and extensional features have similar spacings but also that the widths and spacings of larger scale compressional features be greater than those formed in extension this is consistent with observed differences in the characteristic length scales of larger scale surface features as well as the similarities in spacings of observed smaller scale features

post glacial arid episodes in ethiopia have implications for climate prediction several lines of geomorphological and stratigraphical evidence suggest that the widespread early post glacial wet phase in tropical africa between 12 500 and 5 000 yr bp refs 1 2 was interrupted by severe dry episodes during the intervals 11 000 10 000 8 500 6 500 and 6 200 5 800 yr bp refs 3 9 the magnitude of the second event has recently been questioned10 we present new stratigraphical and dating evidence from the ziway shala lake basin ethiopia indicating the severity and timing of dry phases during the interval 12 000 5 000 yr bp our findings have important implications for predictions of the climatic impact of increasing atmospheric levels of co2

environmental change and human occupation of southern ethiopia and northern kenya during the last 20 000 years abstract our understanding of the impact of climate driven environmental change on prehistoric human populations is hampered by the scarcity of continuous paleoenvironmental records in the vicinity of archaeological sites here we compare a continuous paleoclimatic record of the last 20 ka before present from the chew bahir basin southwest ethiopia with the available archaeological record of human presence in the region the correlation of this record with orbitally driven insolation variations suggests a complex nonlinear response of the environment to climate forcing reflected in several long term and short term transitions between wet and dry conditions resulting in abrupt changes between favorable and unfavorable living conditions for humans correlating the archaeological record in the surrounding region of the chew bahir basin presumably including montane and lake marginal refugia for human populations with our climate record suggests a complex interplay between humans and their environment during the last 20 ka the result may contribute to our understanding of how a dynamic environment may have impacted the adaptation and dispersal of early humans in eastern africa

tectonics and topography for a lithosphere containing density heterogeneities the purpose of this paper is to clarify the dynamic role of lithospheric density heterogeneities in particular with respect to mountain building and other processes of intraplate deformation density anomalies within or just beneath the lithosphere constitute major sources for tectonic stress fields the product of their magnitude by their depth is shown to characterize their ability to induce deformation this rule of the density moment directly yields the lithospheric thickening or thinning rate when applied to structures of large lateral extent for anomalies of lateral extent that is small in comparison with their depth the deformation is vertically inhomogeneous and has been computed with the help of simple physical models of a stratified viscous newtonian lithosphere the analytical treatment is based on fourier transform continent continent collision thickens not only the crust but the entire lithosphere the cold root underlying a mountain chain induces strong regional compressive stresses able to sustain the mountain bulding process without further help from forces transmitted from far away thus the continental lithosphere is in a somewhat metastable mechanical state adiabatic i e rapid thickening or thinning tends to grow further once initiated tectonic phases of strong compression correspond to the climax of such instabilities the response of models with cold lithospheric roots of various intensities has been computed both in two and three dimensions they yield velocity distributions and stress fields instructive comparisons are made with earthquake focal mechanisms and in situ stress measurements in the alpine and appalachian regions in the presence of lateral variations of the mechanical properties of the lithosphere the tectonic style is not only function of the local topography and of the nature of its compensation deformations in neighbouring provinces are coupled as shown by 3 dimensional models for example thickening sustained by a cold lithospheric root may generate extension in peripheral zones of weakness these last results illustrate the point that the computation of regional tectonic stresses requires the knowledge of the density anomalies within the lithosphere on the one hand and of geometrical constraints related to lateral mechanical heterogeneities on the other

venus gravity a harmonic analysis an improved model of venusian global gravity has been obtained by fitting an eighteenth degree and eighteenth order spherical harmonic series to 78 orbital arcs of high altitude 950 1350 km at periapsis tracking data and 351 orbital arcs of lower altitude 150 200 km at periapsis data from the pioneer venus orbiter pvo compared to a recently published tenth degree model mottinger et al 1985 which is based on the 78 high altitude arcs only the current model provides a significant improvement in resolution and fidelity as a measure of this improvement we note that for the low altitude arcs alone the variance of the residuals for the present model is reduced to 19 of the data variance compared to 51 for the tenth degree model venus differs significantly from the earth in that it exhibits a significant correlation between long wavelength topography and gravity the gravity topography spectral admittances are inconsistent with either airy or pratt isostasy but they are consistent with dynamic support by mantle convection

venus gravity anomalies and their correlations with topography this report provides a summary of the high resolution gravity data obtained from the pioneer venus orbiter radio tracking data gravity maps covering a 70 latitude band through 360 of longitude are displayed as line of sight and vertical gravity topography converted to gravity and bouguer gravity maps are also shown in both systems topography to gravity ratios are made over several regions of the planet there are markedly different ratios for the aphrodite area as compared to the beta and atla areas

evidence for divergent plate boundary characteristics and crustal spreading on venus detailed examination of the topography and morphology of western aphrodite terra reveals numerous features that are similar to terrestrial divergent plate boundary characteristics individual domains between fracturezone like discontinuities contain a variety of bilaterally symmetrical topographic elements that suggest that topographic features have been created at rise crests rifted and separated and moved laterally to their present symmetrical positions the topographic and morphologic similarities together with strikingly similar mirror image map patterns on both sides of the rise axis suggest that western aphrodite terra shares the characteristics of oceanic divergent plate boundaries and is the site of crustal spreading on venus topographic profiles are consistent with spreading rates of the order of several centimeters per year

palaeoecological evidence for holocene environmental change from the virunga volcanoes in the albertine rift central africa abstract this study presents two new well dated sedimentary pollen and charcoal records from high altitude crater sites in the virunga volcanoes located in the albertine rift central africa currently part of one of the world s most biodiverse areas here we argue that holocene vegetation changes in a ca 8000 year palaeoenvironmental record from a crater swamp at an altitude of 3474 m and a ca 2800 year record from a crater lake at an altitude of 4127 m are linked to variations in both climate and human activity climatic changes during the mid to late holocene are reflected in the high altitude sites and more widely in adjacent parts of the albertine rift vegetation changes comprising a decline in ericaceous vegetation at ca 5000 cal yrs bp and subsequent expansion of afroalpine vegetation together with a later increase in taxa associated with lower montane forest particularly podocarpus reflect increasing aridity during the mid to late holocene human induced environmental change in the virunga volcanoes is apparent only within the last millennium despite the long history of human occupation of the area both study sites record significant forest clearance at ca 900 cal yrs bp involving a reduction in lower montane forest taxa and increases in disturbance indicators changes in the composition of upper montane forest and particularly the expansion of hagenia are possibly linked to anthropogenic induced changes in the fire regime and are apparent from ca 900 cal yrs bp human induced environmental modification from the early part of the last millennium likely associated with onset of the late iron age appears to have extended to high altitudes the importance of natural long term climate change as a major cause of environmental change in the albertine rift has been eclipsed within the last millennium by human induced environmental effects

viscosity and thickness of the sub lithospheric low viscosity zone constraints from geoid and depth over oceanic swells abstract the medium wavelength geoid to depth anomalies ratio gdr at oceanic hotspot swells has been found to increase from 0 5 m km to 5 m km according to the age of the lithosphere they occur on in order to interpret this trend the geoid and topography anomalies associated with mantle convective plumes crossing a sublithospheric low viscosity zone lvz have been derived from numerical models and a systematic investigation of the gdr dependence on the viscosity and depth extent of the lvz on the thickness and thermal structure of the lithosphere and on the rayleigh number has been conducted it is shown that for viscosity drops across the base of the lvz greater than one order of magnitude the gdr is strongly dependent on the depth of shallow interfaces such as the lithosphere athenosphere boundary and on the lvz s thickness consequently the empirical trend can be accounted for by the thickening of the lithosphere with age provided it occurs at the expense of a lvz whose base is at a fixed depth around 200 km in such a frame no significant variation with age of the lvz s viscosity is required by the gdr data best fit with the empirical trend is found for a lvz about 50 times less viscous than the underlying mantle the mantle flow starts to fluctuate when the local rayleigh number of the low viscosity layer exceeds the rayleigh number of the underlying mantle the fluctuations are initiated in the upper boundary layer in the diverging part of the plume at a distance of a few hundreds of kilometers from the main ascending current for viscosity contrasts in the range of 40 60 deduced from the present study the conditions for the development of these small scale instabilities are realized only where the lithosphere has not yet grown significantly downwards ages

seasat observations of flexure evidence for a strong lithosphere abstract outer rises seaward of subduction zones arise due to flexure of the oceanic lithosphere a mechanical model of this flexure is used to predict the shape of the associated geoid signal these predicted geoid signals are compared with seasat altimeter profiles over outer rises adjacent to nine major subduction zones the nine zones include the aleutian mariana izu bonin puerto rico lesser antilles peru chile middle america south sandwich philippine and kuril the comparisons with seasat data provide estimates of effective elastic thickness he and maximum bending moment mm for each zone it is shown that 1 1 these thicknesses hc are approximately proportional to the square root of lithospheric age 2 2 the maximum bending moments mm increase roughly as the cube of thickness hc these relations 1 and 2 are both very similar to those predicted by a layered rheology model brittle elastic ductile of the lithosphere derived from mechanical experiments on rocks according to this model thermally activated creep is the mechanism of ductile flow in the lower lithosphere hence an isotherm 740 50 defines the bottom of the lithosphere thus old oceanic lithosphere may have significant strength to depths in excess of 60 km or about half that of the thermal lithosphere

tectonics and evolution of venus the global tectonics of venus differs significantly from that of earth most markedly in that the surface is covered predominately by gently rolling terrain there apparently are no features like ocean rises the gravity is positively correlated with topography at all wavelengths and the few highlands are estimated to be supported or compensated at a depth of approximately 100 kilometers the surface of venus appears to be covered mainly by an ancient crust the high surface temperature making subduction difficult it seems likely that well over 1 billion years ago water was destabilized at the surface and soon after plate tectonics ceased the highlands appear to be actively supported presumably as manifestations of long enduring hot spots

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

the younger dryas cold interval as viewed from central greenland abstract greenland ice core records provide an exceptionally clear picture of many aspects of abrupt climate changes and particularly of those associated with the younger dryas event as reviewed here well preserved annual layers can be counted confidently with only 1 errors for the age of the end of the younger dryas 11 500 years before present ice flow corrections allow reconstruction of snow accumulation rates over tens of thousands of years with little additional uncertainty glaciochemical and particulate data record atmospheric loading changes with little uncertainty introduced by changes in snow accumulation confident paleothermometry is provided by site specific calibrations using ice isotopic ratios borehole temperatures and gas isotopic ratios near simultaneous changes in ice core paleoclimatic indicators of local regional and more widespread climate conditions demonstrate that much of the earth experienced abrupt climate changes synchronous with greenland within thirty years or less post younger dryas changes have not duplicated the size extent and rapidity of these paleoclimatic changes

model of the lithosphere with a creep threshold stress abstract laboratory workers have shown that there is a threshold stress which must be exceeded before diffusion creep is possible both in metals and in oxide ceramics although no data are available for silicate ceramics at low enough stress it seems likely that a creep threshold stress will be found to be in effect here too this paper shows that the threshold stress that would be required in the lithosphere to explain bathymetry and gravity observations at the emperor seamounts is about 30 mn m 2 0 3 kbar at a depth of 25 km this value fits in with the laboratory observations supporting the assumption that the threshold stress observed in metals and oxides is operating in the lithosphere too this result is of interest in two ways it may provide additional clues about the physical origin of the creep threshold stress clues that are not available from laboratory studies alone and it suggests a view of the lithosphere whose thickness depends not only on the temperature but also on the state of stress in the earth

implications of oceanic intraplate seismicity for plate stresses driving forces and rheology abstract recent studies of earthquakes occurring within the oceanic lithosphere provide valuable information about stresses in the lithosphere plate tectonic driving forces and the rheology of the lithosphere asthenosphere and mantle focal mechanisms indicate that oceanic lithosphere older than 35 million years is almost entirely in deviatoric compression both compressional and extensional events occur in younger lithosphere in young lithosphere mechanisms generally indicate compressive stress in the spreading direction or extension oblique to the spreading direction extension in the spreading direction is not observed using the constraint of compression in the spreading direction in old lithosphere models of the stresses produced by a combination of ridge push and basal drag forces require the magnitude of the drag be less than a few bars for rapidly moving plates and a few tens of bars for slow moving plates assuming that basal drag results from mantle return flow the upper limits on drag can be converted to constraints on the viscosity structure of the asthenosphere and mantle using simple two dimensional models if return flow occurs in a mantle of viscosity 10 22 poise comparable to the results from glacial rebound studies the predicted basal drag is too high unless a thin asthenosphere with viscosity less than 10 19 to 10 20 poise depending on flow depth is present the intraplate stress data thus are consistent with the idea of oceanic plates largely decoupled from the underlying mantle the strength of the lithosphere is constrained by the maximum depth of oceanic intraplate seismicity which increases with lithospheric age and appears to be bounded by a 700 800 c isotherm this limiting depth is approximately equal to the flexural elastic thickness of the lithosphere and is consistent with experimental olivine rheologies which predict rapid weakening at high temperatures similar phenomena are important for estimating the fraction of seismic and aseismic slip on transform faults and in determining the extent of rupture for large trench normal faulting events

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

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

timing of the younger dryas event in east africa from lake level changes the last deglaciation was interrupted by an abrupt cooling event the younger dryas at 11 000 ndash 10 000 yr bp uncalibrated radiocarbon timescale 1 originally recognized in climate records from northwest europe the younger dryas has now been identified in marine and ice core records worldwide2 6 in the tropics a broadly contemporaneous change in climate is recorded by decreases in water levels and increased salinity of lakes7 9 14 indicating a period of arid climate caused by a reduction in ocean to land moisture flux the exact timing of these changes in relation to the younger dryas event in high latitude records has remained unclear however here we present climate records based on analyses of diatom assemblages geochemistry and magnetic mineralogy of radiocarbon dated sequences of laminated lake sediments from lake magadi in the east african rift these records provide a detailed record of climate change in lowland equatorial africa throughout the last deglaciation 12 800 ndash 10 000 14c yr bp we find that lake level and humidity maxima coincide with the most rapid phases of ice melting in the northern hemisphere and that the climate changes including the younger dryas event were synchronous at low and high latitudes thus the effects of abrupt climate change appear to be felt at both high and low latitudes without a significant time lag

high resolution sedimentary record of the last deglaciation from a high altitude lake in ethiopia sedimentological and geochemical studies conducted on a 15 82 m long core collected from lake garba guracha ethiopia associated with a precise ams 14c time scale document a unique record of the sedimentary processes linked to the progressive retreat of a high altitude glacier in the bale mountains since 17 000 yr cal bp lake sedimentation is interpreted as the result of discharges of meltwater and glaciogenic sediment which progressively filled the accommodation space generated by glacier retreat within the basin monogenic sediment originated from glacial erosion of the trachytic tuff forming the cirque floor ice melting ended progressively between 12 600 and 11 800 cal bp as suggested by the decrease in sedimentation energy followed by a sharp change in sedimentary facies from 11 800 cal bp the lake reached its maximum development and clastic input was replaced by organic rich sedimentation this relates to a major increase of lake productivity which lasted up to 4500 cal bp from this period a lowering in productivity reflects the widespread dryness which occurred throughout the east african tropics

numerical interpretation of a high resolution holocene pollen record from burundi the holocene history of the tropical montane forest is reconstructed using a high resolution pollen record from a valley swamp 3 35 s 29 41 e at 2000 m elevation the post glacial establishment of the montane rain forest is evidenced at ca 10 600 yr b p numerical analysis combines the correspondence analysis with the cluster analysis of both 38 modern and 101 fossil pollen spectra including 125 identified pollen taxa four distinct successionnal stages of forest development have been identified the past vegetation recorded during the early holocene period 10 600 3800 yr b p was more closely related to modern pollen spectra from secondary forest rather than to primary forest samples from burundi at 3800 yr b p a sharp decrease of arboreal pollen percentages and a noticeable increase of celtis a semi decidous tree indicate the opening of montane forest due to a drier climatic event the beginning of the anthropogenic impact evidenced after at 3500 yr b p contemporaneously with the early iron age in burundi follows the late holocene climatic shift

formation of mountain belts on venus evidence for large scale convergence underthrusting and crustal imbrication in freyja montes ishtar terra the freyja mottles venus deformation zone consists of a distinctive linear orogenic belt freyja montes an adjacent plateau a steep scarp separating the plateau from the north polar plains lying about 3 km below the plateau and a volcanically flooded foredeep and rise at the base of the scarp on the basis of mapping of radar images and interpretation of the geologic units and topography the freyja montes deformation zone is interpreted to be the site of large scale crustal convergence and imbrication involving the underthrusting of the north polar plains beneath ishtar terra this deformation is interpreted to have begun with north south regional compressional deformation causing flexure buckling and underthrusting of the crust and lithosphere of the north polar plains beneath the relatively higher proto ishtar lakshmi further convergence and underthrusting caused crustal thickening and uplift and created compressive stresses in the upper part of the underthrusting layer failure of this layer led to a new site of underthrusting outboard of the rise and a new detachment surface the detached layer and the wedge of volcanic flows in the intervening foredeep were further deformed and incorporated sutured onto the plateau and mountain range this crustal underthrusting and imbrication caused crustal thickening and corresponding crustal uplift the continuing process of convergence underthrusting detachment and imbrication produced large scale crustal thickening uplift and the formation of the plateau behind the asymmetrical front of freyja montes the present configuration of topography and units and the distinctive elevation of the scarp and freyja montes suggest that the process continues to operate at the present time

a mechanism for tectonic deformation on venus in the absence of identifiable physiographic features directly associated with plate tectonics alternate mechanisms are sought for the intense tectonic deformation observed in radar images of venus one possible mechanism is direct coupling into an elastic lithosphere of the stresses associated with convective flow in the interior spectral green s function solutions have been obtained for stresses in an elastic lithosphere overlying a newtonian interior with an exponential depth dependence of viscosity and a specified surface density distribution driving the flow at long wavelengths and for a rigid elastic fluid boundary condition horizontal normal stresses in the elastic lid are controlled by the vertical shear stress gradient and are directly proportional to the depth of the density disturbance in the underlying fluid the depth and strength of density anomalies in the venusian interior inferred by analyses of long wavelength gravity data suggest that stresses in excess of 100 mpa would be generated in a 10 km thick elastic lid unless a low viscosity channel occurring beneath the lid or a positive viscosity gradient uncouples the flow stresses the great apparent depth of compensation of topographic features argues against this however thus supporting the importance of the coupling mechanism if there is no elastic lid stresses will also be very high near the surface providing also that the viscosity gradient is negative

integrated marine and terrestrial evidence for abrupt congo river palaeodischarge fluctuations during the last deglaciation we present a high resolution reconstruction of tropical palaeoenvironmental changes for the last deglacial transition 18 to 9 cal kyr bp based on integrated oceanic and terrestrial proxies from a congo fan core pollen grass cuticle pediastrum and dinoflagellate cyst fluxes sedimentation rates and planktonic foraminiferal 18o ratios u37k sea surface temperature and alkane alkenone ratio data highlight a series of abrupt changes in congo river palaeodischarge a major discharge pulse is registered at around 13 0 cal kyr bp which we attribute to latitudinal migration of the intertropical convergence zone itcz during deglaciation the data indicate abrupt and short lived changes in the equatorial precipitation regime within a system of monsoonal dynamics forced by precessional cycles the phases of enhanced congo discharge stimulated river induced upwelling and enhanced productivity in the adjacent ocean copyright 2001 john wiley sons ltd

late glacial holocene diatom record of water chemistry and lake level change from the tropical east african rift lake abiyata ethiopia abstract diatom study of a 12 6 m long sediment core ab95ii from lake abiyata ethiopian rift 8 n revealed large amplitude changes in the water level and chemistry over the past 13 4 cal kyr bp past water conductivity ph and water chemical facies reflected by ionic ratios are quantitatively reconstructed based on diatom transfer functions previously established conductivity has varied from 400 to 21 500 s cm 1 ph from 8 to 10 5 the anion hco 3 co 3 2 cl so 4 2 and cation na k ca 2 mg 2 ratios ranged between 0 9 4 and 0 5 275 respectively a high water salinity oscillation is recorded at the base of the sequence the extrapolated age of this interval 12 7 12 0 cal kyr bp corresponds to the younger dryas chronozone the early middle holocene registers a high stand freshwater lake between 12 and 5 4 cal kyr bp changes in diatom assemblages suggest minor changes in water depth lake mixing and or nutrient content during this interval post 5 4 cal kyr bp shallow saline conditions generally predominate short term regressive events are recorded at 5 6 and 3 0 cal kyr bp our results are roughly consistent with previous works in the region they refine the timing of the hydrochemical changes and quantify their amplitude thus providing inputs for modelling approaches

long term redox evolution in granitic rocks modelling the redox front propagation in the rock matrix advanced plans for the construction of a deep geological repository for highly radioactive wastes from nuclear power plants have evolved during the past decades in many countries including sweden as part of the swedish concept the waste is to be encapsulated in canisters surrounded by low permeability backfill material the copper canisters will be deposited at around 500 metres depth in granitic rock which acts as a natural barrier for the transport of radionuclides to the ground surface these natural and engineered barriers are chosen and designed to ensure the safety of the repository over hundred of thousands of years one issue of interest for the safety assessment of such a repository is the redox evolution over long times an oxidising environment would enhance the corrosion of the copper canisters and increases the mobility of any released radionuclides in the first part of the present thesis the ability of the host rock to ensure a reducing environment at repository depth over long times was studied a model framework was developed with the aim to capture all processes that are deemed to be important for the scavenging of intruding oxygen from the ground surface over long times simplifications allowing for analytical solutions were introduced for transparency reasons so that evaluation of results is straight forward and so that uncertain parameter values easily can be adjusted more complex systems were solved numerically for cases when the analytical simplifications are not applicable and to validate the simplifications underlying the analytical solutions results were presented for prevailing present day conditions as well as for conditions deemed to be likely during the melting phase of a period of glaciation it was shown that the hydraulic properties have a great influence on the oxygen intrusion length downstream along flow paths in the rock an important parameter that determines the extent of interaction between the dissolved oxygen and the reducing minerals in the rock was shown to be the flow wetted surface to flow rate ratio the results show that for an initial period of time depending on the amount of reducing minerals and reaction rates chemical reaction kinetics may control the rate of the overall depletion of oxygen for longer times internal diffusion resistance in large particles or in the rock matrix become rate limiting for the overall process it was found that there are many uncertainties that have to be considered in order to make reliable quantitative predictions on the extent of oxygen intrusion in the second part of the thesis the impact of intruding oxygen on the corrosion of the copper canisters was explored also a mechanism for the production of sulphide close to the deposition holes was studied sulphide is another corroding agent that may be produced microbially in a reducing environment from sulphate in the presence of organic reductants such as methane from calculation results it was found that corrosion of more than 50 kg of copper is not likely over a period of one million years

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

the intercept is a poor estimate of a calibrated radiocarbon age intercept based methods of generating a point estimate of a calibrated radiocarbon date are very popular but exhibit undesirable behaviour they are highly sensitive to the mean of the radiocarbon date and to adjustments of the calibration curve other methods give more stable results the weighted average of the probability distribution function is recommended as the best central point estimate but more consideration should be given to using the full probability distribution rather than a point estimate in developing age depth models

early human impact in the forest ecotone of southern high asia hindu kush himalaya abstract the vegetation of the treeline ecotone of the southern declivity of arid high asia hindu kush northern areas of pakistan himalaya northern central nepal is dominated by hedgehog like open dwarf shrublands of thorny cushions since climatically sensitive ecotones are always also sensitive to human impact the question arises whether the current lack of forests is a result of the subboreal climate decline or of human impact due to inadequate knowledge of the pollen flora and of ecological indicator values of the plants pollen analyses in high asia have mainly been limited to the regional verification of globally known climatic impulses however the role of human impact on regional vegetation patterns has been widely neglected we postulate that today s open dwarf shrublands replace woodlands and forests isolated vigorous juniper trees and successful reforestation appear to confirm our hypothesis an abrupt decline of pinus forests before 5700 and 5400 ka cal yr bp can be demonstrated as the first indicator pollen of human impact appeared at both sites synchronous with the forest pollen decline we infer human impact to be a more decisive cause for this environment change superimposing the effects of a climatic deterioration the forests were displaced by open dwarf shrublands

a 9000 year oxygen and carbon isotope record of hydrological change in a small ethiopian crater lake the stable isotope composition of authigenic calcites in a core from lake tilo a small crater lake in the ethiopian rift valley provides a subcentury scale record of lake response to climatic change over the last 8850 14c yrs 9850 cal yrs an unusually high range of 18o and 13c values 15 is attributed in part to major changes in hydrothermal groundwater flux although hydrothermal groundwater influx to the lake was high during the early holocene its flow rate was relatively stable and thus climatically induced changes to the water budget can be inferred from variations in 18o and 13c ratios a major decline in hydrothermal groundwater input from 5500 14c yrs bp increased lakewater residence time and led to substantial increases in 18o and 13c before the mid holocene transition to more arid conditions during the last 2700 14c years diagenetic processes have resulted in extremely varied 18o and 13c values the holocene isotope record from lake tilo challenges the widely held assumpti

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

pollen and isotopic records in late holocene sediments from lake turkana kenya abstract a 12 m piston core lt 84 8p taken in the northern basin of lake turkana has been dated by 6 ams radiocarbon dates on large ostracods and analysed for its pollen content during the past 2500 years the pollen assemblages were dominated by chenopodiaceae amaranthaceae cheno am and gramineae representing the local pollen signal of the surrounding vegetation under arid conditions percentage fluctuations of cheno am are interpreted as reflecting lake level variations in good correspondance with oxygen isotope from the same core a rise in lake level is recorded at about 1600 yr b p preceded by a relative lowstand during much of the period from 2500 to 1600 yr b p the top of the core is estimated to be about 600 yr old and shows a shift of unknown duration towards a more humid condition the high lake level documented in this paper roughly coincides with that reconstructed from other cores in the lake based on diatom and carbonate analyses

human impact on the environment in the ethiopian and eritrean highlands a state of the art abstract this review analyses the environmental evolution of the ethiopian highlands in the late quaternary the late pleistocene 20 000 12 000 14c years bp was cold and dry with 1 low lake levels in the rift valley 2 large debris fans on the flanks of lake abhe basin and 3 the blue nile transporting coarse bedload then a period with abundant and less seasonal rains existed between 11 500 and 4800 14c years bp as suggested by increased arboreal pollen high river and lake levels low river turbidities and soil formation around 5000 4800 14c years bp there was a shift to more arid conditions and more soil erosion many phenomena that were previously interpreted as climate driven might however well be of anthropic origin thick sediment deposits on pediments as well as an increase of secondary forest scrub and ruderal species in pollen diagrams are witnesses of this human impact one important aspect of the late quaternary palaeoenvironment is unclear changes in nile flow discharges and rift valley lake levels have been linked to changes in precipitation depth most authors do not take into account changes in land use in the highlands nor changes in the seasonality of rain both of which can lead to a change in runoff coefficients tufa and speleothem deposition around 14 000 years ago tend to indicate that at the end of the last glacial maximum lgm conditions might have been wetter than generally accepted the most important present day geomorphic processes are sheet and rill erosion throughout the country gullying in the highlands and wind erosion in the rift valley and the peripheral lowlands based on existing sediment yield data for catchments draining the central and northern ethiopian highlands an equation was developed allowing to assess area specific sediment yields sy 2595a 0 29 n 20 r 2 0 59 where sy area specific sediment yield t km 2 year 1 and a drainage area km2 with respect to recent environmental changes temporal rain patterns apart from the catastrophic impact of dry years on the degraded environment cannot explain the current desertification in the driest parts of the country and the accompanying land degradation elsewhere causes are changing land use and land cover which are expressions of human impact on the environment deforestation over the last 2000 3000 years was probably not a linear process in ethiopia studies on land use and land cover change show however a tendency over the last decades of increasing removal of remnant vegetation which is slowed down or reversed in northern ethiopia by a set aside policy ongoing land degradation requires urgent action at different levels of society soil and water conservation swc structures are now widely implemented local knowledge and farmer s initiatives are integrated with introduced swc techniques at various degrees impact assessments show clear benefits of the soil conservation measures in controlling runoff and soil erosion in high rain areas runoff management requires greater emphasis during the design of soil conservation structures in such areas investment in swc might not be profitable at farm level although benefits for society are positive this pleads in favour of public support the present land degradation in the ethiopian highlands has a particular origin which includes poverty and lack of agricultural intensification causes of these are to be found in the nature of past and present regional social relations as well as in international unequal development this review strengthens our belief that under improved socio economic conditions land husbandry can be made sustainable leading to a reversal of the present desertification and land degradation of the ethiopian highlands

quantitative tests for plate tectonics on venus if plate tectonics exists on venus the rate of topographic dropoff from spreading centers should be about half that on earth due to greater rock fluid density contrast and lower temperature differential between the surface and interior using 1 1 mean elevations 21 000 km of ridge were identified on venus and 33 000 km on earth heights h at distances up to 1000 km to either side of the ridges were analyzed to make least squares determinations of the parameters ho h and n in where s is off crest distance the venus ridges have a much less well defined mode in crest heights ho than the earth ridges and a somewhat smaller exponent n on the average indicating greater concavity fixing n to the theoretical value of 0 5 plate velocities were inferred from the h s using h t from north atlantic and north pacific data on earth these velocity solutions were compared to those from plate rotations which indicate the h t is probably about 40 too high for a global mean the resulting plate creation rates on venus assuming comparable total heat generation and taking into account the density and temperature differences are 0 7 km yr or less and indicate that not more than 15 of venus s energy is delivered to the surface by plate tectonics in contrast to 2 9 km yr and 70 energy delivery on earth

holocene climate and vegetation change in the main ethiopian rift valley inferred from the composition c n and δ13c of lacustrine organic matter abstract in order to track changes in the relative abundance of c3 and c4 plants in savanna vegetation c n and 13c values were measured on bulk organic material in an 8840 14c year record from lake tilo ethiopia between 8840 and 2500 bp high c n ratios suggest that input to the lake was predominantly from terrestrial plants the corresponding 13c values thus provide a proxy for changes in catchment vegetation that are supported by pollen data 13c values in the early holocene are relatively low reflecting the dominance of c3 vegetation woody plants and a more humid climate 13c shows no response to a known regional arid interval at 7800 yr bp suggesting that woody vegetation was able to survive relatively prolonged dry periods a gradual rather than sharp 13c response to the end of the early holocene humid interval at 4500 yr bp further supports this higher 13c values at 2800 2300 and 1000 yr bp correspond to increases in sedge pollen thought to be growing in freshwater springs exposed as lake level fell the c n and 13c composition of bulk organic material complements the pollen evidence and may be useful in other lakes in savanna regions as indicators of terrestrial vegetation change

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

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

the geology and geomorphology of the venus surface as revealed by the radar images obtained by veneras 15 and 16 a region by region condensed description of almost all of the area that was radar photographed by veneras 15 and 16 is presented using some generalizations the diversity of terrain was reduced to a discrete set from which a geological morphological map was constructed the predominant type of terrain of the studied area is a plain that was tentatively subdivided into five morphological types ridge and band patchy rolling plain dome and butte plain smooth plain and high smooth plain stratigraphically the ridge and band plains are the oldest and the smooth plains are the youngest the stratigraphic position of the other types is yet to be determined large sections of the plains show similarities to the mare type basaltic plains of the moon mercury and mars other types of terrain are combinations of ridges and grooves in various patterns linear parallel orthogonal diagonal or chevron like and chaotic in some places the ridge and groove terrain is stratigraphically below the plain material but in other places it appears to be plain material that has been subsequently deformed near the eastern and western boundaries of ishtar terra large several hundred kilometers in diameter ring like features can be seen that are named coronae or ovoids evidence of tectonic deformation and the presence of flow like patterns support their designation as volcano tectonic features beta regio seems to be an uplifted plain showing evidence of rifting and volcanism all types of terrain are sparesely peppered with craters of obvious impact morphology their average density gives the plain an age range of 0 5 to 1 109 years the fact that many impact craters are still in the pristine state indicates a very low rate of surface reworking at least for the last 0 5 to 1 109 years no evidence for water erosion sedimentation processes has been found the tectonic activity of venus has no equivalent on the moon mercury or even mars and can be compared only with that of the earth intensive horizontal deformation previously known only on earth occurs on venus but in a characteristic venusian style

strontium isotope evidence for late pleistocene reestablishment of an integrated nile drainage network evidence of late pleistocene closed basin conditions in equatorial african lakes has far reaching implications for the history of the nile river there has been widespread acceptance that overflow of lakes victoria and albert into the nile occurred some time during the terminal pleistocene but recent suggestions that lake victoria remained a closed basin until ca 7 2 ka imply a much longer period of greatly reduced white nile discharge this hypothesis has major consequences for current ideas about the river9s paleohydrology early human exploitation of riverine resources nile delta sedimentation and late quaternary genesis of sapropels in the eastern mediterranean here we resolve the timing of lake victoria9s overflow by using strontium isotopes as a tracer for water from the principal lakes and rivers of the upper nile our data demonstrate that overflow of lakes victoria and albert into the nile drainage network occurred no later than 11 5 14 c ka

environmental changes in a tropical lake lake abiyata ethiopia during recent centuries lake abiyata is a small closed saline alkaline lake located in the central part of the ethiopian rift valley east africa a multi proxy study of a sediment core 116 cm long and with undisturbed mud water interface was performed to test the sensitivity of the lake system and of different proxies to the changes in climate and human activities that occurred in the catchment during the past few centuries the 210pb analyses suggest that the upper 80 cm of the core represent the past 200 years this study complements millennial scale environmental records available in the region the main freshwater climatic and biological features of the modern lake system and their variations over the past decades as known from observations are first summarised results derived from individual proxies analysed along the core are then presented successively major physical and chemical properties of bulk sediments diatoms and pollen uncertainties on the chronological framework are discussed major limnological stages are finally identified based on the multi proxy interpretation of our record our record shows large variations in the lake water and salt balances in the sediment sources and in the vegetation distribution in the basin using our 210pb chronology major changes observed in the core are tentatively compared with environmental events known from instrumental and historical records the upper 41 cm of the core 210pb age 1940 1998 ad reveal several fluctuations in diatom inferred water depth and salinity which seem to be consistent with known changes in water level human impact on vegetation clearly appears since about 30 years the interval 85 41 cm suggests a period of overall water deficit lake abiyata experienced episodes shallower and more saline than over the past decades especially around 68 66 cm 210pb dated at ca 1890 ad this level may coincide with one of the worst droughts known in the ethiopian history during 1888 1892 the lower part of the core includes a stage 108 85 cm of lake level much higher than today and which ended before 1800 ad although its base is undated so far this stage suggests that conditions much wetter than today have prevailed in the region during at least part of the 18th century lake abiyata appears to be a suitable site for a detailed environmental reconstruction over the recent past although further work is needed to reduce the uncertainties on our record as discussed in the conclusions

styles of tectonic deformations on venus analysis of venera 15 and 16 data analysis of the radar images obtained by veneras 15 and 16 leads to the conclusion that the ridge and groove structures on the surface of venus are the result of tectonic deformation although the mechanism of such deformation cannot yet be unequivocally deduced several styles of deformation can be described areal deformation occurs where horizontal stresses have operated over large areas shear deformation appears in bands showing differential longitudinal deformation transversal stresses operating over long and relatively narrow areas have produced belt deformation circular deformation is related to a specific locus of the stresses the absence of densely cratered areas indicates that the terrains were deformed after the period of heavy bombardment the origin of the stresses could be drag of the lithosphere by asthenospheric currents or gravity induced spreading of surface material over upwellings a general conclusion is that in the surveyed area of venus neither terrestrial plate tectonics nor lunar highland type terrain exists

holocene palaeovegetation of the tigray plateau in northern ethiopia from charcoal and stable organic carbon isotopic analyses of gully sediments abstract a long history of supporting sophisticated but unsustainable kingdoms makes the tigray plateau of the northern ethiopian highlands a promising region for the study of relationships between palaeoenvironmental change and the trajectories of human civilizations the natural vegetation above 2200 m elevation is thought to be forests dominated by juniperus procera nonetheless this hypothesis is not supported in the vegetation cover now and is scarcely studied in the palaeorecord to examine changes in vegetation climate and land use we identified buried charcoalized wood and estimated the percentage of organic carbon from c 4 plants c 4 carbon from 13 c values of bulk organic matter in the soils of gully walls in the boundaries of the ancient aksumite empire charcoal ranged in age from ca 13 700 to 110 cal yr bp juniperus procera occurred in even the youngest samples although at lower percentages of the total charcoal than in older samples nevertheless rapidly regenerating angiosperms usually dominated or co dominated charcoal even in some of the oldest strata a shift towards higher c 4 carbon and total organic carbon toc in soils younger than 3300 cal yr bp began during a period when agricultural uses of land may have increased in order to support the needs of growing societies a shift towards higher c 4 carbon but lower toc began at ca 6000 cal yr bp however during a period when no charcoal was found and no changes in human societal complexity are known these results indicate that juniper forest types have long been present at 2200 m in the tigray plateau but that they have rarely been the dominant natural vegetation furthermore lack of repeatable correspondence between factors suggests that the causes of similar shifts in vegetation composition were not always the same

comment on application of analytical diffusion models to outcrop observations implications for mass transport by fluid flow through fractures by antonellini et al 2017

constraints on yield strength in the oceanic lithosphere derived from observations of flexure summary the wavelength and amplitude of outer rises seaward of sub duction zones and arches surrounding islands and seamounts are used to parameterize flexure profiles in terms of the moment and curvature at the first zero crossing the data show the clear age dependence in the mechanical thickness of the lithosphere up to 60 100myr saturation of moment at large curvature is interpreted in terms of a depth dependent yield strength for the lithosphere using relations adopted from laboratory experiments of rock deformation a comparison of theoretical curves with observed moments indicates that old oceanic lithosphere has no long term strength below about 40 km depth with no difference between 100 and 165 myr old crust moderate axial loading forces 200 mpa can explain most variations in the moment curvature observations except in the case of the kuril trench which appears anomalous given the age of the crust regional tension causes greater variability in moment as compared to regional compression because of the greater slope in the brittle failure envelope under tension the observations point to a lithosphere weaker than the prediction from experimental deformation of rocks of the possible weakening mechanisms elevated pore fluid pressure on faults does not predict the correct age dependence and is incompatible with earthquake focal mechanisms our favoured explanation is that the activation energy q appropriate for ductile flow at geological strain rates is lower than the values derived from laboratory extrapolations of dry olivine data taken at high temperatures if recent oceanic geotherms are reliable q in the lower lithosphere must be lower than 100kcal mol 1 the method used here is most appropriate for trench profiles with curvatures greater than 10 7m 1 for lower curvatures such as along seamount profiles small errors in the curvature estimate cause large changes in rheological parameters

climatic change in northern ethiopia during the past 17 000 years a diatom and stable isotope record from lake ashenge lake ashenge a closed basin lake near the northernmost penetration of summer monsoon rains is well placed to provide a continental record of past changes in the strength of the african monsoon system diatom and oxygen isotope analyses of the lake sediments confirm that the overall trend of climate change during the past 17 000 years was driven by precessional forcing punctuated by abrupt shifts that may be linked to changes in atlantic surface temperatures the lake level was low from at least 17 2 to 16 2 cal kyr bp and then rose between 16 2 and 15 2 cal kyr bp which may represent a temporary reactivation of the monsoonal circulation system following its reduced activity during the last glacial maximum the lake was significantly low between 13 6 and 11 8 cal kyr bp coinciding approximately with the younger dryas but beginning 900 years before its recognised onset in the greenland ice core record a major sedimentary hiatus covering the interval 11 8 to 7 6 cal kyr bp was probably caused by an early holocene lowstand the precise timing of which cannot be determined because pre lowstand sediments were eroded from the core site the lake filled to its overflow from 7 6 cal kyr bp until 5 6 cal kyr bp when the sediments record an abrupt lake response to the regional transition to arid conditions that mark the end of african humid period evidence is also presented for climate changes which may have been associated with the rise and fall of aksum ethiopia s first great civilisation

a heat pipe mechanism for volcanism and tectonics on venus a heat pipe mechanism is proposed for the transport of heat through the lithosphere on venus this mechanism allows the crust and lithosphere on venus to be greater than 150 km thick a thick crust and thick lithosphere can explain the high observed topography and large associated gravity anomalies for a 150 km thick lithosphere the required volcanic flux on venus is 200 km sup 3 yr this is compared with a flux of 17 km sup 3 yr associated with the formation of the oceanic crust on earth a thick basaltic crust on venus is expected to transform to eclogite at a depth of 60 to 80 km the dense eclogite would contribute the lithospheric delamination that returns the crust to the interior of the planet completing the heat pipe cycle topography and the associated gravity anomalies can be explained by airy compensation of the thick crust the principal observation that is contrary to this hypothesis is the mean age of the surface that is inferred from crater statistics the minimum mean age is about 130 ma and this implies an upper limit of 2 km sup 3 yr for the surface volcanic flux if the heat pipe mechanism was applicable on earth in the archean itmore would provide the thick lithosphere implied by isotopic data from diamonds less

last glacial pollen record from lanzhou northwestern china and possible forcing mechanisms for the mis 3 climate change in middle to east asia abstract the vegetation on the northeastern margin of the tibetan plateau is highly sensitive to climatic changes and thus represents a potentially interesting environmental archive pollen samples from the fanjiaping loess section in lanzhou on the western chinese loess plateau clp were analyzed in conjunction with osl dating the results indicate that pollen zone b 60 6 46 0 ka correlative to the early mis 3 had the greatest abundances of cupressaceae tsuga gramineae and cyperaceae of the entire section suggesting a warm phase during the last glacial period these pollen taxa decreased significantly in abundance in the zones c 46 0 39 0 ka and d 39 0 27 0 ka reflecting a substantial climate cooling from the middle mis 3 to mis 2 these results correlate with climate records from the south china sea the clp baikal lake north america north atlantic ocean and other regions and probably correspond with the decline of northern high latitude insolation and the increase of global ice volume from 50 to 20 ka in particular arboreal pollen fern spore and algae abundances declined sharply since 40 ka while shrub and herb pollen reached the highest abundances conifer pollen picea and abies abundance also rose markedly and increased up the section this implies significant climate deterioration and likely corresponded with substantial growth of the polar ice sheets since 40 ka the decreasing temperature caused by an insolation decline during the last glacial period probably reinforced the cooling effect in a snow ice albedo feedback which would result in less climate sensitivity to radiative forcing meanwhile vegetation decline in the northern hemisphere during the last glacial period and tundra development at high latitudes possibly caused additional cooling enhancing the growth of polar ice sheets since 40 ka the development of polar ice sheets increased the polar to equator temperature and pressure gradients strengthening the westerlies and supplying plenty of moisture to northwest china during 40 30 ka lake sediments developed widely on the tibetan plateau during 40 30 ka probably related to an increase in the seasonality of middle to low latitude insolation which caused an enhancement of glacier melting on the plateau

isostasy and the ambient state of stress in the oceanic lithosphere two conditions are normally considered to be characteristic of the state of stress in a system in local isostatic equilibrium the first is that the stress is hydrostatic below some depth the level of compensation and the second is that above that depth there are no vertical shear stresses in the case of a planar two dimensional mid ocean ridge these two conditions suffice to determine the stress completely except for the normal component parallel to the ridge but to determine that component an additional assumption is required it is proposed here that an appropriate additional assumption is that in unflexed regions where the stress is due only to the elevation of mid ocean ridges the deviatoric stress in the oceanic lithosphere is a minimum the unique state of deviatoric stress consistent with this assumption is horizontally compressive in a direction perpendicular to the axes of mid ocean ridges with a magnitude in old ocean basins of a few hundred bars in agreement with oceanic intraplate earthquake mechanisms at any given distance from the ridge the stress is maximum at the seafloor and it decreases with depth at about the same rate as the cooling of the lithosphere so that the entire thermal boundary layer may be regarded as acting to support the global square root of age topography of the oceans

gravity driven deformation of the crust on venus intense horizontal deformation features are observed on the surface of venus despite the apparent absence of terrestrial style plate tectonics high surface temperature low erosion rate and regions of elevated terrain suggest that gravity spreading may be responsible for some of these features gravity spreading on the earth is known to cause extensive deformation in certain thin skinned tectonic regions where the uppermost layers become detached along a decollement while the high pore pressures and soft sedimentary rocks frequently observed along decollements on the earth are not plausible for venus temperature induced regions of ductile deformation may exist simple viscous models of gravity spreading above a ductile region of the crust are developed to examine the effects of rheology thermal gradient surface slope and crustal thickness on deformation for a reasonable range of parameters the models show that surface slope driven ductile deformation at relatively shallow depths in the crust is possible and may be capable of detaching the uppermost rigid portion of the crust resulting in horizontal translation

geoid height versus topography for oceanic plateaus and swells oceanic plateaus and swells are a major component of the seafloor topography yet they remain among the most poorly understood features this is especially true of the oceanic plateaus which show large variations in crustal thickness to determine the depth and mode of compensation for 53 of the largest plateaus and swells we analyzed the relationship between geoid height and topography in polygonal areas containing each feature both geoid height and topography were first band pass filtered 400 km i 4000 km to isolate the signal associated with local compensation from flexural and deep mantle signals the ratio of geoid height to topography was then determined by fitting a straight line to the data except for nine of the smaller features there is a high correlation between geoid height and topography that is positive in accordance with airy and thermal compensation models eighteen features have high geoid topography ratios that cannot be explained by the airy compensation model of crustal thickening these features thermal swells are partially supported by thermal buoyancy forces in the lower half of the lithosphere the ratios are highest for active hot spot swells and decay with the thermal age of the swell to values consistent with airy compensation of the enduring volcanic edifice the remaining features plateaus have lower geoid topography ratios in agreement with the airy compensation model those plateaus with average height greater than 4 km are thought to be continental fragments the shorter plateaus tend to be volcanic features modified continental plateaus presumably small fragments of extended and intruded continental margin crust cluster around heights of n3 km overlapping the range associated with oceanic plateaus since the origin of many plateaus is poorly understood this global geoid topography analysis provides a new technique for comparing the deep structure of oceanic plateaus and swells

thick plate flexure summary we derive analytical expressions for the displacements and stresses due to loading of a floating uniform elastic plate of arbitrary thickness by a plane or axisymmetric harmonic load the solution is exact except for assumptions of small strains and linear boundary conditions and gravitation within the plate is neglected for typical earth parameters its predictions are comparable to those of the usual thin plate theory frequently assumed in studies of lithospheric flexure gravity and regional isostasy even for a very thick lithosphere which may exist in some regions of mars the thin plate theory is a better approximation to the thick plate solution than the elastic half space limit except for short wavelength loads

venus ishtar gravity anomaly ishtar has been detected as a positive gravity anomaly verification of this result has been made with two independent data sets isostatic compensation of the ishtar topography using an airy model is achieved at approximately 150km

pollen inferred climatic fluctuations in ethiopia during the last 3000 years abstract although lake levels fluctuations are known from ethiopia during the late holocene few continuous paleoclimatic records are available from the high plateau a 3000 yr high resolution pollen record from the arsi mountain in arussi province of southeastern ethiopia provides information on recent changes in the vegetation above the upper tree line at 3600 m elevation the organic sediment was deposited into the dega sala swamp ca 3000 yr b p on top of a coarse gravel reepresenting a solifuxion deposit linked to neoglacial the surrounding vegetation was established as that of the ericaceous belt with ericaceae shrub dominant the pollen diagram registers significant changes in the relative frequencies of the local herbaceous pollen component versus the arboreal taxa from the upper forest limit allowing the high altitude of the site such changes indicate downward movements of the vegetation belts that are due mainly to temperature variations an estimated cooling of 2 c is registered at 560 120 yr b p ams date contemporaneous with the little ice age the successive upward regrowth of the forest is rather indicative of a recent warming than linked to recolonization of the forest following anthropogenic deforestation unlikely at altitude above potential cultivation

a high resolution palaeoclimatic record for the last 27 500 years in tropical west africa from the carbon and nitrogen isotopic composition of lacustrine organic matter variations in the carbon and nitrogen isotopic composition of bulk organic matter in the sediments of lake bosumtwi ghana reflect climatically induced changes to the lake and the catchment flora cores spanning the last 27 5 kyr of sedimentation in the lake show large oscillations in 13corg pdb and 15norg air the late pleistocene record is particularly detailed showing changes of ca 20 in carbon and ca 10 in nitrogen isotopic compositions these variations are of complex origin although different in magnitude major isotopic excursions in the two records are generally in phase and reveal the occurrence of two major dry intervals at and immediately following the last glacial maximum the allerod younger dryas period also seems to have been marked by generally dry conditions in this part of tropical west africa nitrogen isotopic evidence suggests that during the period 9 2 3 2 kyr the lake had an extremely stable water column probably due to the absence of a windy or cool dry season or both regular circulation of the water column recommenced during the late holocene and has persisted until the present day

the effect of a shallow low viscosity zone on the apparent compensation of mid plate swells abstract a simple model for mid plate swells is that of convection in a fluid which has a low viscosity layer lying between a rigid bed and a constant viscosity region finite element calculations have been used to determine the effects of the viscosity contrast the layer thickness and the rayleigh number on the flow and on the perceived compensation mechanism for the resulting topographic swell as the viscosity decreases in the low viscosity zone the effective local rayleigh number for the top boundary layer of the convecting cell increases also because the lower viscosity facilitates greater velocities in the low viscosity zone the low viscosity layer produces proportionally greater horizontal flow near the conducting lid causing the base of the conducting lid to appear like a free boundary the change in the local rayleigh number and in the effective boundary condition both cause the top boundary layer to thin through a green s function analysis we have found that the low viscosity zone damps the response of the surface topography to the temperature anomalies at depth whereas it causes the gravity and geoid response functions to change sign at depth counteracting the positive contributions from the shallower temperature variations by increasing the viscosity contrast the conbined effects of the thinning of the boundary layer and the behaviour of the response functions allow the apparent depth of compensation to become arbitrarily small therefore shallow depths of compensation cannot be used to argue against dynamic support of mid plate swells furthermore we compared the distribution of the effective compensating densities which is used to obtain the geoid to that of pratt compensation which is often used to calculate the depth of compensation from geoid and topography data for mid plate swells for all of our calculations including those with no low viscosity layer the effective gravitational mass distribution is more complex than assumed in simple pratt models so that the pratt models are not an appropriate gauge of the compensation mechanism

multi tracer study of continental erosion and sediment transport to the red sea and the gulf of aden during the last 20 ka abstract mineralogical compositions and grain size distributions combined with 87sr 86sr and end values of the detrital fraction were studied on cores recovered from the gulf of aden md92 1002 and the red sea md92 1008 basins in order to document past changes in indian monsoon and northwesterly winds during the last glacial interglacial transition the last 20 ka encompassing the african humid period ahp the end vs 87sr 86sr plot indicates that sediments result from the mixing of two main sedimentary sources corresponding to the afar volcanic rocks in ethiopia and to the arabian nubian shield variations of sediment isotopic and mineralogical composition point to a diminution of the volcanic source contribution during the last deglaciation changes of mineral accumulation rates and grain size distributions denote a decline in the aridity of the source regions during the holocene particularly of the afar volcanic region in this area the reduction of detrital supply from 15 cal ka bp can be explained by an increase of precipitations during the ahp which resulted in an expansion of the vegetation cover and lake extensions in east africa in the arabian peninsula precipitations were confined to the south allowing sediments to be transported even during the holocene our data suggest that the southwest monsoon was not the main carrier of aeolian sediments to the red sea and gulf of aden basins but the northwesterlies in the red sea the isotopic and mineralogical tracers reveal a contribution from saharan dust between 16 and 12 cal ka bp transported from the nile catchment after aridification during heinrich event 1

evidence for progressive holocene aridification in southern africa recorded in namibian hyrax middens implications for african monsoon dynamics and the african humid period abstract presented here are stable nitrogen isotope data from a rock hyrax procavia capensis middens from northwestern namibia that record a series of rapid aridification events beginning at ca 3800 cal yr bp and which mark a progressive decrease in regional humidity across the holocene strong correlations exist between this record and other terrestrial and marine archives from southern africa indicating that the observed pattern of climate change is regionally coherent combined these data indicate hemispheric synchrony in tropical african climate change during the holocene with similar trends characterising the termination of the african humid period ahp in both the northern and southern tropics these findings run counter to the widely accepted model of direct low latitude insolation forcing which requires an anti phase relationship to exist between the hemispheres the combined dataset highlights 1 the importance of forcing mechanisms influencing the high northern latitudes in effecting low latitude climate change in africa and 2 the potential importance of solar forcing and variations in the earth s geomagnetic shield in determining both long term and rapid centennial scale climate changes identifying a possible mechanism for the variations marking the ahp termination in both the southern and northern tropics

glacialanterglacial record from intertropical africa high resolution pollen and carbon data at rusaka burundi abstract in northern intertropical africa many geological data have documented several abrupt climatic fluctuations taking place during the time of the last deglaciation one of them is an aridity event generally placed in the interval 11 000 10 000 bp and synchronous with the younger dryas cold episode of the temperate regions that has been explained by the influence of the atlantic ocean circulation on global climate in this paper we present high resolution 50 to 150 years studies of organic matter and pollen undertaken on a new core from the rusaka peat bog 3 26 south 29 37 east 2070 m altitude dated by the conventional l2 dates and the ams methods 18 dates obtained on bulk sediment and macroremains although the sediment at the bottom of the core was deposited during the last glacial period prior to 21 000 bp the dating provides a reliable chronology for the last 12 000 bp only several vegetational changes are documented in the interval 12 000 9000 bp but their chronological placement can only be achieved with a few years of uncertainties a great dispersion of ams dates is attributed to contamination by charcoal dated between 11 700 and 11 500 bp and to a possible age plateau at ca 10 000 bp therefore the results are preferably presented versus depth to keep synchronism in the stratigraphic placement of the successive events at rusaka increase in carbon storage c n ratio for terrestrial plants and tree cover began at ca 12 000 bp soon after the swamp had been established this date is in good agreement with a major increased hydrological balance well documented throughout the intertropical region the forest installation is progressive rather complex and seen as a two step process the first forest phase 11 700 bp to 10 600 bp is dominated by hagenia a montane taxon in good correspondance with previous records from other authors in east african mountains the second forest regeneration took place post 10 000 bp after a clearly reversed trend of tree cover from 10 600 to 10 000 bp ending with a short episode of minimum trees lasting no more than about 200 years but there is no indication of cooling whatsoever and the opening of the forest is better explained by increased aridity rather than normal pattern of forest succession this will remain a controversal interpretation among palynologists until high resolution pollen data can be obtained for the 13 000 bp and earlier period nevertheless the timing of the arid episode at rusaka is in good agreement with other evidence mentioned from highland and lowland pollen data a detailed comparison with 25 available pollen diagrams is limited by poor or inconsistent dating or low resolution of the pollen data nevertheless pollen from four of them document an arid event in good agreement with its timing at rusaka and high resolution isotopic results from two sites in equatorial lowlands and from the sahel this work shows that the arid younger dryas occurred at the same time both in lowlands and highlands and therefore was most likely a precipitation signal rather than a response to temperature decrease

simulation of an abrupt change in saharan vegetation in the mid holocene climate variability during the present inter glacial the holocene has been rather smooth in compar ison with the last glacial nevertheless there were some rather abrupt climate changes one of these changes the desertication of the saharan and arabian region some 4 6 thousand years ago was presumably quite important for human society it could have been the stimulus leading to the foundation of civilizations along the nile euphrat and tigris rivers here we argue that saharan and arabian de sertication was triggered by subtle variations in the earth s orbit which were strongly amplied by atmosphere vegeta tion feedbacks in the subtropics the timing of this tran sition however was mainly governed by a global interplay between atmosphere ocean sea ice and vegetation

viscous relaxation of impact crater relief on venus constraints on crustal thickness and thermal gradient because of the high surface temperature on venus and the strong temperature dependence of strain rate in silicates solid state creep may be an important mechanism for the reduction of topographic relief on that planet venera 15 16 radar altimetric profiles of impact craters 30 140 km in diameter however indicate crater depths of several hundred meters suggesting that substantial relaxation has not occurred for these features on the basis of models for the viscous relaxation of impact crater topography we derive constraints on the thickness of the crust and the mean lithospheric thermal gradient beneath the craters from the mean and variance of observed crater depth as a function of crater diameter on venus and from estimates of the initial depths of fresh impact craters on venus obtained by scaling from lunar observations we formulate a statistical test for the maximum amount of viscous relaxation that is consistent with the observations at a specified confidence level we develop a general formulation for gravity driven flow in a linearly viscous fluid incorporating the densities and temperature dependent effective viscosities of distinct crust and mantle layers the statistical comparison of predicted and observed depths yields linked upper limits to crustal thickness h and thermal gradient dt dz since dt dz can be estimated from global heat loss arguments or thermal models an upper bound to h may be derived the preserved relief of the largest craters constrain h to be less than or equal to 10 20 km because the craters with measured depths appear to be representative of a larger population of impact structures in terms of regional elevation geological unit and degree of preservation of morphological detail extrapolation of this result to lowlands and rolling plains regions on a global basis is not unreasonable such an extrapolation together with isostatic considerations yields an upper limit to the crustal volume of venus of 1010 km3 this value is an order of magnitude less than the time integrated volume of crust produced on earth and implies either that the average rate of crustal generation has been much smaller on venus than on earth or that some form of crustal recycling has occurred on venus

alpine grassland palaeoecology of the virunga volcanoes east africa a new phytolith record from mt muhavura abstract this study presents a first attempt at using phytoliths from a high altitude crater lake as a palaeoecological tool at a tropical montane site mt muhavura 4127 m asl at the border of uganda and rwanda is part of the virunga volcanoes located in the albertine rift one of the most biodiverse areas in africa to investigate the potential of phytoliths to record grassland history in an afroalpine environment a unique and little studied ecosystem we analysed the phytolith content of 37 sediment samples from a 159 cm sediment core from the crater lake at the summit of mt muhavura representing the last ca 2400 years bp ten plant species from the alpine belt were also analysed for their main phytolith morphotypes phytoliths from the sediments were grouped into 12 main categories and the most frequently occurring individual morphotypes were bilobate 33 oblong 16 elongate 17 and globular psilate 13 the phytolith assemblage consists predominantly of phytoliths characteristic of grasses 70 a literal interpretation of the results would be that panicoideae grasses mainly c 4 that are characteristic of warm and wet climate and c 3 pooideae subfamily grasses characteristic of cold climate high altitude have co existed throughout the late holocene in this belt as only c 3 cold climate grass subfamilies are known to occur at this altitude today the abundance of short cell phytoliths bilobates typical of panicoideae grasses in the lake sediments at this altitude presents a new perspective to the phytolith discussion the c 3 pooideae grasses analysed did not produce bilobates in this study the phytolith index ic that is used as a proxy of past grass subfamily dominance is not representative of the taxonomic significance for which it is known this study suggests that caution needs to be taken when using bilobate morphotypes and the ic as indicators of grass subfamily dominance at high altitude on east african mountains as the bilobates may not be taxonomically representative of the grasses for which they are typically known this highlights the inherent problem of phytolith redundancy even at an altitude where c 3 c 4 grass distribution is better delineated

environmental change in montane southwest uganda a pollen record for the holocene from ahakagyezi swamp the environmental history of montane southwest uganda over the last c 12 300 years is described using data from a 22 84 m long core of sediment from ahakagyezi swamp rukiga highlands these data include plant microfossils and macrofossils charcoal and nine radiocarbon dates according to evidence from plant fossils and radiocarbon dates vegetation in the ahakagyezi catchment was very different to the expected climax today until c 10500 bp the surface of the swamp was dominated by sedges whilst the adjacent hillsides were largely devoid of trees changes in pollen suggest that a form of dry montane scrub was replaced by moist lower montane forest from c 11000bp subsequent fluctuations in pollen indicate that once established the composition of moist lower montane forest varied during the holocene and that unreversed forest reduction commenced in the ahakagyezi catchment at c 800 bp on the surface of the swamp itself syzygium dominated swamp forest increased in extent from c 5800 bp and mor

sensitivity of the african and asian monsoons to mid holocene insolation and data inferred surface changes abstract orbital forcing alone is not sufficient to explain the massive northward penetration of monsoon rains in africa shown by data during the mid holocene 6000 yr ago feedbacks associated with changes in ssts and land surface cover may be necessary to produce a sufficient increase in the monsoon a step toward a better understanding of the respective role of oceans and land surfaces is to design sensitivity studies with prescribed forcings inferred from observations in the first study ssts are lowered in the upwelling regions offshore of west africa and somalia and increased in the bay of bengal and south china sea in the second simulation the modern sahara desert is replaced by a combination of xerophytic woods scrub and grassland in both cases the amount of water vapor advected from oceanic sources is increased north of 10 n in africa in response to the increased land sea temperature contrast thereby enhancing rainfall but the magnitude of the simulated changes is much larger when land s

convection driven tectonics on venus an analysis is presented for convective stress coupling to an elastic lithosphere as it applies to tectonic deformation on venus theoretical solutions are introduced for the response of a mathematically thick elastic plate overlying a newtonian viscous medium with an exponential depth dependence of viscosity a green s function solution is obtained for the viscous flow driven by a harmonic density distribution at a specified depth it is shown that during the transient phase of the solution the field quantities stress velocity displacement in both media depend upon the rheology of both media whereas in steady state the solutions become uncoupled due to this uncoupling stresses obtained from a purely viscous solution can be applied as boundary conditions at the base of an isolated elastic plate i show for such a plate that 1 shear stress imparted at the base of the lower boundary can induce large in plane stretching mode forces and 2 these forces can be estimated directly from free air gravity anomalies without detailed knowledge of the density distribution in the underlying flow free air gravity anomalies for venus imply in plane forces of about 2 3 1012 n m in plane forces of this magnitude are capable of extensive disruption of the venusian lithosphere to demonstrate this an elastic plastic analysis is carried out for the deformation of a model venus lithosphere the calculation is based on a lithospheric strength envelope and uses gravity and topography as a guide to the magnitude of stresses imparted by convective flow the results predict that dynamic uplift of venusian topography must be accompanied by extensive brittle failure and viscous flow in the lithosphere geoid anomalies and geoid to topography ratios for venusian topographic features are large compared to values for thermal swells in earth s ocean basins this suggests that dynamic support of topography must be an important process on venus further a low viscosity zone is probably not present beneath the venusian lithosphere a comparative analysis suggests that tectonic deformation of the lithosphere associated with dynamically supported topography on earth is unimportant

normal faulting and flexure in an elastic perfectly plastic plate abstract an elastic perfectly plastic plate model has been developed to analyze the flexure associated with normal faulting the model consists of a thin layer which is completely cut by a normal fault overlying a fluid substratum for a given applied bending moment at the fault the relationship between the amount of displacement on the fault and the extent of the failure zone can be calculated the model is applied to the wasatch front region in the eastern basin and range province usa to determine the correlation of its parameters with geological and geophysical data in the vicinity of a major normal fault the wasatch fault along which there has been 3 4 km of late cenozoic uplift in this region most seismic activity occurs away from the wasatch fault in a zone 30 km wide roughly centered 30 km east of the fault this activity occurs at depths of 15 km or less in order to match the observations the lithospheric layer must have a flexural rigidity of 0 5 to 1 1 1022 n m and a yield stress of 1 2 kb and must have zero applied bending moment at the fault the effective mechanical thickness of the lithosphere in this region is 20 25 km these results indicate that the lithosphere in long term mechanical studies in the eastern basin and range is thin and weak evaluating these results as compared to the seismic lithospheric thickness and temperature regime of the region produces some interesting correlations with studies in oceanic regions

late quaternary vegetation changes around lake rutundu mount kenya east africa evidence from grass cuticles pollen and stable carbon isotopes woody subalpine shrubs and grasses currently surround lake rutundu mount kenya multiple proxies including carbon isotopes pollen and grass cuticles from a 755 cm long core were used to reconstruct the vegetation over the past 38 300 calendar years stable carbon isotope ratios of total organic carbon and terrestrial biomarkers from the lake sediments imply that the proportion of terrestrial plants using the c4 photosynthetic pathway was greater during the late pleistocene than in the holocene pollen data show that grasses were a major constituent of the vegetation throughout the late pleistocene and holocene the proportion of grass pollen relative to the pollen from other plants was greatest at the last glacial maximum lgm grass cuticles confirm evidence that c4 grass taxa were present at the lgm and that the majority followed the cold tolerant nadp mec4 subpathway copyright 2003 john wiley sons ltd

new evidence for a reduced water balance in east africa during the last glacial maximum implication for model data comparison some syntheses of lake level data for the last glacial maximum lgm in east africa 10 n and 30 s east of 25 e show apparently wetter conditions than present for some basins whereas palaeovegetation reconstruction indicates a generally dry climate pmip gcm simulations for the lgm support both scenarios for this region when run under different boundary conditions here we compare three new records from lakes in the data poor southern part of east africa lake malawi lake massoko and lake rukwa we also re assess previously published lake level data and apply a salinity transfer function to the diatom record from lake manyara our results show that in contrast to previous interpretations these lakes were at least as low as today at the lgm and are thus in agreement with the palaeovegetation data relative drought across east africa is best simulated by gcms that use computed ssts rather than the higher climap values lower ssts and the presence of the northern hemisphere ice sheets must have been dominant over any monsoon precipitation rise caused by astronomically induced summer insolation enhancement in the southern african tropics

late quaternary changes in climate and land cover in the northern horn of africa and adjacent areas abstract most studies of landscape dynamics in the ethiopian highlands offer linear relations of population pressure and periodic droughts as causes of environmental degradation such views rely on limited historical information of one or two centuries which is inadequate for understanding the longer term effects of climatic shifts on landscape dynamics and environmental change in the highlands in this study we conducted a meta analysis of long term climate and biogeographic change and landscape dynamics during the holocene using multi proxy evidence from lacustrine n 10 marine n 5 geomorphic n 13 and vegetation n 7 records from the highlands and the wider horn of africa and western indian ocean region based on the datasets we discern late pleistocene dry phases in the region during the last glacial maximum the heinrich 1 event and the younger dryas additionally we recognize at least six dry phases during the holocene after 6500 bce 4900 bce 3700 bce 1000 bce 450 ce and 1750 ce the hydroclimatic shifts seem associated with changing equatorial atlantic and western indian ocean sea surface temperatures further the results show nonlinear relations between major holocene climate shifts and grassland expansions in the highlands our findings challenge conventional cause effect explanations of human population growth and land degradation and point instead to potentially strong but unexplored feedbacks between climate changes human activity and landscape dynamics over longer and short term periods

geoid anomalies in a dynamic earth in order to obtain a dynamically consistent relationship between the geoid and the earth s response to internal buoyancy forces we have calculated potential and surface deformation love numbers for internal loading these quantities depend on the depth and harmonic degree of loading they can be integrated as green functions to obtain the dynamic response due to an arbitrary distribution of internal density contrasts spherically symmetric self gravitating flow models are constructed for a variety of radial newtonian viscosity variations and flow configurations including both whole mantle and layered convection we demonstrate that boundary deformation due to internal loading reaches its equilibrium value on the same time scale as postglacial rebound much less than the time scale for significant change in the convective flow pattern by calculating relaxation times for a series of spherically symmetric viscous earth models for uniform mantle viscosity the geoid signature due to boundary deformations is larger than that due to internal loads resulting in net negative geoid anomalies for positive density contrasts geoid anomalies from intermediate wavelength density contrasts are amplified by up to an order of magnitude geoid anomalies are primarily the result of density contrasts in the interior of convecting layers density contrasts near layer boundaries are almost completely compensated layered mantle convection results in smaller geoid anomalies than mantle wide flow for a given density contrast viscosity stratification leads to more complicated spectral signatures because of the sensitivity of the dynamic response functions to model parameters forward models for the geoid can be used to combine several sources of geophysical data e g subducted slab locations seismic velocity anomalies surface topography to constrain better the structure and viscosity of the mantle

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