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

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

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

charcoal analysis for paleoenvironmental interpretation a chemical assay a nitric acid digestion and ignition technique developed to measure relative frequencies of charcoal in lake and bog sediments measures the weight of charcoal relative to the dry weight of the sample percentage charcoal and gives results comparable to counting of microscopic charcoal differences in the two techniques caused by changes in sediment type were found especially in postsettlement and late glacial parts of cores charcoal values from sediments less than 100 yr old cannot be compared to older charcoal values unless wood charcoal and charcoal from fossil fuel burning are separated microscopically tests of this chemical assay on mixtures of known weights of charcoal leaves and sand give correlations between observed and expected values of 0 99 to 1 00

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

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

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

age constraints on tarkwaian palaeoplacer and lode gold formation in the tarkwa damang district sw ghana two major epigenetic gold forming events are recorded in the world class gold province of southwest ghana a pre tarkwaian event was the source of the world class tarkwa palaeoplacers whereas post birimian and tarkwaian deformation which was related to the eburnean orogeny gave rise to the world class e g prestea to giant e g obuasi orogenic gold deposits which have made the region famous for more than 2 500 years a maximum age of 2133 4 ma for tarkwaian sedimentation is provided by 71 of 111 concordant shrimp ii u pb dates from detrital zircons in tarkwaian clastic rocks from damang and bippo bin northeast of tarkwa the overall data distribution broadly overlaps the relatively poorly constrained ages of birimian volcanism and associated dixcove type granitoid emplacement indicating syntectonic development of the tarkwaian sedimentary basin these zircon ages argue against derivation of the palaeoplacer gold from an orogenic gold source related to the compressional phase of an orogeny significantly older than the eburnean orogeny instead they suggest that the gold source was either orogenic gold lodes related to an earlier compressional phase of a diachronous eburnean orogeny or ca 2200 2100 ma intrusion related gold lode the co2 rich fluid inclusions in associated vein quartz pebbles are permissive of either source at the damang deposit an epigenetic orogenic lode gold system clearly overprinted and sulphidised low grade palaeoplacer hematite magnetite gold occurrences in the banket series conglomerate within the tarkwaian sedimentary sequence gold mineralisation is demonstrably post peak metamorphism as gold related alteration assemblages overprint metamorphic assemblages in host rocks in alteration zones surrounding the dominant subhorizontal auriferous quartz veins there are rare occurrences of hydrothermal xenotime which give a shrimp u pb age of 2063 9 ma for gold mineralisation the similar structural timing of epigenetic gold mineralisation in tarkwaian host rocks at damang to that in mainly birimian host rocks elsewhere in southwest ghana particularly at obuasi suggests that 2063 9 ma is the best available age estimate for widespread orogenic gold mineralisation in the region argon argon ages of 2029 4 and 2034 4 ma for hydrothermal biotite from auriferous quartz veins appear to represent uplift and cooling of the region below about 300 c as estimates of the temperature of gold mineralisation are higher at around 400 c if peak metamorphism with temperatures of about 550 c is assumed to have occurred at about 2100 ma the biotite ages in combination with the xenotime age suggest a broadly constant uplift rate for the region of about 1 km per 10 million years from about 2100 to 2025 ma

quantifying submerged fluvial topography using hyperspatial resolution uas imagery and structure from motion photogrammetry quantifying the topography of rivers and their associated bedforms has been a fundamental concern of fluvial geomorphology for decades such data acquired at high temporal and spatial resolutions are increasingly in demand for process oriented investigations of flow hydraulics sediment dynamics and in stream habitat in these riverine environments the most challenging region for topographic measurement is the wetted submerged channel generally dry bed topography and submerged bathymetry are measured using different methods and technology this adds to the costs logistical challenges and data processing requirements of comprehensive river surveys however some technologies are capable of measuring the submerged topography through water photogrammetry and bathymetric lidar are capable of reasonably accurate measurements of channel beds in clear water whilst the cost of bathymetric lidar remains high and its resolution relatively coarse the recent developments in photogrammetry using structure from motion sfm algorithms promise a fundamental shift in the accessibility of topographic data for a wide range of settings here we present results demonstrating the potential of so called sfm photogrammetry for quantifying both exposed and submerged fluvial topography at the mesohabitat scale we show that imagery acquired from a rotary winged unmanned aerial system uas can be processed in order to produce digital elevation models dems with hyperspatial resolutions c 0 02 m for two different river systems over channel lengths of 50 100 m errors in submerged areas range from 0 016 m to 0 089 m which can be reduced to between 0 008 m and 0 053 m with the application of a simple refraction correction this work therefore demonstrates the potential of uas platforms and sfm photogrammetry as a single technique for surveying fluvial topography at the mesoscale defined as lengths of channel from c 10 m to a few hundred metres this article is protected by copyright all rights reserved

soil erosion history in central tanzania based on osl dating of colluvial and alluvial hillslope deposits abstract the irangi hills in kondoa district central tanzania are severely degraded by sheet rill and gully erosion using recently developed optically stimulated luminescence osl dating techniques and a detailed study of the hillslope stratigraphy and soils we have determined the sequence of events that gave rise to this highly degraded landscape two major colluvial deposits have been identified on the slopes the oldest colluvium gave osl deposition dates of 14 700 1600 14 200 1500 and 11 400 1300 years ago these dates coincide with the climatic change from dry to wet conditions which took place during the late pleistocene it is possible that the erosion and deposition of the old colluvium occurred as a response to this change this phase of erosion and deposition was followed by a long phase of stability and pedogenetic alteration of the old colluvium a phase of accelerated soil erosion began not later than 900 years ago as indicated by a dated alluvial fan the presence of which indicates that some deeper gullies already existed on upper pediment slopes at that time the second more recent colluvial deposit gave osl dates of 460 40 590 70 and 660 50 years a major period of gully formation and incision with subsequent fan development occurred sometime between 600 and 300 years ago the recent phase of erosion

stable isotope and fluid inclusion studies of gold and silver bearing hydrothermal vein deposits cheonan cheongyang nonsan mining district republic of korea cheonan area the cheonan gold silver mineralized area is located approximately 80 km south southeast of seoul within the precambrian gyeonggi metamorphic belt of the korean peninsula the cheonbo ilbo seonggeo daeheung and chungnam mines which comprise the area are each located along gold bearing hydrothermal quartz veins that crosscut early proterozoic gneiss and jurassic and cretaceous granites mineralization can be separated into three distinct stages i ii and iii which fill preexisting fault breccia zones stages i and ii are terminated by the onset of fracturing and brecciation events fluid inclusion data suggest that quartz sulfide bearing stages i and ii each evolved from initial high temperatures near 400 degrees c to later lower temperatures near 200 degrees c each of these stages represents a separate mineralizing system which cooled and largely abated prior to the onset of the next stage fluid inclusion data from stage iii a postore carbonate stage of mineralization indicate a much cooler 240 degrees 115 degrees c more dilute 3 4 equiv wt nacl hydrothermal system which was the result of increasing influx of meteoric waters sulfur isotope and fluid inclusion evidence suggests electrum sphalerite galena ore mineralization was deposited during stage i at temperatures between 170 degrees and 255 degrees c from fluids with salinities between 3 and 9 equiv wt percent nacl fluid inclusion evidence of boiling suggests pressures of less than 100 to 300 bars this range of pressure is consistent with a depth of mineralization near 1 km under pressure conditions that alternated between hydrostatic and lithostatic sulfur isotope compositions of sulfide minerals are consistent with an igneous source of sulfur with a delta 34 s sub sigma s value near 5 0 per mil measured and calculated delta d and delta 18 o values of hydrothermal fluids in the cheonan area ore deposits are stage i 122 to 127 and 6 2 to 5 9 per mil stage ii 135 to 143 and 3 6 to 3 2 per mil stage iii 134 to 137 and 3 4 to 7 4 per mil respectively these values are consistent with progressive meteoric water involvement increasing water rock ratios with increasing paragenetic time comparison of these values with those of a deeper 2 3 km jurassic gold deposit delta d 78 102 delta 18 o 7 7 3 8ppm suggests a relationship between depth and water rock ratio in korean au ag bearing vein deposits the jurassic au system has ratios of 0 001 to 0 01 the cretaceous au ag system 0 02 to 0 5 the relationships among depth metal contents and water rock ratios in these korean au ag deposits suggest differences in the hydrodynamics and postmagmatic evolutions of granitic hydrothermal systems and may be indicative of the manner in which gold and silver are scavenged from cooling plutons

simulated climate and biomes of africa during the late quaternary comparison with pollen and lake status data new compilations of african pollen and lake data are compared with climate ccm1 ncar boulder and vegetation biome 1 2 gsg lund simulations for the last glacial maximum lgm and early to mid holocene emh the simulated lgm climate was ca 4 c colder and drier than present with maximum reduction in precipitation in semi arid regions biome simulations show lowering of montane vegetation belts and expansion of southern xerophytic associations but no change in the distribution of deserts and tropical rain forests the lakes show lgm conditions similar or drier than present throughout northern and tropical africa pollen data indicate lowering of montane vegetation belts the stability of the sahara and a reduction of rain forest the paleoenvironmental data are consistent with the simulated changes in temperature and moisture budgets although they suggest the climate model underestimates equatorial aridity emh simulations show temperatures slightly less than present and increased monsoonal precipitation in the eastern sahara and east africa biome simulations show an upward shift of montane vegetation belts fragmentation of xerophytic vegetation in southern africa and a major northward shift of the southern margin of the eastern sahara the lakes indicate conditions wetter than present across northern africa pollen data show an upward shift of the montane forests the northward shift of the southern margin of the sahara and a major extension of tropical rain forest the lake and pollen data confirm monsoon expansion in eastern africa but the climate model fails to simulate the wet conditions in western africa

image based measurement of flux variation in distal regions of active lava flows 1 understanding the processes involved with the advance of lava flows is critical for improving hazard assessments at many volcanoes here we describe the application of computer vision and oblique photogrammetric techniques to visible and thermal images of active a flows in order to investigate distal flow processes at mount etna sicily photogrammetric surveys were carried out to produce repeated topographic data sets for calculation of volumetric lava flux at the flow fronts velocity profiles from a distal channel were obtained by rectification of a thermal image sequence and are used to investigate the rheological properties of the lava significant variations of the magma flux were observed and pulses of increased flux arrived within the flow front region on timescales of several hours the pulses are believed to be the distal result of more frequent flux changes observed in the vent region hence they reflect the importance of flow processes which are believed to cause the coalescence of flux pulses along the channel system as well as short period variations in effusion rate in considering advance processes for the individual flow fronts it must be assumed that they were fed by a highly unsteady flux which was volumetrically at least an order of magnitude lower than that observed near the vent

discussion on gold deposits of the tapajos and alta floresta domains tapajos parima orogenic belt amazon craton brazil by santos jos groves di hartmann la moura ma mcnaughton nj mineralium deposita 36 278 299 2001

geological and shrimp ii u pb constraints on the age and origin of the breves cu au w bi sn deposit carajas brazil the breves deposit in the carajas copper gold belt brazil a member of the cu au w bi sn group of deposits contains about 50 mt of 1 22 cu 0 75 g t au 2 4 g t ag 1 200 g t w 70 g t sn 175 g t mo and 75 g t bi it is hosted by sandstones and siltstones of the aguas claras formation minimum age of 2 681 5 ma in the roof zone of a complex highly altered granite intrusion the mineralisation is disseminated in a greisenized zone resulting from alteration of probable monzogranites and syenogranites the ore bearing greisen contains abundant xenomorphic quartz in association with fe chlorite and muscovite the gangue assemblage also includes fluorite tourmaline and minor amounts of monazite xenotime chlorapatite thorite zircon calcite siderite and bastnaesite copper mineralisation is dominated by chalcopyrite associated with pyrite arsenopyrite pyrrhotite and molybdenite gold particles in equilibrium with native bismuth are common as inclusions in chalcopyrite the greisen contains sub economic concentrations of tungsten and niobium that are related to the presence of ferberite qitianlingite and nb rutile shrimp ii zircon dating of the host granites gives 207pb 206pb ages of 1 878 8 and 1 880 9 ma for two phases and a combined age of 1 879 6 ma shrimp ii dating of monazite and xenotime grains in late to post mineralisation veins gives a combined 207pb 206pb age of 1 872 7 ma indistinguishable from the ages of the granites this provides a genetic connection between the breves deposit and the ca 1 88 ga a type granite magmatism that typifies the carajas belt as part of a much larger intracratonic magmatic province that extends over much of the amazonian craton the recognition of this association has exploration implications not only for the geophysical signature of the granite roof zones but also for likely geochemical dispersion around the deposits of this type

from experimental plots to experimental landscapes topography erosion and deposition in sub humid badlands from structure from motion photogrammetry in the last decade advances in surveying technology have opened up the possibility of representing topography and monitoring surface changes over experimental plots 10 m2 in high resolution 103 points m 1 yet the representativeness of these small plots is limited with structure from motion sfm and multi view stereo mvs techniques now becoming part of the geomorphologist s toolkit there is potential to expand further the scale at which we characterise topography and monitor geomorphic change morphometrically moving beyond previous plot scale work using terrestrial laser scanning tls surveys this paper validates robustly a number of sfm mvs surveys against total station and extensive tls data at three nested scales plots 30 m2 within a small catchment 4710 m2 within an eroding marl badland landscape 1 km2 sfm surveys from a number of platforms are evaluated based on i topography ii sub grid roughness and iii change detection capabilities at an annual scale oblique ground based images can provide a high quality surface equivalent to tls at the plot scale but become unreliable over larger areas of complex terrain degradation of surface quality with range is observed clearly for sfm models derived from aerial imagery recently modelled doming effects from the use of vertical imagery are proven empirically as a piloted gyrocopter survey at 50m altitude with convergent off nadir imagery provided higher quality data than an unmanned aerial vehicle uav flying at the same height and collecting vertical imagery for soil erosion monitoring sfm can provide data comparable with tls only from small survey ranges 5 m and is best limited to survey ranges 10 20 m synthesis of these results with existing validation studies shows a clear degradation of root mean squared error rmse with survey range with a median ratio between rmse and survey range of 1 639 and highlights the effect of the validation method e g point cloud or raster based on the estimated quality

paleoenvironmental change in tropical africa during the holocene based on a record from lake kifuruka western uganda a 10 000 year palynological record of vegetation and environmental changes in tropical africa is presented based on sediment data from a 230 cm long core recovered from lake kifuruka in western uganda the lake kifuruka record provides new sedimentary and paleoecological data for human environment interactions and anthropogenic disturbance in the tropical african region during the holocene the record shows evidence for prolonged dry conditions in the region at the terminus of the last glaciation period and the onset of the holocene period the early holocene and mid holocene periods were characterized by largely warm and wet climatic conditions the pollen record shows evidence of forest disturbance that is probably associated with agricultural activities near lake kifuruka shortly after ca 4300bp cal a bp coinciding with a time of increased desiccation in the region the forest disturbance in the record appears to have intensified and continued into the modern period but with no significant change in the overall vegetation diversity suggesting quick environmental recovery the lake kifuruka record is largely devoid of the abrupt climatic changes that have been observed from a couple of sites in the east african region which implies that the changes may be site specific rather than regional scale

human impacts climate change and aquatic ecosystem response during the past 2000 yr at lake wandakara uganda analyses of carbon and hydrogen isotope ratios of terrestrial leaf waxes and the carbon and nitrogen abundance ratio and isotopic composition of bulk sediments from lake wandakara a crater lake in western uganda east africa document human and climatic controls on the aquatic system and on the surrounding terrestrial vegetation during the past two millennia our data indicate that wandakara was a relatively stable productive lake surrounded by c3 vegetation from ad 70 to 1000 abrupt changes in the 13c of terrestrial leaf waxes indicate a series of abrupt shifts in the relative abundance of c3 and c4 vegetation caused by a combination of climate change and human activities around wandakara beginning at ad 1000 abrupt shifts in bulk sediment organic geochemistry particularly c n ratios and 15n indicate that human activities at this time caused permanent changes in the limnology of lake wandakara including eutrophication our results suggest that the biogeochemistry of lake wandakara was more sensitive to shifting human impacts than to climate variations during the past millennium highlighting the importance of understanding the intensity of pre colonial human impacts on africa s aquatic ecosystems

mitigating systematic error in topographic models for geomorphic change detection accuracy precision and considerations beyond off nadir imagery unmanned aerial vehicles uavs and structure from motion photogrammetry enable detailed quantification of geomorphic change however rigorous precision based change detection can be compromised by survey accuracy problems producing systematic topographic error e g doming with error magnitudes greatly exceeding precision estimates here we assess survey sensitivity to systematic error directly correcting topographic data so that error magnitudes align more closely with precision estimates by simulating conventional grid style photogrammetric aerial surveys we quantify the underlying relationships between survey accuracy camera model parameters camera inclination tie point matching precision and topographic relief and demonstrate a relative insensitivity to image overlap we show that a current doming mitigation strategy of using a gently inclined 0 center dot 3 m representing accuracy issues an order of magnitude greater than precision based error estimates for higher relief topography and for nadir imaging surveys of the lower relief topography systematic error was 0 center dot 09 m modelling and subtracting the systematic error directly from the topographic data successfully reduced error magnitudes to values consistent with twice the estimated precision thus topographic correction can provide a more robust approach to uncertainty based detection of event scale geomorphic change than designing surveys with small off nadir camera inclinations and furthermore can substantially reduce ground control requirements c 2020 the authors earth surface processes and landforms published by john wiley sons ltd

a late holocene paleoclimatic history of lake tanganyika east africa abstract a nearshore core lt03 05 from the north basin of lake tanganyika provides diatom pollen and sedimentary time series covering the last ca 3800 yr at 15 36 yr resolution a chronology supported by 21 ams dates on terrestrial and lacustrine materials allows us to account for ancient carbon effects on 14c ages and to propose refinements of the region s climatic history conditions drier than those of today were followed after ca 3 30 ka by an overall wetting trend several century scale climate variations were superimposed upon that trend with exceptionally rainy conditions occurring 1 70 1 40 ka 1 15 0 90 ka 0 70 0 55 ka and 0 35 0 20 ka around 0 55 0 35 ka during the sporer sunspot minimum drier conditions developed in the northern tanganyika basin while more humid conditions were registered at lakes victoria and naivasha this indicates significant variability in the nature and distribution of near equatorial rainfall anomalies during much of the little ice age

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

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

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

quantitative estimates of full glacial temperatures in equatorial africa from palynological data this paper presents a new reconstruction of the mean annual temperature obtained from a high altitude pollen sequence in equatorial africa 3 28 s 29 34 e 2240 m it has been achieved with an extended modern african reference data set by adding spectra from 228 new sites and using another selection for pollen taxa the purpose of this paper is to test how the obtained temperature value depends upon the availability of modern analogues the results are in good agreement with those previously published reinforcing the validity of the method the mean standard error is reduced by 0 3 c the mean temperature for the holocene appears 1 4 c warmer than the present and the last glacial maximum 25 18 kyrs bp cooling is better specified at 3 1 9 c a conservative value more consistent with reconstructed sea surface temperature in the equatorial ocean

letter to the editor climate of east africa 6000 14c yr b p as inferred from pollen data by odile peyron dominique jolly raymonde bonnefille annie vincens and joel guiot

gold deposits of the tapajos and alta floresta domains tapajos parima orogenic belt amazon craton brazil the tapajos region is one domain of a major paleoproterozoic orogenic belt named tapajos parima and is discussed in the context of the evolution of the amazon craton the orogenic belt is composed of a back arc sequence four volcano plutonic arcs intra arc sedimentation and is limited to the east by the cratonic rocks of the central amazon province the evolution and timing of the main events is established by zircon baddeleyite and titanite shrimp u pb geochronology of 29 rock samples while lead and argon isotopes are used to study the age and source of the gold mineralization based on the mesoscopic nature of the orebodies and in some cases on microthermometric and stable isotope data the tapajos gold deposits are classified as 1 orogenic and 2 intrusion related and may be grouped into four deposit type categories 1 orogenic turbidite hosted disseminated and quartz pyrite veinlet deposits hosted by metaturbidites lower greenschist facies jacareacanga group and emplaced in ductile structures 2 orogenic magmatic arc hosted disseminated and pyrite quartz carbonate vein deposits hosted by metamorphic rocks cuiu cuiu complex and formed under a ductile brittle regime with the ouro roxo deposit as a type example 3 intrusion related epizonal quartz vein deposits vertical to subvertical quartz pyrite veins and pyrite disseminations filling extensional brittle faults and 4 intrusion related epizonal disseminated stockwork deposits the type example being the serrinha deposit gold mineralization of type 3 is similar to that of korean type while type 4 mineralization shows some similarities to porphyry type deposits galena pb pb and muscovite ar ar data indicate an age of 1 860 ma for the intrusion related gold mineralization preliminary pb isotope data on k feldspar indicate that the fluid source was more likely to have been from within the jacareacanga cuiu cuiu and tropas units than the creporizao maloquinha and iriri units this study shows the existence of two main types orogenic and intrusion related of gold deposits which are related to specific tectono magmatic events that occurred during a limited period of time in the orogenic belt evolution this information may be useful as a guide for gold exploration along the orogenic belt