the amspoort silts northern namib desert namibia formation age and palaeoclimatic evidence of river end deposits abstract detailed geomorphological and chronological investigations of the nw namibian amspoort silt formation show that the sediments are typical river end deposits this type of endoreic sediment occuring only in desert margin areas provides valuable information about the palaeo environment in the hoanib valley the fine grained deposits have buried riverine trees radiocarbon dating of the wood and luminescence dating of the sediments allow a detailed reconstruction of the aggradation processes accumulation started 10 km downstream of amspoort around the beginning of the 15th century and ended in the 19th century some kilometres upstream of amspoort this upstream shift of sedimentation during the little ice age was caused by gradually decreasing runoff resulting from aridification of the upper part of the hoanib river catchment lying east of the namib desert margin 1 200 m a s l the amspoort silt terrace is evidence of palaeo hydrological fluctuations in nw namibia at present the hoanib river erodes deeply into the silty deposits indicating that nw namibia receives more monsoonal rainfall today than during the little ice age however this contradicts the hypothesis of a continual natural aridification of nw namibia damaraland kaokoveld since the mid 19th century in the course of global climatic change rather deposition and erosion of the amspoort silts indicate that landscape degradation in nw namibia is primarily anthropogenically induced and most probably not accelerated by a decrease in precipitation

late cretaceous cenozoic tectonic development of the southwest pacific region abstract a new model of the plate tectonic development of the southwest pacific integrates the continental geology of new zealand with the age structure of the surrounding oceanic crust revealed previously from magnetic anomaly lineations the model differs from previous ones in that the onland geology of new zealand is used to constrain the tectonic development in two important ways 1 the modern australia pacific plate boundary did not transect the new zealand sector until 23 million years ago 2 there has been a total of only c 500 km of dextral displacement on the plate boundary through new zealand the model is described with reference to a series of paleotectonic maps drawn to represent the setting at the times of anomalies 32 24 7 and 5 novel interpretations based on these reconstructions include the following during the late cretaceous the campbell fault was a continental transform fault between the tasman sea spreading centre and the bounty rift its 330 km of dextral displacement accompanying 25 of counterclockwise rotation of a campbell plateau block when the eastward propagating southeast indian ridge broke into the south tasman sea 57 million years ago it continued to propagate directly into southern south island and generated late eocene oligocene continental rifting through western new zealand at the same time back arc spreading in the norfolk basin caused continental rifting along a similar trend in western north island the australia pacific plate boundary originated as a transform fault between the southeast indian ridge and the pre existing kermadec trench it developed in response to the sudden cessation of spreading on the segment of the southeast indian ridge east of a fracture zone here named fracture zone z two implications arise from this model regarding the integrity of antarctica 1 1 during the cretaceous marie byrd land was probably no more than 200 km northwest of its present position 2 2 the proposition made in some earlier plate tectonic reconstructions that if the alpine fault did not form until after the late eocene there must be a late cretaceous early tertiary plate boundary within antarctica is most probably valid

holocene aeolian activity in the southwestern kalahari desert southern africa significance and relationships to late pleistocene dune building events little is known about the timing of dune building episodes in the southwestern kalahari desert previously arid phases when dune activity occurred have been inferred from gaps in chronologies derived from information that attests to former humid periods to redress this optically stimulated luminescence osl dating was applied to sediment samples taken from a range of aeolian landforms in the sw kalahari four periods of dune building are identified in the late pleistocene and holocene holocene aeolian activity at 6 ka and 1 2 ka led to the construction of localised dune forms in contrast to late pleistocene times when the extensive field of linear dunes that dominates the region was emplaced late holocene aridity was however sufficient to allow rapid emplacement of the lunette dune at witpan the record of aeolian activity is evaluated in the context of humid chronologies and an ocean atmosphere model of holocene climate change with good agreement obtained it is concluded that the application of

several distinct wet periods since 420 ka in the namib desert inferred from u series dates of speleothems abstract the scarcity of numerical dates of the arid areas in southern africa is a challenge for reconstructing paleoclimate this paper presents a chronological reconstruction in the central part of the namib desert namibia for the last 420 000 yr it is based on 230th u dates tims from a large stalagmite and a thick flowstone layer in a small cave located in the hyper arid central namib desert the results provide for the first time evidence of three or possibly four succeeding wet periods of decreasing intensity since 420 ka through which speleothem deposited at approximately 420 385 ka 230 207 ka and 120 117 ka following the 100 ka milankovitch cycle speleothem growth was not recorded for the holocene these wet periods interrupted the predominantly dry climate of the namib desert and coincided with wet phases in deserts of the northern hemisphere in the murzuq basin sahara the negev israel the nafud desert saudi arabia and the arid northern oman arabian peninsula

flash flood sediments and ichnofacies of the late pleistocene homeb silts kuiseb river namibia abstract the homeb silt formation consists of eroded remnants of fine grained alluvium that were deposited within the kuiseb river valley in southern namibia during the late pleistocene in holocene times the river deeply incised its valley fill leaving steep walled castles of relict alluvium the homeb silts this study is based on the 25 m thick type exposure of homeb silts at homeb and describes flash flood sediments that were colonised by successive generations of opportunistic arthropods the stacked flood units each comprise a massive tabular siltstone bed overlain by a narrower interval of rapidly alternating sandstone and siltstone with claystone veneers these flood units are interpreted as having accumulated under semi arid climatic conditions by episodic back flooding of the kuiseb river into embayments and tributary mouths soon after deposition as the floodwaters subsided the sediment was colonised by burrowing sediment ingesting organisms mostly arthropods that produced a taenidium ichnofacies after the floodwaters had drained the exposed sediment was colonised by grasses and burrowed by terrestrial arthropods probably ants and termites resulting in an overprint of termitichnus ichnofacies with associated pelletal chambers thirteen horizons of rhizocretions and root tubules occur toward the top of the succession and indicate a gradual reduction in the frequency of flooding these immature calcic palaeosols suggest that the climate in the central namib desert 20 000 years ago was semi arid being wetter and more seasonal than the hyperarid conditions that prevail today

nd sr and o isotopic constraints on the petrogenesis of mafic intrusions in the proterozoic trans hudson orogen of central canada sm nd isotopic data from five mafic intrusions in the la ronge lynn lake domain indicate crystallization between 1 88 and 1 79 ga the data from a single intrusion in the glennie lake domain suggest a slightly younger age of about 1 71 ga these results indicate a cogenetic origin of the mafic intrusions with the associated volcanic and plutonic rocks initial end values vary from 4 5 to 0 6 and can be modeled by mixing proterozoic mantle derived magmas with 17 archaean crust or subduction of 3 sediment similar to pelites from the wollaston domain the initial end value of 0 for a late proterozoic mackenzie dike is compatible with an origin from mantle sources affected by hudsonian subduction processes rb sr data indicate variable and generally high initial 87sr 86sr isotopic ratios which reflect in part isotopic redistribution during metamorphism 18o values of coexisting clinopyroxene orthopyroxene and plagioclase indicate isotopic disequilibrium plagioclase samples show variable degrees o

the hongqiaocun site the earliest evidence of ancient flood sedimentation of the water conservancy facilities in the chengdu plain china abstract the earliest water conservancy facilities in the chengdu plain were found at the hongqiaocun site of about 4 0 ka before present nearly 2 000 years earlier than the world famous dujiangyan irrigation project in order to understand the background of the construction of the water conservancy project we sampled and analyzed the ancient river profile ts20w13 near the water conservancy facilities sedimentology demonstrated that the 4th to 6th layers of the ts20w13 profile were the strongest stage of hydrodynamic force in the river the zr rb and ba nb values indicated that the 4th to 7th layers might have been those layers of a time of frequent flooding in this ancient river the results of zircon psephicity analysis also showed that the psephicity of the zircon at the 4th to 6th layers were the best indicating that along this stage of the river the average transporting force was the strongest according to the osl dating results and the above analysis at around 4 0 ka bp the hongqiaocun site was in the midst of a flood prone period which was synchronous with the ancient flood records of the yangtze river and yellow river basin the increase in flooding during this period might be related to the aggravation of climatic fluctuation by about 4 0 ka bp in the context of the above environmental background the ancient people at the hongqiaocun site built water conservancy facilities to withstand floods

fission track analysis reveals character of collisional tectonics in new zealand during the late cenozoic the pacific plate has been obliquely converging with the australia plate in south island new zealand this has resulted in westward obduction of continental crust of the pacific plate and consequently a major mountain range the southern alps has formed east of the alpine fault and within the leading edge of the pacific plate we have applied the apatite and zircon fission track dating technique to a suite of outcrop samples from three transects across the southern alps starting at the alpine fault to better understand the uplift age and structure of the continental collision the results show the following 1 uplift was underway at 7 ma in the south at haast pass and by 5 ma in the north at arthur s pass 2 profiles of the total amount of late cenozoic uplift show an exponential increase toward the alpine fault adjacent to the alpine fault there has been 14 5 to 21 5 km of uplift in the vicinity of haast pass 14 5 to 21 km near mt cook and 12 5 to 15 5 km in the arthur s pass transect adjacent to the alpine fault in the mount cook region the average uplift rate from 5 5 ma to 1 ma was 2 3 to 2 5 mm yr between 1 ma and the present it has averaged 6 2 mm yr the strongly asymmetric uplift pattern is responsible for the exposure of a 13 25 km wide belt of greenschist to midamphibolite facies rocks immediately east of the alpine fault known as the alpine schists 3 late cenozoic uplift of torlesse greywackes and alpine schists was preceeded during the jurassic early cretaceous by 5 8 km of uplift the otago schist belt to the south was uplifted much more during this interval and in addition western parts of the otago schists were uplifted 5 6 km during the late cretaceous 4 the fission track data do not support the notion of significant late cenozoic shear heating about the alpine fault as previously suggested from spatial variations in k ar ages of the otago and alpine schists

interdune deposits of the namib sand sea abstract the namib sand sea of southwestern africa is dominated by large north south trending linear dunes interdune areas however comprise about 50 of the area of the sand sea and may consist of exposures of the pre dune surface or sediments deposited between the dunes four major types of interdune deposits are identified and described 1 1 coarse poorly sorted aeolian sands 2 2 localized calcareous lacustrine deposits 3 3 silty deposits of ephemeral rivers that have flooded into the sand sea margins 4 4 sabkhas and salt marshes near the coast the preservation potential of these sediments is poor except where they are buried and protected by dunes or stabilised by high groundwater levels in coastal areas

sedimentary record and luminescence chronology of palaeoflood events along the gold gorge of the upper hanjiang river middle yangtze river basin china abstract palaeoflood slackwater deposits swds along the river banks have important implications for the reconstruction of the past hydro climatic events two palaeoflood swd beds were identified in the holocene loess soil sequences on the cliff river banks along the gold gorge of the upper hanjiang river by field investigation and laboratory analysis they have recorded two palaeoflood events which were dated by optically stimulated luminescence to 3 2 2 8 ka and 2 1 1 8 ka respectively the reliability of the ages obtained for the two events are further confirmed by the presence of archaeological remains and good regional pedostratigraphic correlation the peak discharges of two palaeoflood events at the studied sites were estimated to be 16 560 17 930 m3 s a correlation with the palaeoflood events identified in the other reaches shows that great floods occurred frequently during the episodes of 3200 2800 and 2000 1700 a bp along the upper hanjiang river valley during the last 4000 years these phases of palaeoflood events in central china are well correlated with the climatic variability identified by 18o record in the stalagmites from the middle yangtze river basin and show apparent global linkages palaeoflood studies in a watershed scale also imply that strengthened human activities during the shang dynasty bce 1600 1100 and han dynasty bce206 ce265 may have caused accelerated soil erosion along the upper hanjiang river valley

a paleohydrologic reinterpretation of the homeb silts kuiseb river central namib desert namibia and paleoclimatic implications abstract slack water deposits are usually fine grained fine sand and coarse silt flood sediments deposited in areas of the floodplain that are sheltered from high velocity flood flows slack water deposits have been recognized by many investigators in a wide variety of physiographic and climatic settings late quaternary slack water deposits have not been described from the extremely arid namib desert of southwestern africa fine grained flood sediments in the kuiseb valley near homeb accumulated between 23 000 and 19 000 14c years bp have been interpreted by previous workers as i sediments deposited behind dune dams ii river endpoint accumulations and iii flood deposits of an aggrading river controlled either by a base level change in the lower reaches or a change in the hydrological regime in the catchment area furthermore the so called homeb silts could document more arid climatic conditions in the kuiseb catchment area during the last glacial maximum lgm our research shows that the homeb silts are slack water deposits and that in many valleys of the namib desert slack water sediments are observed most commonly at the junctions of major rivers and minor tributaries in bedrock canyon settings here we present evidence that these slack water deposits accumulated as a result of more humid conditions and or more intense precipitation events in the upper reaches of the river they do not document climatic changes in the desert itself as postulated by many researchers

how dry was dry late pleistocene palaeoclimates in the namib desert abstract evidence for late quaternary changes in climate in the namib desert is geographically scattered and often poorly dated the available evidence suggests periods of increased river discharge and groundwater flow centered on 8 12 20 24 and 26 32 ka bp based on modern analogues these palaeohydrologic changes likely involved increased rainfall and runoff in the escarpment zone east of the desert there is little evidence to suggest significantly increased rainfall within the desert during these periods except perhaps during the interval prior to 26 ka bp these data reinforce earlier conclusions that the namib has experienced mostly arid to hyperarid conditions throughout the quaternary

paleohydrology of southwestern texas current statistical methods may be unable to accurately predict recurrence intervals of rare large magnitude floods especially in semiarid regions having positively skewed annual flood distributions great hydrologic variability and widely spaced gaging stations current approaches rely on historical data but catastrophic floods may have recurrence intervals far greater than the length of historical records in the lower pecos and devils rivers of southwestern texas paleoflood discharge and frequency estimates are extended over 10 000 years by the study of slack water flood sediments slack water deposits are typically fine grained sand and silt that accumulate during floods in areas where current velocity is reduced i e in back flooded tributary mouths channel expansions downstream from bedrock spurs and or slump blocks and in shallow caves along bedrock walls radiocarbon dating of organic detritus in slack water deposits establishes the flood chronology while paleoflood discharges can be estimated by slope area techniques paleoflood information extracted from slack water sediments can greatly extend flood records these floods may be weighted like historical data in log pearson type 3 calculations of flood frequency our morphostratigraphic approach combines recorded data with geomorphic evidence to derive estimates of flood frequency this technique offers an inexpensive and rapid way to assess catastrophic flood risk

nd and sr isotopic geochemistry of mafic layered intrusions in the eastern baltic shield implications for the evolution of paleoproterozoic continental mafic magmas nd and sr isotopic data are presented for the 2449 2441 ma olanga and burakovka layered mafic complexes in the eastern baltic shield these complexes have similar tectonic position but differ in two aspects the age of the enclosing crust and the post crystallization metamorphic history the sm nd isotopic results for the kivakka and lukkulaisvaara intrusions olanga complex are consistent with the model of closed system crystallization of a single magma without significant wallrock assimilation the rb sr systems of minerals were disturbed by late rb addition during 1 75 1 50 ga metamorphism the nd and sr isotopic systems in the burakovka complex show no metamorphic disturbance and indicate mixing of at least four isotopically distinct components isotopic variations in the burakovka complex can be explained by a 4 20 per cent contamination of a primary komatiitic or picritic magma with a mesoarchean crust similar to that exposed in the region a similar model applied to the olanga complex using a neoarchean crustal isotopic composition cannot reproduce the observed isotopic signature the nearly uniform initial nd values between 1 and 2 3 observed in the kivakka and lukkulaisvaara intrusions of the olanga complex as well as in the other 2 50 2 44 ga layered mafic intrusions throughout the eastern baltic shield are better explained by a mantle plume model with small amounts of crustal contamination and minor involvement of asthenospheric material this model is also consistent with the geological observations and the temporal distribution of the paleoproterozoic mafic magmatism in the eastern baltic shield as an alternative the enriched isotopic characteristics may be explained by melting of a metasomatically modified lithospheric mantle source

optical age estimates for hyper arid fluvial deposits at homeb namibia abstract two 25 m sequences of slack water facies sediments have been dated in the kuiseb river in namibia by optical dating of sand sized quartz grains the eight samples indicate the sediments were deposited between 6 3 and 9 8 ka this revises previous age estimates for the homeb silts according to our criteria six out of eight samples displayed characteristics of partial bleaching this result was not unexpected as arid fluvial systems have high suspended sediment concentrations short flow durations and a tendency for flow peaks to occur at night

hydrological response of a dryland ephemeral river to southern african climatic variability during the last millennium a long term flood record from the buffels river the largest ephemeral river of nw south africa 9250 km2 was reconstructed based on interpretation of palaeoflood documentary and instrumental rainfall data palaeoflood data were obtained at three study reaches with preserved sedimentary evidence indicating at least 25 large floods during the last 700 yr geochronological control for the palaeoflood record was provided by radiocarbon and optically stimulated luminescence osl dating annual resolution was obtained since the 19th century using the overlapping documentary and instrumental records large floods coincided in the past within three main hydroclimatic settings 1 periods of regular large flood occurrence 1 large flood 30 yr under wetter and cooler prevailing climatic conditions ad 1600 1800 2 decreasing occurrence of large floods 1 large flood 100 yr during warmer conditions e g ad 1425 1600 and after 1925 and 3 periods of high frequency of large floods 4 5 large floods in 20 30 yr coinciding with wetter conditions of decadal duration namely at ad 1390 1425 1800 1825 and 1915 1925 these decadal scale periods of the highest flood frequency seem to correspond in time with changes in atmospheric circulation patterns as inferred when comparing their onset and distribution with temperature proxies in southern africa

fossil hyrax dung and evidence of late pleistocene and holocene vegetation types in the namib desert abstract pollen was derived from fossil dung of herbivorous hyraxes deposited in a rock shelteron the highest mountain in namibia da ures or brandberg situated on the namib desert margin radiocarbon dating ranging in age between modern times and 30000yrbp showed it representsthe rst empirical pollen evidence of continental palaeovegetation during the late pleistocene alongthe western escarpment of southern africa the initial results indicate last glacial maximum vegeta tion differed totally from the current pattern as vegetation types were dominated by small asteraceaeshrubs in contrast to those of the holocene and modern times which show more succulents grassandwoody elements arboreal pollen the resultssuggest that cape oral communities didnot reachinto thetropics along thewestern escarpment ofafrica despite such pollentypes occurring inmarinecores copyright 2004 john wiley sons ltd keywords palynology namibia restionaceae late pleistocene last glacial maximum

historical palaeohydrology and landscape resilience of a mediterranean rambla castellon ne spain floods and people abstract this paper provides a new methodological approach to analyse secular patterns of flooding magnitude and frequency from sedimentary evidence palaeofloods taking into account changes in channel geometry and their links to historical environmental changes and the inherent social and demographic evolution within the catchment a case study analysis was focused in rambla de la viuda drainage area of 1500 km 2 whose stream flow is related to extreme rainfalls a 500 years sedimentary archive was reconstructed from eight stratigraphic profiles comprising continuous sequences of slackwater flood deposits interbedded with episodic colluvial and edaphic horizons discharge estimates associated to sedimentary flood evidences were obtained from one dimensional hydraulic modelling the stratigraphic units were sampled to characterise their geochemical and paleobotanical phytoliths contents palaeoflood chronology was obtained from radiocarbon and luminescence osl dating supported by documentary data written historical documents a high frequency and high magnitude palaeoflood period took place during the 15 th middle 16th century which seem to correlate in time with general wetter conditions three short term environment stability conditions land use and climatic also made possible the development of three paleosols the lowest flood magnitude and discharges in the sedimentary record was found between the mid 17th to mid 18th centuries under prevailing drier environmental conditions episodic high magnitude flooding took place at late 18th century correlating in time with palaeovegetation and geochemical evidences of important changes on land use deforestation and grazing poorer developed soils were found at upper stratigraphic sequences 19th century characterised by thick units of colluvium deposits usually culminating sequences of short lived continuous slackwater flood units despite of the potential human influence land use on soil hydrology the long term behaviour of high magnitude floods 1000 m 3 s 1 has been stationary over the last 500 years

late oligocene pacific wide tectonic event stratigraphic and structural changes radiometrically and biostratigraphically dated from basins and basement across new zealand indicate that the modern australia pacific plate boundary including the alpine fault sector formed between 28 and 24 ma this age range coincides with changes at about 25 ma in the trends of the hawaiian and louisville hotspot chains and a 27 25 ma reorganization of spreading on the antarctic pacific spreading ridge it also follows closely the 28 5 ma initiation of subduction of the east pacific rise south of mendocino fracture zone that lead to formation of the san andreas continental transform the late oligocene pacific wide tectonic reorganization may have been triggered by this ridge trench collision

paleohydrology of extraordinary floods along the swakop river at the margin of the namib desert and their paleoclimate implications abstract the swakop river is a major ephemeral river drainage basin area about 30 000 km 2 crossing the western namib desert its hydrology and limited water resources depend on storms and floods therefore the hydrology and flow regime in the swakop river basin determines the availability of water for human use in this study we present a millennia long record of extreme floods using paleoflood hydrological methods and optically stimulated luminescence osl ages the record includes 44 large floods ranging in date from the late pleistocene to the present the oldest six to eight floods are dated from the late pleistocene to the mid holocene between 14 900 1700 and 5100 620 years ago with calculated peak discharges of 550 1280 m 3 s 1 eighteen floods eleven of which were between 1000 and 1600 m 3 s 1 occurred between 740 90 and 190 10 years ago additional 17 later floods occurred between about ad 1850 and 1963 with peak discharges between 100 and 1000 m 3 s 1 the historical record which partly parallels the latter period recorded 17 large floods during the period 1893 1963 flood frequency analysis ffa of the combined systematically gaged record 1963 2003 and the paleoflood record since ad 1270 740 90 years the most complete record was applied using the maximum likelihood method a two component extreme value distribution tcev estimates the 1 annual exceedance probability magnitude using only the systematic gaged record at 520 m 3 s 1 the combined paleoflood historical and gage records resulted in a much larger magnitudes for the 1 annual exceedance probability flood magnitude of 1320 1350 m 3 s 1 the 0 1 annual exceedance probability magnitude is estimated at 770 m 3 s 1 and 2100 2150 m 3 s 1 respectively the hydroclimatic interpretation of this paleoflood record is complex due to the discontinuity of the record during the late holocene our flood data show three intervals differing in their flood regime 1 1280 80 to 740 90 years bp with one extraordinary flood 200 m 3 s 1 about every 180 years 2 740 90 to 200 45 14 c years bp with one large flood per 80 years and 3 200 45 14 c years bp to ad1963 with one large flood per 7 years interval 2 represents large floods probably related to the transition from a drier climate to the colder and probably wetter climate of the little ice age lia the third period relates to the end of the lia and to a wetter period of the 19th and early 20th centuries in which more rainfall in the kalahari and namib deserts and many historical floods were documented the presence of three distinct calcretes rich in calcified rhizoliths at one of the sites two of which were dated by osl to 13 2 1 9 ka and 8 8 1 0 ka indicates at least locally higher moisture availability than in the current semiarid climate the presence of denser vegetation can be associated with increased groundwater seepage and wetter climatic conditions

metavolcanic rocks of the la ronge domain in the churchill province saskatchewan geochemical evidence for a volcanic arc origin the la ronge domain in the churchill province of the canadian shield is a volcanic sedimentary plutonic belt of early proterozoic age geochemical data supported by geological evidence strongly suggest that the la ronge metavolcanics were emplaced in an island arc setting the metavolcanic rocks range from basalt through andesite to dacite and rhyolite calcalkaline and tholeiitic types are dominant with minor alkaline varieties classification based on the major elements produces ambiguous results because of mobility of these elements during metamorphism however relatively immobile minor and trace elements provide reliable characterization of the basalts in particular the latter display distinctive trace element properties and morb normalized geochemical patterns similar to those of recent volcanic arc basalts being enriched in incompatible elements of low ionic potential and depleted in those of high ionic potential the calcalkaline basalts are in addition enriched in th p ce and sm a minor within plate component is also displayed by some metabasalts suggesting that magma genesis in parts of the la ronge arc involved sub continental lithosphere rare earth element patterns show enrichment in the lree with little or no eu anomaly interpretation by means of trace element discrimination diagrams incorporating elements such as ti y nb zr cr th yb and ce provides clear evidence for emplacement in a volcanic arc tectonic setting

1 700 myr greenstone volcanic successions in southwestern north america and isotopic evolution of proterozoic mantle 1 700 myr greenstone volcanic successions in southwestern north america and isotopic evolution of proterozoic mantle

analyses of the magnitude and frequency of a 400 year flood record in the fish river basin namibia abstract ephemeral rivers in dryland regions exhibit a high interannual variability of streamflow regime mainly dominated by floods in these environments floods are a water resource and a potential hazard with important socioeconomic implications the fish river 86 600 km2 is the largest ephemeral stream in namibia and recently also the focus of new development plans including construction of the largest dam in namibia the hydrological analysis to support decisions implies large uncertainties owing to spatial limited and short hydrological records since 1962 here we investigate the current and past patterns of extreme floods combining instrumental historical and palaeoflood records palaeoflood studies were performed at two reaches with preserved sedimentary evidence at the upper sector vogelkranz upstream of hardap dam 13 catchment area and in the lower part of the river in the fish river canyon national park 70 catchment area in the hardap reach the palaeoflood record identified at least eight large floods during the last 350 years with the largest flood reaching a minimum discharge of 4800 m3 s 1 150 year return period in the fish river canyon reach the sedimentary record shows at least 12 large floods over the last 400 years the largest with an estimated minimum discharge of 8700 m3 s 1 the elevation of alluvial surfaces without flood evidence provided an upper bound for flood stages associated with discharges of 6400 m3 s 1 and 16 140 m3 s 1 for the upper and mid lower fish river respectively in the upper reach the flood frequency analysis ffa combining systematic and palaeoflood data provided lower discharges 25 of the flood quantiles than the ones using only systematic data sets in the fish river canyon reach incorporation of the palaeoflood data into the ffa results in slightly higher values in the magnitude of the higher flood quantiles 4 7 the ffa analysis using an upper limited lognormal distribution function ln4 fitted with palaeoflood data shows a good performance with a slow behaviour approaching the upper limit our flood frequency results suggest that the hardap dam should increase the spillway capacity and safety check flood of the original design in order to satisfy the dam safety criteria however the projected reevaluation figures calculated from conventional hydrological methods results in an overestimation of the safety floods according to our estimations

the age and origin of the north shore plutons in the rae province goldfields area saskatchewan the north shore plutons are peraluminous granitoids emplaced within the murmac bay group supracrustals exposed on the north shore of lake athabasca saskatchewan 207pb 206pb ages obtained using the single zircon pb evaporation technique indicate emplacement at 1952 18 ma coeval with taltson magmatism that occurred during the 1 9 2 0 ga thelon orogeny which influenced the rae province the granitoids inherited zircons from archean source terranes dated at approximately 2 5 and 3 0 ga detrital zircons from the younger source terrane were also identified in the murmac bay group thus constraining the maximum age of the group to the paleoproterozoic rare earth element and incompatible element data and rb sr and sm nd isotope systematics from the north shore plutons and adjacent host rocks indicate that the plutons are crustal melts generated from melting of murmac bay group rocks and archean crust at deeper levels granitic gneisses described as older granites represent mixtures of variable amounts of

geochemical oxygen and neodymium isotope compositions of metasediments from the abitibi greenstone belt and pontiac subprovince canada evidence for ancient crust and archean terrane juxtaposition abstract the abitibi greenstone belt agb and pontiac subprovince ps in the southwestern superior province are adjacent greenstone plutonic and metasedimentary dominated terranes respectively separated by a major fault zone metasediments from these two contrasting terranes are compared in terms of major and trace element and o and nd isotope compositions and detrital zircon ages the following two compositional populations of metasediments are present in the low grade abitibi southern volcanic zone 1 1 a mafic element enriched population mep characterized by flat depleted ree patterns enhanced mg cr co ni and sc low incompatible element contents and minor or absent normalized negative troughs at nb ta and ti and 2 2 a low mafic element population lmep featuring lree enriched patterns enhanced rb cs ba th and u contents and pronounced normalized negative troughs at nb ta and ti these geochemical features are interpreted to indicate that the mep sediments were derived from an ultramafic and mafic dominated oceanic provenance whereas the lmep sediments represent mixtures of mafic and felsic arc source rocks the ps metasediments are essentially indistinguishable from abitibi lmep on the basis of majorelement and transition metal abundances suggesting comparable types of source rocks and degrees of maturity but are distinct in terms of some trace elements and o isotope compositions the pontiac metasediments are depleted in 18o and enriched in cs ba pb th u nb ta hf zr and total ree and also have higher ratios of rb k cs rb ba rb ta nb th la and ba la relative to the abitibi lmep two subtypes of ree patterns have been identified in ps metasediments both have moderately fractionated distributions with la yb n 3 9 17 and 7 8 13 6 respectively but the first has minor eu anomalies eu eu 0 89 1 16 whereas the second features pronounced negative anomalies eu eu 0 52 0 81 coupled with high concentrations of incompatible elements such as k rb th u nb and ta the first subtype is interpreted to be derived from provenances of mixed mafic and felsic volcanic rocks whereas the eu depleted type has geochemical features that are typical of post archean sediments or archean k rich granites and volcanic equivalents abitibi metasediments have nd 2 7 ga values of 0 71 to 1 83 corresponding to model tchur 2 82 2 55 ga with the majority around 2 65 ga close to the depositional age of 2 7 ga in contrast ps metasediments are characterized by nd 2 7 ga 0 95 to 3 51 corresponding to t dhur 2 7 to 3 1 ga consistent with a detrital zircon age spectrum of 2 69 ga to 3 1 ga pontiac subprovince and abitibi metasediments have contrasting provenances suggesting their separate initial geodynamic settings and later tectonic juxtaposition major and trace elements o and nd isotopic compositions and detrital zircon ages collectively suggest that some of the ps metasediments were derived mainly from juvenile rocks whereas others require various degrees of contribution from older 3 0 3 5 ga evolved crust mixed with juvenile 2 7 ga rocks older components were derived either directly from erosion of an ancient continental arc or from contemporaneous volcanic rocks which may contain recycled older components if the ps represents in part an accretionary prism to the abitibi southern volcanic zone then oblique subduction may have been involved which allowed the pontiac subprovince to juxtapose or sample exotic source rocks

multiple episodes of aridity in southern africa since the last interglacial period there is generally a dearth of evidence of the nature of quaternary climate change within desert systems which has limited previous interpretations of past environmental change at low latitudes the last glacial maximum has previously been identified as the peak of late quaternary aridity when desert systems expanded to five times their present extent1 2 3 and low latitude aridity has been described for previous glaciations4 but little evidence has been derived directly for large desert basins particularly southern africa here we report new chronological optical dating evidence of arid episodes recorded in aeolian sediments from the mega kalahari sand sea episodic aeolian activity is recorded at the northeastern desert margin whereas more sustained activity is evident from the southwestern desert core several significant arid events are apparent since the last interglacial period with dune building arid phases at 95 115 41 46 20 26 and 9 16 kyr before present existing atmospheric general circulation model simulations and independent palaeoclimate data indicate that the changes in aridity are related to changes in the northeast southwest summer rainfall gradient which are in turn related to sea surface temperatures in the southeastern atlantic ocean

late pleistocene wetting and drying in the nw kalahari an integrated study from the tsodilo hills botswana abstract the sediments and landforms at the tsodilo hills in the northwestern kalahari desert provide an opportunity to directly investigate the late quaternary wetting and drying of the region from evidence at a single site lacustrine carbonates including incorporated molluscs and diatoms a lake shoreline feature and stabalised linear dunes were investigated for their constituent palaeoenvironmental signals chronometric control is provided by calibrated 14 c ams and osl dating the evidence suggests that linear dune construction has not occurred since the last glacial maximum with particular development from 36 to 28 ka lake stands indicating wetter regional conditions than present occurred at 40 32 ka with more seasonal conditions from 36 ka and at 27 12 ka with a possible drying out at 22 19 ka data are consistent with other independent studies from the region and with recent evidence obtained from atlantic cores off the coast of namibia it is concluded that careful consideration of multi proxy data from a single location can assist in resolving discrepancies that arise from independent studies of lake cave and dune records in the kalahari

paleoflood hydrology and extraordinary flood events paleoflood hydrology is the study of past or ancient flood events the most accurate technique involves the analysis of slackwater deposits and paleostage indicators swd psi slackwater deposits are sand and silt emplaced from suspension in exceptionally deep high velocity floods that characterize narrow deep canyons in resistant geologic materials these and other paleostage indicators are used to establish hydraulic grade lines for the generating flow events favorable sites for slackwater deposition include channel margin areas where stresses and velocities are reduced below critical values necessary to maintain fine grained bedload material sand and silt in suspension computerized procedures for hydraulic flow modeling are used to tie the elevations of the highest slackwater deposits to surveyed river cross sections the flood discharges calculated by this method can be calibrated through the study of modern floods on gaged rivers correlation of multiple swd psi sites along a river reach is used to identify the maximum paleostage achieved by a given flood advances in the dating of flood deposits permit estimates of flood frequency to be made extending over a data base of thousands of years the major geochronologic tool is radiocarbon dating of various kinds of organic matter intercalated with the slackwater deposits an important development is the use of the tandem accelerator mass spectrometer for direct measurement of 14c tiny blebs of charcoal seeds and other organics can be analyzed in order to date ancient flood deposits of hydrologic significance since swd psi studies yield very accurate determinations of paleoflood ages and magnitudes there is a pressing need for new statistical procedures that make optimum use of the information content in paleoflood records for flood frequency analysis nevertheless swd psi paleoflood hydrology has moved beyond the research phase its use should be encouraged in evaluating past experience of extraordinary floods at appropriate hazardous sites

geochemical and nd isotopic compositions of the metasedimentary rocks in the la ronge domain trans hudson orogen canada implications for evolution of the domain abstract metasedimentary rocks of greenschist to amphibolite facies occur in various belts in the paleoproterozoic la ronge domain of the trans hudson orogen metasedimentary rocks in the central metavolcanic greenstone belt are low to intermediate in sio2 contents have nd values of 2 3 to 4 5 with two rare earth element ree patterns of la yb n 8 5 respectively the predominant component meta arkoses of the overlying mclennan lake belt have distinctly high k2o and sio2 contents high la yb n ratios 11 46 large negative eu anomalies eu eu 0 1 0 58 and uniformly low nd values 0 8 to 0 2 the maclean lake belt is comprised of a variety of metasedimentary gneisses the volumetrically minor amphibole and diopside rich gneisses have high ferromagnesian element sc and co concentrations high nd values of 4 1 to 5 7 and flat ree patterns with no eu anomalies the predominant lithological unit in the maclean lake belt the psammitic gneisses is geochemically more variable the mafic psammitic gneiss samples in the maclean lake belt and the metasedimentary rocks in the central metavolcanic belt are similar in trace and major element geochemistry and nd isotope composition whereas those more felsic psammitic gneisses are similar to the mclennan lake meta arkoses geochemical data combined with lithological and geochronological evidence indicate that sediments in the central metavolcanic belt were derived mainly from contiguous volcanic debris with minor addition of felsic components from exotic sources in contrast the mclennan lake meta arkoses were derived from a distinct felsic source whereas the psammitic gneisses in the maclean lake belt are mixtures of volcanic materials in the central metavolcanic belt and meta arkoses like felsic components nd isotopic data and ages of detrital zircon suggest that proterozoic juvenile materials were dominant sources for metasedimentary rocks in the la ronge domain comparison of geochemical signatures of the metasedimentary rocks with those of the felsic plutonic rocks in the central metavolcanic belt suggests that the felsic materials in the mclennan lake and maclean lake belts could not have been derived predominantly from the plutonic rocks implying more distal sources topographic changes during collision may have resulted in a shift from mixing of both local and distal sources for the rocks in the maclean lake belt to the dominance of a more distal source for the mclennan lake meta arkoses

a record of rapid holocene climate change preserved in hyrax middens from southwestern africa the discovery of sensitive paleoenvironmental proxies contained within fossilized rock hyrax middens from the margin of the central namib desert africa is providing unprecedented insight into the region s environmental history high resolution stable carbon and nitrogen isotope records spanning 0 11 700 cal calibrated yr b p indicate phases of relatively humid conditions from 8700 7500 6900 6700 5600 4900 and 4200 3500 cal yr b p with a period of marked aridity occurring from 3500 until ca 300 cal yr b p transitions between these phases appear to have occurred very rapidly often within 200 years of particular importance are 1 the observed relationship between regional aridifi cation and the decline in northern hemisphere insolation across the holocene and 2 the signifi cance of suborbital scale variations in climate that covary strongly with fl uctuations in solar forcing together these elements call for a fundamental reexamination of the role of orbital forcing on tropical african systems and a reconsideration of what factors drive climate change in the region the quality and resolution of these data far surpass any other evidence available from the region and the continued development of this unique archive promises to revolutionize paleoenvironmental studies in southern africa

a contribution to late middle paleolithic chronology of the levant new luminescence ages for the atlit railway bridge site coastal plain israel abstract the atlit railway bridge arb prehistoric site is located on the northern coastal plain of israel within natural caves which formed in calcareous aeolianites kurkar perhaps during a high sea stand flint artifacts belonging to the levantine later mousterian tradition and faunal remains were found embedded in the kurkar infill of two caves the aeolianites in which the caves had developed were previously constrained by irsl 50 dating of feldspars to be older than the last interglacial highest sea stand frechen m et al 2004 chronology of pleistocene sedimentary cycles in the carmel coastal plain of israel quaternary international 121 1 52 providing a maximum age for the artifacts samples for luminescence dating were collected from the infill of the two caves ii and iii from the same deposits as the archaeological finds both quartz and alkali feldspars kf were extracted and measured using four different luminescence signals optically stimulated luminescence blue osl and violet stimulated luminescence vsl on quartz and the infrared stimulated luminescence irsl post ir ir 290 signal and the ir 50 signal corrected for anomalous fading on kf the ages obtained from analyses of the different minerals and signals mostly agree within errors the new luminescence ages date the sediment infill in caves iii and ii to 90 ka and 70 ka respectively indicating that hominin occupation of this locality is coeval with the nearby skhul cave and layer b in tabun cave

holocene environmental changes in namibia inferred from pollen analysis of swamp and lake deposits spring deposits exposed during building operations in downtown windhoek and lake sediments retrieved from underneath 50 m of water in a sinkhole lake otjikoto contain pollen profiles which reflect environmental changes in namibia during the holocene at windhoek moist local conditions are reflected by pollen in the spring deposits which were radiocarbon dated to between ca 7000 and 6000 bp they remained relatively favourable until 5630 bp despite signs of drying weedy compositae lactucoideae or liguliflorae increased until the end of this record ca 2410 bp indicating local disturbance deposits from lake otjikoto were dated to the late holocene although an accurate chronology could not be estabiished for the sequence due to unexpected results with radiocarbon measurements pollen accumulation values and composition indicate relatively dry conditions after 3500 bp which were followed temporarily by a wetter climate during more recent times

the last millennium largest floods in the hyperarid kuiseb river basin namib desert the largest kuiseb river floods initiate at the basin s semi arid headwater downstream along the hyperarid namib desert these floodwaters are feeding shallow alluvial aquifers the only available water for human activity and for the natural ecology here we characterize the largest floods and their changing frequency and magnitudes using palaeohydrological methods along 120 km of the rivers canyon 35 palaeoflood deposit sites were identified at five of these sites we conducted stratigraphic and geochronological analyses and flood discharge estimations the upper bound of the largest flood over the late holocene is 1475 m 3 s 1 over the last 1300 years more than ten floods have exceeded 1250 m 3 s 1 an additional 33 floods exceeded 400 m 3 s 1 the last millennium was characterized by one large flood every 30 40 years during the periods 1250 1335 1355 1565 and 1715 ad to the present a slight increase in flood frequency not magnitude occurred during 1565 1715 ad one large flood in 20 years and during two short episodes 1185 1205 and 1335 1355 ad seven and four floods in 20 years respectively these episodes of increased flood frequency are associated with other proxy records of higher water availability in the namib copyright 2013 john wiley sons ltd

depositional environment and osl chronology of the homeb silt deposits kuiseb river namibia previous studies suggest that the homeb silts of the kuiseb valley namibia i accumulated in a dune dammed lake ii are end point deposits iii represent an aggrading river bed and iv are slackwater deposits thus they have been used alternatively as evidence of past drier conditions or past wetter conditions lithostratigraphic analysis of two sediment sequences at homeb indicates sedimentation by aggradation of the kuiseb river triggered by a transition from an arid to humid climate osl ages for the sequences were obtained by the sar protocol on aliquots of 9 6 mm and 4 0 mm diameter and on single grains four millimeter aliquot minimum ages closely approximate the single grain minimum ages and are younger than 9 6 mm aliquot minimum and central ages based on these results the small aliquot 4 mm approach appears to provide ages comparable to those obtained by the more laborious and time consuming single grain method minimum ages indicate rapid deposition of the homeb silts in at least two episodes centered at 15 ka and 6 ka during climate transitions from arid to humid flash floods eroded the valley fills during slightly more arid conditions

mass and hyperconcentrated flow deposits record dune damming and catastrophic breakthrough of ephemeral rivers skeleton coast erg namibia channel shaped deposits of well sorted sand with only 2 4 fine grained material being either massive and structureless or upward fining with basal lag are found interbedded with aeolian sand in the skeleton coast erg namibia detailed analyses of the channel shaped deposits suggest that they are formed as hyperconcentrated flows within the erg grain size analysis and whole rock geochemical modeling revealed that some of the fluvial sediments contain up to 70 aeolian sand interpreted as a result of dune collapse into the fluvial system in certain cases this instantaneous supply of sand resulted in generation of sandy mass flows with laminar flow behaviour the presence of smectite as dominant clay mineral proved to be of crucial importance in formation of mass flow deposits these mass flows had no erosional capacity and drape the palaeotopography they are comparable to those generated by catastrophic collapse of dunes described in the literature this paper suggests that all these deposits should be termed intra erg mass flows as several of them carry little if any debris based on their origin intra erg mass flows can be divided into two groups

punctuated aridity in southern africa during the last glacial cycle the chronology of linear dune construction in the northeastern kalahari abstract the mega kalahari of central southern africa is one of the most extensive quaternary desert basins on a regional scale present day aeolian activity is restricted to episodic dune crest reactivation in the most arid southwestern desert core there is however abundant evidence of former periods of both more arid and more humid conditions many of which have little or no chronological control we have employed optical dating of quartz sand grains to develop a chronology of arid intervals as recorded by phases of linear dune construction in the northeastern sector of the mega kalahari we identify repeated phases of aeolian deposition during the last interglacial glacial cycle at ca 95 115 41 46 20 26 and post 20 ka which are separated by depositional hiatuses that we infer to correspond to more humid periods these aeolian depositional events correlate with and are inferred to relate to millennial scale cold sea surface temperature events in the southeast atlantic which have been linked to sub milankovitch climate changes recognised in northern hemisphere oceanic and cryospheric environmental archives covering the same time period while the present landscape is the product of either post 20 ka hwange national park dune field or 20 30 ka victoria falls dune field aeolian activity and subsequent erosion and reworking much of the vertical expression of the larger dune forms corresponds to the earlier periods of activity the linear ridges of the area are rich archives of late quaternary terrestrial climate change

sedimentology of high stage flood deposits of the tagus river central spain abstract this paper details the sedimentology of high stage flood deposits with the definition of sedimentary environments and their characteristic sequences along two bedrock reaches of the tagus river central spain high stage flood deposits accumulated in bedrock canyons include slackwater flood deposits swd and other types of deposits located at flow separation zones and associated with slow moving flow 1 these flood deposits are common indirect indicators of flood stages used in palaeoflood studies for estimating the discharges associated with quaternary floods depositional environments of flood deposits include 1 channel widening 2 canyon expansion 3 bedrock obstacles and 4 backflooded areas along tributary streams these flood deposits can be found associated with other non fluvial environments namely aeolian reworked and slope washflow facies channel widening due to flood stage variations comprises internal and external zones of the channel margins and their characteristic sequences contain similar facies to those of alluvial floodplains canyon expansion environments favour vertical accretion of slackwater units and the development of flood deposit benches which contain four sequences related to bench elevation and distance from the channel s main thread of flow at the lee side of bedrock obstacles characteristic sedimentary sequences are dominated by reverse flow structures e g climbing ripples migrating upstream due to eddies with a high sand concentration flood deposits located within tributary mouths contain typical sequences of reworked floodplain deposits backflooding of tributaries during flood stages produces deposition from suspension of sand silt and clay within three sequences characterised by non structure or parallel lamination and intense bioturbation a better understanding of the flood deposit sequences may contribute to the characterisation of flood magnitudes and flood hydraulics and can also be applied to some ancient depositional environments

evidence of past climatic change in the namib desert abstract a preliminary survey of some areas of the central and northern namib desert was undertaken in order to identify geomorphic and sedimentary features that could potentially provide information on past climatic change in the region wherever possible samples were collected for radiocarbon dating the visits to the skeleton coast area were especially revealing practically every river valley in this tract shows a sequence of terraces and deposits that would warrant a detailed study in all about 120 new radiocarbon dates were obtained for the region these have supplied a wealth of individual results and collectively have enabled the construction of a tentative chronological sequence of the changes in climate for the past 40 000 yr three periods of increased aridity in the hinterland but with decreasing intensity are postulated during the last glacial maximum the early holocene and between a d 1200 and a d 1600 the inter pleniglacial up to 23 000 yr b p the late glacial and the period between about 4000 yr b p and 1200 yr b p on the other hand were more humid than today also with decreasing intensity other indicators of past climate were also identified but since they lie beyond the range of radiocarbon dating they cannot as yet be placed in a chronological context of significance is the fact that convincing evidence was obtained for climatic change within the last millennium and especially the rainfall is found to have been increasing since the 16th century

contrasting styles of ephemeral river systems and their interaction with dunes of the skeleton coast erg namibia abstract the skeleton coast erg forms a prominent nnw trending dune belt 6 22 km wide comprising dunes up to 50 m high sub parallel to the south atlantic margin of namibia to a variable degree along its strike the dune belt dams west southwestward flowing ephemeral river systems on their route towards the atlantic ocean the southern rivers koigab uniab and hunkab affecting the erg are characterized by short infrequent ephemeral flows from restricted catchments whereas northern rivers hoanib and hoarusib are fed by large catchment areas favouring higher discharge and more frequent flows as the intertropical convergence zone and greater likelihood of monsoonal rainfall influence is approached the potential for river damming increases in the same downwind direction through the erg because of increasing dune belt width and height and the change from low barchanoid dune forms in the south to large composite transverse dunes in the north whereas cross sections of the southern part of the erg show considerable asymmetry with low barchans and sinuous crested dune forms eastwards but a high dune wall at the western edge cross sections over the northern erg are more uniformly symmetrical as a consequence the northern rivers are effectively dammed at the eastern erg margin causing flood reservoir basins there in contrast southern rivers are dammed within the erg as the most pronounced and commonly encountered barrier is provided by the dunewall at its western border

dynamics of pacific plate crust in the south island new zealand zone of oblique continent continent convergence here we analyze topographic and fission track data to quantify the response of the surface of the pacific plate in south island new zealand to late cenozoic oblique continental convergence across the alpine fault over the central 350 km length of the southern alps mountain chain we derive and map the rates of mean surface uplift the rates of working associated with mean surface uplift during the late cenozoic mountain building the amounts and rates of denudation and consequent isostatic rebound and the tectonic component of rock uplift the rate of mean surface uplift ranges from 0 3 mm yr over most of the area east of the main divide the highest rates of mean surface uplift occur to the southeast of the regions of highest mean elevation and relief the rate of working against gravity during uplift of the mean surface ranges from 2 5 mw m 2 in the southwest to 10 mw m 2in the central eastern parts of the alps areas of lower mean elevation uplifted most recently have received rates of energy input similar to that of areas of higher mean elevation where uplift started earlier the amount of denudation is large compared with the mean surface uplift and ranges from 18 km adjacent to the alpine fault to 2 km along the southeast margin of the southern alps the rate of denudation ranges from 2 5 to 0 5 mm yr with increasing distance from the alpine fault across the alps to the southeast the amount of isostatic uplift ranges from a maximum of 14 km adjacent to the alpine fault to 2 km along the southeast margin of the alps the tectonic component of uplift varies from 4 km along the alpine fault to 1 km along the eastern margin of the alps

holocene fluvial valley fill sources of atmospheric mineral dust in the skeleton coast namibia western namibia is a significant global source of atmospheric mineral dust we investigate the relationship between dust and source sediments assessing the sustainability of dust flux remote sensing studies vickery and eckhardt 2013 have highlighted specific ephemeral fluvial systems as important contributors to dust flux including highlighting sections of valleys that are the origins of dust plumes in the period 2005 8 little is known however about the specific within valley dust sediment sources particularly whether dust is derived from modern ephemeral channel floors or older valley fill sediments many of which have been reported in the region stone and thomas 2013 as part of a region wide analysis of aeolian dust flux we investigate the sediment properties of atmospheric dust samples and valley sediments from the huab valley one of the principal regional dust sources trapped dust samples contain up to 88 very fine sand and silt when collected samples are disaggregated prior to analysis valley fill surface samples comprise 80 very fine sand and silt and the surface of the modern ephemeral channel 30 valley fill sediments were sampled at depths up to 3 6m below the present surface and reveal holocene depositional ages from 0 6 0 03ka back to 9 79 0 73ka these sediments contain 30 to 6 very fine sand and silt with levels decreasing with depth and age aeolian bedforms in the valley system nebkhas on the fill surface and climbing dunes on valley margins indicate that aeolian processes under the influence of strong seasonal easterly winds likely result in dust being winnowed out of the valley fill surfaces with sandy bedforms being constructed from the coarser component of the fill sediments the volume of valley fill sediment suggests dust sourced from holocene sediments is likely to continue into the future regardless of flow conditions in the modern channel system

environmental changes at the eastern namib desert margin before and after the last glacial maximum new evidence from fluvial deposits in the upper hoanib river catchment northwestern namibia in the upper hoanib river catchment area northwestern namibia fine grained silty deposits are widespread accumulated as river end deposits they form excellent geomorphological archives of a highly sensitive desert margin area different sedimentary complexes are separated sedimentologically and by luminescence dating the deposits give evidence of climate oscillations at the eastern margin of the namib desert during the past 50 000 years in northwestern namibia the dry conditions of the desert margin area with less than 200 300 mm mean annual precipitation prevailed at 60 40 ka and at 34 24 ka when sediment complex i accumulated during the last glacial maximum lgm fluvial activity and sedimentation were most likely interrupted due to arid conditions between 12 ka and the mid holocene higher rainfall and runoff led to the aggradation of sediment complex ii the climate was more humid than before the lgm but drier than at present after 3 ka runoff increased and the hoanib river re eroded the older deposits forming deep channels a prominent more sandy and gravely 4 m terrace in the upper and middle course of the hoanib river dates to the little ice age lia it is likely that this terrace is genetically linked with the amspoort silt formation in the lower hoanib valley d 2005 elsevier b v all rights reserved

iron and nutrient content of wind erodible sediment in the ephemeral river valleys of namibia abstract research concerning the global distribution of aeolian dust sources has principally focussed on salt clay pan and desiccated lacustrine emission areas in southern africa such sources are identified as etosha pan in northern namibia and makgadikgadi pans in northern botswana dust emitting from ephemeral river valleys however has been largely overlooked rivers are known nutrient transport pathways and the flooding regimes of ephemeral river valleys frequently replenish stores of fine sediment which on drying can become susceptible to aeolian erosion such airborne sediment may be nutrient rich and thus be significant for the fertilisation of marine waters once deposited this study investigates the dust source sediments from three ephemeral river valleys in namibia in terms of their particle size distribution and their concentrations of bioavailable n p and fe we compare the nutrient content of these sediments from the ephemeral river valleys to those collected from etosha and makgadikgadi pans and consider their relative ocean fertilising potential our results show that the ephemeral river valleys contain fine grained sediment similar in physical character to etosha and makgadikgadi pans yet they have up to 43 times greater concentrations of bioavailable iron and enriched n and p macronutrients that are each important for ocean fertilisation the known dust emitting river valleys of namibia may therefore be contributing a greater fertilisation role in the adjacent marine system than previously considered and not yet investigated given this finding a re assessment of the potential role of ephemeral river valleys in providing nutrient rich sediment into the aeolian and marine systems in other dryland areas is necessary

timing of lake level changes in etosha pan namibia since the middle holocene from osl ages of relict shorelines in the okondeka region abstract in 2003 examination of aerial photographs revealed a series of previously unknown relict shorelines on the arcuate ridge possibly a clay lunette dune that marks the western boundary of etosha pan in namibia the shorelines are 120 600 m wide and the most prominent extend for tens of km around the lunette dune the shorelines were examined on the ground in 2004 and an attempt was made to date the three lowest levels at ca 5 2 5 and 1 m above the present pan surface the osl ages obtained indicate higher and more prolonged lake conditions than today at ca 6 4 4 0 and 2 1 ka with the youngest shoreline sediments resting on an ancient pan surface dating to ca 13 ka the evidence indicates dry conditions in the pan at ca 13 ka wetter conditions and higher lake levels in the middle holocene followed by a decline in lake levels to the present periods of inundation were of sufficient duration to produce shorelines at the southwestern end of the pan due to the prevailing northeasterly winds that would have maximized wave action along this section of the pan margin the etosha findings together with other regional paleoclimate data suggest four periods of increased wetness in sw africa during the holocene at 7 5 4 5 3 5 2 5 1 7 and ca 1 0 ka there is widespread evidence for the oldest of these periods suggesting that it was a prominent and widespread interval of wetness prior to ca 8 0 ka the climate may have been drier than today

formation of 1 9 ga old trans hudson continental crust pb isotopic data pb isotopic data are reported for whole rock samples and feldspars from two parts of the 1 9 1 8 ga old trans hudson orogen in canada the reindeer zone in saskatchewan and the ottawa and belcher islands in hudson bay the first area contains a variety of volcanic granitoid and sedimentary rocks in several oceanic and magmatic arcs aligned parallel to the margin of the archean hearne craton the second is part of the circum superior belt a belt of oceanic basalts and sedimentary rocks that surrounds the superior craton in several regions pb pb isochrons gave ages generally consistent with available geochronological or geological data flin flon 1822 44 ma 2 peter lake complex 2710 220 ma ottawa islands 1790 80 ma belcher islands 1960 160 ma in other regions the pb pb age was too low reflecting disturbance of the pb system e g la ronge belt 1730 115 ma or too high perhaps because of contamination of certain samples with older crust e g wathaman batholith 2200 200 ma model values and th u ratios calculated from the pb data are interpreted in terms of variations in the compositions of mantle sources complicated by contamination with material from the upper or lower continental crust in the southern part of the reindeer zone and in volcanic rocks from the ottawa islands model values vary from 8 0 to 8 1 and correlate inversely with initial nd values this variation is thought to result from contamination of juvenile material 8 0 with material from archean upper crust which had low nd but relatively high in the northern part of the zone and in eskimo formation basalts from the belcher islands values remain constant or decrease as nd declines and this is attributed to contamination with u depleted lower crust calculated th u ratios vary from 1 34 to 6 67 the lowest values 1 4 1 8 come from mafic to felsic volcanic rocks of the flin flon and western la ronge belts these low th u values are like those in modern island arc volcanics and are attributed to enrichment of u in the source slightly higher values are calculated for oceanic basalts from the circum superior belt morb like basalts from the ottawa islands have a low th u of 2 5 reflecting depletion of th along with other incompatible elements higher values of 3 9 characterize the more enriched flaherty basalts the highest th u values which are found in samples with the lowest nd and low values wathaman batholith and eskimo basalts are thought to reflect the high th u of the u depleted lower crustal contaminant these results have several interesting implications the similarity between the isotopic and chemical characteristics inferred for these 1 9 ga old rocks and those of volcanics in modern convergent margin settings provides further indication that the tectonic and petrogenetic processes peculiar to modern subduction environments operated in the middle proterozoic the apparent presence of material from the lower and upper crust in the sources of the granitoids indicates that these rocks were not derived from subducting oceanic plate or the mantle wedge but probably formed and evolved within the crust itself

sedimentology and paleohydrology of late quaternary lake deposits in the northern namib sand sea namibia abstract the namib sand sea is the largest active desert dunefield in southern africa and is comprised mainly of large north south linear dunes in the interdune areas of the northern sand sea eleven small areas of calcareous lacustrine sediment have been studied these beds are typically less than a metre thick and are dominantly comprised of calcareous sandstones to mudstones and sandy limestones the carbonates are mainly magnesian calcites 1 14 mgco 3 with some protodolomite and aragonite calcified reed casts and fresh to brackish water gastropods diatoms and ostracods are present in some beds 18 o values indicate a hot and dry climate a number of enriched 13 c values may reflect high salinity low organic populations or carbonate recrystallization these carbonate rich lacustrine deposits are indicative of increased periods of moisture availability in this normally hyperarid region during the late quaternary the origin of the water responsible for depositing these sediments may be 1 ponding at the end point of the tsondab river which at one time extended farther west into the sand sea 2 flooding into interdune corridors when water levels rose in rivers such as the kuiseb 3 groundwater seepage into depressions either through dunes that border rivers or from the underlying tsondab sandstone and 4 increased rainfall we do not believe that there is evidence to support a major increase in precipitation over the region however even a small increase in precipitation in the headwaters of valleys that drain toward the sand sea might 1 generate enough additional runoff to extend the terminal point of rivers such as the tsondab farther into the dunes 2 cause lateral flooding from major valleys into interdune corridors and 3 recharge aquifers the sedimentary records at narabeb ancient tracks and west pan which lie along the old course of the tsondab river favor a ponded river origin for them whereas groundwater seepage is favored at other sites the chronology of deposition based on radiocarbon dates suggests that ponding and recharge occurred earlier in the lower western part of the area and later in the east this is in harmony with the view that the end point of the tsondab river progressively retreated eastward between about 30 and 14 ka bp as dunes blocked its route

origin and history of nitrate in confined groundwater in the western kalahari abstract data are presented for nitrate dinitrogen and argon concentrations and 15 n 14 n ratios in groundwater with radiocarbon ages up to 40 000 yr for three confined sandstone aquifers in the western kalahari of south west africa namibia the nitrate is probably generated within the soil of the recharge areas and its production rate during the period 3000 40 000 b p has remained between 0 5 and 1 6 meq no 3 l 1 of recharge water with 15 n between 4 and 8 variations in the amount of nitrate and of excess air in groundwater recharge are found and can only reflect changes in the environmental conditions during recharge they must therefore be caused by the climatic changes that have taken place during the past 25 000 yr

extraordinary flood events and the response to monsoonal climatic change during the last 3000 years along the middle yangtze river valley china abstract well dated long term flood data series is very important in studying of hydrological response to past climatic change along the mainstream of the yangtze river five palaeoflood events recorded by slackwater deposits were identified by the sedimentological criteria and analytical results a detailed chronological framework of palaeoflood events during the late holocene was established by optically stimulated luminescence dating in combination with anthropogenic remains and with pedo stratigraphic correlations with well dated holocene loess soil profiles within the middle yangtze river basin the five palaeoflood events were dated to 2 43 0 18 ka 1 82 0 17 ka 0 94 0 07 ka 0 72 0 06 ka and 0 59 0 05 ka respectively these results are consistent with geological records at other palaeoflood sites and historical flood records along the middle yangtze river valley a regional synthesis of the palaeoflood chronology over the last 3000 years was compiled along the middle yangtze river extraordinary flood events generally seem to be correlated with late holocene climatic variability i e neoglacial cooling roman warm period dark ages cold period medieval climate anomaly and little ice age the high resolution climatic proxies from stalagmites of the dongge cave and heshang cave ice cores of the grip the total solar irradiance variations and sunspot number and enso activities suggest that these hydroclimatic events are possibly related to the weaker asia summer monsoon and the cooling climatic events as well as the stronger enso activities during the late holocene in the yangtze river valley these results provide insights into the response of hydroclimatic system to global change in the large rivers of asia

rb sr study of metavolcanic rocks from the la ronge and flin flon domains northern saskatchewan two whole rock suites of metavolcanic rocks from separate volcanic belts of the churchill province in northern saskatchewan have been dated by rb sr samples from the amisk group of the flin flon snow lake domain provide an isochron date of 1784 44 ma suites from the waddy lake and devil lake areas of the la ronge lynn lake domain yield isochron dates of 1814 26 and 1854 100 ma respectively all are regarded as minima for but close approximations to emplacement ages the maximum crustal age of any suite cannot greatly exceed 1850 ma previous rb sr and u pb isotopic dates together with these new determinations confirm the contemporaneous existence of two volcanic arcs active during the late aphebian 1875 1784 ma in the churchill province low initial 87 sr 86 sr ratios 0 7017 0 7022 are consistent with a petrochemically inferred subduction related origin for the volcanic rocks with no closed system reworking of archean crust and a linear evolution of 87 sr 86 sr ratio in the magmatic arc mantle source region from 4 55 ga to the present

holocene climatic optimum in southwest africa evidence from the marine clay mineral record abstract sediment composition grain size and clay mineral record of a high resolution sediment core from the continental slope off namibia was investigated to gain information on the deposition of terrigenous matter in this part of the southwest african continental margin during the last 18 k y the depositional processes involved are fluvial input by the kunene river and eolian input from the namib and kalahari deserts each supplying characteristic mineral suites during low sea level erosion of the exposed shelf yields additional material the amount of eolian or fluvial matter depends on the strength of the transport process which is a function of aridity or humidity of the source area thus allowing paleoclimatic interpretations arid conditions prevailed during a low sea level from 18 to 15 ka and unconsolidated shelf sediment was mobilized and supplied to the slope by short distance transport by southerly winds a dramatic increase in the accumulation of terrigenous sediment is recorded from 15 to 10 ka without major changes in sediment composition which is attributed to increased runoff of the kunene river and fluvial erosion of shelf sediments this period coincides with a strengthening of the monsoonal system during a precessional minimum which is observed in numerous sites in central africa and indicates an intensified influence of the monsoon at the kunene headwaters a distinct shift in clay mineralogy towards river derived material marks a second period of increased river runoff during high sea level from 9 to 5 ka indicating maximum humidity in the source area from 6 to 5 ka this corresponds to the holocene climatic optimum observed in the arid belts of northern africa the present balance between fluvial and eolian input was reached approximately at 4 ka

u pb geochronology in the trans hudson orogen northern saskatchewan canada we have obtained u pb ages on zircons from volcanic and plutonic units in several lithotectonic domains of the southern trans hudson orogen in northern saskatchewan these data constrain the timing of early proterozoic orogenic events in the region and enhance our understanding of both the relationships among local domains and the relationship of the trans hudson orogen to other early proterozoic orogens in north america with one exception all units studied so far yield zircon ages of 1890 1835 ma most of which are systematically earlier than previously reported rb sr isochron ages on the same or similar units suggesting open system behavior in the rb sr systems five metarhyolites from volcanic sequences in the la ronge domain glennie domain and hanson lake block give ages ranging from 1888 to 1876 ma most of the plutons we dated ranging from gneissic syntectonic tonalites and granodiorites to less deformed late intrusions such as the wathaman batholith and other smaller bodies yield ages of 18