single crystal 40ar 39ar dating of the olorgesailie formation southern kenya rift single crystal laser fusion sup 40 ar sup 39 ar analyses and several conventional bulk fusion sup 40 k sup 40 ar dates have been used to determine the age of volcaniclastic strata within the olorgesailie formation and of associated volcanic and sedimentary units of the southern kenya rift in the principal exposures along the southern edge of the legemunge plain the formation spans the interval from approximately 500 to 1 000 ka deposition continued to the east along the ol keju nyiro river where a tuff near the top of the formation has been dated at 215 ka in these exposures the formation is unconformably overlain by sediments dated at 49 ka a possible source for the olorgesailie tephra the ol doinyo nyokie volcanic complex contains as ash flow dated at approximately 1 ma extending the known age range of this complex to encompass that of virtually the entire olorgesailie formation in the legemunge plain these geologic examples illustrate the importance of the single crystal sup 40 ar sup 39 ar dating technique whereby contaminant altered or otherwise aberrant grains can be identified and eliminated from the determination of eruptive ages for reworked or altered pyroclastic deposits the authors have presented a computer modeling procedure based on an inverse isochron analysis thatmore promotes a more objective approach to trimming sup 40 ar sup 39 ar isotope data sets of this type less

global vegetation dynamics and latitudinal temperature gradients during the mid to late miocene 15 97 5 33 ma a 617 site palaeobotanical dataset for the mid to late miocene is presented this dataset is internally consistent and provides a comprehensive overview of vegetational change from 15 97 to 5 33 ma the palaeobotanical dataset has been translated into the biome4 classification scheme to enable direct comparison with climate model outputs the vegetation change throughout the langhian seravallian tortonian and messinian is discussed the data shows that the langhian which includes the end of the mid miocene climatic optimum represents a world significantly warmer than today the high northern latitudes were characterised by cool temperate forests the mid latitudes by warm temperate mixed forests the tropics by tropical evergreen broadleaf forests and antarctica by tundra shrub vegetation cool temperate mixed forest existed during the seravallian in the high northern latitudes a reduction in warm temperate mixed forests in the mid latitudes and a loss of tundra on antarctica was noted tortonian vegetation distribution indicates that further cooling had occurred since the seravallian the major changes in vegetation include the first evidence for cold taiga forest in the high northern latitudes and a further reduction of warm temperate mixed forests by the messinian this cooling trend had eliminated warm temperate mixed forests from the western usa and australia and had formed mid latitude deserts despite the cooling trend throughout the mid to late miocene the vegetation distribution of all four reconstructed stages reflect the vegetation of a world warmer than the pre industrial conditions the latitudinal distribution of bioclimatic zones suggests that the latitudinal temperature gradient for the langhian would have been significantly shallower than at present and has gradually but asymmetrically become more modern towards the end of the miocene first the southern hemisphere distribution of bioclimatic zones became more modern probably due to the climatic effects of a fully glaciated antarctica the northern hemisphere bioclimatic zone gradient continued to be shallower than modern throughout the miocene and slowly became more modern by the messinian

chronostratigraphy of the baringo tugen barsemoi hspdp btb13 1a core 40ar 39ar dating magnetostratigraphy tephrostratigraphy sequence stratigraphy and bayesian age modeling the baringo tugen barsemoi 2013 drillcore btb13 acquired as part of the hominin sites and paleolakes drilling project recovered 228 m of fluviolacustrine sedimentary rocks and tuffs spanning a 3 29 2 56 ma interval of the highly fossiliferous and hominin bearing chemeron formation tugen hills kenya here we present a bayesian stratigraphic age model for the core employing chronostratigraphic control points derived from 40ar 39ar dating of tuffs from core and outcrop 40ar 39ar age calibration of related outcrop diatomaceous units and core magnetostratigraphy the age model reveals three main intervals with distinct sediment accumulation rates an early rapid phase from 3 2 to 2 9 ma a relatively slow phase from 2 9 to 2 7 ma and the highest rate of accumulation from 2 7 to 2 6 ma the intervals of rapid accumulation correspond to periods of high earth orbital eccentricity whereas the slow accumulation interval corresponds to low eccentricity at 2 9 2 7 ma suggesting that astronomically mediated climate processes may be responsible for the observed changes in sediment accumulation rate lacustrine transgression regression events as delineated using sequence stratigraphy dominantly operate on precession scale particularly within the high eccentricity periods a set of erosively based fluvial conglomerates correspond to the 2 9 2 7 ma interval which could be related to either the depositional response to low eccentricity or to the development of unconformities due to local tectonic activity age calibration of core magnetic susceptibility and gamma density logs indicates a close temporal correspondence between a shift from high to low frequency signal variability at 3 ma approximately coincident the end of the mid piacenzian warm period and the beginning of the cooling of world climate leading to the initiation of northern hemispheric glaciation c 2 7 ma btb13 and the baringo basin records may thus provide evidence of a connection between high latitude glaciation and equatorial terrestrial climate toward the end of the pliocene

paleomagnetism of miocene east african rift sediments and the calibration of the geomagnetic reversal time scale paleomagnetic stratigraphy and k ar age determinations are reported for the type section of the middle miocene ngorora formation found in the kenya rift valley the magnetostratigraphy is well correlated to the geomagnetic reversal time scale grts and spans from the lower part of chron c5 9 to chron c5ab r 14 k ar dates were determined for euhedral sanidines handpicked from seven tuff horizons within the ngorora formation and the underlying turn phonolite flow these dates can therefore be tied directly to the grts the eight dates fall into three discrete groups averaging 12 5 0 22 ma mean and standard deviation of results from four tuffs 11 6 0 06 ma mean and standard deviation from three tuffs and 10 16 0 38 average of three analyses from one tuff we interpret the age groups as resulting from three successive eruptive episodes the stratigraphic positions of which are well constrained in spite of episodic supply at the eruptive source sediment accumulation is continuous at the resolution of the grts this suggests that accumulation is controlled by basin subsidence rather than sediment supply sanidine dates support an age for the older boundary of marine magnetic anomaly 5 of about 10 ma as opposed to 11 12 ma suggested by the most recent results from icelandic basaltic lava flows

the effect of the subglacial water pressure on the sliding velocity of a glacier in an idealized numerical model in order to interpret observed short term variations of the sliding velocity of a glacier the effect of a variable subglacial water pressure on the sliding velocity has been studied using an idealized numerical model in particular the transient stages of growing or shrinking water filled cavities at the ice bedrock interface were analysed it was found that the sliding velocity was larger when cavities were growing than when they had reached the steady state size for a given water pressure the smallest sliding velocities occurred while cavities were shrinking when cavitation is substantial a small drop of water pressure below the steady state value e g by 0 5 bar can temporarily cause backward sliding a limiting water pressure at which sliding becomes unstable is derived the consequences of more realistic assumptions than those of the model are discussed

glaciohydraulic supercooling a freeze on mechanism to create stratified debris rich basal ice ii theory simple theory supports field observations lawson and others 1998 that subglacial water flow out of overdeepenings can cause accretion of layered debris bearing ice to the bases of glaciers the large meltwater flux into a temperate glacier at the onset of summer melting can cause rapid water flow through expanded basal cavities or other flow paths if that flow ascends a sufficiently steep slope out of an overdeepening the water will supercool as the pressure melting point rises and basal ice accretion will occur diurnal occasional or annual fluctuations in water discharge will cause variations in accretion rate debris content of accreted ice or subsequent diagenesis producing layers under appropriate conditions net accretion of debris bearing basal ice will allow debris fluxes that are significant in the glacier sediment budget

test of climate leaf physiognomy regression models their application to two miocene floras from kenya and 40ar 39ar dating of the late miocene kapturo site abstract to test multiple regression models developed to predict seven variables of climate from leaf margin and size characters leaf character data were collected from modern woodland and rainforest plant communities in uganda these localities were chosen for their structural similarity to two fossil sites for which climate values were desired the test indicated that for these communities the model accurately predicts mean annual temperature however the multiple regression models overestimate seasonality of temperature and mean annual and seasonal precipitation amounts for the two modern communities reasons for the models inaccuracies may include no african analog in the non african database on which the model is based too much intraspecific variability in leaf size an important character in precipitation models and relationships between leaf form and climate that may not be well defined by multiple regression mean annual temperatures were reconstructed for two miocene paleobotanical localities from the tugen hills of the eastern rift valley kenya using leaf form data primarily margin characters the results indicate that mean annual temperatures at kabarsero 12 6 ma and kapturo 6 7 7 2 ma were essentially equivalent to temperatures at those localities today and differed little from one another the kapturo paleofloral assemblage is a deciduous woodland interpreted as indicating less available or more seasonal moisture than the kabarsero paleofloral assemblage a wet forest with west african floral affinities laser fusion 40 ar 39 ar dating of volcanic materials at kapturo provides the first control of this paleoflora bracketed by a 7 2 ma trachyte at the base of the sedimentary sequence and an age of 6 7 ma on a reworked tuff overlying the fossiliferous horizon the chronologic data places the paleofloral assemblage in stratigraphic context relative to other tugen hills units containing paleofloras and abundant vertebrate remains including hominoids

post palaeozoic evolution of weathered landsurfaces in uganda by tectonically controlled deep weathering and stripping a model for the evolution of weathered landsurfaces in uganda is developed using available geotectonic climatic sedimentological and chronological data the model demonstrates the pivotal role of tectonic uplift in inducing cycles of stripping and tectonic quiescence for cycles of deep weathering it is able to account for the development of key landforms such as inselbergs and duricrust capped plateaux which previous hypotheses of landscape evolution that are based on climatic or eustatic controls are unable to explain development of the ugandan landscape is traced back to the permian following late palaeozoic glaciation a trend towards warmer and more humid climates through the mesozoic enabled deep weathering of the jurassic mid cretaceous surface in uganda during a period of prolonged tectonic quiescence uplift associated with the opening south atlantic ocean terminated this cycle and instigated a cycle of stripping between the mid cretaceous and early miocene deep weathering on the succeeding miocene to recent african surface has occurred from miocene to present but has been interrupted in the areas adjacent to the western rift where development of a new drainage base level has prompted cycles of stripping in the miocene and pleistocene

a methodology for targeting palaeo proxy data acquisition a case study for the terrestrial late miocene when planning the acquisition of new palaeo proxy data for reconstructing past climates many factors influence the decision of where the proxies are to be collected one such influence is the likelihood of recording a significant climate change relative to the modern another consideration which is less often considered is the desire to target regions in which there is significant uncertainty in numerical model predictions of past climate change new proxy data in these regions enables a more rigorous test of model simulations and results in better constraints on the various boundary conditions used to force the models in this paper we present a methodology for targeting new palaeo proxy data based on model simulations we use the terrestrial late miocene 11 6 to 5 3 ma as a case study the late miocene climate provides insights into future climate behavior as it is believed to have been considerably warmer than the modern we carry out a suite of late miocene atmosphere only general circulation model simulations which we initially evaluate relative to an example dataset of terrestrial temperature and precipitation in terms of targeting future data acquisition we locate those regions in the model which exhibit the largest change in temperature and or precipitation relative to the pre industrial we also locate those regions which display the largest variability between the model simulations because new data in these regions are most likely to provide a strict test of the reliability of the model results among other regions the amazon basin the chad basin central canada and northern india are identified as potentially useful areas to collect data

subglacial conditions during and after stoppage of an antarctic ice stream is reactivation imminent borehole observations from the base of the west antarctic ice sheet wais reveal the presence of a 10 to 15 m thick accretionary basal ice layer in the upstream area of kamb ice stream kis this ice layer has formed over a time of several thousand years by freeze on of subglacial water to the ice base and has recorded during this time basal conditions upstream of its current location analysis of samples and videos sequences from boreholes drilled to the bottom of kis confirms that kis stoppage was due to basal freeze on and that relubrication of the ice stream is well underway these results further suggest that ice stream cyclicity may be shorter than expected 1000s of years and that a restart of kis may be imminent within decades to centuries

neogene biomarker record of vegetation change in eastern africa the evolution of c4 grassland ecosystems in eastern africa has been intensely studied because of the potential influence of vegetation on mammalian evolution including that of our own lineage hominins although a handful of sparse vegetation records exists from middle and early miocene terrestrial fossil sites there is no comprehensive record of vegetation through the neogene here we present a vegetation record spanning the neogene and quaternary periods that documents the appearance and subsequent expansion of c4 grasslands in eastern africa carbon isotope ratios from terrestrial plant wax biomarkers deposited in marine sediments indicate constant c3 vegetation from 24 ma to 10 ma when c4 grasses first appeared from this time forward c4 vegetation increases monotonically to present with a coherent signal between marine core sites located in the somali basin and the red sea the response of mammalian herbivores to the appearance of c4 grasses at 10 ma is immediate as evidenced from existing records of mammalian diets from isotopic analyses of tooth enamel the expansion of c4 vegetation in eastern africa is broadly mirrored by increasing proportions of c4 based foods in hominin diets beginning at 3 8 ma in australopithecus and slightly later kenyanthropus this continues into the late pleistocene in paranthropus whereas homo maintains a flexible diet the biomarker vegetation record suggests the increase in open c4 grassland ecosystems over the last 10 ma may have operated as a selection pressure for traits and behaviors in homo such as bipedalism flexible diets and complex social structure

taphonomy of a middle miocene autochthonous forest assemblage ngorora formation central kenya abstract a 12 6 million year old forest assemblage of leaves twigs and rare fruit is preserved in the ngorora formation of central kenya the assemblage is autochtonous preserved in airfall tuff and represents well the diversity heterogeneity and richness of the original forest the herbaceous component not usually preserved in paleobotanical assemblages is present nearest neighbor analysis and mapping of the vertical distribution of plant organs indicates that leaves and twigs are preserved near the parent plant and that there is no apparent size weight or species bias in deposition of these remains leaves occur isolated in clusters or attached to branches and fell directly to the ground as a result of ashfall most falling very soon after ashfall commenced the absence of upright trunks and large stems is probably due to post depositional weathering which may also be responsible for the absence of a leaf litter layer however leaf litter may have been absent at the time of ash deposition

the zambezi delta mozambique channel east africa high resolution dating combining bio orbital and seismic stratigraphies to determine climate palaeoprecipitation and tectonic controls on a passive margin abstract the zambezi delta draining the southern african plateau and the southern part of the east african rift is one of a the largest delta of africa with a long lasting history starting during early cretaceous with more than 12 km of sediments deposited the zambezi delta is therefore a unique archive of the past topographic evolution of southern and eastern africa and their related deformations but also of the climate changes global and regional consequences of local topographic growths understanding this archive supposes to get a high resolution dating of the sediments our two objectives are here 1 to construct an age model of the zambezi cenozoic delta using a combination of biostratigraphy orbital stratigraphy and sequence stratigraphy and 2 to determine the palaeoprecipitation variations of the zambezi catchment from the oligocene to present day in a known tectonic framework the neogene sequences were dated at high resolution assuming that the third order sequences are of eustatic origin and record long term eccentricity cycles the sequences were correlated in ages on the calculated earth orbital solutions of laskar for the time intervals provided by the biostratigraphy nannofossils planktonic foraminifers the palaeoprecipitation record was based on the definition of a humidity index based on pollen analysis and associated botanical associations the late oligocene was a quite wet period getting dryer in the uppermost chattian the base tortonian 11 ma was a humid period the messinian was a dry period with a slight increase of the humidity during the zanclean and a sharp increase around the zanclean piacenzian boundary the zambezi delta has recorded the uplifts of the southern african plateau around 85 ma and around 25 ma and those of the southward migration of the east african rift since 5 5 ma

a tortonian late miocene 11 61 7 25 ma global vegetation reconstruction for the tortonian age of the miocene epoch 11 6 7 25 ma we present a global palaeobotanical and palaeoecologically based vegetation dataset combined with a best fit late miocene climate vegetation model experiment to create an advanced global data model hybrid biome reconstruction this new palaeoecological database and global vegetation reconstruction can be used both for the purposes of validating future palaeoclimate model simulations as well as a land cover dataset to initialise palaeoclimate modelling experiments our tortonian reconstruction shows significant changes in the distribution of vegetation compared to modern natural vegetation for example in contrast to the modern scenario in the northern hemisphere boreal forests reached 80 n and temperate forests were present above 60 n warm temperate forests covered much of europe coastal north america and south east asia our reconstruction shows a spread of temperate savanna in central usa the middle east and on the tibetan plateau evidence for arid deserts is sparse with the exception of the atacama region south america areas that exhibit arid desert today in the tortonian were instead covered by shrublands grasslands savannas and woodlands the extent of tropical forests in south america was likely reduced but expanded in the indian sub continent and east africa this pattern of global vegetation in the late miocene suggests a warmer and wetter world which is supported by the pattern of climate anomalies predicted by our best fit palaeoclimate vegetation model experiment global mean annual temperature may have been as much as 4 5 c higher than present day with many regions experiencing higher than modern amounts of precipitation over the annual cycle the pattern of temperature and precipitation change reconstructed palaeobotanically and predicted within our climate model experiment infers a global forcing agent on tortonian climate e g such as elevated concentrations of greenhouse gases to explain the observed and modelled climate anomalies this is in contrast to current proxy records of tortonian atmospheric co2 which range from last glacial maximum to mid 20th century levels

early palaeocene palaeoclimatic inferences from fossil floras of the western interior usa abstract twenty one early palaeocene floral assemblages were collected from a variety of alluvial depositional environments from sites in the williston powder river and raton basins of the western interior of the usa leaf fossils were assigned to morphotypes and their physiognomy was quantified for palaeoclimatic analysis two methods were used climate leaf analysis multi variate program clamp and leaf margin analysis lma temperature estimates and trends derived from clamp and lma indicate that early palaeocene palaeotemperatures decreased from the latest cretaceous particularly in northern areas of north america climatic zones shifted southwards from their latest cretaceous positions resulting in a steepened north south temperature gradient and decreased equability in the early palaeocene sub tropical conditions prevailed as far north as the raton basin with the 20 c mean annual temperature isotherm megathermal mesothermal boundary located at approximately 45 n palaeolatitude the 13 c mean annual temperature isotherm mesothermal microthermal boundary was located at approximately 55 n palaeolatitude to the south of the williston basin clamp estimates indicate that precipitation increased significantly in all areas sampled directly after the k t boundary event especially in the southern rockies precipitation levels decreased somewhat after this initial peak but remained high throughout the early palaeocene evidence from this and other studies suggests that seasonality increased from the latest cretaceous into the palaeocene and was most pronounced in central and northerly areas of the north american continent

basal hydraulic system of a west antarctic ice stream constraints from borehole observations pressure and tracer measurements in boreholes drilled to the bottom of ice stream b west antarctica are used to obtain information about the basal water conduit system in which high water pressures are developed these high pressures presumably make possible the rapid movement of the ice stream pressure in the system is indicated by the borehole water level once connection to the conduit system is made on initial connection here also called breakthrough to the basal water system the water level drops in a few minutes to an initial depth in the range 96 117 m below the surface these water levels are near but mostly somewhat deeper than the flotation level of about 100m depth water level at which basal water pressure and ice overburden pressure are equal which is calculated from depth density profiles and is measured in one borehole the conduit system can be modelled as a continuous or somewhat discontinuous gap between ice and bed the thickness of the gap has to be about 2 mm to account for the water level drop on breakthrough and about 4 mm to fit the results of a salt tracer experiment indicating downstream transport at a speed of 7 5 mm s 1 the above gap conduit model is however ruled out by the way a pressure pulse injected into the basal water system at breakthrough propagates outward from the injection hole and also by the large hole to hole variation in measured basal pressure which if present in a gap conduit system with 2 or 4 mm would result in unacceptably large local water fluxes an alternative model that avoids the e objections called the gap opening model involves opening a gap as injection proceeds starting with a thin film the injection of water under pressure lifts the ice mass a round the borehole creating a gap 3 or 4 mm wide at the ice bed interface evaluated quantitatively the gap opening model accounts for the volume of water that the basal watery system accepts on breakthrough which obviates the gap conduit model in order to transport basal meltwater from upstream it is then necessary for the complete hydraulic model to contain also a network of relatively large conduits of which the most promising type is the canal conduit proposed theoretically by walder and fowler 1994 flat low conduits incised into the till 0 1 m deep and perhaps 1 m wide with a flat ice roof the basal water pressure data suggest that the canals are spaced 50 300 m apart much closer than r tunnel would be the deepest observed water level 117 m i the most likely to reflect the actual water pressure in the canals corresponding to a basal effective pressure of 1 6 bar in this interpretation the shallower water levels are affected by loss of hydraulic head in the narrow passageway s that connect along the bed from borehole to canal s once a borehole has frozen up and any passageways connecting with canals have become closed a pressure sensor in contact with the unfrozen till that underlies the ice will measure the pore pressure in the till given enough time for pressure equilibration this pressure varies considerably with time over the equivalent water level range from 100 to 113 m basal pressure sensors 500 m apart report uncorrelated variations whereas sensors in boreholes 25 m apart report mostly but not entirely well correlated variations of unknown origin in part of the record remarkable anticorrelated variations are interspersed with positively correlated one and there are rare abrupt excursions to extreme water levels as deep as 125m and as shallow as 74 m a diurnal pressure fluctuation intermittently observed may possibly be caused by the ocean tide in the ross sea the lack of any observed variation in ice stream motion when large percentagewise variations in basal effective pressure were occurring according to our data suggests that the observed pressure variations are sufficiently local and so randomly variable from place to place that they are averaged out in the process by which the basal motion of the ice stream is determined by an integration over a large area of the bed

kamb ice stream flow history and surge potential a basal zone tens of meters thick of debris laden ice was observed in kamb ice stream west antarctica using a video camera lowered into boreholes made by hot water drilling the debris content varies sometimes abruptly forming a sequence of layers that reflect the complex history of fast ice flow and bed interaction in most parts the concentration of debris is low a few percent by weight with particles often mud clots dispersed in a matrix of clear ice the nature of the debris distribution can be interpreted in terms of specific time intervals in the history of fast motion of kamb ice stream including processes leading up to the termination of its streaming behavior and possible reactivation

basal sliding of ice stream b west antarctica a tethered stake apparatus is used to measure basal sliding in a borehole on ice stream b west antarctica about 300 km upstream east from its grounding line near the head of the ross ice shelf a metal stake emplaced at the top of a layer of unfrozen till underlying the ice is connected by a tether line to a metering unit that measures the tether line as it is pulled out from the borehole by the stake as a result of basal sliding the measured sliding motion includes any actual slip across the ice till interface and may include in addition a possible contribution from shear deformation of till within about 3 cm of the interface this 3 cm figure follows from a qualitative model of the movements of the stake in the course of the experiment based on features of the record of apparent sliding alternative but less likely models would increase the figure from 3 cm to 10 cm or 25 cm in any case it is small compared to the seismically inferred till thickness of 9 m measured apparent sliding averages 69 of the total motion of 1 2 m d 1 over 26 days of observation if a 3 5 day period of slow apparent sliding 8 of the total motion is included in the average the occurrence of the slow period raises the possibility that the sliding motion switches back and forth between c 80 and c 8 of the total motion on a time scale of a few days however it is likely that the period of slow apparent sliding represents instead a period when the stake got caught on the ice sole if the slow period is therefore omitted the indicated average basal sliding rate is 83 of the total motion in either case basal sliding predominates as the cause of the rapid ice stream motion in the last 2 days of observation the average apparent sliding rate reached 1 17 m d 1 essentially 100 of the motion of the ice stream if till deformation contributes significantly to the ice stream motion the contribution is concentrated in a shear zone 3 cm to possibly 25 cm thick at the top of the 9 m thick till layer these observations if applicable to the west antarctic ice sheet in general pose complications in modeling the rapid ice streaming motion

mean annual temperature estimation based on leaf morphology a test from tropical south america abstract are models that predict mean annual temperature mat from leaf morphology applicable globally fifteen models that predict mat from leaf morphology were tested on thirty floral samples from tropical south america to determine the degree to which models based on published data that are primarily from other regions are applicable to floras from tropical south america the models included are based on regional data from north america south america and asia of the fifteen models tested five are simple linear regressions six are multiple linear regressions two are canonical correspondence analyses and two are correspondence analyses followed by nearest neighbor analyses for the seven modern floras with mat 21 c every model overestimates mat for the 23 modern floras with mat 21 c all models produce variable results without a systematic error the range of average model errors is 2 7 7 3 c while the absolute extremes of error are 0 and 15 1 c average 95 predictive confidence intervals range from 1 6 to 6 9 c predicted mat falls within the published standard error of the model for 0 67 of the south american test floras evaluating the seven sites with mat 21 c separately from the 23 sites with mat 21 c shows that no equation accurately estimates mat of the majority of low temperature sites but that four equations accurately estimate 50 of high temperature sites the results suggest that at least for sites of unknown or high elevation mean annual temperature of fossil floras from tropical south america may be better predicted from models based on the leaf morphology of tropical south american floras

glaciohydraulic supercooling a freeze on mechanism to create stratified debris rich basal ice i field evidence debris laden ice accretes to the base of matanuska glacier alaska u s a from water that supercools while flowing in a distributed drainage system up the adverse slope of an overdeepening frazil ice grows in the water column and forms aggregates while other ice grows on the glacier sole or on substrate materials sediment is trapped by this growing ice forming stratified debris laden basal ice growth rates of 0 1 m a 1 of debris rich basal ice are possible the large sediment fluxes that this mechanism allows may have implications for interpretation of the widespread deposits from ice that flowed through other overdeepenings including heinrich events and the till sheets south of the laurentian great lakes

a late miocene climate model simulation with echam4 ml and its quantitative validation with terrestrial proxy data abstract for the tortonian late miocene 11 to 7 ma we present a model simulation using the atmospheric general circulation model agcm echam4 coupled to a mixed layer ml ocean model the tortonian simulation with echam4 ml includes an adjusted weaker than present ocean heat transport a generally lower palaeorography and an appropriate palaeovegetation we analyse the climatic differences between the tortonian run and the present day control experiment and we validate our tortonian model results for the mean annual temperature and the annual precipitation with quantitative terrestrial proxy data globally the tortonian run is slightly warmer 0 6 c and the global precipitation indicates more humid conditions 27 mm a than the pd control run particularly the northern high latitudes are warmer 3 5 c in the tortonian simulation which leads to a weaker meridional temperature gradient than today as well as to a reduction of polar sea ice we attribute the polar warming to the lowering of the greenland elevation 20 c in the miocene and to the effects of the palaeovegetation the reduced altitude of the tibetan plateau causes a warming 20 c as compared to today and leads to a weaker asian monsoon in the tortonian run for europe the mean annual temperature is almost unaffected as compared to today because a cooling effect of the weak palaeocean heat transport is compensated by the palaeovegetation when validating our model results we find that the tortonian run is largely consistent with quantitative proxy data however the high latitudes tend to be generally too cool in the palaeoclimate simulation and the equator to pole temperature gradient is too steep as compared to proxy data for europe the comparison with proxy data indicates that the tortonian simulation underestimates the mean annual temperatures but precipitation patterns are quite consistent between model results and proxy data overall the tortonian run is quite realistic and hence the model simulation contributes to a better understanding of the relevant climate processes in the miocene however the discrepancies to proxy data demonstrate that we still do not sufficiently understand the late miocene climate

development of asian monsoon revealed by marked ecological shift during the latest miocene in northern pakistan carbon isotopes from soil carbonate1 4 and soil organic matter5 6 yield palaeoecological information because the carbon in the soil carbonate forms in isotopic equilibrium with local soil co2 refs 1 7 the isotopic composition of which is in turn determined by local plant cover siwalik group sediments in northern pakistan contain a well exposed palaeosol record spanning the past 18 myr here we report on stable carbon isotope results from associated pedogenic carbonate which indicate a dramatic ecological shift from to dominated floodplain biomass beginning 7 4 7 0 myr ago the earlier c3 floodplain biomasses were probably mainly composed of trees and shrubs whereas c4 grasslands dominated in the plio pleistocene oxygen isotopes also exhibit a shift in the latest miocene probably corresponding to a major climate change which may have induced the forest to grassland transition this dramatic ecological shift in the latest miocene may mark the inception or a marked strengthening of the asian monsoon system

an oligocene microthermal forest dominated by nothofagus in sierra baguales chilean patagonia response to global cooling and tectonic events abstract a large fossil leaf assemblage 3700 specimens is reported from the oligocene rio leona formation in the sierra baguales of chilean patagonia the association comprises 29 species but is dominated by nothofagus genera which constitutes 65 of specimens the collection can be classified as a mixed palaeoflora of the austral antarctic association stratigraphic analysis of the assemblage indicates a decrease in species diversity and richness over time which was accompanied by species turnover quantitative studies of foliar morphology clamp leaf margin analysis indicates cool to cold and dry climatic conditions with a mean annual temperature of 9 2 c a relatively high seasonality in temperature and precipitation and a mean annual precipitation of 931 mm nothofagus only became dominant in southeastern patagonia during the rupelian coinciding with a marked global cooling period linked to the initiation of glaciation in antarctica about 34 ma the decrease in precipitation following this event is attributed to the development of a rain shadow to the east of the rising southern patagonian andes which must have been of the order of 1000 m or more for topographic climate forcing to take effect this contrasts with the rain shadow development east of the andes at lower latitudes which was mainly manifested after the middle miocene

using fossil leaves as paleoprecipitation indicators an eocene example estimates of past precipitation are of broad interest for many areas of inquiry including reconstructions of past environments and topography climate modeling and ocean circulation studies the shapes and sizes of living leaves are highly sensitive to moisture conditions and assemblages of fossil leaves of flowering plants have great potential as paleoprecipitation indicators most quantitative estimates of paleoprecipitation have been based on a multivariate data set of morphological leaf characters measured from samples of living vegetation tied to climate stations however when tested on extant forests this method has consistently overestimated precipitation we present a simpler approach that uses only the mean leaf area of a vegetation sample as a predictor variable but incorporates a broad range of annual precipitation and geographic coverage into the predictor set the significant relationship that results in addition to having value for paleoclimatic reconstruction refines understanding of the long observed positive relationship between leaf area and precipitation seven precipitation estimates for the eocene of the western united states are revised as lower than previously published but remain far wetter than the same areas today abundant moisture may have been an important factor in maintaining warm frost free conditions in the eocene because of the major role of water vapor in retaining and transporting atmospheric heat

measurement of temperature in a margin of ice stream b antarctica implications for margin migration and lateral drag ice temperature was measured in and around the chaotically crevassed south margin of ice stream b antarctica from 1992 to 1994 the temperatures at 30 m depth in the chaotic zone are about 12 k lower than in the adjacent uncrevassed ice due to the ponding of cold winter air at depths greater than 150 m there is clear evidence of internal heating of the ice due to the large shear deformation rate in the marginal zone analysis of the depth of cooling below the crevasses and of the internal heating gives two pieces of information first over the last half century the lateral shear stress averaged 2 0 x 10 5 pa in the top third of the margin and second the margin moved outward at an average rate of 7 3 m a these values do not involve any assumptions about the flow law of ice the uncertainties are roughly 20 the value of lateral shear stress indicates that the most of the drag on the ice stream is along its sides

estimation of rainfall variables from leaf characters in tropical africa abstract regression models are defined for leaf physiognomic and climate variables from 30 modern equatorial african plant communities herbarium specimens provide physiognomic data for published species lists for the 30 sites climate data are from a variety of published sources by limiting the sampled region to equatorial africa where annual amount and seasonality of rainfall are critical determinants of vegetation leaf physiognomy is found to be related linearly to moisture variables principal components analysis demonstrates that the pattern of variation among leaf variables is primarily a function of mean annual and wet months total of all months with 50 mm rainfall multiple regression models are derived for mean annual rainfall wet months rainfall and the number of dry months accuracy of each model is tested by comparing the adjusted predicted values the value for a case when it is not included in calculation of the regression line to the actual climate values at the 30 sites leaf size is the strongest correlate of moisture the mesophyll size class has the most significant positive relationship with annual and wet months rainfall entire margined species increase with wet months rainfall and temperature acuminate tip and length width ratio are also positively related to mean annual and wet months rainfall rounded base emarginate tip and acute tip are inversely related to rainfall variables regression models derived from modern plants and climate can be used to reconstruct seasonal and annual rainfall from cenozoic african leaf fossils

third version of the hazard map of galeras volcano colombia copyright c 1997 elsevier science b v all rights reserved galeras is an andesitic stratovolcano whose activity is generally characterized by vulcanian eruptions based on 14 c dating the active cone is considered to be 5000 years old during that time six major periods of eruptive activity have been identified together with 45 individual events of pyroclastic flows tephra falls lava flows and lahars excluding those reported in historic time 500 years and those which have occurred since the first signs of the current activity 1988 the stratigraphy of the past 5000 years reveals that the most common events are pyroclastic flows constituting the principal volcanic hazard of galeras additionally lava extrusion tephra falls and lahars have also occurred but are lesser hazards the two previous hazard maps of galeras did not account for the studies mentioned above methodology for this revision of the volcanic hazard map of galeras is based on 1 probabilistic zoning of phenomena associated with eruptions that have occurred during the past 5000 years and 2 modeling of other phenomena which did not appear in the geologic record such as shock waves and ballistic projectiles the map consists of three categories of hazard zones high medium and low the zone of high hazard is defined principally by the distribution of pyroclastic flow deposits while the zone of low hazard is defined by tephra fall deposits the medium hazard is a transition zone and ideally if based on the geological record does not include pyroclastic flows however it is possible that a comparatively large pyroclastic flow could extend into this zone additionally this zone defines the possible trajectories of secondary lahars

lapse rates and other variables applied to estimating paleoaltitudes from fossil floras abstract a previous method for determining paleoaltitude from fossil floras has used the difference between sea level and upland paleotemperatures which is then multiplied by the reciprocal of a normal lapse rate the present study examines other variables that can significantly influence such results terrestrial lapse rates have been calculated for 39 areas of the world based upon mean annual temperature mat mean warm month temperature mat and mean cold month ttemperature cmt these show regional and seasonal variability that is explained by factors including continentality elevation of large massifs or high base levels cold air drainage fog mountain barrier effects and the location of the subtropical high pressure zone lapse rates and projected sea level temperatures based on data from the western united states show distinct regional patterns these indicate that within this region the highest lapse rate and temperature values are correlated with 1 continental climates in the subtropical high pressure zone and 2 areas having relief where the lowest elevation land surfaces base levels occur at high altitude the data are applied to evaluate and revise the methodology for estimating paleoaltitudes variables of climatic change sea level fluctuation and paleogeography are also considered limitations in application of the method are emphasized the method is exemplified by application to the latest eocene florissant flora of colorado u s a although the results are conjectural they strongly suggest that the late eocene erosion surface in the southern rocky mountains occurred at a much higher paleoaltitude than previously thought this high elevation may have resulted from earlier laramide crustal thickening

climate impact of high northern vegetation late miocene and present the late miocene belongs to the late phase of the cenozoic climate at that time was still warmer and more humid as compared to today especially in the high latitudes corresponding to the climate situation palaeobotanical evidences support that vegetation in the high northern latitudes changed significantly from the late miocene until today to quantify the climate impact of this vegetation change we analyse how vegetation in the high northern latitudes contribute to climate evolution for that we perform climate modelling sensitivity experiments for the present and for the late miocene tortonian 11 7 ma for our present day sensitivity experiment we introduce the tortonian vegetation in the high northern latitudes for our tortonian sensitivity experiment we introduce the modern vegetation on the same grid cells in the tortonian and in the present the modern vegetation leads to a strong cooling of the northern extratropics up to 4 c nevertheless the meridional heat transports remain nearly unchanged in both cases in general the vegetation impact on climate is similar in the tortonian and in the present however some exceptions occur due to the tethys ocean in the tortonian temperatures decline only weakly in eastern europe and western asia in the tortonian climate temperatures on the sahara realm rise up to 1 5 c while the temperatures do not change remarkably in the present day climate this different behaviour is caused by a stronger and more sensitive hydrological cycle on the sahara region during the tortonian