calibration and testing of upland hillslope evolution models in a dated landscape banco bonito new mexico 1 in this study we tested upland hillslope evolution models and constrained the rates of regolith production colluvial transport and eolian deposition over geologic time scales in a dated volcanic landscape in northern new mexico using field measurements of regolith thickness geochemical analyses of regolith bedrock and regional dust numerical modeling of regolith production and transport and quantitative analyses of airborne light detection and ranging lidar digital elevation models dems within this volcanic landscape many topographically closed basins exist as a result of compressional folding and explosion pitting during eruption the landscape has evolved from an initial state of no regolith cover at 40 5 ka to its modern state which has highly weathered regolith ranging from 0 to 3 m with local thickness values controlled primarily by topographic position our models constrain the maximum rate of regolith production in the study area to be in the range of 0 02 to 0 12 m kyr 1 and the rate of colluvial transport per unit slope gradient to be in the range of 0 2 to 2 7 m 2 kyr 1 with higher values in areas with more aboveground biomass we conclude that a depth dependent colluvial transport model better predicts the observed spatial distribution of regolith thickness compared to a model that has no depth dependence this study adds to the database of estimates for rates of regolith production and transport in the western united states and shows how dated landscapes can be used to improve our understanding of the coevolution of landscapes and regolith cover

organic geochemical proxies of paleoceanographic paleolimnologic and paleoclimatic processes abstract organic matter constitutes a minor fraction of marine and freshwater sediments yet its important contribution to the sedimentary record can be used to reconstruct marine and continental paleoenvironments the organic matter content of sediments is the residue of past biota the amounts and types of organic matter present in sediments consequently reflect environmental conditions that impacted ecosystems at different past times general sources of the organic matter are inferred from bulk properties such as elemental compositions carbon and nitrogen stable isotope ratios rock eval pyrolysis data and organic petrography details of organic matter origins are refined by analyses of biomarker molecular compositions source changes are proxies for fluctuations in sea level oceanic surface currents and continental climates algal paleoproductivity rates are indicated by organic carbon mass accumulation rates and carbon and nitrogen stable isotopic compositions these parameters record past availability of nutrients and therefore are proxies of surface mixing in the oceans and amount of land runoff to lakes sea surface paleotemperatures are recorded by the number of carbon carbon double bonds in lipid biomarkers produced by marine algae larger proportions of the double bonds are proxies for the cooler surface waters that accompanied periods of global glaciation and intervals of enhanced upwelling the 13c and d values of plant organic matter record past concentrations of carbon dioxide in the atmosphere and changes in delivery of atmospheric moisture respectively diagenesis which causes the concentration and composition of organic matter in sedimentary settings to differ from those of the original biologically synthesized materials can bias organic geochemical paleoenvironmental records the magnitude of this potential source of misinformation must always be considered and evaluated comparison of multiple organic geochemical proxies of past conditions helps to compensate for the effects of diagenetic alterations and thereby to improve interpretations of paleoenvironmental change

late pleistocene and holocene palaeohydrology of lake rukwa tanzania inferred from diatom analysis diatom analysis has been used to provide information on the lake level fluctuations of lake rukwa during the last 21 7 ka diatom inferred chemical variables conductivity ph anion and cation ratios indicate that at the lgm lake level was low probably driven by a low p e ratio but with sufficient water to avoid desiccation following the lgm deep water conditions were obtained through a series of steps until maximum water depth was reached by at least 13 5 ka the high lake level phase lasted through the early holocene until after 6 7 ka a shallow saline lake has occupied the basin from ca 5 5 ka until the present day the pattern of reconstructed lake level fluctuation in lake rukwa is similar to that from lake tanganyika and other lakes from the equatorial and northern tropical zone we can confirm that the n tropical mode of millennial scale lake level change since the lgm extended to at least 8 s

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

late quaternary vegetation and fire dynamics on mount kenya abstract pollen and charcoal data generated from a 1469 cm core radiocarbon dated to 26 430 14 c yr bp recovered from rumuiku swamp on the southeast of mount kenya are used to document changes in the distribution and composition of montane vegetation and fire regimes over the late quaternary throughout the transition from the last glacial maximum lgm high resolution sub centennial scale analysis documents a highly dynamic ecosystem and fire regime the pollen record shows that under a cool but rather moist lgm climate ericaceae and stoebe species shifted down slope more than 1000 m relative to the present day rather than simple altitudinal lowering of current vegetation zonation these taxa formed a vegetation assemblage that mixed high altitude components with relatively lowland taxa in particular juniperus that is presently found at altitudes lower than the study site but on the drier side of mount kenya there is noticeable addition and co dominance of hagenia to the ecosystem from 20 500 14 c yr bp until around 14 000 14 c yr bp when a mix of ericaceous belt and upper montane forest taxa such as artemisia polycias schefflera and stoebe dominated the initial development of montane forest reduced levels of hagenia juniperus olea and podocarpus are recorded about the time of the younger dryas with highly variable presence of more mesic taxa such as polyscias and schefflera this development of montane forest over the late pleistocene to holocene transition reflects a significant reorganization of the ecosystem composition that was heavily influenced by a variable fire regime shifts in vegetation composition reflect the onset of a warmer moist climate from the beginning of holocene as mixed montane forest became more established the latter part of the holocene registers human impact and forest clearance with increased anthropogenic impact marked by a transition to open vegetation and increased fire frequency

a simple geomorphic based analytical model for predicting the spatial distribution of soil thickness in headwater hillslopes and catchments 1 soil thickness acts as an important control for headwater hydrologic processes yet its spatial distribution remains one of the least understood in catchment hydrology analytic methods are desirable to provide a simple way for predicting soil thickness distribution over a hillslope or a catchment in this paper a simple geomorphic based analytical model is derived from the dynamic equations of soil thickness evolution in areas with no tectonic uplift or lowering since the recent geological past the model employs terrain attributes slope gradient curvature and upstream contributing area as inputs on grid based dems for predicting soil thickness evolution over time the analytic model is validated first on nine abstract hillslopes through comparing 10 kyr simulation results between our proposed model and the numerical solution the model is then applied to predict soil thickness evolution over 13 kyr in the 7 9 ha shale hills catchment one of the critical zone observatories in the u s located in central pennsylvania field observed and model predicted values of soil thickness are in good agreement with a root mean squared error of 0 39 m r2 0 74 and absolute errors 0 10 m in 70 of 106 sample points moreover our model verifies that terrain shape and position are the first order control on soil thickness evolution in the headwater catchment therefore the derived geomorphic based analytical model can be helpful in understanding soil thickness change over geological time and is useful as a simple tool for deriving spatially distributed soil thickness needed for hydrologic modeling

sediment transport mechanisms on soil mantled hillslopes landscape evolution is modeled widely using a simple creep law for complex processes of sediment transport here field data show how a new transport model combined with an exponential soil production law better captures spatial variations of soil thick ness on hillslopes we combine parameterizations of simple and depth dependent creep with overland flow to predict soil thickness and suggest how soil distribution evolves in response to climatic and tectonic forcing we present an empirical expression for the response time of the system to external forcing that shows strong dependence on relief and is independent of soil production rate we suggest that this parameterization may be used to quantify upland carbon storage and removal and predict impacts of defor estation or rapid climatic changes

late quaternary vegetation dynamics in a biodiversity hotspot the uluguru mountains of tanzania abstract late quaternary vegetation history and environmental changes in a biodiverse tropical ecosystem are inferred from pollen charcoal and carbon isotope evidence derived from a 48 000 yr sedimentary record from the uluguru mountains a component of the eastern arc mountains of kenya and tanzania results indicate that eastern arc forest composition has remained relatively stable during the past 48 000 yr long term environmental stability of the eastern arc forests has been proposed as a mechanism for the accumulation and persistence of species during glacial periods thus resulting in the diverse forests observed today the pollen and isotope data presented here indicate some marked changes in abundance but no significant loss in moist forest taxa through the last glacial maximum thereby providing support for the long term environmental stability of the eastern arc anthropogenic activities including burning and forest clearance were found to play a moderate role in shaping the mosaic of forest patches and high altitude grasslands that characterise the site today however this influence was tempered by the inaccessibility of the mountain

a late holocene record of human impacts on tropical environments from non pollen palynomorphs albertine rift western uganda non pollen palynomorphs and elemental geochemistry data from lake kifuruka in western uganda provide evidence of environmental change in the tropical african region since the beginning of the holocene the multi proxy record presented here shows that dry conditions dominated the end of the pleistocene evidenced by calcium enriched sediments and suppressed fungal taxa activity moist conditions dominated the early holocene and persisted until just after 1960 cal yr bp elevated frequencies of individual fungal spore taxa associated with herbivory and soil erosion including sordaria type sporormiella type chaetomium type and glomus type about 4300 cal yr bp suggests a significant environmental change that could be linked to human activities a convergence of multiple proxy data including microscopic charcoal elemental geochemistry and fungal spores strongly support the occurrence of anthropogenic forest disturbance in the albertine rift about 4300 cal yr bp

analytical solutions to hillslope subsurface storm flow and saturation overland flow we present the analytical solutions to a hillslope based formulation of subsurface storm flow and saturation overland flow here a hillslope is defined as the area between two streamlines originating from two ends of a channel link or from the channel head and ending at the ridgelines the three dimensional soil mantle of a hillslope is reduced to a one dimensional profile along which the soil moisture and discharge are modeled continuity and a kinematic form of darcy s law lead to quasi linear wave equations solvable with the method of characteristics examples are given to illustrate the ability and limitations of the solutions in capturing the essential runoff behavior of different slope types

tropical climates at the last glacial maximum a new synthesis of terrestrial palaeoclimate data i vegetation lake levels and geochemistry palaeodata in synthesis form are needed as benchmarks for the palaeoclimate modelling intercomparison project pmip advances since the last synthesis of terrestrial palaeodata from the last glacial maximum lgm call for a new evaluation especially of data from the tropics here pollen plant macrofossil lake level noble gas from groundwater and delta 18 o from speleothems data are compiled for 18 2 ka c 14 32 degrees n 33 degrees s the reliability of the data was evaluated using explicit criteria and some types of data were re analysed using consistent methods in order to derive a set of mutually consistent palaeoclimate estimates of mean temperature of the coldest month mtco mean annual temperature mat plant available moisture pam and runoff p e cold month temperature mat anomalies from plant data range from 1 to 2 k near sea level in indonesia and the s pacific through 6 to 8 k at many high elevation sites to 8 to 15k in s china and the se usa mat anomalies from groundwater or speleothems seem more uniform 4 to 6 k but the data are as yet sparse a clear divergence between mat and cold month estimates from the same region is seen only in the se usa where cold air advection is expected to have enhanced cooling in winter regression of all cold month anomalies against site elevation yielded an estimated average cooling of 2 5 to 3 k at modern sea level increasing to approximate to 6 k by 3000m how ever neotropical sites showed larger than the average sea level cooling 5 to 6 k and a non significant elevation effect whereas w and s pacific sites showed much less sea level cooling 1 k and a stronger elevation effect these findings support the inference that tropical sea surface temperatures ssts were lower than the climap estimates but they limit the plausible average tropical sea surface cooling and they support the existence of climap like geographic patterns in sst anomalies trends of pam and lake levels indicate wet lgm conditions in the w usa and at the highest elevations with generally dry conditions elsewhere these results suggest a colder than present ocean surface producing a weaker hydrological cycle more arid continents and arguably steeper than present terrestrial lapse rates such linkages are supported by recent observations on freezing level height and tropical ssts moreover simulations of greenhouse and lgm climates point to several possible feedback processes by which low level temperature anomalies might be amplified aloft

late pleistocene and holocene history at mubwindi swamp southwest uganda abstract deposits beneath mubwindi swamp provide a partial record of vegetation history since at least 43 000 yr ago we studied pollen from two cores and obtained nine radiocarbon ages from one of these cores and three radiocarbon ages from the other pollen deposited before and soon after the last glacial maximum represents vegetation very different from the modern vegetation of the mubwindi swamp catchment although species now associated with higher altitudes were dominant some elements of moist lower montane forest persisted possibly because of favorable soils or topography the pollen data provides evidence for a late glacial montane forest refuge near mubwindi swamp moist lower montane forest became much more widespread soon after the glacial maximum the only irrefutably holocene sediments from mubwindi swamp date to the past 2500 yr during this time a combination of climatic and human induced changes in vegetation can be seen in the pollen records

the kashiru pollen sequence burundi palaeoclimatic implications for the last 40 000 yr b p in tropical africa abstract a continuous pollen record from a central african highland site 3 28 s 29 34 e with 15 radio carbon dates spans at least the last 40 000 yr both modern and fossil pollen have been investigated a clear pattern of changes in vegetation and climate is suggested prior to 30 000 yr b p the occurrence of the montane conifer forest including the upper forest limit indicates climatic conditions colder and drier than now but more humid than in late glacial time between 30 000 and 15 000 yr b p grassland with afroalpine indicators expanded down to 2500 2000 m altitude as a result of dry and most probably cool climate the montane forest persisted in refuges although much reduced forest vegetation with the same floristic composition as today appeared after 13 250 200 yr b p suggesting more humid conditions the occurrence of a possible short term humid episode about 21 500 yr b p is correlated with some high lake level stages in east africa and with organic deposits in deep sea cores from the mediterranean and the atlantic ocean

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

estimates of the rate of regolith production using and from an alpine hillslope abstract the production of regolith is a fundamental geomorphic process because most surface processes transport only unconsolidated material we use concentrations of the cosmogenic radionuclides crns 10 be and 26 al in regolith and bedrock to deduce the rate of production of regolith on an alpine hillslope in the wind river range wy these calculations are based on a theoretical model which we develop here this model shows that it is important to consider dissolution of regolith in regolith production and in basin averaged erosion rate studies rates of production of regolith are uniform along the hillslope and the mean rates for the entire hillslope deduced from 10 be and 26 al are 14 3 4 0 and 13 0 4 0 m ma 1 respectively rates of production of regolith deduced from 10 be concentrations in regolith mantled bedrock support the rates deduced from regolith concentrations in the alpine environment examined here the rate of production of regolith beneath 90 cm of regolith is nearly twice as fast as the average rate of production of regolith on bare rock surfaces which small et al small e e anderson r s repka j l finkel r 1997 erosion rates of alpine bedrock summit surfaces deduced from in situ 10 be and 26 al earth and planetary science letters 150 413 425 previously documented rock mantled with regolith probably weathers more rapidly than bare rock because the water required for frost weathering is limited on bare rock surfaces because the hillslope examined here is convex with constant curvature and regolith production and thickness are uniform down the slope the regolith volume flux must be proportional to the local slope of the hillside therefore our results are consistent with gilbert s gilbert g k 1909 the convexity of hilltops journal of geology 17 344 350 steady state hillslope hypothesis if tor height and the difference between rates of weathering on bare and regolith mantled rock provide a fair estimate of the age of summit flats steady state hillslope conditions have been attained in less than several million years

quantification of soil production and downslope creep rates from cosmogenic 10be accumulations on a hillslope profile average soil transport rates over a period of 3500 yr on a convex soil mantled hillslope have been quantified using a mass balance model that incorporates the soil concentration of the cosmogenic isotope 10 be the 10 be model results support the assumption used in most geomorphic models that the soil creep rate is proportional to surface gradient the predicted diffusion coefficient is 360 55 cm 3 yr 1 cm 1 contour length and the average rate of soil production is 0 026 0 007 cm yr within the uncertainty of this technique the data do not reject g k gilbert9s hypothesis that some hillslopes may exist in a condition of dynamic equilibrium with a uniform soil production rate however the model does not require an assumption of dynamic equilibrium and may be an approach that uniquely allows the quantification of a local soil production rate law

reconstruction of a subalpine grass dominated ecosystem lake rutundu mount kenya a novel multi proxy approach palaeoecological reconstructions based on a single proxy are limited but by combining pollen biogeochemistry and grass cuticle analysis ecosystem structure and function can be better understood lake rutundu is a small subalpine lake on the northeast flank of mt kenya during the last glacial pollen evidence suggests a shrub grassland dominated by afroalpine taxa and poaceae representing a dry cold open environment the 13c values of terrestrial biomarkers imply a high proportion of c4 plants grass cuticle analysis allows resolution of the different c4 subtypes and shows that the vegetation was dominated by tall c4 panicoid grasses prone to frequent fires during the holocene poaceae pollen declined while subalpine shrubs increased the 13c values of terrestrial biomarkers imply a c3 dominated vegetation together with an expansion of rainforest at lower altitudes this suggests wetter conditions more favourable to c3 plants increased percentages of c3 pooid grass cuticles confirm a reduction in moisture stress

tracing long term tropical montane ecosystem change in the eastern arc mountains of tanzania pollen charcoal stable carbon and nitrogen isotope and radiocarbon analyses are used to reconstruct a palaeoenvironmental record from the eastern arc mountains eam tanzania dating back 45 000 years a sedimentary hiatus covering the last glacial interglacial transition lgit and early to mid holocene follows the lead up to the last glacial maximum lgm period a late holocene portion of the record is included as a modern analogue before 45 000 cal a bp very low regional pollen counts imply open vegetation surrounding the site subsequently the development of extensive montane forest characterizes a relatively mesic pre lgm the transition into the lgm witnessed a marked shift from c3 t o c4 dominated biomass representing an expansion of open forest dominated by grasses these changes may be attributed to a combination of enhanced moisture stress and co2 limitation factors that may have been exacerbated by an intensive fire regime as evident from the charcoal record this is supported by a decrease in montane forest taxa coupled with an associated expansion of c4 graminoids and ericaceous heathland to form a montane grassland mosaic similar to that previously recorded on other east african mountains around the time of the lgm but not to date having been shown in the eastern arc mountains copyright 2014 john wiley sons ltd

wet phases in tropical southern africa during the last glacial period 1 a 45 000 year high resolution sedimentary record from lake masoko in tanzania shows the climate in this part of east africa to have been characterized by a short and less severe dry season during the last glacial maximum moisture and lake level proxies pollen assemblages and magnetic susceptibility indicate that rainfall in the masoko area was strongly controlled by low latitude insolation i e precessional forcing such observations contrast with the climatic patterns previously reconstructed in east africa for the last glacial indeed widespread dry conditions are generally observed and are attributed to the predominant control of high latitude glacial forcing over insolation forcing on the tropical hydrology however during the younger dryas cold event out of phase with regional insolation behaviour wetter conditions prevailed in this area suggesting that the rainfall belt over africa was probably shifted to the south bringing moisture to the southernmost tropics

an implicit finite difference method for drainage basin evolution 1 in drainage basin evolution models the implementation of sediment transport and sediment balance is constrained by numerical instabilities in order to overcome these difficulties an implicit finite difference method is proposed the method simultaneously adjusts the elevation at each node of the numerical grid is unconditionally stable and significantly reduces the computational time the performance of the method is tested and briefly discussed with a numerical example

the terminal paleozoic fungal event evidence of terrestrial ecosystem destabilization and collapse because of its prominent role in global biomass storage land vegetation is the most obvious biota to be investigated for records of dramatic ecologic crisis in earth history there is accumulating evidence that throughout the world sedimentary organic matter preserved in latest permian deposits is characterized by unparalleled abundances of fungal remains irrespective of depositional environment marine lacustrine fluviatile floral provinciality and climatic zonation this fungal event can be considered to reflect excessive dieback of arboreous vegetation effecting destabilization and subsequent collapse of terrestrial ecosystems with concomitant loss of standing biomass such a scenario is in harmony with predictions that the permian triassic ecologic crisis was triggered by the effects of severe changes in atmospheric chemistry arising from the rapid eruption of the siberian traps flood basalts

co2 and temperature controlled altitudinal shifts of c4 and c3 dominated grasslands allow reconstruction of palaeoatmospheric pco2 abstract during the pleistocene the vegetation changes in the high colombian andes included changes from c3 to c4 plants this is inferred from 13c values of the c31 n alkane from the funza 2 sedimentary record taken from the high plain of bogota at 2550 m elevation the environmental factors thought to be responsible for these changes were investigated using a single point simulation of the biome3 vegetation model including changes in precipitation temperature and atmospheric co2 concentrations the model shows that changes are for a major part caused by these latter two factors the isotopic signature of the n alkanes of several extant c3 and c4 grasses from the area were determined to calibrate the interpretation of the isotopic record from the geochemical record we estimated the altitudinal distribution of c3 and c4 plants using present grass distribution patterns based on floristic data as a template this information in combination with palaeotemperature estimates enabled the reconstruction of atmospheric co2 concentrations the reconstructed co2 concentrations follow the trends of the vostok antarctic ice core through three glacial and two interglacial stages the lowest calculated co2 concentration is ca 210 ppmv for the glacial maxima and within the range of lowest values from vostok our highest value 310 ppmv is for interglacial mis 7 this represents a new method to reconstruct palaeoatmospheric pco2 it is less accurate than measurements from ice cores but has potential to be used for sediments that are much older than the ice cores

the sensitivity of a tanzanian crater lake to catastrophic tephra input and four millennia of climate change diatom genera in many large east african lakes change little throughout the holocene period suggesting relatively stable ecological conditions and some resilience to environmental change ecosystem stability is less common in smaller more sensitive lakes such as those within volcanic craters where external impacts can cause abrupt and rapid fluctuations a 4100 year diatom and cyanobacteria pigment record from lake massoko a volcanic crater lake in southern tanzania is used to illustrate important switches in resource ratios following tephra deposition 1190 years ago it is hypothesized that the tephra reduced the rate of p diffusion from the sediments and increased the si p ratio in the lake a period of acute change in planktonic diatom communities resulted from the tephra impact and lasted c 110 years the magnitude of the change shown by the diatoms and their slow recovery from the tephra may be due in part to a coincident fall in lake level caused by a reduction in regional rainfall the statistical significance of the tephra impact relative to that of catchment and climate change has been tested using variance partitioning and rate of change analysis multiproxy indicators show an important period of positive water balance 1700 ago and a relatively dry episode persisting between 1000 and 400 years ago the lake ecosystem is shown to be highly sensitive to both climate change and tephra deposition

late quaternary vegetation reconstruction from the eastern arc mountains tanzania abstract pollen spore macrofossil and stable isotope c and n analyses from a 266 cm sediment core collected from a swamp on the eastern arc mountains tanzania are used to reconstruct vegetation and environmental history an estimated time scale based on five 14c ages records approximately 38 000 yr this palaeorecord is the first from this biodiversity hotspot and importantly extends through the last glacial maximum lgm the altitudinal transition from montane to upper montane forest shifted from 1700 1800 m 38 000 14c yr bp to 1800 1900 m 35 000 29 000 14c yr bp from 29 000 to 10 000 14c yr bp it shifted from 1850 1950 m across the lgm to 1750 1800 m during 10 000 3500 14c yr bp and to present day elevations at 2000 m during the last 3500 14c yr bp the relative ecosystem stability across the lgm may be explained by the indian ocean s influence in maintaining continuous moist forest cover during a period of east african regional climate aridity during the late holocene presence of abundant coprophilous fungi and algal blooms demonstrates increasing human impact neurospora spores indicate frequent fires coinciding with clear signals of decline in podocarpus and psychotria trees that possibly represent selective logging

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

spatial organization of soil depths using a landform evolution model 1 the evolution of soil depths is investigated by modeling the interaction between soil production and surface erosion within a landform evolution model an enhanced version of the landform evolution model siberia that incorporates a soil evolution module is used to simulate evolving landforms and soils depths over geologic timescales the spatial and temporal evolution of soil depths are examined at the hillslope scale though it is widely accepted among the geomorphology community that soil water enhances chemical physical and biological weathering processes its effect has not been explicitly included in published models of soil production the main scientific questions that we address are 1 what are the implications of incorporating soil moisture dependency in the soil production function and 2 what type of soil production dynamics is needed to generate a bedrock topography that has a different spatial pattern from that of the ground surface a range of physics for the soil production model is explored the effect of soil moisture is included using the wetness index obtained from drainage analysis of either surface elevations or the bedrock topography the results show that the various soil production functions that incorporate either a wetness index or subsurface flow depth based on the bedrock topography give rise to soils that self organize with well defined spatial patterns and bedrock elevations with spatial organization significantly different from that of the surface the model that incorporates the influence of subsurface water on soil production is able to naturally generate a soil production rate with a maximum value for a nonzero soil depth and overcomes an inconsistency of previously published humped soil production models

how well can hillslope evolution models explain topography simulating soil transport and production with high resolution topographic data the morphology of hillslopes is a direct refl ection of tectonic forcing and climatic and biologic processes that drive soil production mobilization and transport soil transport on hillslopes affects river incision by providing tools for channel abrasion and controls the distribution of sediment that infl uences aquatic habitat although numerous hillslope transport relationships have been proposed over the past 60 years a comprehensive analysis of model predictions for a real landscape has not been performed here we use high resolution topographic data obtained via airborne laser swath mapping alsm to simulate the long term evolution of oregon coast range hillslopes and test three published transport models and a new model that accounts for nonlinear depth and slopedependent transport analysis of one dimensional steady state solutions for these four models suggests that plots of gradient curvature may be diagnostic for distinguishing model predictions to evaluate two dimensional model predictions for our fi eld site we assumed local steady state erosion for a 72 000 m 2 sequence of hillslopes and valleys after calibrating each of the four models we imposed constant base level lowering for cells within the valley network simulated 500 000 yr of soil production and transport and determined which transport model best preserved morphologic patterns that describe the current landscape form models for which fl ux varies proportionally with hillslope gradient generated broadly convex hilltops inconsistent with the sharp crested steep sided slopes of our study site whereas the two nonlinear slope dependent models produced convex planar slopes consistent with current hillslope form our proposed nonlinear slope and depth dependent model accounts for how soil thickness controls the magnitude of biogenic disturbances that drive transport this model best preserved the current landscape form particularly the narrow sharply convex hilltops characteristic of the oregon coast range according to our formulation which provides an explicit linkage for relating the distribution of biota to hillslope processes the degree of hilltop convexity varies nonlinearly with the ratio of erosion rate to maximum soil production rate highlighting the profound infl uence of soil depth on hillslope evolution

mangrove ecosystem dynamics and elemental cycling at twin cays belize during the holocene existing at the transition between the terrestrial environment and hydrosphere mangroves are sensitive to environmental change e g sea level rise we present pollen and stable isotope data from a core tcc1 of continuous 10 m mangrove peat from twin cays 12 km off of the coast of belize central america radiocarbon dates on fragments of mangrove leaves preserved in tcc1 show that the core provides an 8600 14c year record of mangrove ecosystem changes variation in the stable carbon and nitrogen isotope composition 13c 30 to 25 15n 6 to 2 of mangrove leaves imply that the stand structure and nutrient status of the mangroves at the site have changed during the holocene pollen data from the same core show that the floral composition of the site has changed at points during the holocene most notably a brief 240 years switch at 3860 14c yr bp to dominance by a species of myrsine not currently present at the site our results are consistent with significant environmental changes either marked disturbance from hurricanes or fluctuations in sea level through the holocene copyright 2004 john wiley sons ltd

lake level history of lake tanganyika east africa for the past 2500 years based on ostracode inferred water depth reconstruction assemblages of ostracodes from sediment cores illuminate lake level history at decadal to centennial timescales during the late holocene at lake tanganyika east africa the ostracode based lake level curves for several cores resemble both each other and the only previously published lake level record of comparable resolution for lake tanganyika during this interval successfully reconstructing known highstands improving the chronology of known lowstands and contributing new information on late holocene lake level variability at this important tropical african location in agreement with other late holocene records from east africa the surface level at lake tanganyika reflects predominantly arid conditions throughout this interval interrupted by relatively brief episodes of higher precipitation and lake level the most pronounced lowstand in the record occurs at v200 0 bc with other significant lowstands dating to the intervals v200 500 ad v700 850 ad the medieval warmperiod mwp v1050 1250 ad at lake tanganyika and the latter part of the little ice age lia v1550 1850 ad the most important wet intervals in the lake level record are centered on v500 ad v1500 ad and v1870 ad the highstands and lowstands reported here for lake tanganyika appear to be fairly coherent with other records of rainfall throughout east africa during the mwp and the lia prior to the mwp paleoclimate records are apparently less coherent although this may be a reflection of the resolution and abundance of recent paleoclimatic data available for this climatically complex region

human environment interactions in an agricultural landscape a 1400 yr sediment and pollen record from north pare ne tanzania abstract a sediment and pollen record from the north pare mountains in north eastern tanzania provides evidence for strong anthropogenic landscape transformations since the 7th century ad and demonstrates that shortly after the arrival of early iron age farmers agricultural land use became the main driving force of vegetation and landscape change predominating over external climatic factors the absence of pollen characteristic for the corresponding altitudinal submontane forest implies widespread forest clearance and agriculture land use prior to the onset of the sedimentary record land clearance soil erosion and corresponding accumulation of colluvial slope deposits on the valley floor triggered paludification and the establishment of the lomwe palaeowetland in the 7th century ad accelerated soil erosion and rapid alluvial burial of the palaeowetland between ad 1200 and ad 1500 represent an environmental tipping point with important repercussions for present day land degradation and resource restraints age control is complicated by radiocarbon date reversals and draws on introduced forest taxa cupressus and eucalyptus as biostratigraphic markers of the 20th century geoarchives from agricultural landscapes that are shaped by long term settlement and land use such as the pare mountains can enhance archaeological occupation histories and provide crucial information for both the identification of ecological thresholds and for the reconstruction of environmental transitions during anthropogenic landscape transformations

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

a 22 kyr bp sedimentological record of lake rukwa 8 s sw tanzania environmental chronostratigraphic and climatic implications abstract the sedimentological study of a 12 8 m long core r96 i from lake rukwa tanzania 8 s provides a new record of past lake level fluctuations that took place in response to changes of the regional climate since the last glacial period from 21 to 15 cal kyr bp nearshore and swamp marsh environments are evidenced from clastic deposition macrophytes debris and ca mg carbonate enrichments a transgressive sequence starting around 15 cal kyr bp lead to the humid lacustrine optimum between 15 and 7 cal kyr bp from 12 to 10 cal kyr bp anoxic lake bottom environments were favored by the concomitant effects of high primary productivity increased subsidence and inputs of volcanic ash in the lake the middle holocene 7 3 cal kyr bp is characterized by high concentrations of silt carbonates and low organic content which indicate the occurrence of relatively oxic shallow and saline depositional environments especially around 7 and 3 4 cal kyr bp although grainsize and toc profiles suggest that shallow environments likely persisted in the upper holocene low mg concentration values for the last 3 kyr may indicate a trend toward relatively more dilute environments the two main lake rukwa low stand periods at 21 15 and 7 3 cal kyr bp correspond remarkably well with the insolation maximum at the equator which occurred at 17 kyr bp for the spring equinox and at 6 kyr bp for the autumn equinox respectively we suggest that during these periods of minimum inter hemispheric insolation gradients monsoon circulation weakened in south equatorial regions and the itcz was located north of 8 s in east africa

vegetation climate and fire dynamics in east africa inferred from the maundi crater pollen record from mt kilimanjaro during the last glacial interglacial cycle abstract the pollen charcoal and sedimentological record from the maundi crater located at 2780 m elevation on the south eastern slope of mt kilimanjaro is one of the longest terrestrial records in equatorial east africa giving an interesting insight into the vegetation and climate dynamics back to the early last glacial period our sediment record has a reliable chronology until 42 ka bp an extrapolation of the age depth model as well as matching with other palaeo records from tropical east africa suggest a total age of about 90 ka bp at the bottom of the record during the last glacial the distribution as well as the composition of the vegetation belts classified as colline savanna submontane woodland montane forest ericaceous belt and alpine vegetation changed the early last glacial is characterized by high amounts of poaceae and asteraceae pollen suggesting a climatically dry but stable phase based on the absence of pollen grains in samples deposited around 70 ka bp we assume the occurrence of distinct drought periods during the pre lgm last glacial maximum a higher taxa diversity of the ericaceous and montane zone is recorded and suggests a spread of forest and shrub vegetation thus indicating a more humid period the taxa diversity increases steadily during the recorded time span the decent of vegetation zones indicate dry and cold conditions during the lgm and seem to have been detrimental for many taxa especially those of the forest vegetation however the early last glacial seems to have been markedly drier than the lgm the reappearance of most of the taxa most importantly alchemilla araliaceae dodonea hagenia ilex myrsine moraceae piperaceae during the deglacial and holocene period suggest a shift into humid conditions an increase in ferns and the decrease in grasses during the holocene also indicate increasing humidity fire played an important role in controlling the development and elevation of the ericaceous zone and the tree line during the holocene no increased anthropogenic impact around the maundi crater can be observed since neither higher fire activity nor a spread of hemerophilic plants is recorded this pollen archive reveals shifts in the upper vegetation zones ericaceous zone and montane forest zone of at least 1100 m but underlines the role of mt kilimanjaro as a glacial refuge for montane forest species similar to that of the eastern arc mountains

pollen based vegetation changes in southern tanzania during the last 4200 years climate change and or human impact the age constrained pollen data of a sedimentary sequence from the crater lake masoko southern tanzania 9 20ps 33 45pe 770 m display a continuous record of vegetation for the past 4200 years this record provides evidence that wetter zambezian woodlands always occupied this area during the late holocene reaching a maximum extent between 2800 and 1650 cal yr bp related to increase in summer monsoon intensity however three main episodes of decline have been detected between 3450 and 2800 cal yr bp between 1650 and 1450 cal yr bp and from 1200 to 500 cal yr bp for which a climatic interpretation decrease in the summer monsoon strength was preferentially advanced the first is synchronous with lowstand of many tropical african lakes and so mainly induced by increased aridity in contrast the abrupt change in the pollen record at 1650 1550 cal yr bp is marked by a large extension of grasslands at the expense of arboreal cover further by an increase in ricinus communis and an intensification of burning it could thus indicate local clearance of vegetation by man however at the same time the decline of montane forest suggests the impact of a more regional change during the last episode between 1200 and 500 cal yr bp dry climatic conditions are inferred from a combination of pollen diatom and magnetic proxies although the occurrence of late iron age settlements in the region means that local human interference cannot be excluded this study illustrates the difficulties in deciphering ecological and anthropological changes from pollen data in african tropical regions a 2003 elsevier b v all rights reserved

an empirical geomorphology based approach to the spatial prediction of soil thickness at catchment scale 1 catchment modeling in areas dominated by active geomorphologic processes such as soil erosion and landsliding is often hampered by the lack of reliable methods for the spatial estimation of soil depth in a catchment soil thickness h can vary as a function of many different and interplaying factors such as underlying lithology climate gradient hillslope curvature upslope contributing area and vegetation cover making the distributed estimation of h challenging and often unreliable in this work we present an alternative methodology which links soil thickness to gradient horizontal and vertical slope curvature and relative position within the hillslope profile while the relationship with gradient and curvature should reflect the kinematic stability of the regolith cover allowing greater soil thicknesses over planar and concave areas the distance from the hill crest or from the valley bottom accounts for the position within the soil toposequence this last parameter is fundamental points having equal gradient and curvature can have significantly different soil thickness due to their dissimilar position along the hillslope profile the proposed model has been implemented in a geographic information system environment and tested in the terzona creek basin in central italy results are in good agreement with field data mean absolute error is 11 cm with 8 5 cm standard deviation and average errors are lower than those obtained with other topography based methods where mean absolute error ranges from 47 cm for a model based on curvature position and slope gradient to 94 cm for a model based solely on slope gradient as a further test the predicted soil thickness was used to determine derived quantities such as the factor of safety for landsliding potential our model when compared to other empirical methods returns the best results and therefore can improve the prediction of soil losses and sediment production when utilized in conjunction with hydrological and landsliding models

regolith production rates calculated with uranium series isotopes at susquehanna shale hills critical zone observatory abstract in the critical zone where rocks and life interact bedrock equilibrates to earth surface conditions transforming to regolith the factors that control the rates and mechanisms of formation of regolith defined here as material that can be augered are still not fully understood to quantify regolith formation rates on shale lithology we measured uranium series u series isotopes 238 u 234 u and 230 th in three weathering profiles along a planar hillslope at the susquehanna shale hills observatory ssho in central pennsylvania all regolith samples show significant u series disequilibrium 234 u 238 u and 230 th 238 u activity ratios range from 0 934 to 1 072 and from 0 903 to 1 096 respectively these values display depth trends that are consistent with fractionation of u series isotopes during chemical weathering and element transport i e the relative mobility decreases in the order 234 u 238 u 230 th the activity ratios observed in the regolith samples are explained by i loss of u series isotopes during water rock interactions and ii re deposition of u series isotopes downslope loss of u and th initiates in the meter thick zone of bedrock that cannot be augered but that nonetheless consists of up to 40 clay silt sand inferred to have lost k mg al and fe apparent equivalent regolith production rates calculated with these isotopes for these profiles decrease exponentially from 45 m myr to 17 m myr with increasing regolith thickness from the ridge top to the valley floor with increasing distance from the ridge top toward the valley apparent equivalent regolith residence times increase from 7 kyr to 40 kyr given that the ssho experienced peri glacial climate 15 kyr ago and has a catchment wide averaged erosion rate of 15 m myr as inferred from cosmogenic 10 be we conclude that the hillslope retains regolith formed before the peri glacial period and is not at geomorphologic steady state both chemical weathering reactions of clay minerals and translocation of fine particles colloids are shown to contribute to mass loss of u and th from the regolith consistent with major element data at ssho this research documents a case study where u series isotopes are used to constrain the time scales of chemical weathering and regolith production rates regolith production rates at the ssho should be useful as a reference value for future work at other weathering localities

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

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

cosmogenic nuclides topography and the spatial variation of soil depth abstract if the rate of bedrock conversion to a mobile layer of soil depends on the local thickness of soil then hillslopes on uniform bedrock in a landscape approaching dynamic equilibrium should be mantled by a uniform thickness of soil conversely if the depth of soil varies across an actively eroding landscape then rates of soil production will also vary and consequently the landscape will not be in morphologic equilibrium the slow evolution of hillslopes relative to the tempo of climatic variations and tectonic adjustments would suggest that local morphologic disequilibrium may be expected in many landscapes here we explore this issue of equilibrium landscapes through a previously developed model that predicts the spatial variation in thickness of soil as a consequence of the local balance between soil production and erosion first we confirm the assumption in the model that soil production varies inversely with the thickness of soil using two independent methods one method uses the theoretical prediction that at local steady state soil production equals removal the depth of soil should vary inversely with hillslope curvature the second method relies on direct measurements of in situ produced concentrations of cosmogenic 10 be and 26 al in bedrock at the base of the soil column for our study site in northern california the two methods agree and yield the expression that the rate of soil production declines exponentially with the thickness of soil from 0 077 mm year with no soil mantle to 0 0077 mm year under 1 m of soil we then use this function of soil production in a coupled production and diffusive model of sediment transport to explore the controls on the spatial variation of the depth of soil on four separate spur ridges noses where we measured the data for the function of soil production model predictions are sensitive to boundary conditions grid scale and run time nonetheless we found good agreement between predicted and observed depths of soil as long as we used the observed function of soil production the four noses each have spatially varying curvature and consequently have varying depths of soil implying morphologic disequilibrium we suggest that our study site has been subjected to a wave of incision and varying intensities of erosion because of tectonic and climatic oscillations that have a frequency shorter than the morphologic response time of the landscape

mauritius since the last glacial environmental and climatic reconstruction of the last 38 000 years from kanaka crater a 10 m long peat core from the kanaka crater 20 25 s 57 31 e located at 560 m elevation in mauritius was analyzed for microfossils eight radiocarbon ages show the pollen record reflects environmental and climatic change of the last ca 38 cal ka bp the record shows that the island was continuously covered by forest with erica heath philippia in the uplands cyperaceous reedswamp with pandanus trees was abundant in the coastal lowlands as well as locally in the waterlogged crater the record shows changes in climatic humidity wet from 38 0 to 22 7 cal ka bp drier from 22 7 to 10 6 cal ka bp and wetter again from 10 6 cal ka bp to recent as the main response to climate change a high turnover in montane forest species is evidenced at 22 7 cal ka bp and at the start of the holocene the limited altitudinal ranges in the mountains of mauritius maximum altitude 828 m and changing humidity being more important than changing temperature suggests that in response to climate change a reassortment in taxonomic composition of montane forests might be equally important as displacement of forest types to new altitudinal intervals we found weak impact of the latitudinal migration of the intertropical convergence zone and data suggest that the indian ocean dipole is a more important driver for climatic change in the southwest indian ocean

rapid succession of plant associations on the small ocean island of mauritius at the onset of the holocene the island of mauritius offers the opportunity to study the poorly understood vegetation response to climate change on a small tropical oceanic island a high resolution pollen record from a 10 m long peat core from kanaka crater 560 m elevation mauritius indian ocean shows that vegetation shifted from a stable open wet forest last glacial state to a stable closed stratified tall forest holocene state an ecological threshold was crossed at w11 5 cal ka bp propelling the forest ecosystem into an unstable period lasting w4000 years the shift between the two steady states involves a cascade of four abrupt 150 years forest transitions in which different tree species dominated the vegetation for a quasi stable period of respectively 1900 1100 and 900 years we interpret the first forest transition as climatedriven reflecting the response of a small low topography oceanic island where significant spatial biome migration is impossible the three subsequent forest transitions are not evidently linked to climate events and are suggested to be driven by internal forest dynamics the cascade of four consecutive events of species turnover occurred at a remarkably fast rate compared to changes during the preceding and following periods and might therefore be considered as a composite tipping point in the ecosystem we hypothesize that wet gallery forest spatially and temporally stabilized by the drainage system served as a long lasting reservoir of biodiversity and facilitated a rapid exchange of species with the montane forests to allow for a rapid cascade of plant associations

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

effects of land use change on aquatic biodiversity a view from the paleorecord at lake tanganyika east africa population growth and watershed deforestation in northwestern tanzania threaten the biodiversity of lake tanganyika through erosion and habitat degradation we used cores collected offshore from gombe stream national park and a deforested watershed to reconstruct how land use changes in the gombe stream area since a d 1750 have affected lake biodiversity paleoenvironmental and paleoecological data reveal substantial changes in mass accumulation rates for sediment and organic matter nitrogen stable isotope values and benthic species composition offshore from the deforested watershed since 1880 comparable changes were not observed offshore from the park

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

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

lake level and paleoenvironmental history of lake tanganyika africa as inferred from late holocene and modern stromatolites fossil and living stromatolites are abundant around the margins of lake tanganyika africa and provide a wealth of paleolimnologic and paleoclimatic information for the late holocene six lines of evidence show that stromatolites and cements are precipitating in the lake today 1 carbonate saturation state calculations 2 documentation of living stromatolites and their depth distribution 3 new stable isotope data showing the lake s present mixing state and ancient evaporation and inflow balance 4 new radiocarbon data and a reevaluation of apparent 14 c ages derived from lake tanganyika carbonates 5 the presence of modern mg calcite cements derived from lake waters and 6 the presence of modern biologically mediated mg calcite precipitates in the lake lake tanganyika s lake levels have been remarkably stable over the past 2800 yr fluctuating around the marginally open to marginally closed level through most of this time period lake lowstands and high 18 o values from the ninth century b c to the early fifth century a d indicate that the lake basin was comparatively dry during this time however the period prior to the most recent opening of lake kivu into the lake tanganyika basin ca a d 550 was not marked by major lake lowstands nor was this opening accompanied by a dramatic lakelevel rise the kivu opening was roughly coincident with a significant shift toward isotopically lighter 18 o and 13 c lake water which persists today the lake remained close to its outlet level between the sixth and thirteenth centuries a d lake levels rose between the fourteenth and sixteenth centuries at some time between the late sixteenth and early nineteenth centuries lake level fell to perhaps its lowest level in the past 2800 yr by the early nineteenth century lake level had begun to rise to the overflow level apparently the result of a regional increase in precipitation evaporation ratios weak 18 o 13 c covariance for late holocene carbonates suggests that the surface elevation of the lake has remained near the outlet level with only occasional periods of closure however there is no simple relationship between solute input from lake kivu isotope input from lake kivu and lake levels in lake tanganyika lake kivu waters are the primary source of major ions in lake tanganyika but are much less important in controlling the 18 o and the lake level of lake tanganyika because the ruzizi river s discharge into lake tanganyika is largely derived from sources other than lake kivu the overflow events in the two lakes have been uncoupled during the late holocene

farmers smelters and caravans two thousand years of land use and soil erosion in north pare ne tanzania abstract slope deposits in north pare provide evidence of two millennia of anthropogenically driven land clearance soil erosion and land degradation drawing on deposit stratigraphy soil magnetic parameters stable carbon isotope composition and radiocarbon dating three phases of soil erosion are distinguished characterized by distinct surface processes and increasing levels of agricultural land use onset of slope deposit formation in pare since about 300 bc documents soil erosion as an immediate consequence of new land use practices associated with the spread of agriculture and iron working across northern tanzania by ad 500 slope deposits extended into valley bottoms and to middle slopes suggesting catchment wide land clearance and soil erosion in the 15th century ad progressive anthropogenic soil erosion had exhausted the topsoil resource and material changes of the slope deposits reflect widespread subsoil erosion the exposure of subsoils represents an ecological tipping point and triggered the transition to a new morphodynamic framework dominated by runoff based erosion processes that are recorded as sand lenses and sand layers the most recent deposits show ongoing accelerated erosion and severe land degradation whilst cessation of sand lens preservation indicates pre colonial intensification of agricultural land use land use changes and socioeconomic transitions associated with the establishment of the ugweno chiefdom and the 19th century caravan trade are discussed as possible responses to imperceptible long term land degradation in pare the study demonstrates that anthropogenic soil erosion and not external climatic drivers shaped landscape development in pare and shows that the identification of environmental thresholds is essential for the assessment of resilience in human dominated ecosystems

centennial to millennial changes in maar lake deposition during the last 45 000 years in tropical southern africa lake masoko tanzania abstract the masoko maar southern tanzania provides one of the most continuous late quaternary lacustrine sedimentary records from africa a detailed chronostratigraphic framework coupled with sedimentological and magnetic measurements allows us to construct a 30 year resolution continuous sedimentary sequence covering the last 45 000 years and to address local depositional environment and climate variability in the tropical southern africa based on present day observations and measurements our results indicate that the low field magnetic susceptibility of the sediment is highly controlled by climate driven processes wind stress and or lake level amplitude changes acting on the titanomagnetite rich shoreline reservoir the tephra and turbidite free magnetic susceptibility record is strongly modulated by a persistent multi decadal to centennial variability 80 to 200 years probably linked to the gleissberg and suess cycles of solar activity at lower frequency the variability of deposition is controlled by the precessional cycle and its harmonics suggesting a dominant multi millennial forcing of low latitude insolation on climatic changes in tropical southern africa inferred wetter conditions during the last glacial maximum and the younger dryas at masoko 9 s indicate southward shifts of the intertropical convergence zone associated with the north atlantic glacial dynamics and or contrasted hydrological changes in the rungwe highlands compared to the neighbouring areas finally former regional transfer function between diatom assemblages and water chemistry suggested drier conditions during the last glacial maximum at lake masoko barker p williamson d gasse f gibert e 2003 climatic and volcanic forcing revealed in a 50 000 year diatom record from lake massoko tanzania quaternary research 60 368 376 in this context further climate proxy data such as pollen and hydrobiological studies in small deep lakes are needed to support our alternative interpretation of the masoko record

coupled atmosphere ocean model simulations of el nino southern oscillation with and without an active indian ocean an atmospheric general circulation model gcm was coupled with an ocean gcm covering the pacific this coupled model pac was integrated over a 30 year period the pac model stimulates well the mean seasonally varying atmospheric and ocean fields and reproduces interannual variations corresponding to enso el nino southern oscillation the same atmospheric gcm was coupled with an ocean gcm covering the indian ocean and the tropical pacific this coupled model ipc was integrated over a 35 year period the model climate in ipc is fairly reasonable and its pacific part is very similar to the pacific climate of the pac model enso is the major interannual variability in the ipc model the dynamics of enso in ipc are essentially the same as that in pac in the pacific the subsurface ocean heat content anomalies are formed by wind anomalies and show westward propagation centered off the equator after they reach the western pacific they show eastward propagation along the equator they produce changes in the thermocline structure in the eastern equatorial pacific resulting in anomalies in ssts the sst anomalies provide wind anomalies the sign of which is opposite to that of the wind anomalies in the first stage so that thismore chain will continue enso in the pac and ipc models can be regarded as the open quotes delayed oscillator close quotes operating in the pacific although the major interannual variability in the indian ocean is linked to enso in the pacific the indian ocean does not play any active role in the enso cycle in the ipc model interannual variability of monsoon activity in the ipc model is more reasonable than that in the pac model however any definite mechanism for the relationship between monsoon activity and enso does not emerge in the present study 31 refs 14 figs less

a process based model for colluvial soil depth and shallow landsliding using digital elevation data a model is proposed for predicting the spatial variation in colluvial soil depth the results of which are used in a separate model to examine the effects of root strength and vertically varying saturated conductivity on slope stability the soil depth model solves for the mass balance between soil production from underlying bedrock and the divergence of diffusive soil transport this model is applied using high resolution digital elevation data of a well studied site in northern california and the evolving soil depth is solved using a finite difference model under varying initial conditions the field data support an exponential decline of soil production with increasing soil depth and a diffusivity of about 50 cm 2 yr the predicted pattern of thick and thin colluvium corresponds well with field observations soil thickness on ridges rapidly obtain an equilibrium depth which suggests that detailed field observations relating soil depth to local topographic curvature could further test this model bedrock emerges where the curvature causes divergent transport to exceed the soil production rate hence the spatial pattern of bedrock outcrops places constraints on the production law the infinite slope stability model uses the predicted soil depth to estimate the effects of root cohesion and vertically varying saturated conductivity low cohesion soils overlying low conductivity bedrock are shown to be least stable the model may be most useful in analyses of slope instability associated with vegetation changes from either land use or climate change although practical applications may be limited by the need to assign values to several spatially varying parameters although both the soil depth and slope stability models offer local mechanistic predictions that can be applied to large areas representation of the finest scale valleys in the digital terrain model significantly influences local model predictions this argues for preserving fine scale topographic detail and using relatively fine grid sizes even in analyses of large catchments

filling in the gaps the contribution of non pollen palynomorphs to knowledge about the local environment of the sierra de atapuerca caves during the pleistocene abstract for several decades conventional pollen analyses have been supplemented with information provided by non pollen palynomorphs npp generally the taxa included in this group are microalgae fungi insects and arachnid s chitinous remains as well as plant debris and other organic palynomorphs of undetermined origin that are valuable tools with which to characterise the original strata as well as the local environmental features that influence deposit formation processes in this work we present the results of the npp analysis of the archaeological and palaeontological deposits of sima del elefante gran dolina and galeria all of which are located in the trinchera area sierra de atapuerca burgos spanning a period from 200 ka to almost 1 5 million years ago the npp analysis of the three deposits has provided information about the presence of decayed organic matter accumulations erosion processes and moisture conditions knowledge of palynomorphs contributes to the understanding of the biological physical and chemical dynamics that resulted in different sedimentary sequences in almost all cases there is a clear connection between the major stratigraphic trends established for the three deposits and the npp assemblages identified however the eminently local character of the dispersion of most of the palynomorphs identified has prevented the interrelation between the three sequences analysed