stable carbon and oxygen isotopic evidence for late pleistocene to middle holocene climatic fluctuations in the interior of southern africa stable carbon and oxygen isotope analyses of ungulate grazers from four archaeolog ical sites located in different environs within the caledon river valley have provided a relatively well dated proxy palaeoenvironmental and palaeoclimatic sequence for the period between 16 000 and 6000 calendar cal yr bp within the overall trend towards hot mid holocene temperatures and a summer rainfall pattern stable carbon isotope results show that there were three periods when growth season temperatures were cool enough for c3 grasses to be present 16 000 14 000 10 200 9600 and 8400 8000 cal yr bp similar trends were recorded in stable oxygen isotope values reflecting shifts in either temperature or available moisture although having a similar pattern to that of the lower altitude site sites situated in foothills and montane portions of the valley consistently maintained lower temperatures until the mid holocene altithermal at this time growth season temperatures warmed sufficiently for a 100 c4 grassland to expand in altitude from the warmer low lying localities in relation to present understanding of synoptic and global climatic patterning these findings suggest that the early to middle holocene transition was not a gradual warming trend but rather it was marked by a series of climatic fluctuations of particular note is the possible global rather than regional occurrence of the 8200 cal yr bp event copyright 2002 john wiley sons ltd journal of quaternary science

vegetation history and climate in the savanna biome south africa since 190 000 ka a comparison of pollen data from the tswaing crater the pretoria saltpan and wonderkrater abstract the long term quaternary vegetation and climate history in south africa s savanna biome since ca 190 000 yr bp is elucidated by the palynological record of lake sediments in the tswaing crater the pretoria saltpan and spring deposits from wonderkrater despite hiatuses in both sequences palaeotemperature trends from the two places are presented by means of principal components analysis pca of pollen data allowing a new regional palaeoenvironmental reconstruction correlation in the older sections of the sequences beyond radiocarbon dating is tentative suggesting that the wonderkrater sequence is roughly 4 times younger than the tswaing sequence combined temperature indications from the two sites suggest cooling at ca 14 000 yr bp and slight warming at ca 15 000 yr bp although no data are available between ca 15 000 and 21 000 and 25 000 and 31 000 yr bp generally cool conditions are suggested between ca 11 000 and 33 000 yr bp between ca 33 000 and an extrapolated age of ca 80 000 yr bp at least three warmer pulses of varying intensity occurred no data are available between ca 80 000 and 150 000 yr but between 150 000 and ca 190 000 yr bp two cool and two moderately warm phases are suggested

persistent millennial scale climatic variability over the past 25 000 years in southern africa abstract data from stalagmites in the makapansgat valley south africa document regional climatic change in southern africa in the late pleistocene and holocene a new tims u series dated stalagmite indicates speleothem growth from 24 4 to 12 7 ka and from 10 2 to 0 ka interrupted by a 2 5 ka hiatus high resolution oxygen and carbon stable isotope data suggest that postglacial warming was first initiated 17 ka was interrupted by cooling probably associated with the antarctic cold reversal and was followed by strong warming after 13 5 ka the early holocene experienced warm evaporative conditions with fewer c4 grasses cooling is evident from 6 to 2 5 ka followed by warming between 1 5 and 2 5 ka and briefly at ad 1200 maximum holocene cooling occurred at ad 1700 the new stalagmite largely confirms results from shorter holocene stalagmites reported earlier the strongest variability superimposed on more general trends has a quasi periodicity between 2 5 and 4 0 ka also present are weaker 1 0 ka and 100 year oscillations the latter probably solar induced given similarities to the antarctic records the proximate driving force producing millennial and centennial scale changes in the makapansgat record is postulated to be atmospheric circulation changes associated with change in the southern hemisphere circumpolar westerly wind vortex

late quaternary paleotemperatures derived from a speleothem from cango caves cape province south africa abstract an oxygen isotope temperature record over a large part of the past 30 000 yr has been obtained for the southern cape province of south africa by combining data on the isotopic composition of a stalagmite from a deep cave with that of a confined groundwater aquifer in the same region results show that temperatures during the last glacial maximum were on average about 6 c lower than those today with peaks up to 7 c lower a detailed analysis of the past 5000 yr suggests multiple fluctuations with generally lower temperatures 1 2 c around 4500 and 3000 yr b p the carbon isotopic composition of the stalagmite indicates significant vegetation changes between the late pleistocene and today and also during the second half of the holocene

late glacial stage and holocene tropical ice core records from huascaran peru two ice cores from the col of huascaran in the north central andes of peru contain a paleoclimatic history extending well into the wisconsinan wurm glacial stage and include evidence of the younger dryas cool phase glacial stage conditions at high elevations in the tropics appear to have been as much as 8 to 12 c cooler than today the atmosphere contained about 200 times as much dust and the amazon basin forest cover may have been much less extensive differences in both the oxygen isotope ratio 18o 8 per mil and the deuterium excess 4 5 per mil from the late glacial stage to the holocene are comparable with polar ice core records these data imply that the tropical atlantic was possibly 5 to 6 c cooler during the late glacial stage that the climate was warmest from 8400 to 5200 years before present and that it cooled gradually culminating with the little ice age 200 to 500 years before present a strong warming has dominated the last two centuries

the validity of the asteraceae poaceae fossil pollen ratio in discrimination of the southern african summer and winter rainfall zones abstract situated at the transition between the mid latitudes and the sub tropics southern africa has a climatic dichotomy between winter and summer rainfall zones wrz and srz the latitudinal extent of the winter rainfall zone during the late pleistocene remains contentiously debated within the regional palaeoscience literature one method posited to reflect the seasonality of rainfall at a given location throughout late pleistocene records for south africa is the ratio of fossil pollen asteraceae to poaceae although adopted for a range of southern african locations the veracity of this method has not been tested this study explores the extent to which this ratio can discriminate between the srz and wrz and the extent of the region subject to fluctuation during the late quaternary the ratio is found to successfully discriminate regions which are and would have remained during the past 20 000 cal yr bp in the srz and wrz exclusively on the basis of these statistics it appears that wrz conditions can be inferred from ratio scores 0 6 and srz conditions from scores

palynological evidence for late quaternary warming episodes in southern africa abstract polleniferous sediments from southern africa associated with past warm episodes before the last glacial maximum are rare interpretation of environmental conditions during these phases are complicated by difficulties of dating in sediments from the northern areas north of 28 s the prominence of tropical woodland pollen suggests that the period between 7000 and 6500 yr b p was associated with optimal temperatures during the holocene identifying the warmest phase in the southern high lying grasslands and semi arid karoo shrublands south of 28 s is difficult because unlike the savanna areas of the north they do not yield good pollen indicators for changes in temperature such as those of frost sensitive trees however some pollen sequences from further south along the southern coast of africa 34 s and at marion island in the southern ocean 47 s suggest that the holocene temperature optimum occurred at about the same time in the sub antarctic area in the south and the subtropical regions in the north the advent of moister conditions in southern africa during the early to middle holocene is generally recorded earlier ca 7500 6500 yr b p in the north at ca 26 s than around 31 s ca 5000 yr b p this change is provisionally associated with a relative shift in seasonality from a predominance of all season precipitation to a greater proportion of summer rainfall which apparently reached the southern karoo areas two thousand years later than the northern bushveld region

hydroclimate in africa during the medieval climate anomaly the medieval climate anomaly mca is a recognized period of distinct pre industrial climate change with a core period of 1000 1200 ce the field of palaeoclimatology has made major progress over the past 15 years during which a great number of high and medium resolution case studies were published reconstructing climate change of the past millennia in many parts of the world regional data coverage has now reached a point which allows compiling palaeoclimate maps for well defined time intervals here we present hydroclimatic trend maps for the mca in africa based on 99 published study locations key hydroclimatic proxy curves are visualized and compared in a series of 16 correlation panels proxy types are described and possible issues discussed based on the combined mca dataset temporal and spatial trends are interpreted and mapped out three areas have been identified in africa in which rainfall seems to have increased during the mca namely tunisia western sahel and the majority of southern africa at the same time a reduction in precipitation occurred in the rest of africa comprising of nw and ne africa west africa eastern africa and the winter rainfall zone of south africa mca hydroclimate change in africa appears to have been associated with characteristic phases of ocean cycles as also supported by modern climate observations aridity in morocco typically coincides with the positive phase of the north atlantic oscillation nao whilst increased rainfall in the western sahel is often coupled to the positive phase of the atlantic multidecadal oscillation amo reduction in rainfall in the region gulf of aden southern red sea to eastern africa could be linked to a negative indian ocean dipole iod or a derived long term equivalent indian ocean cycle parameter the intertropical convergence zone itcz appears to have been shifted pole wards during the mca for both the january and july positions mca hydroclimate mapping revealed major data gaps in the sahara south sudan somalia central african republic democratic republic of congo angola northern mozambique zambia and zimbabwe special efforts are needed to fill these gaps e g through a dedicated structured research program in which new multiproxy datasets are created based on the learnings from previous african mca studies

hindered settling of sand grains the sedimentation rate of sand grains in the hindered settling regime has been considered to assess particle shape effects the behaviour of various particulate systems involving sand has been compared with the widely used richardson zaki expression the general form of the expression is found to hold in as much as remaining as a suitable means to describe the hindered settling of irregular particles the sedimentation exponent n in the richardson zaki expression is found to be significantly larger for natural sand grains than for regular particles the hindered settling effect is therefore greater leading to lower concentration gradients than expected the effect becomes more pronounced with increasing particle irregularity at concentrations around 0 4 the hindered settling velocity of fine and medium natural sands reduces to about 70 of the value predicted using existing empirical expressions for n using appropriate expressions for the fluidization velocity and the clear water settling velocity a simple method is discussed to evaluate the sedimentation exponent and to determine the hindered settling effect for sands of various shapes

holocene paleo climatic record from the south african namaqualand mudbelt a source to sink approach variations in the sediment input to the namaqualand mudbelt during the holocene are assessed using an integrative terrestrial to marine source to sink approach geochemical and sr and nd isotopic

persistent solar influence on north atlantic climate during the holocene surface winds and surface ocean hydrography in the subpolar north atlantic appear to have been influenced by variations in solar output through the entire holocene the evidence comes from a close correlation between inferred changes in production rates of the cosmogenic nuclides carbon 14 and beryllium 10 and centennial to millennial time scale changes in proxies of drift ice measured in deep sea sediment cores a solar forcing mechanism therefore may underlie at least the holocene segment of the north atlantic9s 1500 year cycle the surface hydrographic changes may have affected production of north atlantic deep water potentially providing an additional mechanism for amplifying the solar signals and transmitting them globally

vertical fluxes of sediment in oscillatory sheet flow time series of vertical sediment fluxes are derived from concentration time series in sheet flow under waves while the concentrations c z t vary very little with time for z 10d50 the measured vertical sediment fluxes qzs z t vary strongly with time in this vertical band and their time variation follows to some extent the variation of the grain roughness shields parameter 2 5 t thus sediment distribution models based on the pickup function boundary condition are in some qualitative agreement with the measurements however the pickup function models are only able to model the upward bursts of sediment during the accelerating phases of the flow they are so far unable to model the following strong downward sediment fluxes which are observed during the periods of flow deceleration classical pickup functions which essentially depend on the shields parameter are also incapable of modelling the secondary entrainment fluxes which sometimes occur at free stream velocity reversal the measured vertical fluxes indicate that the effective sediment settling velocity in the high 0 3 c z t 0 4 concentration area is typically only a few percent of the clear water settling velocity while the measurements of richardson and jeronimo chem eng sci 34 1979 1419 from a different physical setting lead to estimates of the order 20 the data does not support gradient diffusion as a model for sediment entrainment from the bed that is detailed modelling of the observed near bed fluxes would require diffusivities that go negative during periods of flow deceleration an observed general trend for concentration variability to increase with elevation close to the bed is also irreconcilable with diffusion models driven by a bottom boundary condition

pollen taphonomy of cave sediments what does the pollen record in caves tell us about external environments and how do we assess its reliability cave sediments may contain important long term records of past environments and human activity pollen provides key evidence since it disperses widely and is relatively durable we still know relatively little about the dispersal of pollen into caves and its preservation within cave sediments compared with our relatively detailed knowledge of pollen taphonomy in other sedimentary environments pollen taphonomy in caves is dependent on a variety of transport pathways and seems to be very contingent on local circumstance the airfall component of cave pollen assemblages often seems comparable with airfall spectra in the landscape outside the cave but bees birds and bats may transport considerable quantities of pollen into caves and the entrance flora may also be significant cave sediments are rarely waterlogged and pollen within them can be subject tomicrobial and chemical degradation sedimentation in caves is often episodic with episodes of storage and deposition sometimes redeposition of sediment which means that biostratinomic preservational factors become very significant comparison with sequences outside caves is difficult because few caves are found in landscapes where there are comparable pollen records from lakes and bogs here we review the factors affecting cave pollen taphonomy and hence the reliability of palynological analysis of sediments from caves with suggestions for future investigation

teleconnections between the subtropical monsoons and high latitude climates during the last deglaciation the major deglacial intensification of the southwest monsoon occurred at 11 450 150 calendar years before present synchronous with a major climate transition as recorded in greenland ice an earlier event of monsoon intensification at 16 000 150 calendar years before present occurred at the end of heinrich layer 1 in the atlantic and parallels the initial rise in global atmospheric methane concentrations and the first abrupt climate changes in the antarctic thus the evolution of the monsoonal and high latitude climates show teleconnections but hemispheric asymmetries superimposed on abrupt events the monsoonal climate shows high frequency variability of 1785 1450 and 1150 year oscillations and abrupt climate change seems to occur when at least two of these oscillations are in phase

interhemispheric synchrony of the last deglaciation inferred from alkenone palaeothermometry the relative timings of the last deglacial warming in the southern and northern hemispheres are not well constrained but are a crucial component in understanding the mechanisms of deglaciation1 a clearer picture of the degree of interhemispheric synchrony has been obscured by a dearth of high resolution temperature records that can be tied to the absolute calendar timescale moreover the quantification of tropical temperatures during the last glacial cycle is controversial2 8 here we apply the alkenone method of sea surface temperature reconstruction9 10 to several high resolution sediment cores recovered from the tropical indian ocean between 20 n and 20 s the inferred initial sea surface temperature warming 15 000 calendar years ago at 20 s is in phase with northern hemisphere sea this study and air11 temperature changes but lags antarctic warming12 14 by several millennia this finding along with the results of recent modelling studies15 16 provides strong support for the idea that changes in the ocean s global thermohaline circulation were not the only cause of interhemispheric climate teleconnection during the last deglaciation

permian phytogeographic patterns and climate data model comparisons abstract the most recent global icehouse hothouse climate transition in earth history began during the permian warmer polar conditions relative to today then persisted through the mesozoic and into the cenozoic we focus here on two permian stages the sakmarian 285 280 ma and the wordian 267 264 ma also known as the kazanian integrating floral with lithological data to determine their climates globally these stages postdate the permo carboniferous glaciation but retain a moderately steep equator to pole gradient judging by the level of floral and faunal differentiation floral data provide a particularly useful means of interpreting terrestrial paleoclimates often revealing information about climate gradations between dry and wet end member lithological indicators such as evaporites and coals we applied multivariate statistical analyses to the permian floral data to calibrate the nature of floral and geographical transitions as an aid to climate interpretation we then classified sakma

the first record of the permian glossopteris flora from sri lanka implications for hydrocarbon source rocks in the mannar basin strata exposed near tabbowa tank tabbowa basin western sri lanka have yielded thefirst representatives of the distinctive permian glossopteris flora from that country the assemblageincludes gymn

onset of neoglaciation in the southern hemisphere a review of evidence documenting an early neoglacial mid holocene advance of mountain glaciers in south america and new zealand reveals that of 16 glaciers for which radiocarbon age control exists only two have associated dates that may approximate the culmination of an advance advances of the remaining glaciers are controlled only by minimum or maximum limiting ages additional dates for several new zealand glaciers are based on weathering rind measurements having large potential errors twelve of the glaciers now terminate or have terminated in lakes or in tidewater and are subject to instabilities associated with calving termini an extensive debris cover in the ablation zone of nearly half of the glaciers makes their moraine succession potentially suspect as a record of detailed climatic variation the two glaciers with seemingly reliable age control are located in valleys where large rock avalanches could affect glacier mass balance and lead to random advances or where rock avalanche deposits might easily be mistaken for moraines and tills the early neoglacial interval has been defined primarily based on this set of data the ages taken at face value suggest that the early neoglacial advance culminated between ca 5400 and 4900 cal yr ago ca 4600 4400 14c yr bp this chronology however should remain provisional until improved age control is obtained for a large population of glaciers based on closely bracketing radiocarbon dates and or an adequate number of cosmogenic isotope ages data from calving glaciers and those subject to rock avalanche activity should be avoided or assessed critically copyright 2000 john wiley sons ltd

hindered settling of silt abstractsediment particles smaller than 64 m with a noncohesive base mineral are referred to as silt the settling velocity of suspended sediment decreases with increasing concentration due to hindered settling effects compared with clay and sand hindered settling of silt is poorly documented and understood a formulation is proposed for the hindered settling of silt water mixtures derived from physical reasoning the effect of silt on the mixture s viscosity is addressed in particular the formulation compares favorably with laboratory experiments on the settling of silt sized sediment this study shows that effective settling velocities can be derived from a single concentration time series measured within the settling mixture provided that the sediment is initially homogeneously distributed

a late quaternary pollen record from the transvaal bushveld south africa pollen spectra from cores of organic spring deposits from the transvaal provide evidence for the climatic evolution of the province during the last 35 000 yr b p or more the past climatic phases are derived from palynological reconstructions of past vegetation types by comparison of fossil pollen data with modern surface pollen spectra from various localities evidence is provided for an early moist cool phase with relatively mesic bushveld and expanded montane forest in the central transvaal followed by a drier period with drier bushveld which probably lasted until approximately 25 000 yr b p during the next phase which at the latest ended about 11 000 yr b p the temperatures were probably 5 6 c cooler than at present at that time bushveld vegetation in the central transvaal was replaced by open grassland with macchia elements climatic amelioration came and semiarid savanna returned to the plains at first gradually and then developing into a warm kalahari thornveld type vegetation after 6000 yr b p it apparently became slightly wetter and a more broad leafed bushveld developed about 4000 yr b p it again became cooler and slightly wetter and the bushveld vegetation on the central and northern plains was comparable to present open upland types after 2000 yr b p conditions gradually became warner until about 1000 yr b p when the modern climate of the central transvaal bushveld originated

rapid climate shifts in the southern african interior throughout the mid to late holocene a detailed climate proxy record based on 18o 13o and grey index of a well dated stalagmite from cold air cave in the makapansgat valley of north eastern south africa suggests that regional precipitation temperatures and vegetation oscillated markedly and rapidly over the last 6500 years on centennial and multi decadal scales the mid holocene prior to 5200 years ago was humid and warm a fundamental transition occurred 3200 years ago leading to drier and cooler conditions that culminated at 1750 ad comparisons with ice core records suggest synchronous changes implicating rapid global teleconnections

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

dissolved gas paleotemperatures and 18o variations derived from groundwater near uitenhage south africa abstract under the near ideal hydrologic conditions provided by the uitenhage artesian aquifer reliable estimates of paleotemperatures were obtained by measurement of the dissolved n2 and ar in groundwater confidently dated in the range 28 000 yr b p to the present using 14c the mean annual air temperature in this area during the last glaciation is calculated to have been on average 5 1 2 c lower than during the holocene the change in the 18 o 16 o ratio of the groundwater corrected for changes in ocean water amounted to 0 4 per c

a high latitude gondwanan lagerstatte the permian permineralised peat biota of the prince charles mountains antarctica abstract the toploje member chert is a roadian to wordian autochthonous parautochthonous silicified peat preserved within the lambert graben east antarctica it preserves a remarkable sample of terrestrial life from high latitude central gondwana prior to the capitanian mass extinction event from both mega and microfossil evidence that includes cryptic components rarely seen in other fossil assemblages the peat layer is dominated by glossopterid and cordaitalean gymnosperms and contains moderately common herbaceous lycophytes together with a broad array of dispersed organs of ferns and other gymnosperms rare arthropod plant and fungal plant interactions are preserved in detail together with a plethora of fungal morphotypes peronosporomycetes arthropod remains and a diverse coprolite assemblage comparisons to other palaeozoic ecosystems show that the macroflora is of low diversity the fungal and invertebrate plant associations demonstrate that a multitude of ecological interactions were well developed by the middle permian in high latitude forest mires that contributed to the dominant coal deposits of the southern hemisphere quantitative analysis of the constituents of the silicified peat and of macerals within adjacent coal seams reveals that whilst silicified peats provide an unparalleled sample of the organisms forming permian coals they do not necessarily reflect the volumetric proportions of constituents within the derived coal the toploje member chert lagerstatte provides a snapshot of a rapidly entombed mire climax ecosystem in the closing stages of the palaeozoic but prior to the onset of the protracted crisis that engulfed and overthrew these ecosystems at the close of the permian

sediment pick up functions experimental research has been performed to determine the sediment pick up by a uniform flow without initial sediment load a sediment lift was designed by which the sediment particles were moved upwards at a constant rate through a circular opening in the flume bottom five types of bed material sand have been used with d d5 d0 values in the range 130 1500 m the mean flow velocities were in the range 9 5 1 0 m s the experimental results have been used to derive an empirical pick up function the results have also been used to evaluate several existing pick up functions finally the proposed empirical pick up function has been applied to predict the bed load and suspended load transport in the latter application the proposed function has been used as a bed boundary condition in a mathematical model for suspended sediment transport

abrupt cooling of antarctic surface waters and sea ice expansion in the south atlantic sector of the southern ocean at 5000 cal yr b p antarctic surface waters were warm and ice free between 10 000 and 5000 cal yr b p as judged from ice rafted debris and microfossils in a piston core at 53 s in the south atlantic this evidence shows that about 5000 cal yr b p sea surface temperatures cooled sea ice advanced and the delivery of ice rafted detritus ird to the subantarctic south atlantic increased abruptly these changes mark the end of the hypsithermal and onset of neoglacial conditions they coincide with an early neoglacial advance of mountain glaciers in south america and new zealand between 5400 and 4900 cal yr b p rapid middle holocene climate changes inferred from the taylor dome ice core antarctica cooling and increased ird in the north atlantic and the end of the african humid period the near synchrony and abruptness of all these climate changes suggest links among the tropics and both poles that involved nonlinear response to gradual changes in northern hemisphere insolation sea ice expansion in the southern ocean may have provided positive feedback that hastened the end of the hypsithermal and african humid periods in the middle holocene

southeast african records reveal a coherent shift from high to low latitude forcing mechanisms along the east african margin across last glacial interglacial transition abstract late quaternary climate variability in the southern african subtropics is still only poorly resolved with significant complexity and apparent contradictions in the regional dataset to more effectively interpret and synthesize key regional records we reanalysed the data from 13 pollen sequences from the summer rainfall zone of south africa spanning the last 45 000 years obtaining directly comparable quantitative reconstructions of mean annual temperature and summer rainfall temperature reconstructions from across the region provide consistent results with all sites reflecting trends observed in southwest indian ocean sea surface temperatures in the adjacent mozambique channel precipitation reconstructions are more heterogeneous with two distinct subregions being identified in the northeast long term trends in precipitation are determined by sea surface and continental temperature trends revealing a positive relationship between temperature and rainfall this long term pattern appears to be primarily driven by high northern latitude mechanisms with direct local insolation being subordinate their relative impact reversed during terminal glacial period early holocene at which time direct insolation forcing became the main driver of rainfall variability further south in central south africa precipitation variability appears also to be influenced by the latitudinal position of the southern hemisphere westerlies which combine with tropical flow to create tropical temperate trough advecting moisture into the interior in this region periods of maximum precipitation coincide with periods of elevated ssts and equatorward expansions of the westerly storm track this study allows for a fully constrained understanding of climate dynamics along the eastern african margin for the last 45 000 years linking dynamics to drivers and describing how the climate systems evolved across the last glacial interglacial transition

linkage between turbulent kinetic energy waves and suspended sediment concentrations in the nearshore zone abstract knowledge about the tradeoffs among turbulent kinetic energy tke waves and suspended sediment concentrations sscs in the nearshore zone is relevant for understanding beach morphodynamics at different temporal and spatial scales a field measurement lasting for nearly three tidal cycles was conducted to holistically discern couplings between tke waves and sscs with further evaluation of the relative significance of tke and waves on sscs under various wave conditions over a meso macro tidal beach yintan to the north of beibu gulf china the results showed a dramatic increase in wave groupiness intensity from the shoaling wave condition to the breaking wave condition and a clear decrease further into the surf bore condition the near bed tkes under the surf bore condition were an order of magnitude larger than those under the breaking and shoaling wave conditions at the measurement position the averaged sscs in the near bed approximately 10 cm under the surf bore condition were 1 5 and 4 5 times larger than those under breaking and shoaling wave conditions respectively the variations in relative wave height were a decisive indicator for the differences in tke intensities among different wave conditions while the occurrence of peak tkes at the wave front within the intrawave cycle was associated with flow acceleration regardless of wave conditions mean sscs were well correlated with waves in terms of both incident wave scale and wave group scale which was limited to the shoaling wave condition and the occurrence of near bed intrawave peak sscs was always related to the offshore wave phase further tke contributed more effectively to sediment suspension at the wave group scale than at the incident wave scale especially under broken wave condition among the hydrodynamic factors tke played the most important role in the variations in sscs for all wave conditions flow acceleration served as the second most important factor under the broken wave condition while wave group single wave and advection were equivalent and less important factors for sscs

holocene climatic instability a prominent widespread event 8200 yr ago the most prominent holocene climatic event in greenland ice core proxies with approximately half the amplitude of the younger dryas occurred 8000 to 8400 yr ago this holocene event affected regions well beyond the north atlantic basin as shown by synchronous increases in windblown chemical indicators together with a significant decrease in methane widespread proxy records from the tropics to the north polar regions show a short lived cool dry or windy event of similar age the spatial pattern of terrestrial and marine changes is similar to that of the younger dryas event suggesting a role for north atlantic thermohaline circulation possible forcings identified thus far for this holocene event are small consistent with recent model results indicating high sensitivity and strong linkages in the climatic system

influence of agulhas forcing of holocene climate change in south africa s southern cape this paper analyses a series of high quality continuous records from southeastern africa to study the spatiotemporal patterning of holocene hydroclimatic anomalies in the region results indicate dominant frequencies of variability at millennial time scales and a series of anomalies broadly common to all records of particular interest data from the southern cape coast exhibit periods of wetter drier conditions that are out of phase with the sites less than 150 km away in the adjacent interior but in phase with sites in tropical regions over 1000 km to the northeast to explain such spatial patterns and gradients we propose that the agulhas current may be a critical vector by which tropical climatic signals are propagated along the littoral zone exerting a dominant highly localized influence on near coastal environmental conditions limitations in the data available do not allow for a detailed examination of the climatic dynamics related to these phenomena but this paper highlights a series of avenues for future research to clarify the spatial extent and stability of the patterns observed

sediment transport in oscillatory sheet flow motivated by an interest in high rates of sediment transport an experimental investigation was conducted to examine sediment motion in conditions of intense sediment transport fluid velocity and sediment concentration were measured simultaneously in a sheet flow regime generated in a u shaped oscillatory water tunnel for two types of noncohesive sediments a probe was developed to measure the instantaneous variations in sediment concentration based on the electrical conductivity of the fluid grain mixture unlike previous concentration devices this probe is nonintrusive and is capable of measuring a wide range of sediment concentrations from extremely dense concentrations within the bed to low concentration suspended load well above the moving layer of sediment horizontal fluid velocities were measured simultaneously using laser doppler anemometry in backscatter mode the sediment concentration measurements provide a comprehensive data set for sediment transport in sheet flow and near sheet flow regimes the temporal variation of sediment transport over the half cycle of oscillation was examined most of the sediment was transported within the moving bed layer with the total transport occurring between the stationary bed height and a distance about twice the thickness of the moving bed layer the temporal variation in sediment transport was found to be proportional to cos t the magnitude of the sediment transport increased with increasing bottom shear stress fluid velocity and sediment concentration measurements were used to calculate instantaneous and mean rates of sediment transport which were compared with predictions from existing models

field observations of turbulence sand suspension and cross shore transport under spilling and plunging breakers measurements of wave orbital velocity near bed turbulence levels and sediment suspension were obtained under plunging and spilling breakers in the outer surf zone on the beach at vejers denmark for the same range of relative wave heights and indicators of wave nonlinearity we observed significantly larger suspended sediment concentrations and onshore directed rates of suspended sediment transport under long period plunging breakers compared to short period spilling breakers this is consistent with the long held understanding that for a given beach slope onshore transport and beach accretion are associated with longer period waves and offshore transport and erosion are associated with shorter period waves an intra wave analysis of hydrodynamics and sediment suspension revealed that the main reason for the larger suspended sediment transport rates under plunging waves was i larger time averaged suspended sediment loads under plunging waves and ii a larger difference in cumulated sediment load under the wave crest phase compared to the wave trough phase for plunging breakers this latter difference was due to an earlier arrival at the seabed of higher levels of turbulent kinetic energy under plunging waves compared to spilling waves hence both magnitude and timing within the wave cycle of turbulent kinetic energy production are important in a quantification of sediment transport under breaking waves for ensemble averaged intense suspension events contributing roughly 50 of the total sediment suspension during individual records we found that instantaneous near bed sediment load was linearly related to instantaneous levels of froude scaled turbulent kinetic energy

the gisp2 δ18o climate record of the past 16 500 years and the role of the sun ocean and volcanoes measured 18o 16o ratios from the greenland ice sheet project 2 gisp2 ice core extending back to 16 500 cal yr b p provide a continuous record of climate change since the last glaciation high resolution annual 18o 16o results were obtained for most of the current millennium a d 818 1985 and record the medieval warm period the little ice age and a distinct 11 yr 18o 16o cycle volcanic aerosols depress central greenland annual temperature 1 5 c maximally and annual 18o 16o for about 4 yr after each major eruptive event on a bidecadal to millennial time scale the contribution of solar variability to holocene greenlandic temperature change is 0 4 c the role of thermohaline circulation change on climate problematic during the holocene is more distinct for the 16 500 10 000 cal yr b p interval analogous to 14c age calibration terminology we express time in calibrated cal yr b p a d 1950 0 cal yr b p the oldest dryas bolling older dryas allerod younger dryas sequence appears in great detail bidecadal variance in 18o 16o but not necessarily in temperature is enhanced during the last phase of lateglacial time and the younger dryas interval suggesting switches of air mass transport between jet stream branches the branched system is nearly instantaneously replaced at the beginning of the bolling and holocene at 14 670 and 11 650 cal yr b p respectively by an atmospheric circulation system in which 18o 16o and annual accumulation initially track each other closely thermodynamic considerations of the accumulation rate temperature relationship can be used to evaluate the 18 o 16o temperature relationship the gisp2 ice layer count years of major gisp2 climate transitions also support the use of coral 14c ages for age calibration

a two phase approach to wave induced sediment transport under sheet flow conditions a numerical model for the general description of the sediment transport under oscillatory sheet flow conditions is developed based on a two fluid representation of the two phase turbulent flows the governing equations of the model are the reynolds averaged continuity equations and equations of motion for both the fluid and the sediment phases the two phases are coupled by the interphase forces including the resistance force the inertia force and the lift force turbulence closure of the fluid phase is based on a slightly modified k epsilon model while an algebraic particle turbulence model is applied to the sediment phase the numerical method is based on the modified simple scheme and an improved time stepping technique the model is validated by the published data for the symmetrical oscillatory sheet flows generated in an oscillatory flow tunnel at the university of tokyo and for both the symmetrical and the asymmetrical oscillatory sheet flows generated in an oscillatory flow tunnel at university of aberdeen the numerical results on the temporal and spacial variation of the sediment concentration the horizontal velocities of the two phases the horizontal and vertical fluxes of the sediment phase as well as the thickness of the sheet flow layer all show satisfactory agreement with the laboratory data the model is also shown to predict the net sediment transport rate with a reasonably good accuracy c 2011 elsevier b v all rights reserved

late quaternary linear dune accumulation and chronostratigraphy of the southwestern kalahari implications for aeolian palaeoclimatic reconstructions and predictions of future dynamics abstract the linear dunes of the kalahari now largely inactive have long been identified as having potential palaeoenvironmental significance the application of optically stimulated luminescence osl dating to these dunes in the 1990s provided the first chronology of aeolian accumulation in this region though field methodologies and time consuming multiple aliquot laboratory protocols limited both the depth of sampling in dune bodies and the total number of samples dated in order to permit a more thorough investigation of the potential of these dunes to preserve long chronological records this intensive study presents 71 osl ages from the linear dunes of the southwestern kalahari at witpan south africa sampled with coring equipment at regular and frequent intervals down to bedrock the earliest sand accumulation recorded at witpan is at 104 ka and in spatially discrete locations other evidence of dune activity is recorded at 77 76 57 52 and 35 27 ka although an inherently discontinuous archive the linear dunes of the southwestern kalahari have the potential to record multiple phases of dune construction following the last glacial maximum there is near continuous evidence of dune building with a peak of accumulation recorded from 15 to 9 ka at five individual sites this latter period is generally recognised from other proxy evidence as being unusually arid in this region and such periods of dune activity are likely to be related to intensification of the continental anticyclone during the holocene accumulation has continued at most sites sampled albeit at a lesser intensity this may imply that these dunes are presently not far from thresholds of activation

pollen indications of holocene palaeoenvironments at florisbad spring in the central free state south africa mid to late holocene pollen data from florisbad in the central free state south africa reveal a number of moisture fluctuations commencing at c 6500 bp the pollen sequence indicates an arid climate but shows more availability of moisture under grassy conditions from summer rain at c 6300 bp and again at c 4420 bp the interval between these dates is characterized by a dry episode with karroid shrubs some time between 5500 and 4500 bp possibly as a result of less marked summer seasonality after 4220 bp moisture conditions gradually fluctuated in intensity or seasonal availability until c 2100 1700 bp when strong summer evaporation is indicated at the upper end of the sequence the data are complemented by previously published results on environmental change from the nearby deelpan site to the west although some distinct events correlate regionally over south africa summer rains appear to have been significant at florisbad at an earlier stage in the mid holocene than in the karoo to the south

dune activity as a record of late quaternary aridity in the northern kalahari new evidence from northern namibia interpreted in the context of regional arid and humid chronologies abstract sediments from presently vegetated linear dune ridges in the caprivi strip of northern namibia are optically dated to assess the timing of dune building phases and aridity in the late quaternary the dunes have been constructed from sediments likely to have been initially transported into the area by fluvial processes dune sediments do not preserve obvious evidence of hiatuses in deposition in the dated period 48 121 ka but continuous deposition and therefore continuous aridity is considered unlikely given differences in dated phases compared with chronologies from western zimbabwe to the southeast and western zambia to the north that include ages derived from the same equivalent sampling depths laboratory experiments provide evidence that dune sediments have not been bioturbated since deposition such that sample palaeodoses determined from different sampling depths are a true reflection of the duration of sediment burial independent chronologies from cave sediments provide evidence that humid episodes have occurred in the region within the dated range of dune construction it is concluded that dune construction in the late quaternary has been punctuated that the preserved record reflects the attributes of the aeolian processes that affect linear dune development and that the late quaternary record of dune building in the middle and northern kalahari was spatially complex

paleoenvironmental implications of fixed dune systems in southern africa extensive systems of fixed linear dunes occur in the kalahari region of southern africa from 28 s to 16 30 s they provide evidence for large scale expansion of arid climates in the subcontinent during the quaternary on the basis of differing dune morphologies and alignments three periods of upper pleistocene aridity are recognised patterns of dune alignments reflect paleosand flow regimes and permit deduction of past winds and circulation patterns leading to the conclusion that arid episodes were associated with and probably caused by greater strength and persistence of the south african anticyclone a tentative chronology of the arid periods tends to support the hypothesis that subtropical aridity was contemporaneous with glacial conditions in high latitudes

holocene pollen from swamp cave and hyrax dung deposits at blydefontein kikvorsberge karoo south africa abstract pollen analysis of material from a variety of sediment types including those from ponds streams a rock shelter and hyrax dung accumulations in the blydefontein basin 31 09 s 25 05 e fig 1b provide a record of vegetation change in the eastern karoo over the last 10 000 years the pollen composition fluctuated as a result of different taphonomic processes inherent in the various sampled deposits results further demonstrate that long term vegetation changes alternated between karoo shrub and grassland plant communities karoo shrubs suggesting relatively dry conditions were generally prominent in the early holocene until ca 5400 yr bp when more grassy vegetation began to flourish presumably in response to increased summer rain conditions pollen representation of the early holocene is not detailed enough to trace millennial scale variations during this period but more detailed middle to late holocene data show millennial or shorter scale shifts between grassland and drier karroid veld

patterns of insect mediated damage in a permian glossopteris flora from patagonia argentina abstract a total of 2523 fossil plant specimens from the la golondrina formation permian santa cruz province argentina were examined in order to assess the diversity frequency and representation of insect damage although no evidence of arthropod bodies have been found in the unit their past presence is recorded through various types of plant insect interactions involving oviposition external feeding piercing and sucking galling and potential mine structures results show that at least 186 specimens all of them plant leaves suffered from some type of insect herbivory among the many damage types identified some of them recognized for the first time in gondwanan floras oviposition and external feeding occurred most frequently followed by piercing and sucking and galling glossopterid foliage was the preferred target over the rest of plant groups and it was common to find more than one type of damage on the same leaf suggesting these were exploited for multiple purposes by the insects compared to the northern hemisphere permian floras the la golondrina formation herbivory levels were low and similar to those of other glossopteris floras from gondwana

a 3000 year high resolution stalagmitebased record of palaeoclimate for northeastern south africa high resolution stable isotope variations and growth structure analyses of the last three millennia of a 6600 year stalagmite record at cold air cave makapansgat valley south africa are presented growth layers which are measurable over the last 250 years are shown to be annual the correlation between the width of growth layers and precipitation is strongly positive changes in 18o and 13c are positively correlated and inversely correlated to changes in the colour of the growth layers in the stalagmite variations in colour are directly correlated with mean annual temperature dark colouration is the product of increased temperature and mobilization of organic matter from the soil and is associated with wetter summers and enhanced growth of c4 grasses darker colouring and enriched 18o and 13c reflect a warmer wetter environment whereas lighter colouring and depleted isotopic values are indicative of cooler drier conditions the dominant episode in the 3000 year record is the cool dry 500 year

permian phytogeography in gondwana abstract phytogeographic patterns based on multivariate techniques multidimensional scaling are proposed for the permian vegetation of gondwana this approach requires a data set with broad geographic coverage based on generic composition and with good time control in general strong phytogeographic differentiation can be observed throughout the permian in order to identify short term phytogeographic patterns the permian period was informally divided into five time slices in the permian stage i earliest permian the vegetation was strongly influenced by glacial conditions a very impoverished vegetation is seen except in patagonia and parts of south america four units were identified patagonia nothosouthamerica australia and indo africa by far patagonia was the most diverse and had the most favourable climatic conditions perhaps as a result of palaeogeographic separation from western gondwana nothosouthamerica which included plant assemblages from nw argentina and brazil perhaps as two subunits is a mid latitude area with glacial influence in the brazilian sector indo africa had a much lower diversity occupying high latitudes whereas australia had the lowest diversity of all the vegetational regions in stage i the early permian stage ii shows a more diversified and expanded plant cover vegetation of eastern and western gondwana was differentiated patagonia and south america including a brazilian and nw argentinean unit had the richest plant assemblages whereas africa indo australia and antarctica as separate units are less diverse probably due to late glacial influence by the late early permian stage iii gondwana shows much more ameliorated climatic conditions basically due to drifting into more temperate latitudes as well as increased moisture resulting in expansion of peat forming areas this is immediately reflected in the vegetation as shown by diversified plant assemblages even in antarctica nonetheless a differentiation is again observed between eastern gondwana patagonian and south american units and the western sector indo australian and antarctic units whereas africa would probably represent an ecotone beginning in the late permian stage iv there was a decrease in the number of plant assemblages and diversity indicating a deterioration in climate resulting from generally drier conditions latitudinal differentiation can be observed with antarctica as a polar unit afro australia at high latitudes whereas the south american indian and patagonian units are located in middle latitudes finally in permian stage v latest permian a south american unit can be differentiated clearly from the rest africa and indo australia antarctica is interpreted again as a discrete unit in the stage v time slice eastern gondwana returns to more humid climatic conditions allowing the formation of extended peat swamps whereas the western sector is affected by seasonal and strong droughtness this kind of phytogeographic approach has proven useful for detecting some palaeogeographic problems such as those involving the patagonian subcontinent in this regard palaeogeographic reconstructions and palaeoclimatic models should be tested against these phytogeographic patterns

a 40 000 yr pollen and diatom record from lake tritrivakely madagascar in the southern tropics abstract links between southern and northern hemisphere climates during the late quaternary are poorly known partly due to the scarcity of continuous climatic records in the southern tropics pollen and diatom evidence from lake tritrivakely 19 47 s provides information on vegetational and hydrological changes in the central highlands of madagascar over the past 40 000 yr most of the record reflects natural environmental variability since humans arrived on the island ca 2000 yr b p during glacial times the migration of mountain plants toward lower altitudes is consistent with a temperature decrease and with reduced atmospheric co2levels in the lake a positive mean annual hydrologic balance from 38 000 to 36 000 and from 17 500 to 9800 cal yr b p coincided with periods of decreasing summer insolation and preceded by several millennia lake rises in the northern tropics a negative hydrologic budget during periods of maximum seasonal contrast in solar radiation is partly attributed to high summer evaporation rate the last glacial maximum was cool and dry the deglacial warming occurred in two steps the first step accompanied by an increase in wetness occurred abruptly at ca 17 000 cal yr b p about two millennia earlier than in the northern hemisphere it is abundantly documented in southern terrestrial data the second step at 15 000 cal yr b p was in phase with the first major temperature change in the northern hemisphere

breaker turbulence and sediment suspension in the surf zone field measurements of fluid velocities and suspended sediment concentrations were used to investigate the relative role of coherent vortices related to wave breaking in suspended sediment dynamics the measurements were obtained from a barred intermediate type beach and the instrument rig was exposed to a range of wave types during the experiment including shoaling waves and both spilling and plunging breakers the time scale of the observed breaker vortices was 5 hz and thus in the frequency range of wave orbital motion coherent breaker vortices were separated from wave orbital motions and incoherent higher frequency turbulence using a threshold magnitude for the vertical velocity component plunging breakers displayed the largest root mean square rms and maximum vertical velocities the latter reaching approximately 1 m s 1 these measurements were made at an elevation of 0 15 m above the bed given the observed scale of the breaker vortices and the phase match between suspended sediment concentrations and fluid velocities the turbulence injected into the water column by the plunging breakers played a significant role in suspending bed sediment on the other hand the role of coherent vortex structures in suspending sediment under spilling breakers and shoaling waves is more limited rms vertical velocities under the spilling breakers and shoaling waves in this experiment were respectively a factor 2 and 4 smaller than for plunging breakers different phase relationships were observed between the horizontal fluid velocity and vortex induced sediment suspension for each of the wave types which may contribute to an explanation of net offshore and onshore sediment transport observed during episodes of beach erosion and recovery respectively

parameterization of intrawave ripple averaged sediment pickup above steep ripples near bed sediment pickup is critical for predictions of intrawave suspension and in turn net sediment transport in coastal models in the present study numerical results from a two dimensional reynolds averaged navier stokes model are used to assess the functional relationship of intrawave ripple averaged sediment pickup above steep ripples the numerical model provides intrawave time histories of ripple averaged near bed velocities and turbulence which are qualitatively interrogated to determine pickup functional relationships several specific sediment pickup formulations are implemented within the numerical model expressions relating pickup to near bed velocity or near bed turbulent kinetic energy via the bed shear stress and expressions relating pickup to near bed shear production of turbulent kinetic energy these are then tested via model data comparisons of near bed suspended sediment concentration the results show that the traditional functions relating sediment pickup to near bed velocity cannot lead to reasonable intrawave suspension predictions above vortex ripples under a ripple averaged framework instead relating sediment pickup to near bed turbulence quantities such as turbulent kinetic energy or shear production of turbulent kinetic energy significantly improves the numerical predictions for these conditions this article is protected by copyright all rights reserved

a high resolution paleoclimate record spanning the past 25 000 years in southern east africa high resolution profiles of the mass accumulation rate of biogenic silica and other geochemical proxies in two piston cores from northern lake malawi provide a climate signal for this part of tropical africa spanning the past 25 000 years the biogenic silica mass accumulation rate was low during the relatively dry late pleistocene when the river flux of silica to the lake was suppressed millennial scale fluctuations due to upwelling intensity in the late pleistocene climate of the lake malawi basin appear to have been closely linked to the northern hemisphere climate until 11 thousand years ago relatively cold conditions in the northern hemisphere coincided with more frequent north winds over the malawi basin perhaps resulting from a more southward migration of the intertropical convergence zone

holocene periodicity in north atlantic climate and deep ocean flow south of iceland climate fluctuations during the past millennium are relatively well documented1 on a longer timescale there is growing evidence of millennial scale variability of holocene climate at periodicities of 2 500 and 950 years possibly caused by changes in solar flux 2 3 and 1 500 years maybe related to an internal oscillation of the climate system 4 5 6 but the involvement of deep water masses in these holocene climate changes has yet to be established here we use sediment grain size data from the iceland basin to reconstruct past changes in the speed of deep water flow the study site is under the influence of iceland scotland overflow water isow the flow of which is an important component of the thermohaline circulation that modulates european climate flow changes coincide with some known climate events the little ice age and the mediaeval warm period and extend over the entire holocene epoch with aquasi periodicity of 1 500 years the grain size data indicate afaster isow flow when the climate of northern europe is warmer however a second mode of operation is observed in the early holocene when warm climate intervals are associated with slower isow flow at that time the melting remnant of land based glacial age ice may have provided a sufficient source of fresh water to the ocean to reduce isow flow south of iceland

a two phase numerical model for sediment transport prediction under oscillatory sheet flows to predict sediment transport under oscillatory sheet flow condition especially for fine sand is still a challenging research subject in coastal engineering this paper describes a newly developed numerical model based on two phase theory with the use of a one equation turbulence closure and its applications in predicting fine sediment suspension in near prototype oscillatory sheet flow conditions model results were compared with comprehensive laboratory measurements of flow velocity and sediment concentration under both symmetrical and asymmetrical oscillatory sheet flows from a large scale water tunnel good agreements between the model results and measurements were achieved and the results demonstrated that the model is capable of reproducing detailed characteristics of sediment entrainment process in the sheet flow regime the comparisons also revealed the fact that the concentration peaks at flow reversal is associated with the strong vertical sediment transport flux in the pickup layer which has been widely observed in many laboratory experiments the effects of flow reversal events on total sediment transport were also discussed

holocene climate change and its influence on early agriculture in the nile delta egypt abstract to investigate how holocene climate patterns influenced the development of early egyptian agriculture at the archaeological site of sais on the nile delta we conducted a palynological study seven palynological zones were defined to reveal holocene climate fluctuations zone i ca 9000 7000 cal yr bp was a period of warming climate succeeded by a progressive drying climate phase in zones ii vii ca 7000 2000 cal yr bp pollen records indicate nile flooding prevailed before 7000 years ago making the study area uninhabitable before it later developed into a wetland setting the earliest farming started at ca 6700 cal yr bp as indicated by the abrupt appearance of cereal poaceae 35 m during a time of climate warming zone iii this material can be differentiated from wild poaceae

palynological analysis of hyrax middens from southern africa abstract hyrax middens in relatively dry parts of africa preserve plant material which includes pollen the bottoms of two stratified hyrax dung middens from blydefontein basin in the karoo shrubland of south africa were dated by radiocarbon to ca 300 yr b p and ca 1130 yr b p comparison of midden pollen spectra with pollen from alluvial sediments near one of the hyrax middens and in modern surface samples indicates that middens contain pollen which reflects local vegetation and that diet probably does not have a major influence on pollen spectra changes in pollen composition probably reflect climatic conditions during the early history of the older midden profile grass dominated spectra suggest moist conditions existed between 1200 and 300 yr b p and an interval of slightly more shrubby karoid vegetation possibly points to a minor dry spell at 1000 yr b p a sharp increase of karoid shrub vegetation and a decrease of grass since 300 yr b p is reflected in both middens the change was apparently triggered by relatively dry conditions and later during the last 150 years it was enhanced by overgrazing from european domestic stock

settling velocity of sediments at high concentrations new data on the settling velocity of artificial sediments and natural sands at high concentrations are presented the data are compared with a widely used semiempirical richardson and zaki equation trans inst chem eng 32 1954 35 which gives an accurate measure of the reduction in velocity as a function of concentration and an experimentally determined empirical power n here a simple method of determining n is presented using standard equations for the clear water settling velocity and the seepage flow within fixed sediment beds the resulting values for n are compared against values derived from new and existing laboratory data for beach and filter sands for sands the appropriate values of n are found to differ significantly from those suggested by richardson and zaki for spheres and are typically larger corresponding to a greater reduction in settling velocity at high concentrations for fine and medium sands at concentrations of order 0 4 the hindered settling velocity reduces to about 70 of that expected using values of n derived for spheres at concentrations of order 0 15 the hindered settling velocity reduces to less than half of the settling velocity in clear water these reduced settling velocities have important implications for sediment transport modelling close to and within sheet flow layers and in the swash zone

minimal insect herbivory for the lower permian coprolite bone bed site of north central texas usa and comparison to other late paleozoic floras abstract an examination of 598 leaves from the lower permian late sakmarian coprolite bone bed cbb site in north central texas usa reveals low herbivory levels based on analyses of foliar surface area removed 0 25 and frequency of herbivorized leaves 15 6 these values contrast with a similar study from a somewhat younger mid artinskian nearby site where analogous values are 2 55 and 31 8 respectively as at the younger site when compared to all other co occurring plant taxa cbb pteridosperms were overwhelmingly herbivorized particularly the peltasperm autunia cf conferta and the medullosan odontopteris cf lingulata the latter preserving an exceptionally high herbivory level of 3 31 these two host plant taxa accounted for 96 8 of all herbivorized leaves and 96 4 of all leaf surface area removed by insects for the bulk flora four subgroups of feeding were documented margin feeding 70 1 of all occurrences hole feeding 24 1 and minor skeletonization earliest occurrence and galling distinctive gall types one on the midveinal region of a cf conferta and the other on a branchlet of walchia piniformis also occurring in the european early permian add to an existing but depauperate insect gall record for the pennsylvanian and permian emerging evidence indicates that herbivory intensity was spatiotemporally heterogeneous across fluvially associated landscapes during the early permian in both western euramerica texas usa and western gondwana parana basin brazil differing by an order of magnitude within each of these areas localities with the highest percentages of bulk leaf surface area removed by herbivores in both realms ranged from 2 39 to 2 97 this range approximates about a third of modern surface area removal values by insect herbivores in both subtropical to tropical floras

holocene floodplain formation in the southern cape region south africa abstract the sediments of holocene floodplain banks in several river catchments in the southern cape region south africa were sedimentologically investigated and radiometrically dated the study resulted in a differentiation into two sedimentation phases the sedimentation of the older phase starts directly above the bedrock or above coarse gravels these sediments are composed of 2 5 3 m of interbedded sand silt and clay in part they are stratified by organic horizons and inclusions the radiocarbon dating of numerous organic horizons as well as fossil wood shows that the sedimentation during the older phase occurred between 1215 and 875 years bp at the base and 670 and 15 years bp at the top of this sequence the sediments of the younger phase mainly consist of homogeneous fine sand and are at least 3 m thick stratigraphically above the sediments of the older deposition phase however the sediments of the younger layer can also comprise the entire holocene deposits situated above the current riverbed these sediments are mainly of modern age and are partly deposits of centennial flood events the context between the onset of sedimentation and the start of pastoral farming by settlers after ad 400 which has been archaeologically verified supports the hypothesis that the first sedimentation phase was set off or favoured by the degradation of the natural vegetation cover as a result of livestock farming later increased sedimentation as well as an increase in peak flows resulting from increased landscape degradation due to intensified pasture farming by the european settlers has to be assumed

modelling cross shore intensive sand transport and changes of bed grain size distribution versus field data abstract the study is focused on the cross shore transport of size graded sediment under combined wave current flow in the intensive flow regime the mathematical method for the description of hydrodynamic processes and sediment transport is adapted from the approach of kaczmarek and ostrowski coast eng 45 2002 1 for non graded sediment the theoretical model of vertical structure of sediment concentration as well as vertical mixing and sorting is presented the model of transport of graded sediment accounts for two directional cross shore sand flux the first component of this flux is directed onshore and results from wave asymmetry effects while the second one is directed seawards and is caused by undertow the vertical structure of this two directional sediment flux and its variability along a cross shore profile is a reason for segregation of grains in a horizontal direction this horizontal sorting of sand is theoretically modelled using the present approach a concept of the carpet of sorted sediments is implemented to reproduce the segregation of sediment in time and to assess the influence of grain size changes on short term evolution of the cross shore profile the results of computations are compared with the large scale laboratory data from the literature and with field measurements carried out by the authors the agreement between the model results and the experimental data is found to be very good particularly it has been theoretically shown that the observed in situ bimodal grain size distributions result from two directional sediment fluxes on the cross shore profile

suspended sediment concentrations under waves abstract a predictive formula is derived for calculation of the magnitude of time averaged suspended sediment concentrations under waves by taking into account the effect of flow contraction near ripple crests the formula applies equally well for rippled beds and sheet flow conditions the formula is in qualitative agreement with the bed load flux formula of madsen and grant 1976

complexity of holocene climate as reconstructed from a greenland ice core glaciochemical time series developed from summit greenland indicate that the chemical composition of the atmosphere was dynamic during the holocene epoch concentrations of sea salt and terrestrial dusts increased in summit snow during the periods 0 to 600 2400 to 3100 5000 to 6100 7800 to 8800 and more than 11 300 years ago the most recent increase and also the most abrupt coincides with the little ice age these changes imply that either the north polar vortex expanded or the meridional air flow intensified during these periods and that temperatures in the mid to high northern latitudes were potentially the coldest since the younger dryas event

onshore offshore sediment transport on beaches abstract the onshore offshore transport of sediment on beaches in analyzed by considering the time averaged behaviour of a control volume bounded by the wave breaking zone and by the beach face interactions are determined between the wave radiation stress the wave set up the shear stress on the beach face and the beach permeability simple theoretical relationships are derived which specify beach slope in terms of incident wave attack and beach material properties these relationships indicate that coarser sediment beaches are steeper because of the higher beach permeability it is also shown that any particular beach will decrease its slope as the wave attack increases according to an inverse function of the wave height not only the beach permeability but also the sediment grading curve is shown to be a key factor in determining beach slope the theory is used to discuss and explain some observed behaviour on real beaches and the possible implications for beach failure under storm attack

stochastic resonance in the north atlantic large abrupt changes between warm and cold modes of north atlantic climate exhibit spectral power at 1500 years yet some climatic changes were linked to outburst floods and other events that are unlikely to have been truly periodic we hypothesize that a weak periodic forcing has combined with noise from ice sheet related events to cause the observed mode switches this stochastic resonance hypothesis predicts a recurrence pattern between warmings that is distinct from the predictions of simple periodic and stochastic models and at least some other models the ice isotopic data from central greenland ice cores are consistent with the stochastic resonance hypothesis but not with other models we have tested we thus support arguments for the existence of a periodicity of 1500 years in the north atlantic climate system and for the importance of ice sheet events in forcing north atlantic changes

evolving southwest african response to abrupt deglacial north atlantic climate change events abstract climate change during the last deglaciation was strongly influenced by the bipolar seesaw producing antiphase climate responses between the north and south atlantic however mounting evidence demands refinements of this model with the occurrence of abrupt events in southern low to mid latitudes occurring in phase with north atlantic climate improved constraints on the north south phasing and spatial extent of these events are therefore critical to understanding the mechanisms that propagate abrupt events within the climate system we present a 19 400 year multi proxy record of climate change obtained from a rock hyrax midden in southernmost africa arid anomalies in phase with the younger dryas and 8 2 ka events are apparent indicating a clear shift in the influence of the bipolar seesaw which diminished as the earth warmed and was succeeded after 14 6 ka by the emergence of a dominant interhemispheric atmospheric teleconnection

coherent tropical subtropical holocene see saw moisture patterns in the eastern hemisphere monsoon systems abstract the concept of a global monsoon gm has been proposed based on modern precipitation observations but its application over a wide range of temporal scales is still under debate here we present a synthesis of 268 continental paleo moisture records collected from monsoonal systems in the eastern hemisphere including the east asian monsoon easm the indian monsoon im the east african monsoon eafm and the australian monsoon aum covering the last 18 000 years the overall pattern of late glacial to holocene moisture change is consistent with those inferred from ice cores and marine records with respect to the last 10 000 years 10 ka i e a period that has high spatial coverage a fuzzy c means clustering analysis of the moisture index records together with xie beni index reveals four clusters of our data set the paleoclimatic meaning of each cluster is interpreted considering the temporal evolution and spatial distribution patterns the major trend in the tropical aum eafm and im regions is a gradual decrease in moisture conditions since the early holocene moisture changes in the easm regions show maximum index values between 8 and 6 ka however records located in nearby subtropical areas i e in regions not influenced by the intertropical convergence zone show an opposite trend compared to the tropical monsoon regions aum eafm and im i e a gradual increase analyses of modern meteorological data reveal the same spatial patterns as in the paleoclimate records such that in times of overall monsoon strengthening lower precipitation rates are observed in the nearby subtropical areas we explain this pattern as the effect of a strong monsoon circulation suppressing air uplift in nearby subtropical areas and hence hindering precipitation by analogy to the modern system this would mean that during the early holocene strong monsoon period the intensified ascending airflows within the monsoon domains led to relatively weaker ascending or even descending airflows in the adjacent subtropical regions resulting in a precipitation deficit compared to the late holocene our conceptual model therefore integrates regionally contrasting moisture changes into the global monsoon hypothesis

holocene hydrologic and vegetation developments in the orange river catchment south africa and their controls the summer rainfall zone srz in the south african interior experienced pronounced hydrological and vegetation changes during the holocene inferred to be driven mainly by shifts in atmospheric and

el nino like pattern in ice age tropical pacific sea surface temperature sea surface temperatures ssts in the cold tongue of the eastern equatorial pacific exert powerful controls on global atmospheric circulation patterns we examined climate variability in this region from the last glacial maximum lgm to the present using a sst record reconstructed from magnesium calcium ratios in foraminifera from sea floor sediments near the galapagos islands cold tongue sst varied coherently with precession induced changes in seasonality during the past 30 000 years observed lgm cooling of just 1 2 c implies a relaxation of tropical temperature gradients weakened hadley and walker circulation southward shift of the intertropical convergence zone and a persistent el nino like pattern in the tropical pacific this is contrasted with mid holocene cooling suggestive of a la nina like pattern with enhanced sst gradients and strengthened trade winds our results support a potent role for altered tropical pacific sst gradients in global climate variations

two phase flow modelling of sediment motions in oscillatory sheet flow in this paper a two phase flow model is presented which simulates the fluid and sediment motions in the sheet flow regime on a flat bed under oscillatory flow conditions the model is developed based on the continuity equations and linearised momentum equations for the fluid and sediment phases respectively all major forcing terms such as the intergranular stresses and the turbulent stress are included in the model from the detailed computations and comparison with the available laboratory data it has been demonstrated that the model is capable of predicting fairly accurately both flow kinematics and sediment concentrations in particular the model predicts that the well known phenomenon of fluid velocity over shoot that exists in clear water also appears in the case of lighter sediments but vanishes when the materials are heavier which is in perfect accord with the experimental observations considered

sand transport beneath waves the role of progressive wave streaming and other free surface effects recent large scale wave flume experiments on sheet flow sediment transport beneath stokes waves show more onshore directed sediment transport than earlier sheet flow experiments in oscillating flow tunnels for fine sand this extends to a reversal from offshore tunnels to onshore flumes directed transport a remarkable hydrodynamic mechanism present in flumes with free water surface but not in tunnels rigid lid is the generation of progressive wave streaming an onshore wave boundary layer current this article investigates whether this streaming is the full explanation of the observed differences in transport in this article we present a numerical model of wave induced sand transport that includes the effects of the free surface on the bottom boundary layer with these effects and turbulence damping by sediment included our model yields good reproductions of the vertical profile of the horizontal mean velocities as well as transport rates of both fine and medium sized sediment similar to the measurements the model reveals the reversal of transport direction by free surface effects for fine sand a numerical investigation of the relative importance of the various free surface effects shows that progressive wave streaming indeed contributes substantially to increased onshore transport rates however especially for fine sands horizontal gradients in sediment advection in the horizontally nonuniform flow field also are found to contribute significantly we therefore conclude that not only streaming but also inhomogeneous sediment advection should be considered in formulas of wave induced sediment transport applied in morphodynamic modeling we propose a variable time scale parameter to account for these effects

holocene palaeoenvironments inferred from a sedimentary sequence in the tsoaing river basin western lesotho abstract the paper presents a sedimentological palynological and phytolith record from a 13 m deep holocene sedimentary sequence located in the tsoaing river valley southwestern lesotho six conventional radiocarbon and two ams dates provide a relatively high resolution holocene record for the sedimentary sequence ranging from ca 12 000 to 4000 years bp pollen is absent in the upper section but present in the lower 2 m confirming terminal pleistocene holocene conditions reported in previous published pollen and charcoal records from the region the absence of pollen in the upper layers of ca 9000 years bp and younger suggests that conditions over southwestern lesotho throughout much of the holocene was typified by a seasonal climate that prevented long term preservation of plant remains although other plant material like robust spores microscopic charcoal and phytoliths withstood oxidation sedimentological and phytolith results suggest that the period from ca 8600 to 8450 years bp experienced rapid environmental change towards drier conditions phases of chemical disintegration with organic input including local swamp phytoliths are suggested at ca 7000 years bp and again after 4500 years bp

vegetation history inferred from pollen in late quaternary faecal deposits hyraceum in the cape winter rain region and its bearing on past climates in south africa abstract pollen analysis shed light on vegetation community structure over the last 23 000 year in the western cape the pollen from dated hyrax faecal accumulations hyraceum relates to the evolution of climate and contributes to proxy records in southern africa principal components analyses of the pollen data and 13c values of the hyrax dung samples show millennial and shorter scale temperature moisture and seasonality variations in the winter rain region the moisture availability at times do not parallel that in previously studied proxy records in the summer rain region showing an asynchronous moist event in the early holocene and drier conditions in the middle holocene anomalies in climate between the two regions may depend on the degree of northward or southward shifting of winter and summer rain circulation systems scenarios with winter rain or cool growing seasons mostly typify the dung sequence but do not exclude the possibility of southward displacement of the westerly belt under precessional strength with slight summer rain increases during the last glacial maximum limited southward displacement is also possible during the mid holocene

shear stress and sediment transport calculations for swash zone modelling it is shown that the observed difference in sediment transporting efficiency by the swash uprush compared with the downrush could be mainly due to greater bed shear stress for a given velocity in the more abruptly accelerated uprush the bed shear stress generated by an arbitrary free stream velocity time series is modelled in terms of usual wave boundary layer models plus a phase lead phi tau of the bed shear stress compared with the free stream velocity at the peak frequency with this approach the total transport amounts in uprush and downrush can be modelled satisfactorily with the same sediment transport formula without the need for different uprush and downrush coefficients while the adaptation of sediment transport formulae from steady flow can thus lead to the right total amounts of sediment moved by this method the timing of the instantaneous sediment transport rates are probably not accurately modelled due to the highly unsteady nature of the swash and the presence of pre suspended sediment in the uprush nevertheless the proposed method is a useful intermediate step before we have a complete understanding of sediment transport under very rapid accelerations and of the relative contribution of pre suspended sediment to the onshore sediment transport in swash zones c 2002 published by elsevier science b v

earliest record of exophytic insect oviposition on plant material from the latest pennsylvanian gzhelian stephanian c of the saale basin germany abstract the late palaeozoic fossil record of plant insect interactions contains few records of oviposition here we report the earliest evidence of exophytic oviposition where eggs are laid directly on the outer surface of plants the fossil structures consist of small circular impressions on plant fossils from the late pennsylvanian gzhelian stephanian c of the saale basin in germany their interpretation as exophytic insect eggs is supported by the oviposition pattern their three dimensional preservation and the undamaged nature of the plant host tissue the tight arrangement of the eggs in curved arcs suggests a producer with a short abdomen such as roachoids the ancestral group of the dictyoptera sensu stricto a diagnostic feature of roachoids is their long ovipositor which was well suited for deposition of single eggs in a distinct pattern