middle to late holocene moisture changes in the desert of northwest namibia derived from fossil hyrax dung pollen new pollen results and radiocarbon dating from fossil hyrax middens derived from the edge of the northern namib desert address the shortage of continental palaeobotanical evidence in arid namibia by providing evidence for the environmental conditions during the mid to late holocene in the region the results obtained reflect long term stability in the area throughout most of the sequence higher than modern moisture availability is suggested between c 6 and 1 ka bp by the increased abundance of poaceae cyperaceae or chenopodiaceae which respond rapidly by flowering after modest quantities of rainfall around 1 ka and recently arid conditions seem to have prevailed with a decrease in poaceae and an increase in acanthaceae other palaeoecological evidence from the local and regional surroundings that focused on fluvial deposits marine palynology and mineralogy confirms this mid holocene increase in more effective rainfall

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

palynology of hyrax middens 2000 years of palaeoenvironmental history in namibia abstract well preserved and dateable faunal middens in rock shelters of dry areas in southern africa occur as desiccated urine rich masses under local conditions which exclude microbial decomposition and oxidation urine cements together faecal pellets and other inclusions nine radiocarbon dated hyrax procavia capensis dung middens from the kuiseb river basin of the namib desert were sub sampled and analysed for fossil pollen some contain small proportions of rat droppings but not many macro fossils comparative modern pollen in faecal samples were also studied most plant macro fossils are severely digested but seeds are well preserved some insect inclusions were identified small scale changes in vegetation over the last two millennia were observed representing environmental fluctuations in cycles of ca 100 200 years the fluctuations partly agree with other palynological data from southern africa and with geomorphologic evidence for environmental changes in the desert region

effect of the formation of the isthmus of panama on atlantic ocean thermohaline circulation the late cenozoic closure of the seaway between the north andsouth american continents is thought to have caused extensive changes in ocean circulation and northern hemisphere climate1 2 but the timing and consequences of the emergence of the isthmus of panama which closed the seaway remain controversial1 2 3 4 5 here we present stable isotope and carbonate sand fraction records from caribbean sediments which when compared to atlantic and pacific palaeoceanographic records indicate that the closure caused a marked reorganization of ocean circulation starting 4 6 million years ago shallowing of the seaway intensified the gulf stream and introduced warm and saline water masses to high northern latitudes these changes strengthened deep water formation in the labrador sea over the next million years as indicated by an increased deep water ventilation and carbonate preservation in the caribbean sea and favoured early pliocene warming of the northern hemisphere the evaporative cooling of surface waters during north atlantic deep water formation would have introduced moisture to the northern hemisphere although the pronounced intensification of northern hemisphere glaciation between 3 1 and 2 5 million years ago substantially lagged the full development of north atlantic deep water formation we propose that the increased atmospheric moisture content was a necessary precondition for ice sheet growth which was then triggered by the incremental changes in the earth s orbital obliquity

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

hoanib river flood deposits of namib desert interdunes asanalogues for thin permeability barrier mudstone layers inaeolianite reservoirs abstract the ephemeral braided hoanib river of nw namibia ows for a few days ayear and only high discharges enable the river to pass through interdunaldepressions within the northern namib desert dune eld to the atlantic thedune eld comprises mainly large transverse dunes resulting frompredominant ssw winds river ood deposits between aeolian dunes areanalogous to mudstone layers conformably interbedded with ancientaeolianite dune foresets deep oods pond laterally to considerable depths metres to 10 m in adjacent interdunes depositing mud layers 1 50 cm toconsiderable heights on avalanche and stoss faces of bounding dunes fairlypassive ooding only disturbs aeolian strati cation minimally floodwaterclay in ltrates and settles as an impermeable seal with a ood pond on top perched above regional groundwater flood ponds evaporate slowly for longperiods 3 years early emergence desiccates higher parts of a mud layer subsequent oods can re ll a predecessor pond bene ting from the existingimpervious seal potential preservation of such mud layers is lower on thestoss face but high on the avalanche face after burial by subsequent dunereactivation and migration the leeward right hoanib bank a dune stoss face is river and wind eroded to exhume fossil interdune pond mud layers of anearlier hoanib channel the highly inclined layers are interbedded with duneavalanche foresets and represent the edges of two successive fossil pondsexposed in plan ancient ood pond mudstones occur in the permian triassichydrocarbon reservoir the sherwood sandstone group of the cheshire basin kinnerton formation and irish sea basin and were previously usederroneously to argue against the aeolian origin of cross bed sets hoanibstudies show that primary river interaction with a dune eld might preserveonly localized erosional omission surfaces in ancient aeolianites with littlesandy barform preservation prone to aeolian reworking around the main uvial channel locus however ood pond mudstone layers should form apredictable halo within which uid permeability will decrease keywords aeolian dunes aeolianite reservoir ash ood interdune namibdesert namibia permeability introductionhigh discharge oods of the hoanib river occa sionally break through the skeleton coast dune eld which forms part of the extensive namibdesert that parallels the western margin of thesouthern african subcontinent this arid regionis 140 km wide and over 2000 km long fromsouth western angola to the northern capeprovince south africa the northern namib dune eld termed the skeleton coast dune eld bywilson 1973 lancaster 1982 and ward sedimentology 2002 49 719 736

spatial and temporal analysis of hillslope channel coupling and implications for the longitudinal profile in a dryland basin author s michaelides katerina hollings rory singer michael bliss nichols mary h nearing mark a

aridity is expressed in river topography globally it has long been suggested that climate shapes land surface topography through interactions between rainfall runoff and erosion in drainage basins1 4 the longitudinal profile of a river elevation versus distance downstream is a key morphological attribute that reflects the history of drainage basin evolution so its form should be diagnostic of the regional expression of climate and its interaction with the land surface5 9 however both detecting climatic signatures in longitudinal profiles and deciphering the climatic mechanisms of their development have been challenging owing to the lack of relevant global data and to the variable effects of tectonics lithology land surface properties and human activities10 11 here we present a global dataset of 333 502 river longitudinal profiles and use it to explore differences in overall profile shape concavity across climate zones we show that river profiles are systematically straighter with increasing aridity through simple numerical modelling we demonstrate that these global patterns in longitudinal profile shape can be explained by hydrological controls that reflect rainfall runoff regimes in different climate zones the most important of these is the downstream rate of change in streamflow independent of the area of the drainage basin our results illustrate that river topography expresses a signature of aridity suggesting that climate is a first order control on the evolution of the drainage basin a global dataset of river longitudinal profiles shows that river profiles become straighter with increasing aridity and numerical modelling suggests that this can be explained by rainfall runoff regimes in different climate zones

how is topographic simplicity maintained in ephemeral dryland channels the lack of topographic complexity in ephemeral dryland channels despite large variations in hydraulic controls and sediment texture presents an unexplained paradox that limits understanding of their long term evolution in dryland basins spatially and temporally discontinuous channel flow transports and sorts sediment along the bed intermittently and irregularly the cumulative effect of these processes counterintuitively produces simple topography manifest in straight longitudinal profiles and symmetrical cross sections in contrast with perennial channels this paper presents numerical modeling experiments based on field measurements to investigate dryland channel topographic development through the responses of bed material flux and net sediment storage to variations in channel hydrology we show that spatially variable flow creates and subsequently destroys incipient topography along ephemeral reaches and that large flood events above a threshold overcome hydraulic and grain size controls to dampen fluctuations in longitudinal sediment flux through a smoothing of the incipient channel bar forms the results provide a physical explanation for emergent topographic simplicity in ephemeral dryland channels despite higher variability in streamflow and sedimentary characteristics compared to perennial systems

a ground penetrating radar survey of late holocene fluvial sediments in nw namibian river valleys characterization and comparison fine grained terraces and coarse grained channel fills that represent episodes of late holocene valley aggradation are used as geoarchives for palaeohydrological reconstructions in many arid regions of the world e g southwestern usa australia israel and the namib desert in namibia previous work on the sediment texture and facies of these valley fills has led to diverse interpretations of the depositional environment including river end palaeoflood and floodout here we show using ground penetrating radar gpr profiles across four complex river channels to image sedimentological and stratigraphical structures that the gpr images can be grouped into eight radar facies reflection patterns a combination of radar facies analysis and groundcheck exposures boreholes has been used to interpret the radar profiles by considering the geomorphological sedimentological features and age determinations of the fluvial deposits described by earlier researchers and supplemented by our own investigations we explain the late holocene fine grained terraces and coarse grained channel fills as deposits accumulated during floods although the gpr results alone cannot conclusively resolve the fine grained terrace deposits either as river end or as slackwater accumulation the novel combination of gpr geomorphological findings and results from the literature provides evidence for our interpretation

late holocene environmental change in the northwestern namib desert margin new fossil pollen evidence from hyrax middens abstract the lack of anoxic environments in arid lands makes well preserved fossil pollen difficult to find the scarcity of continental palaeobotanical data in tropical arid zones restricts the understanding of aridification processes in these endangered ecosystems it is essential to improve the knowledge about their environmental histories during the holocene and therefore we attempt to investigate the causes and describe the patterns of vegetation change in northern namibia with that aim we analyzed pollen from fossil hyrax dung that accumulated over long periods of time by sampling stratigraphically coherent sequences in five radiocarbon dated middens the fossil hyrax middens were found in rock shelters on the eastern desert edge in the northwestern kaokoveld within the so called nama karoo biome this is an ecotonal area between the namib desert and the savanna biomes which reflects features from both systems and its life form composition largely depends on an erratic rainfall pattern thirty three samples were analyzed for pollen and the pollen record reflects a non continuous vegetation history over the last 5200 yr with a hiatus between ca 4200 and 1690 yr bp the pollen spectra reflect arid savanna vegetation with a marked increase in the tree grass ratio from ca 1300 cal yr bp the most likely cause for this change is a decline in moisture that we relate to intrinsic savanna dynamics with early pastoralism megahervibore migration and changes in the hyrax diet playing a minor role

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

southward shifting of the tasman front at 4 4 ma early pliocene paleobiogeographic and oxygen isotopic evidence abstract the tasman front represents a thermal boundary across the north tasman sea located between 31 s and 35 s over the tasman sea ridge system in the south west pacific census data of planktic foraminiferal assemblages and oxygen isotopic variations of planktic and benthic foraminifera obtained from three dsdp sites 284 590a and 588 were used to investigate the pliocene history of the tasman front the results indicate that at about 4 4 ma the tasman front shifted to the south of 31 s as reflected by a major change in the biogeographic distribution of planktic foraminifera this change caused warming of the sea surface particularly in the area of dsdp site 590a temperature gradients between surface water and intermediate water indicated by 18 o planktic benthic at dsdp590a also increased suggesting that the site has been subsequently located beneath a more vertically stratified water column than prior to 4 4 ma the southward shifting of the tasman front might have been a response to the intensification of the east australian current and the enlargement of the west pacific warm pool in the western equatorial pacific both of these oceanographic developments may have been caused by the curtailing of the westerly flowing south equatorial current due to tectonic closure of the new guinea seaway involving northward motion of the australian plate and an associated pliocene orogeny in central new guinea

opal sedimentation shifts in the world ocean over the last 15 myr biogenic silica opal accumulation records were used to trace mechanisms consequence and geographic pattern of shifts in the main locus of opal deposition of the world ocean over the last 15 myr over this time interval the main opal sink seems to have moved from the north atlantic to the pacific equatorial pacific eastern equatorial pacific eastern boundary current upwelling systems california namibia peru and finally to the southern ocean the interplay between opal deposition and a series of climatic tectonic oceanographic and biologic events has been analyzed and discussed these events include the cenozoic global cooling trend intensified glaciation in antarctica late miocene early pliocene biogenic bloom development of northern hemisphere glaciation nhg closing of the panama seaway transition of the climate system from a monopolarto a bipolar glaciated world mid pleistocene revolution mpr nutrient availability evolution of diatoms and c4 plants and changes in continental weathering rates while the observed shifts are mostly traceable to oceanic reorganizations and global climatic evolution conditions favorable to opal deposition involve the above mentioned complex mix of processes for this reason the interpretation of opal deposition records might not always be straightforward we however believe that it can still provide clear indications of large scale oceanographic reorganizations in the geological past d 2004 elsevier b v all rights reserved

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

late quaternary dynamics of southern africa s winter rainfall zone abstract variations in the nature and extent of southern africa s winter rainfall zone wrz have the potential to provide important information concerning the nature of long term climate change at both regional and hemispheric scales positioned at the interface between tropical and temperate systems southern africa s climate is influenced by shifts in the intertropical convergence zone the westerlies and the development and position of continental and oceanic anticyclones over the last glacial interglacial cycle substantial changes in the amount and seasonality of precipitation across the subcontinent have been linked to the relative dominance of these systems central to this discussion has been the extent to which the region s glacial climates would have been affected by expansions of antarctic sea ice equatorward migrations of the westerlies more frequent intense winter storms and an expanded wrz this paper reviews the developing body of evidence pertaining to shifts in the wrz and the evolution of ideas that have been presented to explain the patterns observed dividing the region into three separate axes along the western and southern margins of the continent and across the interior into the karoo and the kalahari a range of evidence from both terrestrial sites and marine cores is considered and potential expansions of the wrz expansions are explored despite the limitations of many of the region s proxy records a coherent pattern has begun to develop of a significantly expanded wrz during phases of the last glacial period with the best documented being between 32 17 ka while more detailed inferences will require the recovery and analysis of longer and better dated records this synthesis provides a new baseline for further research in this key region

sedimentation in lake malawi east africa during the past 10 000 years a continuous paleoclimatic record from the southern tropics a suite of piston cores recovered from lake malawi 9 14 s 34 35 e east africa in 1986 has been analyzed for major and minor elements organic c and n calcium carbonate and diatoms an internally consistent stratigraphy was constructed from calcium carbonate abundance and variations in the two most abundant diatom genera stephanodiscus and melosira with age control obtained primarily from 14c dating of the carbonate differences with time in fe abundance in a transect of cores from different water depths have been interpreted to reflect changes in chemocline depth the depth to the chemocline was on the order of 100 m shallower than present prior to 3500 yr b p indicating less seasonality carbonate production and preservation appears to be related to climatically induced changes in both salinity and chemical distributions in the water column the carbonate which precipitates from surface waters is most abundant during the interval from about 10 000 to 6000 yr b p this micrite most likely represents periods of low lake level when salinity increased and carbonate precipitation was enhanced sedimentary evidence suggests that lake levels were 100 150 m lower than present during this period this record is different from climatic trends in northern intertropical africa but appears to also be related to changes in insolation and monsoon circulation this is the northernmost basin in africa reported to exhibit a southern hemisphere response to the early holocene northern hemisphere summer insolation maximum the climatic hingeline north of lake malawi 9 s implied by our results is significantly south of that indicated by general circulation model simulations however the cores show evidence for periods of abrupt climate change during the interval of generally arid climate

the evidence for cainozoic aridification in southern africa abstract the namib and kalahari deserts preserve evidence of a long history of aridification in southern africa the earliest sedimentation within the kalahari basin dates to the late cretaceous but the first onset of arid conditions probably occurred in the early tertiary other arid episodes recurred throughout the tertiary but the development of the kalahari sand sea with its linear dune system is unlikely to predate the late pliocene this the largest single body of aeolian sand in the world is now largely stabilized by vegetation but evidence exists for its periodic reactivation throughout the quaternary although episodes of dune building are evident within the namib region as early as the eocene the namib sand sea represented by the now consolidated tsondab sandstone formation and the overlying active dunes of the sossus sand formation does not predate the upper miocene evidence from the upper pleistocene suggests however that there were significant fluctuations in the degree of aridity which prevailed during and after the last glacial period

the north equatorial countercurrent and heat storage in the western pacific ocean during 1982 83 a sequence of extreme climate anomalies known as an el nino southern oscillation episode was observed in the tropical pacific ocean during 1982 831 2 models suggest that oceanic circulation has an important role in the formation of such anomalies1 3 4 by altering the pattern of sea surface temperature through advection the baroclinic structure of the upper 500 m of the ocean has been monitored routinely in the western pacific since 1979 providing indirect current measurements by the geostrophic method these observations reported here show large changes in the near equatorial currents during 1982 83 which are consistent with currents observed in the central and eastern pacific5 6 in particular the north equatorial countercurrent necc flowed with 25 50 increased strength during the early phase then weakened almost to zero flow the west pacific heat pool cooled by more than 1 c observed changes in circulation were large enough to alter surface heat storage by advection although other potentially important processes may not be negligible

establishment of the western pacific warm pool during the pliocene evidence from planktic foraminifera oxygen isotopes and mg ca ratios abstract this study presents new evidence of when and how the western pacific warm pool wpwp was established in its present form we analyzed planktic foraminifera oxygen isotopes and mg ca ratios in upper miocene through pleistocene sediments collected at deep sea drilling program dsdp site 292 these data were then compared with those reported from ocean drilling program odp site 806 both drilling sites are located in the western pacific ocean dsdp site 292 is located in the northern margin of the modern wpwp and odp site 806 near the center of the wpwp three stages of development in surface water conditions are identified in the region using planktic foraminferal data during the initial stage from 8 5 to 4 4 ma site 806 was overlain by warm surface water but site 292 was not as indicated by the differences in faunal compositions and sea surface temperature sst between the two sites in addition the vertical thermal gradient at site 292 was weak during this period as indicated by the small differences in the 18 o values between globigerinoides sacculifer and pulleniatina spp during stage two from 4 4 to 3 6 ma the sst at site 292 rapidly increased to 27 c but the vertical thermal gradient had not yet be strengthened as shown by mg ca ratios and the presence of both mixed layer dwellers and thermocline dwellers finally a warm mixed layer with a high sst ca 28 c and a strong vertical thermal gradient were established at site 292 by 3 6 ma this event is marked by the dominance of mixed layer dwellers a high and stable sst and a larger differences in the 18 o values between g sacculifer and pulleniatina spp thus evidence of surface water evolution in the western pacific suggests that site 292 came under the influence of the wpwp at 3 6 ma the northward expansion of the wpwp from 4 4 to 3 6 ma and the establishment of the modern wpwp by 3 6 ma appear to be closely related to the closure of the indonesian and central american seaways

the latest miocene early pliocene biogenic bloom a revised indian ocean perspective abstract numerous investigations of neogene sediment sequences from the indian and pacific oceans have led to a paleoceanographic hypothesis that recently has been coined the biogenic bloom this hypothesis suggests that primary productivity increased substantially at indo pacific upwelling zones during the latest miocene early pliocene although it is clear that recognition of a widespread biogenic bloom provides a fundamental framework for understanding late neogene global change it also is apparent that investigations of this phenomena require syntheses of a variety of information from time correlated sediment sequences that span large oceanic regions late neogene polarity chron boundaries and nannofossil events are placed on a common and current time scale for 12 ocean drilling program odp sediment sequences in order to construct a coherent description of the biogenic bloom in the indian ocean carbonate mass accumulation rates mars were significantly higher than present day in the north and west indian ocean between 9 0 and 3 5 ma sediment deposited during this time interval also is dominated by planktic faunal assemblages that are indicative of high productivity and redox conditions that are explained by an increase in the supply of organic carbon further sediment of this age contains benthic faunal assemblages and chemical characteristics that are consistent with low dissolved o 2 in intermediate waters these observations collectively suggest that between 9 0 and 3 5 ma productivity in the north and west indian ocean was significantly higher than present day and the intermediate water oxygen minimum zone omz was more extensive than present day this paleoceanographic condition was most intense between 6 0 and 5 0 ma in the indian ocean comparison of the indian ocean sites with two locations in the pacific ocean suggests that the biogenic bloom was time coincident in the indian and pacific oceans the biogenic bloom therefore implies an important change in global nutrient cycling occurred during the latest miocene early pliocene although a satisfactory explanation for the change remains elusive

the role of rare rainstorms in the formation of calcic soil horizons on alluvial surfaces in extreme deserts abstract soils in similar geomorphic settings in hyperarid deserts 1 should have similar characteristics because a negative moisture balance controls their development however reg soils in the hyperarid southern negev and namib deserts are distinctly different soils developed on stable alluvial surfaces with only direct input of rainfall and dust depend heavily on rainfall characteristics annual rainfall amount can be similar 15 30 mm but storm duration can drastically alter reg soil properties in deserts the cooler fall winter and dry hot summers of the southern negev desert with a predominance brief 1 day rainstorms result in gypsic saline soils without any calcic soil horizon although the namib desert receives only 50 60 of the southern negev annual rainfall its rainstorm duration is commonly 2 4 days this improves leaching of the top soil under even lower annual rainfall amount and results in weeks long grass cover the long term cumulative effect of these rare rain grass relationships produces a calcic gypsic saline soil the development of these different kinds of desert soils highlights the importance of daily to seasonal rainfall characteristics in influencing soil moisture regime in deserts and has important implications for the use of key desert soil properties as proxies in paleoclimatology

coupled productivity and carbon isotope records in the southwest pacific ocean during the late miocene early pliocene biogenic bloom biogenic components of sediment accumulated at high rates beneath frontal zones of the indian and pacific oceans during the late miocene and early pliocene the 13c of bulk and foraminiferal carbonate also decreased during this time interval although the two observations may be causally linked and signify a major perturbation in global biogeochemical cycling no site beneath a frontal zone has independent records of export production and 13c on multiple carbonate phases across the critical interval of interest deep sea drilling project dsdp site 590 lies beneath the tasman front tf an eddy generating jetstream in the southwest pacific ocean to complement previous 13c records of planktic and benthic foraminifera at this location late neogene records of caco3 mass accumulation rate mar ca ti ba ti al ti and of bulk carbonate and foraminiferal 13c were constructed at site 590 the 13c records include bulk sediment bulk sediment fractions 63 m and 5 25 m and the planktic foraminifera globigerina bulloides globigerinoides sacculifer with and without sac and orbulina universa using current time scales caco3 mars ca ti al ti and ba ti ratios are two to three times higher in upper miocene and lower pliocene sediment relative to overlying and underlying units a significant decrease also occurs in all 13c records all evidence indicates that enhanced export production the biogenic bloom extended to the southwest pacific ocean between ca 9 and 3 8 ma and this phenomenon is coupled with changes in 13c the chron c3ar carbon shift however caco3 mars peak ca 5 ma whereas elemental ratios are highest ca 6 5 ma foraminiferal 13c starts to decrease ca 8 ma whereas bulk carbonate 13c begins to drop ca 5 6 ma temporal discrepancies between the records can be explained by changes in the upwelling regime at the tf perhaps signifying a link between changes in ocean atmosphere circulation change and widespread primary productivity

bedrock topography modeling and calcrete uranium prospectivity analysis of central erongo region namibia abstract the tertiary palaeodrainage system in the namibian calcrete uranium province is deeply buried under cenozoic sediments such as fluvial alluvial and desert flash flood terrace sediments duricrusts and or aeolian and fluvially reworked aeolian sands and has no obvious surface expression because of which this province remains under explored in spite of hosting the world s largest calcrete uranium deposit at langer heinrich this paper presents first pass bedrock geology and bedrock topography models for the namibian calcrete uranium province generated using air borne magnetic data which are further used to delineate subsurface tertiary palaeochannels in the province conjunctive interpretations of surface geology bedrock geology and bedrock topography indicated that the major underlying controls on the spatial distribution and configuration of the tertiary palaeochannels in the namibian province are both lithological and structural however the basement structural fabric that formed during the damaran orogeny possibly exerts a fundamental control on both palaeodrainage as well as present drainage the analysis also indicated that the carnotite deposition in the langer heinrich palaeochannel is possibly a consequence of westward reorientation of the course of the proto swakop river finally the paper presents a manual prospectivity analysis of the province for calcrete uranium deposits which resulted in the demarcation of five prospective sections of palaeochannels in the tumas basin swakop river and khan river terraces and in the gawib flats west of the langer heinrich deposit

amalgamated accumulations resulting from climatic and eustatic changes akchar erg mauritania the akchar erg of the sahara of western mauritania shows a morphology and stratigraphy that can be recognized as the amalgamation of late pleistocene and holocene deposits that reflect eustatic and climatic events mapping trenching and dating by 14c methods and artefacts show that the prominent complex linear dunes draas of the akchar erg are actually composite features showing at least three constructional and two destructional phases the constructional phases are represented by three convex up layers i a modern veneer moulded into superimposed crescentic dunes which partially mantle the larger linear bedforms ii a middle partly root turbated sand deposited sometime during the last 4000 years and iii a core of linear dune sand formed during the last glacial period 13 000 20 000 yr bp which today shows relict relief intense root turbation and pedogenesis these constructional phases are separated by super bounding surfaces that coincide with erg destructional phases surface 2 bounds the middle aeolian sand and is marked by a lag surface of small granules surface 1 is a very prominent surface with an abundance of neolithic artefacts and represents stabilization of the linear dunes during the humid interglacial period 4000 11 000 yr bp interdraa deposits originated during the interglacial period and consist of continental lacustrine limestones and sandstones humic sands deposited in marshes and sabkhas on the coast the sabkhas originated during interglacial highstand of sea level when interdraa areas were marine embayments and subsequently dried during regression the draa and interdraa sequences therefore in spite of being adjacent facies actually represent different events and were not formed simultaneously the upwind sand sheet margin of the akchar erg shows exposures of the middle and core aeolian sands which were previously protected from deflation by vegetation being progressively cannibilized in the current phase of erg construction and revealing a crystalline basement rock in this proximal area conditions are not favourable for the incorporation of these aeolian accumulations into the stratigraphic record

dust and loessic alluvial deposits in northwestern namibia damaraland kaokoveld sedimentology and palaeoclimatic evidence based on luminescence data abstract in northwestern namibia silty deposits are widespread the silts consist of two major components locally produced weathering detritus and allochthonous dust blown in from the western kalahari to the eastern margin of the namib desert sedimentological analysis and luminescence age estimations allow identification of distinct sedimentological and geomorphological processes associated with two different climatic periods between about 30 and 8 ka loessic alluvial deposits several metres in thickness accumulated in valleys and basins the deposition was generated by weak slope wash and or endoreic drainage of the ephemeral rivers the dust input and the accumulation of silt in the valleys indicate semi arid climatic conditions with weak summer rains and long dry seasons since about 9 8 ka silt terraces caused by fluvial incision and erosion have been formed these indicate more humid conditions than had occurred before the change from a dry to a moister semi arid climate at about 9 8 ka is confirmed by proxy data from drill cores off the namibian coast and by dune studies in the south western kalahari

spatial distribution pattern of living polycystine radiolarian taxa baseline study for paleoenvironmental reconstructions in the southern ocean atlantic sector the horizontal and vertical distribution of living polycystine radiolarian taxa was investigated at seven locations on a n s transect between the antarctic zone 54 s in the southern ocean and the subtropical zone 30 s in the southeastern south atlantic during austral autumn four to five depth intervals from 0 to 1000 m depth were sampled with 55 m mesh opening closing nets in combination with a hydrographic ctd survey a factor analysis for the delineation of specific assemblages and the relation between different taxa and taxa groups resulted in eight factors of which five represent surface and subsurface factors and three deep water factors the spatial distribution pattern of the factors shows that distinct radiolarian assemblages are closely related to water depth distribution of water masses hydrographic boundaries such as frontal systems and nutrients this documents that the distribution of individual radiolarian assemblages is controlled by specific hydrographic conditions and water depth and allows the definition and consolidation of the autecological demands these data are crucial for the paleoenvironmental interpretation of fossil polycystine radiolarian assemblages the comparison of living polycystine radiolarian taxa with those preserved in surface sediments shows general similarities this study includes the first documentation on the distribution of living cycladophora davisiana in circumpolar deep water and supports previous studies stating that c davisiana also lives in deep water masses

late miocene through early pliocene deep water circulation and climate change viewed from the sub antarctic south atlantic benthic foraminiferal stable isotope records for the past 11 myr from a recently drilled site in the sub antarctic south atlantic site 1088 ocean drilling program leg 177 41 s 15 e 2082 m water depth provide for the first time a continuous long term perspective on deep water distribution patterns and southern ocean climate change from the late miocene through the early pliocene i have compiled published late miocene through pliocene stable isotope records to place the new south atlantic record in a global framework carbon isotope gradients between the north atlantic south atlantic and pacific indicate that a nutrient depleted watermass probably of north atlantic origin reached the sub antarctic south atlantic after 6 6 ma by 6 0 ma the relative proportion of the northernprovenance watermass was similar to today and by the early pliocene it had increased to greater than the modern proportion suggesting that thermohaline overturn in the atlantic was relatively strong prior to the early pliocene interval of inferred climatic warmth site 1088 oxygen isotope values display a two step increase between v7 4 ma and 6 9 ma a trend that parallels a published n 18 o record of a site on the atlantic coast of morocco this is perhaps best explained by a gradual cooling of watermasses that were sinking in the southern ocean i speculate that relatively strong thermohaline overturn at rates comparable to the present day interglacial interval during the latest miocene may have provided the initial conditions for early pliocene climatic warmth the impact of an emerging central american seaway on atlantic pacific ocean upper water exchange may have been felt in the north atlantic beginning in the latest miocene between 6 6 and 6 0 ma which would be v1 5 myr earlier than previously thought 9 2002 elsevier science b v all rights reserved

neogene stratigraphy paleoceanography and paleobiogeography in northwest south america and the evolution of the panama seaway abstract the atrato basin in the pacific coastal region of nw south america colombia exhibits stratigraphic and biostratigraphic characteristics similar to other neogene pacific coastal basins of southern central america and northern south america an evaluation of the neogene stratigraphy and foraminiferal biostratigraphy with emphasis on paleoceanographic and paleobiogeographic phenomena indicate the following 1 previous to the middle miocene well aerated deep and open oceanic conditions and free and active water circulation prevailed along the steep continental margins of nw south america 2 during the early middle miocene right after the neogene hiatus nh 2 15 2 ma tectonic disturbances triggered the initial uplift of the panama sill bottom water circulation foraminiferal and sedimentation changes are observed in both the pacific and caribbean coastal areas of nw south america 3 during the middle miocene coinciding with the neogene hiatus nh 3 12 9 11 8 ma an abrupt paleobathymetric change from lower to middle bathyal depths indicates a major uplift of the panama sill to about 1000 m reflecting the middle miocene tectonic disturbances in nw south america 4 during the late middle miocene immediately following this uplift a distinctive pacific benthic foraminiferal fauna abruptly appeared and extended from ecuador to california because these assemblages do not occur in the adjacent caribbean region a circulation barrier between the atlantic and pacific oceans as a result of intensification of the cool marginal california current is proposed anoxic middle to upper bathyal environments prevailed under well aerated cool surface waters in the pacific coastal areas of nw south america 5 during the latest miocene immediately following the neogene hiatus nh 6 7 0 6 3 ma surface water circulation between the caribbean and the pacific was re established and the previous influence of the california current disappeared a rapid filling and shallowing of the basin to mostly neritic depths is indicated along the pacific and caribbean coastal basins of colombia as well as a progressive increase in similarity between the coastal pacific and caribbean benthic biotas this shallowing in the early late miocene also appears to have been associated with the earliest terrestrial interchange of ground sloths to north america and raccoons and their allies to south america 6 immediately following early pliocene time the panamanian isthmus became completely emergent providing a terrestrial environment favourable for intermingling of terrestrial faunas and floras between north and south america i correlate this pliocene uplift with the neogene hiatus nh 8 3 7 3 1 ma four regional unconformities associated with tectonic disturbances in the nw corner of south america have also been recognized and correlated with neogene hiatuses recorded from the oceans early middle miocene middle miocene late miocene and early pliocene

evolution of eastern pacific warm pool and upwelling processes since the middle miocene based on analysis of radiolarian assemblages response to indonesian and central american seaways a quantitative racholarian study at ocean drilling program site 1241 in the eastern tropical pacific enables us to reconstruct paleoceanographic changes that occurred since the latest middle miocene today this site is located just under the eastern pacific warm pool epwp based on the abundance variations of radiolarian characteristic species which are indicators of upwelling and thermocline changes it is suggested that three notable changes occurred at 10 6 9 8 and 4 2 ma in the region four distinct periods of oceanographic conditions bounded by these notable changes were characterized on the basis of the following 1 stratified seawater 12 0 to 10 6 ma 2 a shallowing of the thermocline and an increasing of upwelling 10 6 to 9 8 ma 3 significant inflow of warm water to the eastern tropical pacific caused by an intensified northern equatorial countercurrent necc resulting in the formation of epwp 9 8 to 4 2 ma and 4 the reduction of the epwp and the necc and an increase in upwelling 4 2 to 0 ma the timing of these paleoceanographic events indicated the strong relations with the opening and closing of the indonesian and central american panama seaways the reduction of the epwp this study and the deepening of the thermocline in western pacific at about 4 2 ma cannariato and ravelo 1997 chaisson and ravelo 2000 indicated a change from a state resembling el nino in the late miocene and the early pliocene time to a state resembling la nina by the late pliocene c 2009 elsevier b v all rights reserved

the late miocene onset of high productivity in the benguela current upwelling system as part of a global pattern abstract we have examined the history of the elevated primary productivity associated with the benguela current upwelling system off southwest africa using sediments from 7 5 to 4 8 ma at ocean drilling program site 1085 in the middle cape basin sedimentation rates are low until 6 9 ma low accumulation rates of benthic foraminifers and organic carbon indicate that biological productivity was also low paleoproductivity dramatically increased at 6 7 6 5 ma and was highly variable until 4 8 ma with productivity maxima during cooler periods the presence of radiolarian opal only between 5 8 and 5 2 ma suggests an interlude of silica rich intermediate water in the cape basin the onset of heightened productivity under the benguela current is mirrored by similar increases reported between 6 9 and 6 7 ma in the tropical eastern pacific the western and northern pacific and the indian ocean the similarity between the patterns at site 1085 and in the pacific and indian oceans suggests that the dramatic productivity increase off southwest africa is part of a global response to paleoceanographic changes

investigation of the probable wastewater infiltration and its impact on groundwater quality by electrical resistivity tomography and hydrochemistry a case study of the pleistocene aquifer at el sadat city egypt electrical resistivity tomography ert and hydrochemical investigations were conducted at the wastewater ponds oxidation ponds in el sadat industrial city egypt to study the possibility of wastewater percolation to the pleistocene aquifer and its effect on the groundwater chemistry pleistocene aquifer is the main groundwater reservoir in this area where el sadat city and its vicinities use it as a main source for water supplies required for drinking agricultural and industrial activities in this study seven ert profiles were measured around the wastewater ponds and ten water samples were collected from the ponds and the nearby groundwater wells the water samples were analyzed for major cations ca2 na k and mg2 major anions cl text co 3 2 text hco 3 text so 4 2 nutrients text no 2 1 text no 3 1 and text po 4 3 and heavy elements cd v cr zn ni cu fe mn and pb also the physical parameters ph alkalinity ec and tds of the water samples were measured the ert sections exhibited low resistivity values towards the wastewater ponds and high values in opposite sides besides the water table was detected at shallow depths towards the oxidation ponds these indications could verify the possibility of wastewater infiltration to the groundwater aquifer around the oxidation ponds comparison of the physicochemical parameters of the wastewater samples with those of the groundwater samples indicates that the ionic levels are randomly varying and no specific trend could be obtained also the wastewater samples show some ionic levels lower than those detected in the nearby groundwater samples besides the nitrate level is higher in the water samples collected from the cultivated land than the wastewater samples from this study it could be concluded that the wastewater infiltrated from the oxidation ponds is not the main controller of the groundwater chemistry of the pleistocene aquifer in this area rather the variable ionic concentrations could be attributed to local natural and anthropogenic processes

radiolarian assemblage distributions and paleoceanography of the eastern equatorial pacific ocean during the last 127 000 years abstract mathematical treatment q mode factor analysis of radiolarian species abundance data in equatorial pacific core tops yields four oceanographically meaningful assemblages the distribution of these assemblages in surface sediments can be related to the major surface and subsurface characteristics of the equatorial circulation system western tropical mixed layer flow coastal upwelling and the oxygen minimum layer eastern equatorial divergence upwelling and the equatorial undercurrent and the subtropical water masses of both hemispheres changes in the distribution of radiolarian assemblages during the glacial interglacial cycle spanning the last 127 000 yr are observed in two equatorial pacific piston cores these changes reflect fluctuations in the boundaries of equatorial water masses in response to variations in the position and strength of the tradewind system and tend to precede major glacial interglacial transitions by several thousand years a history of atmospheric circulation in the equatorial pacific has been developed for the last 127 000 yr fluctuations in tradewind intensity north south shifts in the latitudinal position of the tradewind maxima and perhaps a change in the zonality of the tradewinds during the last 127 000 yr have been inferred from downcore quartz abundance and radiolarian assemblage distribution patterns oxygen isotopic fluctuations during the last 127 000 yr indicate that continental ice volume was much greater during glacial stage 2 29 000 b p to 11 000 b p than during glacial stage 4 73 000 b p to 61 000 b p large changes in atmospheric and oceanic circulation patterns also are indicated by the distribution of radiolarian assemblages during the two glacial periods during stage 2 oceanic circulation was more intense the southeast tradewind maximum was closer to the equator and wind stress was more meridional than during stage 4

oxygen minimum zones in the eastern tropical atlantic and pacific oceans within the eastern tropical oceans of the atlantic and pacific basin vast oxygen minimum zones omz exist in the depth range between 100 and 900 m minimum oxygen values are reached at 300 500 m depth which in the eastern pacific become suboxic dissolved oxygen content 4 5 mol kg 1 with dissolved oxygen concentration of less than 1 mol kg 1 the omz of the eastern atlantic is not suboxic and has relatively high oxygen minimum values of about 17 mol kg 1 in the south atlantic and more than 40 mol kg 1 in the north atlantic about 20 40 of the north pacific volume is occupied by an omz when using 45 mol kg 1 or 90 mol kg 1 respectively as an upper bound for omz oxygen concentration for ocean densities lighter than 27 2 kg m 3 the relative volumes reduce to less than half for the south pacific 7 and 13 respectively the abundance of omzs are considerably smaller 1 and 7 for the south atlantic and only 0 and 5 for the north atlantic thermal domes characterized by upward displacements of isotherms located in the northeastern pacific and atlantic and in the southeastern atlantic are co located with the centres of the omzs they seem not to be directly involved in the generation of the omzs omzs are a consequence of a combination of weak ocean ventilation which supplies oxygen and respiration which consumes oxygen oxygen consumption can be approximated by the apparent oxygen utilization aou however aou scaled with an appropriate consumption rate aour gives a time the oxygen age here we derive oxygen ages using climatological aou data and an empirical estimate of aour averaging oxygen ages for main thermocline isopycnals of the atlantic and pacific ocean exhibit an exponential increase with density without an obvious signature of the omzs oxygen supply originates from a surface outcrop area and can also be approximated by the turn over time the ratio of ocean volume to ventilating flux the turn over time corresponds well to the average oxygen ages for the well ventilated waters however in the density ranges of the suboxic omzs the turn over time substantially increases this indicates that reduced ventilation in the outcrop is directly related to the existence of suboxic omzs but they are not obviously related to enhanced consumption indicated by the oxygen ages the turn over time suggests that the lower thermocline of the north atlantic would be suboxic but at present this is compensated by the import of water from the well ventilated south atlantic the turn over time approach itself is independent of details of ocean transport pathways instead the geographical location of the omz is to first order determined by i the patterns of upwelling either through ekman or equatorial divergence ii the regions of general sluggish horizontal transport at the eastern boundaries and iii to a lesser extent to regions with high productivity as indicated through ocean colour data

improved chronostratigraphic reference curve of late neogene seawater 87sr 86sr we present a reference curve of seawater 87 sr 86 sr variation through the past 7 m y based on 455 samples of planktonic foraminifera from ocean drilling program odp site 758 in the indian ocean this single site curve is superior to extant curves because of several features 1 a continuous sedimentary section free of disturbances 2 a well determined and precise chronostratigraphy that is calibrated to numeric age on the basis of continuous magnetostratigraphy and a refined astronomical time scale 3 a high temporal resolution sample interval averages 15 ka 4 an improved analytical reproducibility and 5 a uniformly high sample quality the gross structure of the site 758 curve is generally similar to that observed in previous work but the increased temporal resolution and decreased analytical scatter of the curve provide improved chronostratigraphic resolution at the 95 confidence level uncertainties on age estimates range from 0 60 m y in the lower pleistocene and upper miocene to 2 03 m y in the middle pliocene moreover this detailed record identifies two periods of steplike increases in seawater 87 sr 86 sr 6 10 to 5 80 ma and 1 46 to 1 13 ma that are characterized by rates of change of 150 10 6 m y these features if verified at other sites indicate abrupt changes in the sr mass balance of the ocean and may be used as precise event markers in late neogene marine sections

closing of the central american gateway and its effects on the distribution of late pliocene radiolarians in the eastern tropical pacific abstract analysis of radiolarians in late pliocene sediments at dsdp sites 158 476 and 490 off the pacific coasts of mexico and central america was undertaken to evaluate the paleoceanographic impact on the tropical eastern pacific ocean of the closing of the central american gateway which occurred ca 3 2 ma results of the analysis indicate that 1 the relative abundance of radiolarian tests is extremely low in late pliocene sediments relative to their abundance in older and younger sediments and 2 the relative abundance of nassellarians is also significantly depressed in this interval tectonism associated with the uplift closing and subsequent development of the gateway and the pacific margins of central america and mexico apparently triggered an increased flux of terrigenous sediment into the eastern tropical pacific this increased flux is thought to be responsible for the late pliocene dilution of radiolarian abundance and a coincident change in habitat of nassellarians in this region closing of the central american gateway was also responsible for the reorganization of surface circulation in the tropical eastern pacific the time transgressive appearance of radiolarian species noted in pliocene sediments at dsdp sites along the pacific coasts of central america and mexico likely reflects development of the costa rican coastal current

depositional environment and luminescence chronology of the hoarusib river clay castles sediments northern namib desert namibia abstract the clay castles in the hoarusib river valley are fine grained deposits in a tributary immediately upstream of a narrow gorge sedimentological analysis reveals three lithofacies associations 1 channel facies association near the mouth of the tributary 2 back flow facies association in the middle section and 3 silt and massive sand facies association in the head of the tributary overall sedimentary architecture shows that aggradation of the main hoarusib channel led to back flooding into the tributary and the deposition of massive fine grained deposits decreased discharge competency increased sediment load and the presence of narrow gorges within the main valley are suggested as reasons for the aggradation inter fingering of desert derived sediments channeled down the tributary and backflow sediments from the main river indicate an increase in both upland and desert rainfall during the course of sedimentation optically stimulated luminescence osl dating of these deposits indicates formation between 44 and 20 ka with the possibility that deposition occurred during two phases of rapid sedimentation at ca 42 and 25 ka

late quaternary paleoenvironments in the southwestern kalahari abstract landforms of the southwestern kalahari in southern africa include linear dune systems pans and associated lunette dunes and the valleys of ephemeral rivers present day climates in the region are transitional from arid to semi arid making the area sensitive to climatic change the linear dunes are part of an extensive system of fixed dunes developed in the surface kalahari sands the dunes were in existence prior to 32 000 yr b p but have been reactivated on a number of occasions since then in conditions of increased wind velocities more nw winds and reduced rainfall especially in northern and eastern parts of the region periods of dune activity probably occurred at 19 000 17 000 10 000 6000 and 4000 3000 yr b p the development of pans in the region has been strongly influenced by changes in groundwater conditions deflation from pans and formation of lunettes occurred in periods when groundwater levels were higher and fluctuated seasonally more than they do today such periods probably occurred 25 000 20 000 yr b p around 10 000 yr b p and just prior to 1000 yr b p still higher groundwater discharges promoted by increased regional rainfall resulted in the formation of spring tufas and seasonal or permanent lakes in the pans which were also fed by surface runoff during the intervals between 33 000 28 000 23 000 21 000 17 000 15 000 and 12 000 11 000 yr b p the headwaters of the major ephemeral streams lie outside the region but within the same climatic zone increased flow in the molopo river occurred prior to 19 000 and between 15 000 and 13 000 yr b p

late neogene changes in diatom sedimentation in the north pacific abstract during the late neogene north pacific diatom sedimentation underwent major changes in response to high latitude cooling and changes in surface and deep water circulation at 9 ma diatom mass accumulation rates mars increased in the nw pacific and off northeast japan possibly due to shoaling of the isthmus of panama which lead to an enrichment of nutrients in north pacific deep waters during the latest miocene diatom mars increased progressively off southem california 6 5 ma at high latitudes of the north pacific 6 2 ma and off northeastern japan 5 5 ma presumably in response to high latitude cooling at about 4 5 ma diatom sedimentation abruptly increased in the nw pacific but declined off japan and california coincident with the onset of a prolonged period of high latitude warmth enhanced upwelling of nutrient rich deep waters in the nw pacific probably stimulated diatom production there a major step in high latitude cooling at 2 7 ma caused a reversal of these mid pliocene diatom sedimentation patterns upwelling of deep nutrient rich waters waned at higher latitudes leading to a decline in diatom productivity while wind driven coastal upwelling increased off southern california and stimulated diatom growth

pliocene pleistocene evolution of eastern tropical pacific surface water circulation and thermocline depth we evaluated the response of eastern equatorial pacific upper water column hydrography to the panama seaway closure and the initiation of large scale northern hemisphere glaciation the 18o gradient between globorotalia tumida and globigerinoides sacculifer indicates a general shoaling of the thermocline between 4 2 and 3 0 ma which may be related to a recorded decrease in southeast trade wind strength the 13c gradients suggest that surface water nutrient concentrations have increased during the last 5 myr and that upwelling intermediate waters began to change sources at 3 2 ma changes in the trans pacific east west temperature and nutrient gradients at 4 0 and 1 5 ma are coincident with the seaway closure and a major phase of northern hemisphere glaciation changes in the sensitivity of surface circulation to milankovitch forcing 18o records linearly related to orbital variations only after 3 2 ma are associated with the seaway closure pleistocene initiation of 100 and 41 kyr cycles in the thermocline depth proxy may suggest that the hydrographic response to milankovitch forcing is enhanced by air sea interactions which maintain the relatively steep pleistocene trans pacific gradients the 13c records are dominated by the 41 kyr period and are usually coherent with high latitude climate suggesting that associated changes in the global carbon reservoir dominate the tropical 13c signals

late miocene development of the western pacific warm pool planktonic foraminifer and oxygen isotopic evidence abstract the disappearance at 10 ma of the deep dwelling planktonic foraminifer globoquadrina dehiscens from the western pacific including the south china sea was about 3 myr earlier than its final extinction elsewhere accompanying this event at 10 ma was a series of faunal turnover characterized by increase in mixed layer warm water species and decrease to a minimum in deepwater species paleobiological and isotopic evidence indicates sea surface warming and a deepened local thermocline that we interpret as related to the development of an early western pacific warm pool the stepwise decline of g dehiscens and other deep dwelling species from the nw and sw pacific suggests more intensive warm water pileup than equatorial localities where surface bypass flow through the narrowing indonesia seaway appears to remain efficient during the late miocene planktonic 18o values from the south china sea consistently lighter than the tropical western pacific during the miocene also suggest similar to today more variable hydrologic conditions along the periphery than in the core of the warm pool stronger hydrologic variability affected mainly by monsoons and increased thermal gradient along the western margin of the late miocene warm pool may have contributed to the decline of deep dwelling planktonic species including the early extinction of g dehiscens from the south china sea region the late miocene warm pool became influential and paleobiologically detectable from 10 ma but the modern warm pool did not appear until about 4 ma in the middle pliocene

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

phased evolution of the south north hydrographic gradient in the south china sea since the middle miocene abstract the hydrography of the south china sea scs is characterized by a south north s n thermocline gradient in the upper water column from the northern margin of the western pacific warm pool wpwp to the sea area largely controlled by the east asian monsoon here we examine the records of planktonic foraminifers from ocean drilling program odp sites 1143 and 1146 in the southern and northern parts of the scs respectively that identify three stages of evolution of the s n hydrographic gradient in the scs since the middle miocene first the s n thermocline gradient possibly occurred in the scs for the first time during the period 11 5 10 6 ma reflected by an opposite change in the relative abundance of deep dwelling planktonic foraminiferal species in the south and north next the s n thermocline gradient weakened or even disappeared during the period 10 6 4 0 ma indicated by similar changes in the relative abundance of deep dwelling species in the south and north last the s n thermocline gradient substantially increased from about 4 0 3 2 ma marked by a major increase in abundance of deep dwelling species in the northern scs and a decrease in the southern scs based on the results of oceanic and coupled ocean atmosphere model experiments and previous studies on planktonic foraminifers in the pacific ocean it is inferred that the initial wpwp was probably formed during the period of 11 5 10 6 ma in response to the closure of the indonesian seaway the wpwp then weakened or became extremely unstable before developing its present expression about 4 0 3 2 ma induced by the emergence of the isthmus of panama our observations and model experiments support the argument that the stages in evolution of the wpwp are linked to tectonic changes in ocean gateways

can 234u 230th dating be used to date large semi arid tufas challenges from a study in the naukluft mountains namibia the large extensive tufa deposits of the semi arid naukluft mountains namibia are potentially important palaeoenvironmental indicators in an area with few proxy records tufas are reliable indicators of increased moisture availability and have been shown to be amenable to 234u 230th dating although two challenges are detrital contamination and open system behaviour densely cemented tufa facies are good candidates for dating minimising these problems we report attempts to date five densely cemented units which are only found rarely within the naukluft deposits we applied a detailed methodology using multiple subsample analysis measurement of insoluble residues application of isochron mixing lines and attempted open systems modelling alongside observations of micromorphology and cathodoluminescence in order to assess the validity of any obtained dates surprisingly densely cemented tufas were found not always to be suitable for dating two units contained detrital contamination which could not be corrected for using a single leachate correction or isochron methods two units contained excess 230th this could result under a closed system if initial 234u 238u was sufficiently high alternatively this may be the result of open system behaviour and loss of uranium or incorporation of initial unsupported 230th which render samples unsuitable for 234u 230th dating micromorphological appearance and cathodoluminescence behaviour are used to explore these possibilities this study exemplifies the need for careful sample selection and highlights the importance of analysing multiple subsamples from any tufa sample the detailed methodology applied proves to be a powerful tool for identifying the range of problems that can be encountered when selecting suitable candidate samples for successful dating it also shows that semi arid tufa sequences may contain very little material suitable for dating a reliable age of c 80 ka was obtained for a banded unit within a large fluvial barrage with less reliable dates suggesting tufa deposition during times since 350 ka through to the late holocene copyright 2010 john wiley sons ltd

groundwater residence time and paleorecharge conditions in the deep confined aquifers of the coastal watershed south east tanzania summary analysis of radiocarbon and stable isotopes of c h and o in groundwater samples from the coastal watershed of southeastern tanzania is used to decipher the age of groundwater and the climatic conditions at the time of recharge radiocarbon dating was corrected on the basis of stable carbon isotope ratios and hydrochemical data two main recharge periods are recognised 1 a recent recharge period with residence times dated to younger than 1 3 ka bp for the shallow aquifer and some deep groundwater aquifers and 2 a paleorecharge occurring during the late pleistocene to holocene transition between 6 and 13 ka bp for the lower confined aquifer the climatic excursions arid and humid phases identified in this study match well with those identified from studies of paleo climatic records in tanzania and east africa the oxygen 18 and radiocarbon results show that there is a potential mixing of groundwater of varying ages along flow paths in the coastal aquifer system this has also been shown by redox parameters in some of the deeper aquifer samples the data also revealed that there is relatively simple pattern of lateral groundwater flow in the deeper confined aquifer isotopic evidence from this study and related studies in paleolimnology improves our knowledge of the past climates the data suggests that the paleorecharge to the confined aquifer was from inland under wetter climatic conditions than present therefore paleorecharge from the adjacent mountain regions to coastal area like ifakara morogoro and udzungwa mountains could be important parts of recharge area of the kimbiji aquifer however the hinterland mountains of dar es salaam mainly pugu mkuranga ruvu and kisarawe could also play an important role in replenishing the kimbiji coastal aquifers in contrast recharge in the shallow unconfined aquifer is characterized by local rainfall and river water infiltration this study is the first of its kind assessing fingerprinting of paleoclimates in southeastern coastal tanzania using the isotopic composition of deep groundwater aquifers the freshwater resources in the neogene aquifer contain paleo groundwater aquifers that have been mixed with recently recharged flow systems hence the resources may be sustainable this study also provides information that may be useful for designing optimum strategies for a sustainable management of groundwater resources in the coastal aquifer and it testifies to the importance of groundwater as a paleoclimatic archive such information is also essential for consideration by modellers in their attempts to simulate the hydrogeological processes in confined aquifers over very long time spans

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

sea surface temperature changes in the north pacific during the late miocene abstract twenty seven radiolarian species and species groups are identified and used in this study their stratigraphic ranges span from late miocene to modern times and they appear to have distributions which have changed little over this interval of time census data for these 27 taxa in surface sediment samples are used to define six assemblages q mode factors the distribution of these assemblages are similar to modern water mass distributions and can be statistically related to modern sea surface temperatures census data on these same 27 taxa have been collected in the upper miocene intervals of six pacific sites these faunal data are described in terms of the modern radiolarian assemblages and based on the multiple regression equations relating the modern assemblages to modern temperatures estimates of late miocene temperatures are made the estimated temperatures indicate that there is an overall cooling trend in the later part of the miocene magnetic chron 8 to chron 5 punctuated by several distinct cooling events in august the low latitude sites appear to be slightly cooler and the mid latitude sites warmer than in modern times in february both the low and mid latitude sites appear to be warmer than modern temperatures at the same locations the relatively warm temperatures near the base of the interval studied may be associated with the passage of warm tropical atlantic waters into the eastern pacific via the isthmus of panama the general cooling trend seen in these records is thought to be associated with the gradual closure of these straits and the increased influence of the high southern latitudes

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

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

isotopic fingerprints of paleoclimates during the last 30 000 years in deep confined groundwaters of southern india isotopic and geochemical evidence of paleoclimates especially for the last glaciation has been obtained from deep confined groundwaters of southern india the 13c 18o chloride and deuterium analyses of groundwaters show distinct excursions inferred to be related to climatic variations the arid climatic episode associated with the last glaciation 18 000 2000 yr b p is conspicuously identified by signatures of relatively enriched 13c 10 to 12 pdb and 18o 5 3 to 4 8 smow values and high chloride concentration 80 to 160 mg l the transition from an arid to humid period ca 12 000 8000 yr b p is shown by a decreasing trend in the 13c 9 5 to 17 and 18o 4 5 to 6 3 contents of groundwaters the late holocene since 4000 yr b p marked by a more humid but unstable climate is identified by further depletion of 13c 13 to 20 and 18o 5 2 to 6 3 similar variation between 18o and chloride values in confined groundwaters further demonstrates two distinct climatic excursions arid and humid governed by the amount effect this is the first time that isotopic and geochemical signatures related to changing paleoclimates have been identified in the confined groundwaters of the southern indian landmass

isotopic paleoceanography of the caribbean and east pacific role of panama uplift in late neogene time comparisons of carbon isotopic data on benthic foraminifera from deep sea drilling project sites 502 western caribbean and 503 eastern pacific indicate that the difference between the atlantic and the pacific in the per mil enrichment in carbon 13 of total dissolved carbon dioxide increased about 6 million years ago and again 3 million years ago when the difference reached the modern level 1 per mil comparisons of planktonic foraminiferal oxygen isotopic data for the caribbean and the pacific suggest that the salinity of caribbean surface waters began increasing 4 million years ago possibly in response to shoaling of the panama isthmus these results suggest that modern circulation patterns in the caribbean and eastern pacific developed by 3 million years ago in concert with changing tectonic climatic and biogeographic patterns

pacific ocean intermediate and deep water circulation during the pliocene abstract new benthic foraminiferal stable isotopic records of northeast pacific intermediate water odp site 1014 1177 m and mid depth water odp site 1018 2476 m were compared to isotopic records of deep water in the tropical pacific odp site 849 3851 m for the reconstruction of vertical profiles of nutrient and physical properties from the early pliocene to the early pleistocene approx 5 1 4 ma our data indicate that for the entire interval there was enhanced north pacific intermediate water ventilation relative to today and a similar to modern circulation pattern with northward flowing pacific bottom water pbw beneath its southward flowing return flow however the core of maximally aged return flow resided as deep as 2500 m as compared to 1500 m today probably due to the strengthened intermediate water flow less apparent aging of deep water along its path before 2 7 ma indicates that thermohaline overturning may have been more rapid in the warm period of the early pliocene in addition prior to 2 7 ma foraminiferal oxygen isotopic values at mid depth sites are higher than at deep sites a reversed vertical gradient relative to today in both the atlantic and pacific oceans we suggest that nadw was warmer and more saline than today and that it influenced mid depth waters throughout the atlantic and pacific oceans enhanced pliocene formation of warmer saltier intermediate water in the north pacific and deep water in the north atlantic may have been a result of higher than modern high mid latitude sea surface temperatures evaporation and salinity

palaeoenvironments of deposition of neogene laminated diatom mat deposits from the eastern equatorial pacific from studies of benthic foraminifera sites 844 849 851 abstract massive sedimentation of mats of the diatom thalassiothrix longissima forming laminated diatom mat deposits ldm occurred intermittently in the equatorial pacific throughout the neogene from at least 15 to 4 8 m y ago the background deposition was otherwise calcareous nannofossil diatom ooze no benthic foraminifera have been used to reconstruct the benthic environment of deposition and the role of both surface waters as a source of food and bottom waters including their corrosivity during ldm deposition three ldm events were studied site 844 11 4 ma early tortonian site 849 4 8 ma early zanclian and 6 6 6 8 ma early messinian a control section of no spanning the 4 8 ma event was studied from site 851 in addition the carbonate high no immediately preceding the 4 8 ma event was examined in site 849 from the foraminiferal evidence it can be shown that the ldm was deposited under conditions of normal oxygenation because infaunal taxa are present throughout and there was an increase in epifaunal taxa such as cibicides spp however there was a reduction in test size probably because only those smaller than 250 m were able to move through the restricted sediment pore spaces caused by the diatom mat meshwork the rate of accumulation of tests is highest in ldm interval and may reflect reduced predation from macrofaunal endobenthos among the epifaunal taxa nuttallides spp show a decrease in abundance in the ldm and even immediately preceding it this may be linked to a greater influence of corrosive aabw species known to feed opportunistically on phytodetritus epistominella exigua and alabaminella weddellensis are common in both no and ldm the increased relative abundance of a weddellensis in the ldm may be due to this species being smaller than e exigua and better able to exploit the food resource within the restrictive mat meshwork the foraminiferal results corroborate the previous interpretation that preservation of lamination in the ldm is due to the physical exclusion of macro endobenthos rather than oxygen depletion of the bottom waters

radiolarian assemblages in surface sediments along longitude 175 e in the pacific ocean abstract this study of 24 surface sediment samples along longitude 175 e between latitudes 48 n and 14 s in the western pacific ocean documents the relationship between the modern distribution of radiolarian assemblages and present oceanographic conditions ninety two species were classified into eight clusters by using r mode cluster analysis q mode cluster analysis was used to distinguish five geographical zones which were interpreted to represent the tropical southern subtropical northern subtropical transitional and subarctic climatic zones species richness diversity and equitability are highest in the tropical and transitional zones we attempted to formulate new regression equations considering ecological information on the vertical distribution of radiolarian species transfer functions were derived from regression analysis of radiolarian assemblages to observed sea surface temperature sst values the transfer functions have a standard error of 1 16 c in the estimate of winter sst 0 81 c in that of summer sst

unlocking the ice house oligocene miocene oxygen isotopes eustasy and margin erosion oxygen isotope records and glaciomarine sediments indicate at least an intermittent presence of large continental ice sheets on antarctica since the earliest oligocene circa 35 ma the growth and decay of ice sheets during the oligocene to modern ice house world caused glacioeustatic sea level changes the early eocene was an ice free greenhouse world but it is not clear if ice sheets existed during the middle to late eocene doubt house world benthic foraminiferal 18o records place limits on the history of glaciation suggesting the presence of ice sheets at least intermittently since the earliest oligocene the best indicator of ice growth is a coeval increase in global benthic and western equatorial planktonic 18o records although planktonic isotope records from the western equatorial regions are limited subtropical planktonic foraminifera may also record such ice volume changes it is difficult to apply these established principles to the cenozoic 18o record because of the lack of adequate data and problems in stratigraphic correlations that obscure isotope events we improved oligocene to miocene correlations of 18o records and erected eight oxygen isotope zones oi1 oi2 mi1 mi6 benthic foraminiferal 18o increases which are associated with the bases of zones oil circa 35 8 ma oi2 circa 32 5 ma and mil circa 23 5 ma can be linked with 18o increases in subtropical planktonic foraminifera and with intervals of glacial sedimentation on or near antarctica our new correlations of middle miocene benthic and western equatorial planktonic 18o records show remarkable agreement in timing and amplitude we interpret benthic planktonic covariance to reflect substantial ice volume increases near the bases of zones mi2 circa 16 1 ma mi3 circa 13 6 ma and possibly mi5 circa 11 3 ma possible glacioeustatic lowerings are associated with the 18o increases which culminated with the bases of zone mi4 circa 12 6 ma and mi6 circa 9 6 ma although low latitude planktonic 18o records are required to test this these inferred glacioeustatic lowerings can be linked to seismic and rock disconformities for example we link 12 oligocene early late miocene inferred glacioeustatic lowerings with 12 of the sequence boundaries inferred eustatic lowerings of haq et al 1987

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

late quaternary palaeohydrological changes in the northern namib sand sea new chronologies using osl dating of interdigitated aeolian and water lain interdune deposits abstract ephemeral rivers today terminate in playas adjacent to the eastern part of the namib sand sea although there is evidence that these fluvial systems penetrated further west in the past there are water lain sediments exposed in interdune areas of the northern namib sand sea associated with the former course of the tsondab river which are central to palaeoenvironmental reconstruction in the region these deposits indicate that there was increased moisture availability in this presently hyper arid region and or the catchments of the rivers to the east of the sand sea in the great escarpment until now the chronology for the deposition of these sediments has been based on radiocarbon 14 c dates on carbonate materials the validity of these ages is problematic and 14 c limits the timescale to which these sediments can be dated the water lain units are bracketed by aeolian sand units which are highly suitable for optically stimulated luminescence osl dating here 24 osl ages from three sites associated with the former course of the tsondab river are presented allowing a reassessment of the timing of sediment deposits and the west to east progression of the terminus of the tsondab river during the late quaternary narabeb which is 60 km north west of the current end point of the tsondab river at tsondab vlei contains eight mud units six of which are shown to be of marine oxygen isotope stage mis 5 128 75 ka age water lain units at hartmut pan 55 km north west of tsondab vlei are dated by osl to between 16 9 0 9 ka and 12 7 0 1 ka at ancient tracks closest to the tsondab vlei 6 km west mud units are younger and placed between 12 0 0 7 and 12 8 0 8 ka 11 5 0 5 10 5 0 5 ka and after 10 5 0 5 ka this new chronology is compared with other key regional records to identify potential regional relationships and forcing factors there is greatest correspondence with the records in the north of the summer rainfall zone a revision of both old and young 14 c ages on carbonate materials calls for revision of other 14 c chronologies in the region and highlights the problems with the continued used of histograms of 14 c ages to indicate periods of increased humidity in southern africa

archaeology palaeoenvironment and chronology of the tsodilo hills white paintings rock shelter northwest kalahari desert botswana abstract excavations conducted at the white paintings rock shelter wps have uncovered 7 m of deposits ranging in age from the historic period to at least 100 000 years at the base eleven stratigraphic units are described in relation to palaeoenvironmental conditions inferred from sediments and other data these units contain seven major divisions in the cultural sequence highlighted by a lengthy record of later and middle stone age deposits a wide variety of mammals as well as other animals were found in the upper 3 m numerous fish bones wetland mammals and barbed bone points make this site especially interesting because of its desert location the highest frequencies of fish bones are found between c 80 90 130 cm upper fish deposits and between 210 280 cm below the surface lower fish deposits most of the barbed bone points were recovered in the upper fish deposits the lower fish deposits contain extinct equus capensis and a microlithic industry as well as some bone points a large blade industry is found beneath the lower fish deposits this blade industry shows continuity with the underlying middle stone age

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

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

possible gyre gyre interaction in the pacific ocean the subtropical anticyclonic gyres of the north and south pacific ocean are among the largest circulation systems found in the ocean the gyres contain the warm waters of the warm water sphere1 2 and the rotation of these water masses is subject to low frequency variations3 the presence in these gyres of baroclinic rossby waves with periods of many years has been documented4 5 and this seems to be sufficient proof that the gyres undergo vacillations using sea level data we demonstrate here that the subtropical gyres of the north and south pacific oscillate with a period of about 4 years and that these oscillations are probably induced by the major el nino events

role of panama uplift on oceanic freshwater balance comparison between planktic foraminiferal oxygen isotope records from the caribbean sea ocean drilling program odp site 999 and the equatorial east pacific odp site 851 suggests an increase in caribbean surface water salinity between 4 7 and 4 2 ma the modern atlantic pacific salinity contrast of about 1 became fully established at 4 2 ma as reflected by a 0 5 planktic foraminifera 18o enrichment in the caribbean sea this is interpreted as the result of restricted surface water exchange between the tropical atlantic and pacific in response to the shoaling of the central american seaway as a consequence the atlantic and pacific surface ocean circulation regime changed as did the freshwater balance between the major ocean basins simultaneous shifts in benthic carbon isotope records in the caribbean sea suggest an intensification in north atlantic thermohaline circulation these results indicate that the panamanian isthmus formation caused several new ocean atmosphere feedback mechanisms that have affected climate since the early pliocene

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

characteristics and osl minimum ages of relict fluvial deposits near sossus vlei tsauchab river namibia and a regional climate record for the last 30 ka the present end point of the tsauchab river is at sossus vlei 30 km into the namib sand sea interdune deposits in three depressions west and southwest of the vlei include channel and interdune lithofacies associations but no deposits typical of river end points or of groundwater seepage into interdune areas the two lithofacies associations show that the tsauchab river extended further into the sand sea in the past it had a well developed channel and a higher flow than today that caused flooding of adjacent interdune areas osl 4 mm aliquot minimum ages indicate that the tsauchab river reached 2 3 km beyond its present end point at ca 25 ka and ca 9 7 ka and that the river was more active from 0 9 0 3 ka the eastward migration of the river end point since ca 7 ka suggests a reduction in flood magnitude accompanied by the gradual invasion of the sossus vlei area by dunes the regional data indicate an additional wet interval at ca 15 ka that is so far not recorded in the sossus area copyright 2006 john wiley sons ltd

southwest african climate independent of atlantic sea surface temperatures during the younger dryas abstract to investigate land sea interactions during deglaciation we compared proxies for continental pollen percentages and accumulation rates and marine conditions dinoflagellate cyst percentages and alkenone derived sea surface temperatures the proxies were from published data from an ams radiocarbon dated sedimentary record of core geob 1023 5 encompassing the past 21 000 years the site is located at ca 2000 m water depth just north of the walvis ridge and in the vicinity of the cunene river mouth we infer that the parallelism between increasing sea surface temperatures and a southward shift of the savanna occurred only during the earliest part of the deglaciation after the antarctic cold reversal southeast atlantic sea surface temperatures no longer influenced the vegetation development in the kalahari stronger trade winds during the antarctic cold reversal and the younger dryas period probably caused increased upwelling off the coast of angola a southward shift of the atlantic anti cyclone could have resulted in both stronger trade winds and reduced impact of the westerlies on the climate of southwestern africa