the stratigraphic and sedimentological significance of late devensian ice sheet surging in holderness yorkshire u k amino acid analyses of marine mollusc valves show that the basement till of holderness yorkshire is of late devensian age ca 20 000 bp and not pre ipswichian 125 000 bp as is traditionally supposed together with the overlying skipsea and withernsea tills the basement till is argued to be a deformation till resulting from the repeated onshore surging of late devensian ice over a muddy sea floor and the subglacial transport attenuation and mixing of marine sediment silts on the surface of the basement till yielding 14c dates of 18 500 and 18 240 bp have been regarded as minimum ages for late devensian glaciation in eastern england e g the dimlington stadial of rose 1985 1989 the amino acid data presented here indicates that the maximum of the late devensian glaciation in eastern england occurred earlier sedimentological interpretation of the late devensian tills of holderness as glacially reworked marine sediments supports previous glaciological models of the british ice sheet involving surging of ice lobes along the coast of eastern england the basement skipsea and withernsea tills are separated by shallow marine sediments and may be representative of deformable bed till facies deposited below pleistocene ice sheets moving over soft fine grained sediments

pollen taphonomy in a canyon stream abstract surface soil samples from the forested chuska mountains to the arid steppe of the chinle valley northeastern arizona show close correlation between modern pollen rain and vegetation in contrast modern alluvium is dominated by pinus pollen throughout the canyon it reflects neither the surrounding floodplain nor plateau vegetation pollen in surface soils is deposited by wind pollen grains in alluvium are deposited by a stream as sedimentary particles clay size particles correlate significantly with pinus quercus and populus pollen these pollen types settle as clay does in slack water chenopodiaceae amaranthus artemisia other tubuliflorae and indeterminate pollen types correlate with sand size particles and are deposited by more turbulent water fluctuating pollen frequencies in alluvial deposits are related to sedimentology and do not reflect the local or regional vegetation where the sediments were deposited alluvial pollen is unreliable for reconstruction of paleoenvironments

late archaean to early proterozoic granitoid magmatism and high grade metamorphism in the central limpopo belt south africa the central zone of the limpopo belt in southern africa has previously been interpreted as a segment of archaean crust which experienced its main deformation and metamorphism around 2 7 ga ago we report new single zircon u pb and pb pb ages for granitoid gneisses supracrustal rocks and anatectically derived granitic melt patches in the area around messina south africa the sand river gneiss is a composite suite of tonalitic to trondhjemitic rocks with protolith ages between 2 6 and 3 2 ga the singelele gneiss a heterogeneous granodioritic to quartz monzonitic rock has protolith ages between 2 55 and 2 58 ga since both the sand river and singelele gneisses experienced polyphase high strain ductile deformation this must have occurred later than 2 55 ga ago granulite facies pelitic gneisses of the beit bridge complex contain abundant spherical multifacetted zircons which reflect new zircon growth near or at the peak of metamorphism these zircons provide ages with a mean at 2026 5 6 3 ma which is interpreted as reflecting a high p t event 10 kbar 825 25 c granitic melt patches in the metapelites as well as in the sand river gneiss and anatectic granites are probably related to rapid near isothermal decompression to below 3 5 kbar and 600 750 c these rocks contain new magmatic zircons which yielded a mean age of 2005 6 4 4 ma and probably reflect a crustal melting event resulting from rehydration of the granulitic assemblage our zircon data support previous suggestions for only one single granulite facies event in the central zone and we suggest that this event occurred c 2027 ma ago since most of the deformation seen in the gneisses of the messina area must have occurred later than 2 55 ga ago it is likely that the limpopo orogeny at least in the central zone is not an archaean event but took place in the early proterozoic

deposition models for chronological records abstract long term sedimentary sequences provide a wealth of useful information for research into the palaeo environment especially in relation to past climate change shorter records provide similar information in many archaeological contexts however if such records are to be used to provide precise timing of events and more critically the relative timing between different records methods are needed to provide accurate and precise age depth models for these sequences given the imprecision of individual calibrated radiocarbon determinations it is necessary to use the information we have about the deposition process to refine our chronologies and also to provide interpolation between dated levels in any sequence even with layer counted chronologies the uncertainties are sometimes hard to quantify this paper outlines a range of bayesian models for deposition which have been implemented in the computer programme oxcal these models can be used to combine information from the sediments themselves with radiocarbon or other direct dating information such models have the potential to integrate information between different records and provide a coherent chronology on which to base environmental or archaeological research

new radiometric dating of water management features at the prehistoric purron dam complex tehuacan valley puebla mexico abstract recent investigations at the prehistoric purron dam complex in the tehuacan valley of southern puebla mexico applied radiometric dating to more securely date the complex ceramic based dating in the 1960 s placed the dam s origin to the formative period while formative period origins are widely accepted the chronology lacks resolution and direct dates samples from impounded sediments behind the dam and from prehistoric canal fill were dated through optically stimulated luminescence osl as well as both standard 14 c and accelerated mass spectrometry ams assays these samples directly date the functioning of the water management features within the complex the results confirm the formative period origins of the purron dam and offer new insights into the construction and functioning of the dam and irrigation system

soil geomorphology and wet cycles in the holocene record of north central mexico abstract the distinction between the impact of climatic periodicities or land use practices on soil erosion is an important issue for pre hispanic and colonial mexico that question can best be addressed by first documenting the dynamics of changing wet cycles during the holocene in the central mexican region between the northern limits of pre hispanic agriculture and its southern margins in northwestern chihuahua consequently the laguna project targeted a 125 000 km 2 sector of north central mexico 250 km from north to south and 500 km from east to west from saltillo to durango some 40 sedimentary profiles with multiple cumulic soils were studied in the field and laboratory supported by 163 conventional 14 c and ams dates on charcoal and soil humates we distinguish 1 wet floodplains with humic paleosols redox phenomena reflecting high water tables channel ponding sequences and interbedded tufas that imply aquifer recharge sustained base flow and mainly low energy conditions and 2 high energy pulses of discharge that mobilized cobble gravels or forced channel entrenchment gullying and were tied to episodic excessive rains that promoted valley and slope instability in between such wet cycles and recurrent disequilibrium events climate was similar to today probably less humid with limited geomorphologic change or slow soil formation wet cycles were rare at the end of the pleistocene but prominent during the holocene disequilibrium proxies became common and dramatic after 2500 bp the drainages from the eastern and western sierra madres responded in phase but varied in detail around ad 1050 1200 natural erosion led to loss of soil organic carbon as alternating severe droughts and heavy rains destroyed the ground cover and led to ecological aridification well before arrival of spanish miners and settlers the evidence that human activity triggered pre hispanic or colonial erosion in central mexico should therefore be re evaluated global comparisons and interpretations are discussed but with caution since no single theory can explain the whole of the record the soil geomorphology geoarchive of north central mexico primarily is an environmental history of alternating wet cycles rather than of sustained wet or dry climates the critical differences between soaking and excessive rains with their respective impacts may be due to switching between winter and summer storm categories

the selection of sites for paleovegetational studies the judicious selection of sites for paleovegetational and paleoclimatic studies permits paleoecologists to answer specific research questions that go beyond primary descriptions of past vegetation we present a model that describes the relationship between basin size and pollen source area and predicts the proportions of local extralocal and regional pollen sampled by lake basins of different size the distinctive sampling properties of lakes peats and small hollows can be exploited to provide details of pattern in paleovegetation so long as attention is given to the limitations and problems of these types of sites combinations of site types in a single study most fully exploit the information contained in sediments

oriented lake development in the context of late quaternary landscape evolution mckinley bay coastal plain western arctic canada abstract oriented lakes characterized by elongate forms central basins and shallow littoral shelves are common features of circum arctic coastal lowlands the environmental conditions geological processes and chronology associated with the development of oriented lakes however are little known but essential for understanding how such arctic lowlands evolve using combined techniques of field and drill log stratigraphy and sedimentology luminescence and radiocarbon dating methods and geomorphic mapping we reconstruct the landscape evolution leading toward oriented lake formation on the mckinley bay coastal plain of western arctic canada a region with over 900 oriented lakes most lakes with deep central basins are inherited from a preglacial braidplain ca 73 27 ka and alluvial braided channel network that extended beyond the glacial limit ca 18 6 14 3 ka eolian erosion active during the lateglacial and postglacial period ca 12 8 1 9 ka reworked fluvial deposits eolian processes modified existing basins and created other shallow deflationary basins as small barchanoid dunes migrated under cold dry paraglacial conditions between about 12 8 and 10 7 ka vegetation cover developed at the onset of the early holocene climatic optimum ca 10 7 ka and parabolic dunes were active between 9 6 and 4 6 ka thus oriented lakes developed in basins conditioned by fluvial and eolian processes in the absence of much near surface ground ice lateral expansion of deep basin lakes and shallow stabilized deflationary basins predominated during the late holocene through wind induced wave and current processes overall this sequence of oriented lake formation does not support a thaw lake cycle but rather small basin evolution of a periglacial landscape

wet conditions during the last glaciation in the chihuahuan desert alta babicora basin mexico abstract a 467 cm core b94 3 from the alta babicora basin in the chihuahuan desert northern mexico 29 n 108 w documents lake level and climate changes over the past ca 65 000 yr chronological control is provided by four accelerator mass spectrometry 14 c dates and five u series dates on diatom silica the core has been analyzed for magnetic susceptibility loss on ignition carbonate content sediment chemistry and mineralogy and pollen and diatom composition the basin was occupied by a deep freshwater lake throughout the late pleistocene which based on shoreline evidence was at least 19 m deeper than today the lake shallowed after ca 57 000 yr b p high variability typified the period between 54 600 and 38 000 yr b p probably with periodic desiccation and deflation a deep water lake was reestablished after 38 000 yr b p and persisted until ca 29 000 yr b p shallowing occurred through the last glacial maximum although the lake was still deeper than at present the pleistocene holocene transition was marked by a distinctive change in the diatom flora there is no diatom record for much of the holocene but other proxies indicate generally dry stable catchment conditions the babicora record is more akin to those from the southwest united states than it is to central america and northern south america wet conditions in the late pleistocene are attributed to winter rainfall from midlatitudes in contrast to the modern tropical summer rainfall regime

dating the oldest terrestrial rocks fact and fiction abstract this paper is a review of well documented geochronological and related isotopic evidence for the age and total crustal residence time of the oldest known terrestrial rocks mainly using rb 1bsr sm 1bnd and u 1bpb decay schemes the oldest known stable sialic crust of true continental character predominantly composed of calc alkaline orthogneisses was produced during major mantle differentiation some 3 7 3 5 ga ago and is represented on several continents enclaves of older supracrustal rocks as well as a major remnant of a volcano sedimentary sequence of greenstone belt affinity dated at 3 8 ga are preserved within the surrounding gneisses in southern west greenland recent ionmicroprobe evidence for 4 1 ga old detrital zircon grains in a younger early archaean metasedimentary sequence in western australia is also discussed several cases are reviewed where claims for early archaean 3 4 ga dates are believed to be erroneous and to result from oversimplistic and incorrect interpretation of isotopic data some of the geological and geochemical mechanisms which have given rise to claims for spuriously old sm 1bnd u 1bpb and pb pb dates are critically discussed to supplement published data we present several new sets of whole rock sm 1bnd and pb pb isotopic data from west greenland labrador zimbabwe and eastern india relevant to the overall topic of this review

ecosystem responses to climate and disturbances in western central mexico during the late pleistocene and holocene abstract central mexico is a complex area with late quaternary volcanic activity climatic diversity and a long history of human occupation pollen microcharcoal and magnetic susceptibility analyses of two sedimentary sequences from lake zirahuen in western central mexico spanning the last 17 000 years provide evidence of a highly dynamic environment during the late glacial pinus forest developed around the lake indicating cold and dry conditions during the last deglaciation climatic amelioration was recorded at 13 5 ka driving a strong and rapid change in vegetation composition and increase in lake levels from the latest pleistocene to early holocene a hiatus probably related to an erosive event is recognized in the northern sequence the central sequence spans the entire holocene and reveals three periods of important ecological changes during the early holocene a first episode between 9 5 and 9 0 ka with a decrease in pine forest seems to have been associated with summer insolation increases a second peak of forest change at 8 2 ka and was probably associated with the cold oscillation documented in the north atlantic a third abrupt change was evident from 7 5 to 7 1 ka with an anomalous plant community related to wetter holocene climates and possible to a volcanic event an episode of dry conditions was recorded from 4 5 to 4 2 ka which was related to an increase in enso activity human influence over the landscape was evident from 3 5 ka to the present the zirahuen records offer a complex history of a landscape characterized by short and long term vegetation changes associated with factors ranging from global climate to local disturbances

late quaternary coastal evolution and aeolian sedimentation in the tectonically active southern atacama desert chile abstract analyses of aeolianites and associated dune surficial carbonate and marine terrace sediments from north central chile 27 54 s yield a record of environmental change for the coastal southern atacama desert spanning at least the last glacial interglacial cycle optically stimulated luminescence dating indicates phases of aeolian dune construction at around 130 111 98 77 69 and 41 28 ka thin section and stable carbon and oxygen isotope analyses suggest a predominantly marine sediment source for the three oldest dune phases aeolianites appear to have accumulated mainly from tectonically uplifted interglacial marine sediments that were deflated during windier and or stormier intervals bedding orientations indicate that sand transporting winds varied in direction from s ese during mis 5e and wnw ese during mis 5c 5a winds from the southeast quadrant are unusual today in this region of the atacama suggesting either major shifts in atmospheric circulation or topographic airflow modification thin section evidence indicates that the aeolianites were cemented by two phases of vadose carbonate tentatively linked to wetter periods around 70 and 45 ka tectonic uplift in the area has proceeded at an average rate of 305 542 mm kyr 1 the study illustrates the complexity of understanding onshore offshore sediment fluxes in the context of late quaternary sea level fluctuations for an area undergoing rapid tectonic uplift

aeromagnetic interpretation in the south central zimbabwe craton reappraisal of crustal structure and tectonic implications regional aeromagnetic data from the south central zimbabwe craton have been digitally processed and enhanced for geological and structural mapping and tectonic interpretation integrated with gravity data to constrain previous interpretations based on tentative geologic maps and provide new information to link these structural features to known tectonic events the derived maps show excellent correlation between magnetic anomalies and the known geology and extend lithological and structural mapping to the shallow near subsurface in particular they reveal the presence of discrete crustal domains and several previously unrecognised dykes faults and ultramafic intrusions as well as extensions to others five regional structural directions ene nne nnw nw and wnw are identified and associated with trends of geological units and cross cutting structures the magnetic lineament patterns cut across the 2 7 ga greenstone belts which are shown by gravity data to be restricted to the uppermost 10 km of the crust therefore the greenstone belts were an integral part of the lithosphere before much of the upper crustal brittle deformation occurred significantly the observed magnetic trends have representatives craton wide implying that our interpretation and inferences can be applied to the rest of the craton with confidence geological tectonic correlation suggests that the interpreted regional trends are mainly 2 5 ga great dyke age and younger and relate to tectonic events including the reactivation of the limpopo belt at 2 0 ga and the major regional igneous dyking events at 1 8 2 0 ga mashonaland 1 1 ga umkondo and 180 ma karoo thus their origin is here inferred to be inter and intra cratonic collisions and block movements involving the zimbabwe and kaapvaal cratons and the limpopo belt and later lithospheric heating and extension associated with the break up of gondwana the movements produced structures or reactivated older fractures that were exploited by late archaean and proterozoic mafic intrusions there was interplay between vertical and horizontal tectonics as seen in similar terrains worldwide

the growth of the zimbabwe craton during the late archaean an ion microprobe u pb zircon study abstract u pb zircon ages obtained by ion microprobe are presented for late archaean granitoids from the zimbabwe craton and the north marginal zone of the limpopo belt similar ages of crust formation in the felsic gneisses of the zimbabwe craton and the felsic granulites of the north marginal zone support the view previously argued on geological grounds that they represent the middle and lower crustal layers respectively of the zimbabwe craton zircons from the southern margin of the zimbabwe craton and the northern part of the north marginal zone contain old cores confirming the presence of an old c 3 5 ga nucleus to the zimbabwe craton and indicating that the areal extent of this old nucleus is greater than has been hitherto supposed thus late archaean crustal growth involved both the reworking of and the addition of new magmas to this old crustal nucleus at 2 7 ga crustal growth was contemporaneous with plume volcanism in the greenstone belts in contrast crustal growth at 2 74 2 67 2 64 and 2 62 ga was focused at the margins of the old craton9 and may be related to andean style continental margin magmatism lower crustal melting at 2 61 2 59 ga led to the emplacement of granitoids in the upper crust supplementary material a summary of previously published u pb zircon geochronology for the zimbabwe craton and the north marginal zone of the limpopo belt field relationships and sample localities for the samples used in this study and u th pb secondary ionization mass spectrometry data are available at www geolsoc org uk sup18466

late holocene inland dune activity in the uk a case study from breckland east anglia inland dune fields found in breckland east anglia uk have previously been correlated with the widespread late devensian coversands of the region and a seventeenth century sandflood which is known to have inundated the village of santon downham a programme of optically stimulated luminescence and radiocarbon dating undertaken on the dunes at wangford warren and santon downham reveals that episodic aeolian activity has taken place during the last 7000 years five sand depositional phases on breckland dunes are identified c 6500 c 1600 1100 c 500 c 400 335 and from c 200 30 years before present between c 2120 1840 years ago and within the period 1100 300 years ago quiescent periods allowed the development of fen peat and soils local controlling factors on aeolian activity in breckland appear to be related to anthropogenic activities and livestock sheep and rabbits reducing vegetation cover and decreased precipitation and sea levels which led to lowering of water tables breckland aeolian activity when compared to the national and sub continental holocene aeolian activity record shows a high level of concordance the underlying forcing factor for late holocene dune activity in the uk and elsewhere in europe appears to be climatic instability in the north atlantic associated with both the little ice age and previous similar events which led to increased storminess climatic variability and lower temperatures

zircon geochronology of late archean komatiitic sills and their felsic country rocks south central zimbabwe a revised age for the reliance komatiitic event and its implications abstract the 2 8 2 6 ga bulawayan supergroup of the zimbabwe craton is inferred to be a passive margin and plume related conformable sequence comprising in part the felsic volcanic koodoovale sedimentary manjeri komatiitic reliance basaltic zeederbergs units the reliance unit is spatially associated with subjacent komatiitic sills of the mashaba ultramafic suite the inferred subvolcanic intrusive phase of the reliance flow fields we report u pb zircon ages for several units the pangani sill in the filabusi belt yielded an age of 2912 ma indicating it may belong to the 3 0 2 9 ga belingwean supergroup the koodoovale unit appears to range in age from 2866 to 2788 ma inferred syn eruptive mass flow deposits within the manjeri basinal shallow water sub facies in the midlands se belt yield ages of 2877 and 2870 ma sills of the mashaba ultramafic suite together with a previous result of 2743 ma for the mashaba igneous complex yield a mean age of 2746 ma which we propose as a proxy age for the reliance unit and for the reliance komatiitic event as a whole the 2 80 ga age for the top of the koodoovale unit in the western filabusi and gweru nw belts confirms with greater stratigraphic control earlier precise geochronology and provides an age for the start of sedimentation in the overlying manjeri basinal deep water sub facies the 2 79 ga felsic sample from within the reliance unit in the shangani n belt is likely from a koodoovale derived post eruptive mass flow deposit in the midlands se belt the new felsic rock ages support the conformable relationship between the uppermost koodoovale unit and overlying manjeri unit but place the transition at c 2 87 ga significantly earlier than in belts further west the new ages for the koodoovale unit and the revised age for the reliance unit introduce new perspectives on the development of the bulawayan supergroup 1 the koodoovale manjeri transition is diachronous across the zimbabwe craton 2 the duration of manjeri sedimentation varied from 50 ma to 120 ma in the basinal deep water and shallow water sub facies respectively to 80 ma in the platformal facies in the east where sedimentation is estimated to have begun at 2 83 ga such prolonged sedimentation is controversial but can be explained by extended periods of non deposition in the basinal deep water sub facies 3 using all published precise ages reliance zeederbergs volcanism may have lasted 70 ma 2 75 2 68 ga much longer than phanerozoic plume systems and 4 the previously proposed correlation and late archean nearest neighbour relationship between the zimbabwe and slave cratons are provided further support

first direct radiometric dating of archaean stromatolitic limestone here we report the first direct dating of the depositional age of a sedimentary carbonate rock using long lived radioactive decay schemes the mushandike stromatolitic limestone from the masvingo formerly fort victoria greenstone belt of southern zimbabwe1 2 yields a pb pb isochron age of 2 839 33 myr published claims3 for an age of 3 5 gyr for the mushandike limestone were based on its supposed intrusion by the mushandike granite for which a rb sr whole rock age of 3 445 260 myr had been reported4 new rb sr and pb pb isotope data for the mushandike granite indicate a probable emplacement age of 2 900 myr and new field evidence j l o p t j f w in preparation suggests that the granite forms part of the basement on which the limestone was deposited thus the new dates do not support an age of 3 5 gyr for the algal stromatolites at mushandike the pb pb isochron method may have wide applicability to dating the time of deposition of precambrian fossiliferous and unfossiliferous limestones and thus to the problem of the antiquity of life

holocene aridification vegetation change sedimentation regime and limits of carbon isotope data as indicated by the alluvial pollen sites of arroyo grande and el molino in north central mexico abstract a multi proxy record of palynological sedimentological archaeological and carbon isotopic data of alluvial sites in the upper nazas basin arroyo grande durango 1855 m amsl and the parras basin el molino coahuila 1245 m amsl is introduced describing environmental changes across a short grassland desert ecotone from the eastern margins of the sierra madre occidental smo these sites reconstruct holocene vegetation history in semi arid north central mexico and integrate fluvial geomorphology and palynology data in particular pollen data links a rise of xeric flora with higher energy deposition and increased erosion in upland basins of the eastern smo according to archaeological finds at examined sites human occupation is linked to mesic or afforested conditions however limitations of carbon isotope data for environmental historical reconstruction are indicated by somewhat stable values at arroyo grande durango especially while pollen sites show a replacement of short grasses and park woodlands by a more xeric flora at higher elevations with a development of chihuahuan desert flora at low elevations el molino coahuila vegetation reconstructions are supported by pollen analog data collected from the cuatro cienegas region in coahuila mexico

single zircon ages for granitoid gneisses in the central zone of the limpopo belt southern africa and geodynamic significance abstract ion microprobe evaporation and vapour digestion single zircon emplacement ages for granitoid rocks throughout the central zone cz of the limpopo mobile belt south africa define three broad age groups namely at 3188 3314 2637 2734 and ca 2500 ma respectively all these granitoids including the youngest group display polydeformational features suggesting that much of the tectonic history of the cz occurred in the late archaean and early proterozoic the oldest group of predominantly tonalitic trondhjemitic granodioritic gneisses was found in relatively small domains from the extreme w to the e of the cz and probably represents remnants of a once more widespread early archaean terrain the most abundant granitoid gneisses belong to the 2 6 2 7 ga age group and were emplaced into already ductilely deformed gneisses of the oldest group several of these rocks were derived from intracrustal melting rare metamorphic zircons with ages of 2560 2575 ma reflect a high grade metamorphic event in the cz that has also been postulated by other workers a strongly deformed orthogneiss with a protolith emplacement age of 2510 ma proves beyond doubt that at least part of the structural history in the cz occurred in the early proterozoic an anatectic granite in the extreme w of the cz intruded at 2022 ma and is probably related to the peak of granulite facies metamorphism previously dated at 2027 6 ma in the messina area the second period of granitoid magmatism is broadly contemporaneous with charnockite and enderbite emplacement and granulite facies metamorphism in the northern and southern marginal zones of the limpopo belt broad contemporaneity of extensive granitoid magmatism in all three zones raises doubts about the exotic nature of the cz and suggests a revised evolutionary model for the limpopo belt

climate agriculture and cycles of human occupation over the last 4000 yr in southern zacatecas mexico scholars attribute the growth and decline of classic period ad 200 900 settlements in the semi arid northern frontier zone of mesoamerica to rainfall cycles that controlled the extent of arable land however there is little empirical evidence to support this claim we present phytolith organic carbon and magnetic susceptibility analyses of a 4000 yr alluvial record of climate and human land use from the malpaso valley the site of one such classic frontier community the earliest farming occupation is detected around 500 bc and appears related to a slight increase of aridity similar to the level of the modern day valley by ad 500 the valley s classic period mesoamerican settlements were founded under these same dry conditions which continued into the postclassic period this indicates that the la quemada occupation did not develop during a period of increased rainfall but rather an arid phase the most dramatic changes detected in the valley resulted from the erosion associated with spanish colonial grazing and deforestation that began in the 16th century the landscape of the modern malpaso valley is thus primarily the product of a series of intense and rapid transformations that were concentrated within the last 400 yr

past vegetation influence on the hydrological cycle in the chihuahuan desert north mexico as indicated by data of four pollen sites pollen studies of four sites in the laguna mayran region of north central mexico along with sedimentary data reconstruct changes of vegetation and hydrology of both upland and lowland zones in the southern chihuahuan desert region of the past 13 000 years dual palynology sedimentology data sets presented here are indicative of major vegetation influence on the hydrological cycle that represents a factor in the interpretation of climate changes most significant reconstructions include the development ulmus quercus woodland along edges of the laguna mayran and increased evapo transpiration in contrast late holocene environments contain opportunistic wetland plants in a desert setting this suggests that higher lake levels may result at least from lower transpiration and that major precipitation events as such are episodic in the late holocene ponding also occurs at higher elevation with lower transpiration according to a prominence of cool steppe plants at the site of la angostura in the late pleistocene when evaporation levels are also lower carbon isotope data at examined sites is also suggestive of an importance of local factors in the prominence of c3 versus c4 plants

sr and nd isotope systematics of francistown plutonic rocks botswana implications for neoarchaean crustal evolution of the zimbabwe craton the francistown plutonic rocks at the south western margin of the zimbabwe craton consist of three igneous suites sanukitoid tonalite trondhjemite granite ttg suites and high k granites the ttg suite is subdivided into high aluminum ttg ha ttg and low aluminum ttg la ttg sub suites their rb sr isotope systems were partially homogenized by post crystallization thermo tectonic events in which hydrothermal solutions and migmatization played an important role therefore the rb sr isochron age of 2427 54 ma can only be regarded as a lower limit to the francistown plutonic rock age the large errors in the sm nd isochron dates of francistown granitoids indicate that these dates are not really constrained in this study we compared the rock types of francistown and adjacent areas adopting the precise u th pb single zircon shrimp ages from the vumba area as references for ttg and sanukitoid suites the age we adopted is ca 2 7 ga which is close to their depleted mantle sm nd model ages tdm for high k granites the age adopted is ca 2 65 ga which is also close to their sm nd isochron age the highest endt values of sanukitoids and ttg are 2 1 and 2 3 respectively the positive endt values and trace element geochemistry support partial melting of a depleted mantle and young oceanic crust for the genesis of sanukitoid and the ttg suites respectively the lowest endt values of sanukitoids and ttgs are 1 0 and 1 1 respectively indicating contamination by continental crust up to 10 and 14 respectively the endt values of ttg decrease with decreasing al2o3 and sr contents and increasing eu negative anomalies eu eu suggesting that the ttg magmas underwent a coupled fractionation crystallization and crustal contamination and that the la ttg was the product of the fractionation and contamination of the ha ttg sub suite in contrast negative endt values for the high k granites from 0 4 to 3 5 indicate the involvement of la ttg and some materials from an old continental crust in their genesis the products of partial melting of both oceanic and continental crusts at the south western margin of the zimbabwe craton occurred within a short time interval from 2 7 to 2 65 ga ago suggesting that the francistown plutonic rocks were formed in a active continental margin environment where a young ocean plate limpopo oceanic plate subducted underneath an old continental plate zimbabwe craton

phylogeny and paleobiogeography of triassic sauropterygia problems solved and unresolved abstract this comparison of the geographic and stratigraphic distribution of the secondarily marine stem group sauropterygian reptiles from the triassic with the phylogenetic relationships within this group indicates an incompleteness of the fossil record for the clade of almost 50 which limits the reconstruction of the biogeographical history of its component taxa given their restriction to nearshore habitats or shallow epicontinental seas it is unlikely that stem group sauropterygians had the capacity for trans oceanic dispersal dispersal along coastlines may have been a component of their biogeographical history however as is indicated by the immigration of sauropterygians into the anglo germanic basin at the transition of the lower to middle triassic and by the population of the alpine triassic by sauropterygians after the evolution of the appropriate intraplatform basin habitat during the middle triassic conversely trans panthalassan relationships of pistosaur sauropterygians appear best explained as a consequence of vicariance due to the accretion of exotic terranes at opposite margins of the pacific basin a vicariant event resulting from the opening of the southern branch of the neotethys may also account for the sister group relationships of nothosaurs from the northern gondwanan shelf and the anglo germanic basin

growth of subcontinental lithospheric mantle beneath zimbabwe started at or before 3 8 ga re os study on chromites re os isotope results on chromites separated from age constrained archean 2 7 to 3 5 ga ultramafic intrusions in the zimbabwe craton show that chromites average a present day 187os 188os ratio of 0 1066 significantly below the bulk silicate earth value the source reservoir is interpreted to reflect the zimbabwean subcontinental lithospheric mantle sclm archean re depletion of the sclm has previously been postulated on the basis of xenolith studies from southern africa siberia and the wyoming craton because our samples are age constrained however the re os data also shed light on the evolution of the archean sclm through time they indicate that the zimbabwean sclm began to be separated from asthenospheric mantle before 3 8 ga and grew quasi continuously throughout the archean this result is reconciled with present day sclm data in forward modeling

high resolution stratigraphy with strontium isotopes the isotopic ratio of strontium 87 to strontium 86 shows no detectable variation in present day ocean water but changes slowly over millions of years the strontium contained in carbonate shells of marine organisms records the ratio of strontium 87 to strontium 86 of the oceans at the time that the shells form sedimentary rocks composed of accumulated fossil carbonate shells can be dated and correlated with the use of high precision measurements of the ratio of strontium 87 to strontium 86 with a resolution that is similar to that of other techniques used in age correlation this method may prove valuable for many geological paleontological paleooceanographic and geochemical problems

the mushandike granite further evidence for 3 4 ga magmatism in the zimbabwe craton the conflict between independently published ages for the mushandike granite zimbabwe 2 92 0 17 ga and 3 45 0 13 ga has been resolved in favour of the older age by shrimp u pb analyses of zircon two samples yield indistinguishable estimates of 3374 7 and 3368 11 ma for the crystallization age of the magma together with published data from elsewhere in southern zimbabwe the results imply a widespread magmatic event at about 3 35 ga a single zircon core giving 3 46 ga together with the granite s previously measured nd model age suggests that the mushandike magma could have incorporated remobilized basement similar to the c 3 5 ga tokwe gneisses which crop out 30 km to the west the published rb sr and pb pb datasets show evidence of late archaean disturbance of sr and pb isotope systematics in the absence of exposed contacts between the mushandike granite and the neighbouring mushandike stromatolitic limestone the new u pb emplacement age suggests that the limestone is unconformable on the granite

zircon geochronology of archaean felsic sequences in the zimbabwe craton a revision of greenstone stratigraphy and a model for crustal growth u pb ion microprobe shrimp work on zircon populations from 13 zimbabwean archaean felsic rocks are presented and interpreted samples were extracted from felsic volcanic sequences from most of the major greenstone belts and represent the first zircon geochronological data from within the greenstone belts themselves the data demonstrate a late archaean volcanicity spanning 250 ma which began at least 2900 ma ago and ended at 2650 ma the intrusion of extensive granitoid sills of the chilimanzi suite at c 2 6 ga marks the widespread stabilization of the craton based on the new zircon data and a re evaluation of published mapping a new stratigraphic subdivision is presented for the late 3 3 ga fragment of archaean crust in zimbabwe these data suggest that the early archaean crust is restricted to the tokwe segment in the south of the country since even the oldest of the felsic volcanics 2 90 ga lower belingwean have zircons which are 50 ma older it is suggested that remnants of earlier cyclic greenstone granitoid events must underlie the present craton and that all of the currently exposed greenstone belts of zimbabwe were developed on sialic crust

large perturbations of the carbon cycle during recovery from the end permian extinction high resolution carbon isotope measurements of multiple stratigraphic sections in south china demonstrate that the pronounced carbon isotopic excursion at the permian triassic boundary was not an isolated event but the first in a series of large fluctuations that continued throughout the early triassic before ending abruptly early in the middle triassic the unusual behavior of the carbon cycle coincides with the delayed recovery from end permian extinction recorded by fossils suggesting a direct relationship between earth system function and biological rediversification in the aftermath of earth s most devastating mass extinction

tectonic eustatic and climatic significance of raised beaches of val de saire cotentin normandy france detailed mapping morphostratigraphical and geochronological analyses were undertaken in the val de saire cotentin normandy northern france these studies have identified a series of middle and upper pleistocene age deposits overlying four wave cut platforms pf iv pf i which can be associated with temperate high sea level events and equated with mis 11 9 7 and mi substage 5e a sea level stand recorded at 20 m ngf at the la mondree site seems to correspond to mi substage 5c or 5a the completeness of the val de saire record and particularly the width of platform iv mis 11 has so far no equivalent on the south coast of the english channel this study provides contribution to an understanding of the impact of the interglacial glacial cycles in a coastal intraplate situation the cyclicity of the morphogenesis is explained by interaction between eustatic variations and strongly expressed activity of slow uplift ca 60 mm kyr through the middle pleistocene several osl dates from the gravel to sand sequence of the eemian weichselian cycle indicate that these deposits represent the marine regression of the end of the last interglacial

tectonic evolution of a laramide transverse structural zone sweetwater arch south central wyoming structural anisotropy of magnetic susceptibility ams and paleomagnetic data record patterns of layer parallel shortening lps vertical axis rotation and regional fault fold evolution across the sweetwater arch a major west to wnw trending basement cored laramide uplift in wyoming the southern arch flank is bounded by a wnw striking reverse fault zone that imbricated basement and cover rocks the northern flank is bounded by a west striking fault zone with a component of strike slip and nw trending en echelon folds and the eastern plunge transitions into an area of multiple trending faults and folds synorogenic strata record major arch uplift from maastrichtian to early eocene time followed by arch collapse lps with development of systematic minor fault sets and ams lineations preceded large scale folding lps directions estimated from both minor fault and ams data were oriented wsw along the northern flank subparallel to laramide regional shortening but were refracted to the ssw along the southern flank and to the west along the eastern arch plunge additional minor faults developed along steep fold limbs during continued shortening with directions remaining ssw along the southern flank but becoming more variable along the eastern plunge where an increasingly heterogeneous stress field developed as additional faults were activated along basement heterogeneities vertical axis rotation was limited along the arch flanks whereas the eastern plunge underwent counterclockwise rotation deflections in shortening directions were partly related to basement heterogeneities including weak supracrustal belts on the arch flanks a strong granitic core and local reactivation of precambrian shear zones

prehistoric and colonial land use in the chihuahuan desert as inferred from pollen and sedimentary data from coahuila mexico abstract pollen along with context sedimentary data of two desert bottom sites in south coahuila mexico reconstruct holocene environment in the chihuahuan desert in relation to possible human influence on vegetation during the holocene examined pollen sites here derive from the margins and bottoms of the playa lake of the laguna mayran lm first at the reported site of cuota 4 lm4 archaeologically dated deposits of the initial spanish colonial period here are correlated with a xeric environment followed by an early modern mesic period of increased rainfall with wetland plants second at the reported site of coahuila highway 30 ch30 unconformable discontinuous deposits with intermittent representation of early middle and late holocene sedimentation are analyzed showing periodic changes in hydrology at ch30 possible cultivation based on cucurbitaceae pollen occurs in a defined wet phase dating to circa 1500 bp finally origins of pollen taphonomy at ch30 are defined by analog samples from the chihuahuan desert zone in the menchacas basin near cuatro cienegas are analyzed the pilot pollen analog study is based on inferred pollen recruitment mechanics in different sediments as well as moss pollsters in the menchacas basin in the chihuahuan desert in middle coahuila

u pb zircon evidence for an extensive early archean craton in zimbabwe a reassessment of the timing of craton formation stabilization and growth u pb single zircon analyses provide direct evidence for an enlarged early archean craton forming the core to the present zimbabwe craton virtually identical dates from the south central tokwe segment 3455 2 ma and midlands 3456 6 ma parts of the craton strongly suggest their synchronous formation during an event that formed a single early cratonic nucleus which we propose to call the sebakwe protocraton this is considered to underlie most of the current zimbabwe craton parts of the craton are at least 3565 21 ma a rock age reported here that represents the oldest rock dated from zimbabwe a ca 3350 ma relatively undeformed and unmetamorphosed intrusive granitic phase constrains the timing of the high grade metamorphism and the stabilization of the protocraton comparison with published re os data for the zimbabwe craton strongly indicates a depleted subcontinental lithospheric mantle underlying the entire sebakwe protocraton subsequent intrusive and volcanic activity from 3 0 to 2 6 ga represents a second major period of magma genesis and crustal formation within which the predominant rocks of the exposed zimbabwe craton were generated

aeolian sediments in lower to middle triassic rocks of central wyoming abstract the lower to middle triassic red peak and crow mountain formations of central wyoming have commonly been interpreted as being of shallow marine or shelf origin this paper presents conclusive evidence of an aeolian origin for parts of the two formations the interpretation is based on highly indicative sedimentary structures including wind ripple grainfall and grainflow strata lag deposits and granule ripples in the sediments of these formations facies in the upper red peak and lower crow mountain formations represent aeolian dune sandsheet wet interdune and sabkha environments emphasis is placed on the recognition and interpretation of correlative surfaces of regional extent which not only allow subdivision of the aeolian sediments into different genetic sequences but also aid in the reconstruction of the depositional history the recognition of an aeolian deposystem and of regional bounding surfaces adds valuable information to the efforts of reconstructing the triassic paleogeography of the western interior

age constraints on the palaeoproterozoic lomagundi jatuli event in zimbabwe zircon geochronology of the magondi supergroup

luminescence dating of sediments using single aliquots new procedures abstract the application of luminescence dating to quaternary sediments has been hampered by the labour intensive procedures involved determination of equivalent doses using single aliquots measured using an automated luminescence reader offers a significant reduction in labour equivalent dose determinations using two single aliquot additive dose methods are compared with equivalent doses measured using standard multiple aliquot methods for a suite of 12 dune sand samples from southwest north island new zealand the single aliquot correction method based upon adjusting the magnitude of the measured luminescence signal is found to give underestimates for those samples whose equivalent dose is greater than 80 gy and whose luminescence response to dose is non linear a second correction method based upon adjusting the value of the total added dose that the sample has received is described this second method gives equivalent dose results similar to those measured using standard multiple aliquot methods over the range from 0 1 to 400 gy

selective extinction of triassic marine reptiles during long term sea level changes illuminated by seawater strontium isotopes abstract the relationship between cycles of sea level change and marine diversity has long been the subject of debate large predators may be particularly sensitive to changes in habitat availability and marine productivity driven by changes in sea level especially those dependent on nearshore benthic food resources to test this relationship we compared the proportional diversity of differing marine reptile ecotypes through the triassic with the isotopic composition of seawater strontium 87 sr 86 sr a geochemical index linked to tectonically controlled sea level change the proportional abundance of marine reptiles adapted toward a diet of shelled prey rose during times of rapid sea level rise and fell during times of rapid sea level fall while open water forms were more resistant to these changes our results indicate that the rate of sea level change rather than the absolute magnitude of sea level or flooded shelf area played a role in shaping patterns of ecological diversification and ecologically selective extinction during the triassic the link between the isotopic composition of seawater strontium and the evolution of marine reptiles demonstrates that sea level change played an important role in the structuring of marine ecosystems over geologic time

seasonal control on coastal dune morphostratigraphy under a monsoon climate mui ne dunefield se vietnam abstract having developed in the later quaternary period the mui ne dunefield representing the upper most portion of a major coastal barrier complex in se vietnam formed by wind regimes associated with the asian monsoon climate the barrier complex is 100 km long and higher than 150 m in elevation but despite its large dimensions research on its geomorphological and geological characterization is limited several active dune fields extend 2 10 km landwards from the coast while the rest of the barrier surface is stabilized we applied ground penetrating radar gpr and quartz optically stimulated luminescence osl dating to the transverse dunes of the mui ne dunefield to characterize the morphostratigraphy and recent aeolian sedimentation history currently the dunes have a relief of typically 5 10 m and a wavelength of 50 100 m they are driven by the northeasterly winter monsoon winds and reversed with minor relief in response to the southwesterly summer monsoon gpr profiles and osl ages define three morphostratigraphic units the modern holocene and pleistocene dune units the gpr data of the modern dune unit is dominated by a series of steep reflections that dip exclusively towards the west reflecting the winter monsoon and is punctuated by reactivation surfaces evidence for the wind reversal is not preserved in the sediment successions but is represented by reactivation surfaces thus a subunit bounded by the reactivation surfaces defines annual deposition the preservation potential of a subunit varies with the migration rate of the transverse dune and in the fastest migrating dunes approximately one subunit was preserved every year 1 1 0 6 subunit yr the modern dunes are generally younger than 100 years and have migrated wsw at net rates of 0 6 2 5 m yr the holocene dune unit is as thick as the modern unit although it spans a period ten times longer reflecting the lower preservation potential of the transverse dunes as sedimentary records due to erosion associated with their migration the rate of dune migration suggests that an aeolian contribution of coastal sands has occurred following the sea level highstand of the middle holocene accounting for the extent of the active dunefields this mode of the barrier accumulation is considered as an analogue for earlier highstand events however the dunefields only cover a fraction of the entire barrier it is thus inferred that barrier formation occurred over a long period probably encompassing several glacial interglacial cycles sand accumulation during the lowstand either by transport of sand from the subaerially exposed shelf and or reworking of sand emplaced during the prior highstand may contribute significantly to the barrier development

juvenile crust formation in the zimbabwe craton deduced from the o hf isotopic record of 3 8 3 1 ga detrital zircons hafnium and oxygen isotopic compositions measured in situ on u pb dated zircon from archaean sedimentary successions belonging to the 2 9 2 8 ga belingwean bulawayan groups and previously undated sebakwian group are used to characterize the crustal evolution of the zimbabwe craton prior to 3 0 ga microstructural and compositional criteria were used to minimize effects arising from pb loss due to metamorphic overprinting and interaction with low temperature fluids 207 pb 206 pb age spectra concordance 90 reveal prominent peaks at 3 8 3 6 3 5 and 3 35 ga corresponding to documented geological events both globally and within the zimbabwe craton zircon delta o 18 values from 4 to 10 point to both derivation from magmas in equilibrium with mantle oxygen and the incorporation of material that had previously interacted with water in near surface environments in epsilon hf time space 3 8 3 6 ga grains define an array consistent with reworking of a mafic reservoir 176 lu 177 hf similar to 0 015 that separated from chondritic mantle at similar to 3 9 ga crustal domains formed after 3 6 ga depict a more complex evolution involving contribution from chondritic mantle sources and to a lesser extent reworking of pre existing crust protracted remelting was not accompanied by significant mantle depletion prior to 3 35 ga this implies that early crust production in the zimbabwe craton did not cause complementary enriched and depleted reservoirs that were tapped by later magmas possibly because the volume of crust extracted and stabilised was too small to influence asthenospheric mantle isotopic evolution growth of continental crust through pulsed emplacement of juvenile chondritic mantle derived melts into and onto the existing cratonic nucleus however involved formation of complementary depleted subcontinental lithospheric mantle since the early archaean indicative of strongly coupled evolutionary histories of both reservoirs with limited evidence for recycling and lateral accretion of arc related crustal blocks until 3 35 ga c 2017 elsevier ltd all rights reserved

a new age constraint for the early triassic alcova limestone chugwater group wyoming abstract the alcova limestone is a widely exposed but poorly studied triassic unit found throughout wyoming although it is used as a regional lithostratigraphic marker bed the age of the limestone is not well constrained and contention regarding its depositional nature marine vs non marine saline lake has not been adequately resolved although the marine hypothesis is more widely accepted currently its age is based on the occurrence of a single basal pistosaurid corosaurus alcovensis we provide the first 87 sr 86 sr analysis of the alcova limestone the data strongly support a marine origin for this unit furthermore the data provide an age range based on their placement on the marine strontium curve that limits the deposition of the alcova limestone to the latest olenekian to earliest anisian early triassic middle triassic boundary this placement is consistent with the timing of the spathian regression recorded in other transitional marine to non marine strata of the western usa

holocene fluctuations of lake ngami middle kalahari chronology and responses to climatic change abstract lake ngami a 3000 km 2 basin at the distal end of the okavango delta botswana has a complex suite of shorelines ranging from the lake sump at 919 m asl up to 945 m asl linked to topographic thresholds and inflows current and fossil in the okavango system the lake has been ephemeral throughout the 20th century and completely dry since 1982 yet held a substantial body of water up to a level of 930 m asl when visited by european travellers in the mid nineteenth century the historical decline in the lake has been well documented and used as evidence for regional desiccation but the holocene record based on archaeological studies and radiocarbon dating of inorganic carbonates is more speculative a study of a 30 km 2 diatomite at the eastern end of the basin indicates that a substantial freshwater lake at a level of at least 932 m asl was present throughout the holocene linked to a now abandoned inflow of the kunyere river a programme of 18 luminescence age determinations of the peripheral dautsa and magotlwanen shoreline sequences suggests that they are old features that have been constructed on a lacustrine substrate dating back to 90 100 ka or earlier and have been modified by holocene fluctuations in lake level and subsequent aeolian reworking the water level peaked at 936 m asl between 4 and 3 ka returned to 934 m at 2 8 2 ka ponding water into the phatane gap and fell rapidly thereafter to 932 m and thence to historical levels the 936 m level can be correlated with increased throughflow in the okavango delta hence increased rainfall in the catchment the subsequent decline can be attributed to episodic closure and rerouting of okavango distributaries in particular the thaoghe perhaps aided by anthropogenic activities as such the desiccation of the lake is not indicative of regional climatic change

ipswichian fauna of victoria cave and the marine palaeoclimatic record the lack of age dating methods which can be applied beyond the limit of radiocarbon dating makes the global correlation of continental climatic events and stratigraphic sequences with the continuous palaeoclimate record obtained by isotopic and faunal analysis of deep sea sediment cores1 3 a difficult task often only a relative time scale can be obtained using complex and perhaps tenuous litho and biostratigraphical data speleothems may assist in this correlation for embodied in their calcium carbonate structures are elemental and isotopic variations which indicate both their age and climatic conditions prevailing during their deposition speleothems may be reliably dated by the 230th 234u method to an age of 350 kyr bp providing they consist of non porous unrecrystallized calcite which is essentially free of clastic detrital sediments4 6 they may also provide palaeoclimatic information for previously glaciated areas 1 because their presence indicates that groundwater movement was not prevented by ice formation at the surface and co2 was produced in the soil zone by root respiration and plant decay limestone bedrock may then be dissolved and reprecipitated as speleothems in caves below the surface and 2 because of the variations in stable isotopic content and fluid inclusion waters contained therein4 7 8 additional climatic and chronological information may be obtained where speleothems are interstratified with deposits characteristic of a particular climatic regime and with deposits containing organic remains we describe here the results of dating speleothems encrusting the remains of mammals from victoria cave in northern england and discuss their significance in terms of the correlation between continental and oceanic palaeoclimatic records

holocene sedimentary and environmental history of an in channel wetland along the ecotone of the sonoran and chihuahuan desert grasslands abstract the paleoenvironmental record of sedimentation and development of an in channel wetland that spans the border of southeast arizona usa and northeast sonora mexico suggests a complex history between climate change and regional groundwater levels from the late pleistocene to early holocene the channel occupied by the san bernardino cienega was largely aggradational with cobbles to sands being deposited in the oldest and deepest sediments changes in grain size indicate lowered stream power through the early to middle holocene around 7200 years ago sediments changed from sand and cobble dominance to significant proportions of silts and clays suggesting standing or still waters along with changes in sediment texture preservation of organic material initiated providing a record of regional and local vegetation change the upland transitioned from woody grassland co dominance between 7000 and 5400 cal bp to grassland dominated conditions after 5400 cal bp locally wetland vegetation established within the stream channel after 7000 cal bp and persisted until ca 4100 cal bp from 4100 to 2400 cal bp the cienega remained stabilized but was covered with grasses and forbs wetland vegetation returned after 2400 cal bp the change in sediment characteristics and inferred establishment of wetland vegetation is interpreted as increased importance of regional groundwater hydrology from the middle holocene to present the timing of this change is coincident with increasing el nino variability and suggests a causal mechanism for increasing regional groundwater namely increase in winter precipitation in the american southwest

geochemistry of late archaean stromatolites from zimbabwe evidence for microbial life in restricted epicontinental seas we report comprehensive trace element and sr isotope data for microbial carbonates from the archaean mushandike limestone masvingo greenstone belt zimbabwe the stromatolites have very coherent ree y patterns and share the essential shale normalised characteristics of other archaean marine precipitates positive la and gd anomalies absence of a negative cc anomaly and a strongly superchondritic y ho ratio mixing models constrain the maximum amount of shale contamination to 0 25 1 and calculated detritus free carbonate ree y systematics require precipitation from seawater in terms of light ree over heavy ree depletion however the studied samples are very different from all other known archaean marine precipitates in shale normalised plots the mushandike samples yield a negative slope a very restricted regional input source of the dissolved load is indicated because normalisation with locally occurring tonalite gneiss ree y data yields a pattern closely resembling typical shale normalised archaean marine chemical sediments the disappearance of a negative eu anomaly when patterns are normalised with local tonalite gneiss strengthens this interpretation sr isotope ratios are strongly correlated with trace element contents and ratios which explains the modest scatter in sr isotope ratios as representing minor clastic contamination importantly even the least contaminated samples have very radiogenic initial sr 87 sr 86 ratios 0 7184 implying sr input from an ancient high rb sr source such as the early archaean gneisses of south central zimbabwe a local ancient 3 5 3 8 ga source is also indicated by previously published pb isotope datasets for the mushandike stromatolites this is entirely compatible with the occurrence of 3 7 3 8 ga zircons in quartzites and metapelites from comparably old greenstone belts within less than 150 km of the studied locality comparison of the pb isotope ratios of the mushandike stromatolites with 2 7 and 2 6 ga old stromatolites from the neighbouring belingwe greenstone belt demonstrates differences in initial isotope composition that relate to the extent of exchange with the open ocean the development of numerous basins on old continental crust with water masses variably restricted from the open ocean suggests a lack of strong vertical topography on this late archaean craton c 2004 elsevier b v all rights reserved

an absolute chronology for the raised beach and associated deposits at sewerby east yorkshire england the raised beach sediments revealed in the cliffs at sewerby east yorkshire are widely regarded as ipswichian in age previously reported evidence for this dating is reviewed and new results from a range of luminescence dating techniques applied to the blown sand deposit overlying the raised beach are reported these provide the first absolute date for the sewerby site of 120 84 11 82 ka which places the blown sand at the boundary between oxygen isotope stages ois 5e and 5d as the underlying raised beach is little older than the blown sand it probably formed during ois stage 5e as suggested by the faunal evidence

ancient and modern cold climate aeolian sand deposition a review although partly active aeolian sand sheets and dunes cover large areas in the zones of dis continuous permafrost little precise information is available about the influence of cold climate conditions on modern aeolian processes this means that palaeoenvironmental reconstructions in the stabilised mainly late pleistocene dune fields and cover sand regions in the sand belts of the european lowlands and the northern great plains of the usa and canada are necessarily still based on ancient evidence cold climate wind deposits are typically derived from areas of abundant sediment supply like unvegetated flood plains glacial outwash plains till plains and lake shores the common parabolic and transverse dune forms resemble those observed in temperate regions although a variety of periglacial features has been identified in late pleistocene dune and cover sands none of them indicate that permafrost is crucial to aeolian activity specific structures in aeolian strata permit tentative interpretation of the moisture content of depositional sand surfaces the nature of annual sedimentation cycles and the processes by which strata were deposited and or contorted but surprisingly little is known about the role of vegetation in the process of sand accumulation dunes are most informative with respect to reconstructions of past wind regimes which offer important data for verification of palaeoclimatic simulations

methodological considerations in the optical dating of quartz abstract the optically stimulated luminescence from quartz grains extracted from sediments at two archaeological sites has been studied the sites were the upper palaeolithic site at hengistbury head dorset u k and the upper aterian site at chaperon rouge morocco problems associated with the technique of optical dating are discussed particularly the thermal treatment that is necessary to remove thermally unstable charge following artificial irradiation and the extrapolation of growth curves to estimate the equivalent dose the technique used is described and some preliminary dates are presented

coarse sand beach ridges at cowley beach north eastern australia their formative processes and potential as records of tropical cyclone history storm surges generated by tropical cyclones have been considered a primary process for building coarse sand beach ridges along the north eastern queensland coast australia this interpretation has led to the development of palaeotempestology based on the beach ridges to better identify the sedimentary processes responsible for these ridges a high resolution chronostratigraphic analysis of a series of ridges was carried out at cowley beach queensland a meso tidal beach system with a 3 m tide range optically stimulated luminescence ages indicate that 10 ridges accreted seaward over the last 2500 to 2700 years the ridge crests sit 3 5 to 5 1 m above australian height datum ca mean sea level a ground penetrating radar profile shows two distinct radar facies both of which are dissected by truncation surfaces hummocky structures in the upper facies indicate that the nucleus of the beach ridge forms as a berm at 2 5 m australian height datum equivalent to the fair weather swash limit during high tide the lower facies comprises a sequence of seaward dipping reflections beach progradation thus occurs via fair weather wave accretion of sand with erosion by storm waves resulting in a sporadic sedimentary record the ridge deposits above the fair weather swash limit are primarily composed of coarse and medium sands with pumice gravels and are largely emplaced during surge events inundation of the ridges is more likely to occur in relation to a cyclone passing during high tide the ridges may also include an aeolian component as cyclonic winds can transport beach sand inland especially during low tide and some layers above 2 5 m australian height datum are finer than aeolian ripples found on the backshore coarse sand ridges at cowley beach are thus products of fair weather swash and cyclone inundation modulated by tides knowledge of this composite depositional process can better inform the development of robust palaeoenvironmental reconstructions from the ridges this article is protected by copyright all rights reserved

strontium isotope evolution of late permian and triassic seawater the 87sr 86sr values based on brachiopods and conodonts define a nearly continuous record for the late permian and triassic intervals minor gaps in measurements exist only for the uppermost brahmanian lower part of the upper olenekian and middle norian and only sparse data are available for the late permian these 219 measurements include 67 brachiopods and 114 conodont samples from the tethyan realm as well as 37 brachiopods and one conodont sample from the mid european middle triassic muschelkalk sea the late permian lower triassic interval is characterized by a steep 1 3 10 3 rise from 0 7070 at the base of the dzhulfian to 0 7082 in the late olenekian a rate of change comparable to that in the cenozoic in the mid triassic anisian and ladinian the isotope values fall to 0 7075 followed again by a rise to 0 7081 in the middle late norian the 87sr 86sr values decline again in the late norian sevatian and rhaetian to 0 7076 the sharp rise in the 87sr 86sr values during the late permian early triassic was coincident with widespread clastic sedimentation because of the paucity of tectonic uplifts the enhanced erosion may have been due to intermittent humid phases during mainly an arid interval coupled with the absence of a dense protective land plant cover following the mass extinction during the latest permian the apex of the 87sr 86sr curve at the olenekian anisian boundary coincides with cessation of the large scale clastic sedimentation and also marks the final recovery of land vegetation as indicated by the renewed onset of coal formation in the middle triassic the rising 87sr 86sr values from the middle carnian to the late norian coincide with the uplift and erosion of the cimmeride indosinian orogens marking the closure of the palaeotethys the subsequent rhaetian decline that continues into jurassic pliensbachian toarcian boundary on the other hand coincides with the opening of the vardar ocean and its eastern continuation in the izmir ankara ophiolitic belt samples from the upper muschelkalk are more radiogenic than the global trend this may reflect separation of the basin from the open ocean due to strong meteoric influx from a large land mass in the north the germanic basin became increasing brackish up section in the north and east but because of the high evaporation rates the salt content was not much reduced in the southern and central basin where a rich but increasingly endemic marine fauna survived

environmental and diagenetic variations in carbonate associated sulfate an investigation of cas in the lower triassic of the western usa an integrated stable isotope elemental and petrographic analysis of early triassic spathian carbonates and evaporites along a proximal to deep environmental transect reveals significant variations in 34s composition of carbonate associated sulfate cas the variations in the 34s of cas are strongly correlated with the ca mg composition of carbonates suggesting that the variations are driven by the degree of dolomitization the 34s of dolostones and evaporites are similar to one another and exhibit lower 34s values than limestones from all localities three hypotheses may explain the differences in 34s between proximal dolostones evaporites and inner middle shelf limestones 1 limestones experienced anaerobic sulfate reduction and subsequent incorporation of 34s enriched sulfate into cas during diagenesis while dolostones did not this is unlikely because of the lack of correlation between 34scas and toc as well as other indicators of diagenesis 2 dolomitization controlled the 34scas in proximal paleoenvironments where the source of the 34s depleted fluids was either continentally derived or the result of rayleigh distillation during evaporite formation and 3 a 34s depth gradient existed during the early triassic such that limestones formed in distal waters are more enriched in 34s versus evaporites and dolostones formed in proximal settings we do not favor this hypothesis because the strong correlation between ca mg and 34scas implies that dolomitization controls the 34scas in these samples results from subtidal well preserved non dolomitized limestones suggest that the 34s of spathian seawater sulfate may have been heavier than previously suggested from analyses of evaporite deposits alone

age dating and the orbital theory of the ice ages development of a high resolution 0 to 300 000 year chronostratigraphy using the concept of orbital tuning a continuous high resolution deep sea chronostratigraphy has been developed spanning the last 300 000 yr the chronology is developed using a stacked oxygen isotope stratigraphy and four different orbital tuning approaches each of which is based upon a different assumption concerning the response of the orbital signal recorded in the data each approach yields a separate chronology the error measured by the standard deviation about the average of these four results which represents the best chronology has an average magnitude of only 2500 yr this small value indicates that the chronology produced is insensitive to the specific orbital tuning technique used excellent convergence between chronologies developed using each of five different paleoclimatological indicators from a single core is also obtained the resultant chronology is also insensitive to the specific indicator used the error associated with each tuning approach is estimated independently and propagated through to the average result the resulting error estimate is independent of that associated with the degree of convergence and has an average magnitude of 3500 yr in excellent agreement with the 2500 yr estimate transfer of the final chronology to the stacked record leads to an estimated error of 1500 yr thus the final chronology has an average error of 5000 yr