a physical basis for the frictional melting of some rock forming minerals abstract under conditions of coseismic slip 0 1 2 0 m s 1 within the upper crust differences in the shear yield strengths fracture toughnesses melting points and to a lesser extent thermal conductivities of rock forming minerals result in a hierarchy of friction melting susceptibilities in many so called pseudotachylytes it is observed that phyllosilicates and amphiboles are preferentially consumed to form the melt phase whilst quartz and feldspar tend to survive as clasts this can be explained in terms of the disequilibrium flash melting of those mineral phases possessing the lower shear yield strengths fracture toughnesses and thermal conductivities rather than by eutectic melting that would otherwise generate equilibrium minimum melts indicative of igneous processes at high strain rates typical of coseismic faulting the mechanical behaviour of minerals is primarily controlled by their fracture toughnesses with progressive comminution during slip leading to fusion the friction melting dependence on the mechanical properties of minerals indicates that there may be an important relationship between wall rock lithology and earthquake magnitude if frictional melting results in fault lubrication

intercalibration of astronomical and radioisotopic time the 40ar 39ar radioisotopic dating technique is one of the most precise and versatile methods available for dating events in earth s history but the accuracy of this method is limited by the accuracy with which the ages of neutron fluence monitors dating standards are known calibrating the ages of standards by conventional means has proved difficult and contentious the emerging astronomically calibrated geomagnetic polarity time scale apts offers a means to calibrate the ages of 40ar 39ar dating standards that is independent of absolute isotopic abundance measurements seven published 40ar 39ar dates for polarity transitions nominally ranging from 0 78 to 3 40 ma are based on the fish canyon sanidine standard and can be compared with apts predictions solving the 40ar 39ar age equation for the age of the fish canyon sanidine that produces coincidence with the apts age for each of these seven reversals yields mutually indistinguishable estimates ranging from 27 78 to 28 09 ma with an inverse variance weighted mean of 27 95 0 18 ma normalized residuals are minimized at an age of 27 92 ma indicating the robustness of the solution

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

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

holocene climatic variability indicated by a multi proxy record from southern africa s highest wetland the eastern lesotho highlands experience climate patterns distinct from those of surrounding lower altitude regions representing a niche environment with a unique biodiversity leading to well adapted but restricted vegetation this study explores changes in the holocene composition of diatoms and pollen at southern africa s highest altitude wetland mafadi 3390 m a s l the palaeoenvironmental record for mafadi wetland indicates fluctuations between cold wet conditions prevalent between 8140 and 7580 cal yr bp and between 5500 and 1100 cal yr bp and warmer drier periods between 7520 and 6680 cal yr bp and between 6160 and 5700 cal yr bp marked climatic variability is noted from 1100 cal yr bp with colder conditions at 150 kyr bp notably the first of these cold periods occurs soon after the northern hemisphere 8 2 kyr event while a second period of notably cold conditions occurs around 1100 cal yr bp variability exists between the moisture reconstructions presented in this study and those from adjacent lower altitude sites which is hypothesised to reflect variations in the strength and extent of the westerlies throughout the holocene

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

cretaceous to neogene cooling and exhumation history of the oetztal stubai basement complex eastern alps a structural and fission track study the central austroalpine oetztal stubai basement complex osk of the eastern alps together with its overlying sedimentary brenner mesozoic bmz and tectonic units upper austroalpine nappes underwent extension and exhumation during the cretaceous before its incorporation into the hanging wall of the neogene brenner normal fault during this late cretaceous to early tertiary deformation the brenner mesozoic and the immediately underlying uppermost part of the osk developed a strong stretching lineation within a near horizontal and often mylonitic foliation related to consistent top to ese directed transport quartz and calcite behaved ductilely throughout the area whereas dolomite shows a transition from brittle fracture in the north to plastic flow in the south consistent with the known cretaceous metamorphic temperature gradient the transition to crystal plastic flow of dolomite occurs at 480 c quartz textures indicate an increase in the contribution of prism glide together with a decrease in basal glide toward the south also consistent with the temperature gradient in the osk and bmz fission track analysis of both zircon and apatite along a n s profile in combination with existing data from the literature allows a continuous cooling path to be constructed for the osk cooling of the eastern osk started at 90 ma the thermal peak of metamorphism and ended some 30 myr later at near surface temperatures at 60 ma the northern region had already cooled below the upper limit of the apatite partial annealing zone whereas in the south cooling related to this exhumation phase stopped at temperatures between 100 and 200 c the ese directed shearing is interpreted as late cretaceous to early tertiary normal faulting that exhumed the osk and brought the upper austroalpine blaser and steinacher nappes into contact with the underlying brenner mesozoic the pattern of alpine metamorphic isograds established in the osk directly reflects the geometry and kinematics of this late cretaceous early tertiary exhumation due to low angle normal faulting

siliceous microfossils as late quaternary paleo environmental indicators at braamhoek wetland south africa this thesis contributes with information on past climate and environmental changes in south africa s summer rainfall region the study is based on multi proxy analyses on wetland peat cores and analyses of stable isotope composition 13c 18o and wood anatomy in cross sections from subtropical trees breonadia salicina the peat archive covers the last 16 ka ka 1000 cal yrs bp and was analysed in terms of fossil pollen charcoal diatoms phytoliths and stable isotope composition the peat record infers relatively wet climate conditions at c 13 7 12 8 ka 10 5 9 5 ka and 2 5 0 5 ka and drier conditions at c 16 13 7 ka 12 8 10 5 ka and during mid holocene tentatively cooler late pleistocene temperatures shifted towards warmer after c 9 5 ka the study of b salicina demonstrates the paleo climatic value of subtropical trees despite absence of annual tree rings an age model was constructed from radiocarbon dating and calibration by wiggle matching 13c in b salicina shows a co variation with annual rainfall amounts suggesting that it may be considered a regional climate proxy 18o is mainly influenced by local factors but acts as a useful complement when interpreting 13c together with other regional high resolution records the 600 year long 13c record suggests dry climate conditions in northern south africa during the 1700s and mid 1500s ad inferred climate and environmental changes are suggested being a response to expansion contraction and latitudinal shifts of the tropical subtropical and mid latitude atmospheric circulation cells an observed inverse humidity pattern between southern and equatorial africa suggests that enso like teleconnections may be a possible forcing mechanism in a decadal to centennial time perspective

palaeogeography and geodynamic evolution of the gosau group of the northern calcareous alps late cretaceous eastern alps austria abstract the sedimentation of the late cretaceous gosau group of the northern calcareous alps records the complex geodynamic evolution of the northern part of the austroalpine microplate the tectonically controlled sedimentation and facies distribution were mainly governed by the oblique subduction of the pennic ligurian ocean below the austroalpine margin most of the gosau successions comprise two distinct sedimentary complexes the lower gosau subgroup upper turonian campanian is characterized by terrestrial to shallow marine facies associations alluvial fan and fan delta deposits shallow marine sandstones and sandy limestones and storm influenced nearshore and shelf deposits during the early santonian sea level highstand a great part of the northern calcareous alps was covered by a shallow shelf sea palaeotransport directions point to source areas situated both to the north and to the south of the northern calcareous alps the upper gosau subgroup upper santonian eocene comprises deep water hemipelagic and turbiditic deposits rapid subsidence into bathyal to abyssal depths started diachronously from the northwest santonian to the southeast maastrichtian after a short phase of deformation and erosion abyssal deposits in the northern parts of the northern calcareus alps and north directed palaeotransport indicate a tilting of the slope towards the north this subsidence pulse is attributed to an event of subcrustal tectonic erosion probably triggered by the subduction of a topographic high of the south penninic ocean the relationships of the gosau group of the austroalpine unit to late cretaceous deposits of the carpathians and southern alps are discussed in the framework of a plate tectonic model

pollen analysis of hyena coprolites and sediments from equus cave taung southern kalahari south africa equus cave in quaternary tufa near taung in the semiarid woodland of the southern kalahari yielded 2 5 m of sediment in which a rich assemblage of bones and coprolites was preserved the fossils were accumulated mainly by hyenas during the late pleistocene and holocene pollen from coprolites reflects diet as well as vegetation over relatively large areas visited by hyenas while pollen from sediments represents more local sources the pollen sequence derived from coprolites and sediments demonstrates how the vegetation evolved from open grassland with small shrubs and occasional trees during the late pleistocene to open savanna with more small shurbs then during the last 7500 yr to modern savanna temperatures were not more than 4 c cooler and it was slightly moister than today during the late pleistocene phase it became gradually warmer but relatively dry before optimal temperature and moisture conditions developed around 7500 yr b p climatic conditions slightly less favorable for woodland occurred during part of the late holocene

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

a late quaternary record of monsoon variability in the northwest kimberley australia abstract understanding of the late quaternary environment of australia s vast kimberley region has to date been hindered by the region s lack of classic palaeoenvironmental archives such as deep lake sediments however mound spring peat deposits in the region have been found to be a potentially rich archive of palaeoenvironmental data here we present a high resolution record from black springs mound spring in the kimberley s northwest filling some of the current gaps in knowledge of the region s environmental history this builds on a 6000 year record developed from the same site and indicates that since the last glacial interglacial transition the australian summer monsoon has varied greatly in intensity with an increase in monsoonal precipitation from 14 000 yr bp and pronounced drying in the late holocene despite some chronological uncertainties thought to be due to the inclusion of younger microscopic root fragments changes in the record compare well with other records of climatic change from the kimberley and across tropical northern australia

a multi proxy analysis of late quaternary palaeoenvironments sekhokong range eastern lesotho the eastern lesotho highlands host an array of periglacial and glacial geomorphic features their analysis has provided past climate interpretations predominantly for cold periods yet no multi proxy temporally continuous palaeoenvironmental records exist this study presents a palaeoenvironmental reconstruction based on sedimentary characteristics fossil pollen and diatoms from an alpine wetland located in the sekhokong mountain range the record commences in the late pleistocene with a wet period from 16 450 to 14 440 cal a bp interrupted by dry conditions from 16 350 to 15 870 cal a bp from 14 150 to 8560 cal a bp drier conditions are inferred slowly transitioning to warmer wetter conditions warmer dry conditions are inferred for 8560 7430 cal a bp followed by cold wet conditions from 7280 to 6560 cal a bp a dry warmer period occurs from 6560 to 3640 cal a bp indicated by pollen diatom and sedimentary records followed by cool wet conditions from 3400 to 1200 cal a bp the period from 1110 cal a bp to the present is characterized by progressive drying pronounced cold events are detected from the diatom record moisture records appear relatively specific to the topographic setting of sekhokong near the great escarpment edge probably driven by orographically constrained synoptic controls

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

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

radiogenic isotopes in fluid inclusions radiogenic isotopes studied in fluid inclusions are still a limited field with great potential for expansion as analytical techniques improve the main limitation for sr ar and he isotope work is the very small number of radiogenic atoms produced in a typical fluid inclusion the requirements to analysts are correspondingly high examples show that isotopic tracing on fluid inclusion fluids can be a decisive tool in solving geological problems

vegetation and environmental changes at the middle stone age site of wonderkrater limpopo south africa wonderkrater a middle stone age site in the interior of south africa is a spring and peat mound featuring both paleoclimatic and archaeological records the site preserves three small msa lithic assemblages with age estimates of 30 ka 45 ka and 138 01 7 7 ka here we present results of the pollen analysis of a core retrieved from the middle of the peat mound which covers with hiatuses the timespan between ca 70 10 ka and 30 ka pollen percentages of terrestrial local aquatic and semi aquatic plants reveal changes in the regional climate and in the water table of the spring results identify regional wet conditions at ca 70 10 ka followed by a dry and a wet period between 60 ka and 30 ka superimposed on these three phases recurring changes in the size and depth of the water table are observed between 45 ka and 30 ka wet conditions at 70 ka and 30 ka are tentatively correlated here with marine isotope stage 4 and heinrich stadial 3 respectively a warm and dry savanna landscape was present during human occupation older than 45 ka and a wet phase was contemporaneous with the final occupation dated at 30 ka

a late triassic age for the rochechouart impact structure france argon 40 argon 39 laser spot fusion dating of pseudotachylyte from the 25 km diameter roche chouart impact structure of western central france yields a matrix age of 214 c 8 ma 20 field evidence indicates that the pseudotachylyte was generated during the modification stage of the impact process prob ably during transient cavity collapse this new age is considerably older than the previously accepted age of 186 8 ma for this structure which was obtained from hydrothermally altered melt sheet samples the new age is in accordance with earlier paleomagnetic and fission track data which indicated that rochechouart was formed during the late triassic moreover the new determination is in agreement with the regional geolog ical setting and field relations of the structure the new age of 214 2 8 ma falls within the norian stage of the triassic system

pseudotachylyte controversy fact or friction high speed slip experiments performed on westerly granite using friction welding apparatus reveal that comminution is an essential precursor to melting by friction observations of slip surfaces via analytical scanning electron microscopy sem document the following sequence of events occurring in 2 s with increasing velocity up to 2 m s fracture progressive comminution surface melting of mineral fragments fragment to fragment adhesion and finally production of a fragment laden melt supported suspension explosive dehydration and melting of the epidote group mineral allanite indicates that temperatures of at least 1000 c were realized at the interface this is corroborated by calculation of the temperature rise for the known operating conditions contrary to earlier proposals these results show that comminution and frictional melting are complementary and not mutually exclusive processes depending on the velocity shear stress displacement relations prevailing during frictional slip rocks produced in seismogenic zones can be predominantly comminuted wall rock or fragment melt mixes pseudotachylytes

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

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

ar diffusion in hydrous silicic melts implications for volatile diffusion mechanisms and fractionation the effect of dissolved water on the diffusivity of ar in glasses and melts of rhyolitic and albitic compositions was investigated experimentally at pressures up to 1500 mpa and water contents of 0 1 5 wt the data for water poor rhyolitic composition at 500 mpa can be described in the whole temperature range of 480 1102 c by a simple arrhenius relationship dar 2 14u10 36 m 2 s exp 318 883 t a 4 0 wt increase in water content increases the ar diffusivity by approximately one order of magnitude in both rhyolitic and albitic melts at 1000 c in contrast to viscosity and total water diffusion an exponential dependence of ar diffusivity on water content was observed for the rhyolitic composition in the whole range of water contents for water poor rhyolite ar diffusivity depends on pressure with an apparent activation volume of 13 15 cm 3 mol at pressures up to 800 mpa for water rich rhyolite v5 wt water there is no significant pressure effect at 1000 c in the range 500 1500 mpa combining our data with previous data from carroll m r carroll earth planet sci lett 103 1991 156 168 ar diffusivity in 10 312 m 2 s in rhyolitic melts can be expressed as

40ar 39ar age gradients in micas from a high temperature low pressure metamorphic terrain evidence for very slow cooling and implications for the interpretation of age spectra estimates of the time temperature history of geologic samples are often based on 40 ar 39 ar ages obtained through incremental heating experiments on minerals with different ar retentivities laser incre mental heating of single crystals of muscovite and biotite from an 1700 ma monzogranite from a low pressure high temperature metamorphic terrain in central arizona suggested an essentially uniform distribution of radiogenic 40 ar in each crystal and yielded ages of 1412 5 ma for the muscovite and 1410 10 ma for the biotite the similarity in these ages implies rapid 25 k m y cooling of the monzogranite after a reheating episode slightly before 1412 ma in contrast detailed age mapping of single crystals from the same samples with the laser microprobe revealed large radiogenic 40 ar gradients indicative of very slow 400 m y evidence for such low cooling rates has important implications for the longevity of high thermal gradients in the early middle proterozoic lithosphere in addition inconsistency between the laser mapping and incremental heating results for these micas suggests caution when interpreting incremental heating data for slowly cooled samples

frictional melting processes and products in geological materials introduction and discussion this special issue is the outcome of a theme session convened by the above authors entitled frictional melting processes and products in geologic materials held at the 1990 geological society of america annual meeting in dallas texas u s a the goals of the session were threefold to bring together researchers working on various aspects of frictional melting in rock to provide a forum for the exchange of new ideas and to encourage the production of a thematic set of papers dedicated to the topic of the eight papers presented here the first two spray swanson are primarily processand mechanism oriented the next magloughlin maddock o hara deal primarily with chemical aspects of host rock pseudotachylyte relations the last three mainly provide descriptions of naturally occurring koch and masch techmer et al and artificially produced kennedy and spray friction melts natural pseudotachylytes are described from strike slip zones magloughlin swanson thrusts and their listric counterparts koch and masch techmer et al and connecting lateral ramps o hara thus it appears that there exists an association between pseudotachylyte generation and relatively long lived largedisplacement faulting and shearing the prime objective of the meeting was to promote the integration of different approaches

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

strontium diffusion kinetics in plagioclase feldspars abstract a pronounced systematic variation by a factor of 104 in strontium tracer diffusion kinetics has been measured in four feldspar specimens that span the range of plagioclase compositions the experiments were carried out in air using isotopically enriched 86sr and analyzed with an ion microprobe the arrhenius relations for strontium diffusion are plagioclase ab d 0 m 2 s q kj g atom sr temperature range c albite 98 1 05e 279 550 1080 oligoclase 73 7 4e 7 261 750 1100 labradorite 36 1 1e 6 295 800 1300 anorthite 4 5 7e 9 267 900 1300 no difference was found within the measurement uncertainty between these results and those for albite under hydrothermal conditions at one kbar 100 mpa strontium diffusion rates parallel to and normal to the albite twin lamellae are the same there is no effect on the strontium diffusion rate in albite between an oxygen partial pressure of 0 2 bar air and 10 19 bar graphite buffer a relation is developed that permits calculation of the strontium diffusion arrhenius parameters for any normal plagioclase composition the large variation in diffusion rate with composition has a major effect on the ease of strontium isotope exchange at high temperatures between plagioclases and coexisting phases in rocks which may prove useful in determining thermal histories

time calibration of a pt path from the western tauern window eastern alps the problem of closure temperatures new hornblende k ar and 39ar 40ar and mica rb sr and k ar ages are used to place specific timemarks on a well constrained pressure temperature path for the late alpine metamorphism in the western tauern window after identification of excess 40ar the closure behavior of ar in hornblende is compared with that of sr and ar in phengite and biotite samples were collected in three locations whose maximum temperatures were 570 c zemmgrund 550 c pfitscher joch and 500 540 c landshuter hutte

geochronological constraints on the evolution of the periadriatic fault system alps fault rocks from various segments of the periadriatic fault system paf alps have been directly dated using texturally controlled rb sr microsampling dating applied to mylonites and both stepwise heating and laser ablation 40ar 39ar dating applied to pseudotachylytes the new fault ages place better constraints on tectonic models proposed for the paf particularly in its central sector along the north giudicarie fault oligocene e se directed thrusting 29 32 ma is currently best explained as accommodation across a cogenetic restraining bend within the oligocene dextral tonale pustertal fault system in this case the limited jump in metamorphic grade observed across the north giudicarie fault restricts the dextral displacement along the kinematically linked tonale fault to 30 km dextral displacement between the tonale and pustertal faults cannot be transferred via the peio fault because of both late cretaceous fault ages 74 67 ma and sinistral transtensive fault kinematics in combination with other pseudotachylyte ages 62 58 ma widespread late cretaceous paleocene extension is established within the austroalpine unit coeval with sedimentation of gosau group sediments early miocene pseudotachylyte ages 22 16 ma from the tonale pustertal jaufen and passeier faults argue for a period of enhanced fault activity contemporaneous with lateral extrusion of the eastern alps this event coincides with exhumation of the penninic units and contemporaneous sedimentation within fault bound basins

preliminary uvlamp determinations of argon partition coefficients for olivine and clinopyroxene grown from silicate melts an ultra violet laser ablation microprobe uvlamp has been applied for the first time to investigate argon partition coefficients for olivines and clinopyroxenes grown from silicate melts at 1 bar argon pressure these preliminary measurements yield crystal melt partition coefficients ranging from 0 138 0 01 to 0 013 0 003 for olivine and 0 589 0 003 to 0 0016 0 0005 for clinopyroxene the higher values may indicate sub microscopic melt inclusions or some other heterogeneous distributions of non equilibrium argon in the crystals the lower values are probably more representative of true partition coefficients and fall at least an order of magnitude below the previously reported experimental data the possibility of anomalous high argon contents for crystals in previous studies is discussed in terms of surface adsorption trapped argon and early partial melting

isotopic equilibration during partial melting an experimental test of the behaviour of sr abstract experiments using isotopically enriched sr doped minerals designed to test for isotopic equilibrium between source and melt during partial melting reveal that 87 sr 86 sr ratios of the liquids are primarily determined by the proportions of minerals consumed and vary with the advancement of the melting reaction the experiments were performed at 1 atm on model crustal assemblages composed of pairs of natural plagioclase an 68 87 sr 86 sr 0 701 and synthetic fluorphlogopite doped with 90 ppm sr having 87 sr 86 sr 4 2 sims traverses showed that during the initial stages of the reaction liquids are isotopically zoned all the analyzed melts have 87 sr 86 sr markedly higher than that of the bulk starting assemblage i e the source because of the faster melting rate of fluorphlogopite at 1200 c and 1250 c melting occurs above a critical temperature wherein the dissolution rates of the crystals are controlled by diffusion of species in the melt and reactants and reaction products are out of isotopic equilibrium this is due to faster melt crystal boundary migration when compared to sr diffusion in the crystals equilibration is possible only if melting stops calculations show that total equilibration between melt and residue by sr tracer diffusion in the crystals takes 10 4 10 6 yr for temperature and grain size ranging respectively from 800 c to 1000 c and 0 1 to 1 cm when compared to the proposed residence time of crustal magmas at their sources this result strongly suggests that magmas that do not reflect the bulk isotopic characteristics of their source regions can be produced

dating fault generated pseudotachylytes comparison of 40ar 39ar stepwise heating laser ablation and rb sr microsampling analyses three different geochronological techniques stepwise heating laser ablation 40ar 39ar rb sr microsampling have been evaluated for dating fault generated pseudotachylytes sampled along the periadriatic fault system paf of the alps because pseudotachylytes are whole rock systems composed of melt clast and alteration phases chemical control from both ar isotopes cl k ca k ratios and empa analyses is crucial for their discrimination when applied to stepwise heating 40ar 39ar analyses this approach yields accurate melt related ages even for complex age spectra the spatial resolution of laser ablation 40ar 39ar analyses is capable of contrasting melt clast and alteration phases in situ provided the clasts are not too fine grained the latter of which results in integrated mixed ages without geological information elevated cl k and ca k ratios were found to be an invaluable indicator for the presence of clast admixture or inherited 40ar due to incomplete isotopic resetting during frictional melting rb sr microsampling dating did not furnish geologically meaningful ages on the basis of isotopic disequilibria among pseudotachylyte matrix phases and independent rb sr microsampling dating of cogenetic ultra mylonites the concordant 40ar 39ar pseudotachylyte ages are interpreted as formation ages the investigated pseudotachylytes altogether reveal a cretaceous to miocene history for the entire paf consistent with independent geological evidence individual faults however consistently reveal narrower intervals of enhanced activity lasting a few million years electronic supplementary material to this paper can be obtained by using the springer link server at http dx doi org 10 1008 s00410 002 0381 6

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

fossil oribatid mites acari oribatida from the florisbad quaternary deposits south africa in a pioneer application of acarology to quaternary fossil bearing sediments in southern africa the oribatid composition in the florisbad quaternary sediments was determined and compared to the currently known distribution of those species nine species of oribatid mites were recorded in the holocene aeolian deposits of the third test pit three species from the middle stone age msa horizon sediments of the third test pit and thirteen species from the holocene spring sediments the florisbad results indicate a better agreement between the oribatid fauna of the last interglacial msa horizon of the third test pit and the organic rich mid holocene deposits near the spring than between either of these and early and late holocene aeolian sediments of the third test pit suggesting some similarity in microsedimentary environments the majority of the species recorded in the sediments are parthenogenetic and can be regarded as pioneer species

the nature distribution and genesis of the coesite and stishovite associated with the pseudotachylite of the vredefort dome south africa abstract the vredefort dome represents the uplift centre of a well known 2 00 ga old impact structure of unusually large magnitude shock features like shatter cones planar features and high pressure silica polymorphs are common the present study deals with the description mode of occurrence field distribution and post shock metamorphic alteration of coesite and stishovite which were poorly documented up to now these minerals occur as unusually large crystals in the quartzite of the wall rock in contact with very thin pseudotachylite veins in the pseudotachylite itself which can be interpreted as a friction and or a shock recovery melt fine needles of kyanite are ubiquitous it is proposed that these thin pseudotachylites a type formed during the transit of the shock wave they preceded the development of thick pseudotachylite and microbreccia veins b type which formed during the tensional period which immediately followed from comparison with the model of formation of high pressure polymorphs in porous sandstone it is suggested that higher pressure and stress was concentrated along the a type veins at the arrival of the shock front at this time the quartz was transformed into a high pressure phase which was probably poorly crystalline behind the shock front that is during the rarefaction the pressure was progressively released and the polymorphs crystallized from this initial high pressure phase stishovite crystallized first followed by coesite the high pressure conditions may have lasted an unusually long time due to the magnitude of the vredefort impact this long time probably about one second may account for the large size of the coesite and stishovite crystals most of the high pressure silica polymorphs are corroded to a variable degree by secondary quartz and preserved only in a restricted area of the impact structure this alteration is attributed to post shock metamorphism due to the temperature of the rock before impact plus the heat left after shock recovery some of the evidence put forward by authors against the impact model is discussed according to the facts arising from this investigation it is concluded that an origin by impact remains the simplest and most realistic model to explain the origin of the vredefort dome

reconstruction of environmental and climate changes at braamhoek wetland eastern escarpment south africa during the last 16 000 years with emphasis on the pleistocene holocene transition abstract a paleo environmental record covering the last 16 ka 16 000 cal yrs bp from the eastern areas of the summer rainfall region in south africa is presented this area is until now sparsely investigated due to the lack of well preserved natural archives for this study we used a peat section from a wetland situated close to the drakensberg escarpment where the high annual rainfall amounts supported a continuous peat accumulation since c 16 ka one peat core was analysed in terms of fossil pollen composition carbon and nitrogen content isotope composition 13c 15n and microscopic charcoal concentration the greatest degree of temporal resolution was achieved from the late pleistocene and early holocene section where proxy records indicate relatively dry conditions between ca 16 13 7 ka 12 8 10 5 ka 9 5 8 2 ka and wet conditions between c 13 7 12 8 ka and 10 5 9 5 ka a weak moisture signal is also evident at c 8 2 7 5 ka the late pleistocene to early holocene period was relatively cool while conditions became generally warmer after 11 8 ka the interpretation of the mid and late holocene sequence is limited due to a slow accumulation and low sample resolution but the available data suggest relatively dry conditions until c 1 5 ka followed by more humid conditions until c 0 5 ka we suggest that the millennial scale variability within the record can be attributed to shifts in the circulation systems dominating the region i e the latitudinal movements of the inter tropical convergence zone itcz and the dynamics of the mid latitude low pressure belts

carbon and oxygen isotope stratigraphy of the english chalk and italian scaglia and its palaeoclimatic significance a detailed carbon and oxygen isotope stratigraphy has been generated from upper cretaceous coastal chalk sections in southern england east kent culver cliff isle of wight eastbourne and seaford head sussex norfolk coast and the british geological survey bgs trunch borehole norfolk data are also presented from a section through the scaglia facies exposed near gubbio italian apennines wherever possible the sampling interval has been one metre or less both the chalk and scaglia carbon isotopic curves show minor positive excursions in the mid cenomanian mid and high turonian basal coniacian and highest santonian lowest campanian there is a negative excursion high in the campanian in chalk sections that span that interval the well documented cenomanian turonian boundary spike is also well displayed as is a broad positive excursion centred on the upper coniacian a number of these positive excursions correlate with records of organic carbon rich deposition in the atlantic ocean and elsewhere the remarkable similarity in the carbon isotope curves from england and italy enables cross referencing of macrofossil and microfossil zones and pinpoints considerable discrepancy in the relative positions of the turonian coniacian and santonian stages the oxygen isotope values of the various chalk sections although showing different absolute values that are presumably diagenesis dependent show nonetheless a consistent trend the east kent section which is very poorly lithified indicates a warming up to the cenomanian turonian boundary interval then cooling thereafter regional organic carbon burial documented for this period is credited with causing drawdown of co 2 and initiating climatic deterioration inverse greenhouse effect data from other parts of the world are consistent with the hypothesis that the cenomanian turonian temperature optimum was a global phenomenon and that this interval represents a major turning point in the climatic history of the earth

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

a late quaternary record of seasonal sea surface temperatures off southern africa the southern cape coastal region is important for understanding both the behavioural history of modern humans and regional and global climate dynamics because it boasts a long archaeological record and occupies a key geographical location near the intersection of two major oceans the western boundary agulhas current implicated in global heat exchange dynamics is an important modulator of southern african climates and yet we understand its past behaviour only broadly as the current itself scours the coastal shelf and marine sediment core records necessarily provide little detail numerous archaeological sites from both the late pleistocene and holocene provide the opportunity for reconstruction of near shore seasonal sst records which respond both to localized wind driven upwellings and agulhas temperature shifts corresponding in turn with terrestrial precipitation trends in the near coastal and summer rainfall regions here we present a record of seasonal ssts extending over mis5 mis4 and the holocene from serial 18o measurements of a single gastropod species turbo sarmaticus the results show that mean sst shifts accord well with global sst trends although they are larger than those recorded in the agulhas current from coarser scale marine sediment records comparison with a record of antarctic sea ice suggests that annual sst amplitude responds to antarctic sea ice extent reflecting the positioning of the regional wind systems that drive upwelling dynamics along the coast thus near shore sst seasonality reflects the relative dominance of the westerly and easterly wind systems these data provide a new climate archive for an important but understudied climate system

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

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

isotopic dating of strain fringe increments duration and rates of deformation in shear zones the time scales over which deformation in the earth9s crust remains localized in shear zones are poorly known as are the associated strain rates we have determined the longevity and rates of deformation using rubidium strontium rb sr microsampling dating of increments of fibrous strain fringes from a pyrenean shear zone the fibers grew quasi continuously through a protracted deformation history between 87 and 50 million years ago over a period comparable to that of an orogeny during a short interval between 66 and 62 million years a rise in strain rate from 1 1 10 15 to 7 7 10 15 seconds 1 occurred this acceleration correlates with an abrupt change in fiber growth direction and a stress field inversion from gravitational collapse to renewed horizontal crustal shortening

pseudotachylytes generated during seismic faulting and eclogitization of the deep crust pseudotachylytes are typically interpreted to have formed by frictional melting during coseismic faulting within the upper to middle crust pseudotachylytes in the bergen arcs of western norway contain microlites including omphacite garnet plagioclase and quartz this eclogite facies assemblage is stable at temperatures of about 800 c and pressures of 18 to 19 kilobars corresponding to depths of 60 kilometers or more the pseudotachylytes are exposed in grenvillian granulites that locally underwent fluid induced eclogitization and corresponding volume reduction of approximately 10 percent during the caledonian continental collision the pseudotachylytes may have formed as a result of the rapid relaxation of stresses caused by the eclogitization process