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

the influence of continental ice atmospheric co2 and land albedo on the climate of the last glacial maximum the contributions of expanded continental ice reduced atm9spheric co2 and changes in land albedo to the maintenance of the climate of the last glacial maximum lgm are examined a series of experiments is perform ed using an atmosphere mixed layer ocean model in which these changes in boundary conditions are incorporated either singly or in combination the model used has been shown to produce a reasonably reajistic simulation of the reduced temperature of the lgm manabe and broccoli 1985 by comparing the results from pairs of ex periments the effects of each of these envirornnental changes can be determined expanded continental ice and reduced atmospheric co2 are found to have a substantial impact on global mean temperature the ice sheet effect is confined almost ex clusively to the northern hemisphere while lowered co2 cools both hemispheres changes in land albedo over ice free areas have only a minor thermal effect on a global basis the reduction of co2 content in the atmosphere is the primary contributor to the cooling of the southern hemisphere the model sensitivity to both the ice sheet and co2 effects is characterized by a high latitude amplification and a late autumn and early winter maximum

stable isotope based paleoaltimetry abstractthe quantitative estimation of paleoaltitude has become an increasing focus of earth scientists because surface elevation provides constraints on the geodynamic mechanisms operating in mountain belts as well as the influence of mountain belt growth on regional and global climate the general observation of decreasing 18o and 2h values in rainfall as elevation increases has been used in both empirical and theoretical approaches to estimate paleoelevation these studies rely on the preservation of ancient surface water compositions in authigenic minerals to reconstruct the elevation at the time the minerals were forming in this review we provide a theory behind the application of stable isotope based approaches to paleoaltimetry we apply this theory to test cases using modern precipitation and surface water isotopic compositions to demonstrate that it generally accords well with observations examples of the application of paleoaltimetry techniques to himalaya tibet and the andes are discussed

timing of post glacial reinhabitation and ecological development of two new england usa drainages based on trace fossil evidence abstract trace fossils as paleoecological and paleobiogeographical tools in pleistocene glaciolacustrine sediments have been largely ignored combining high resolution varve stratigraphy with trace fossil data can lead to refined paleoenvironmental interpretations during times of rapid climate change based on trace fossils from glaciolacustrine varves in the connecticut and merrimack river valleys a new timeline for the reinhabitation of new england by fish and associated invertebrates has been constructed in addition it appears that colonization of the recently deglaciated environments occurred in at least four successional stages if these stages can be recognized elsewhere it may be possible to accurately model other postglacial migrations using trace fossils and varve chronostratigraphy

tropical boundary layer equilibrium in the last ice age a radiative convective boundary layer model is used to assess the effect of changing sea surface temperature pressure wind speed and the energy export from the tropics on the boundary layer equilibrium equivalent potential temperature it remains difficult to reconcile the observations that during the last glacial maximum 18 000 yr bp the snowline on the tropical mountains fell 950 m while the tropical sea surface temperatures fell only 1 2 k

surculichnus bifurcauda n igen n isp a trace fossil from late pleistocene glaciolacustrine varves of the connecticut river valley usa attributed to notostracan crustaceans based on neoichnological experimentation abstract new trace fossils found in the late pleistocene glaciolacustrine varves of the connecticut river valley vermont usa represent the first known notostracan presence in glacial lake hitchcock these unique trace fossils warrant a new ichnogenus and ichnospecies surculichnus bifurcauda the new england varve chronology nevc constrains the initial presence of s bifurcauda at 13 3 13 2 kyr the morphology of s bifurcauda correlates well with notostracan characteristics and behavior sieving of bedding planes that contained s bifurcauda produced one chitinous fossil that is suggestive of a notostracan telson neoichnological experimentation was conducted with the species triops longicaudatus during the subadult stage t longicaudatus produced traces representative of locomotion and feeding behaviors and at the adult stage reproduced s bifurcauda as well as rusophycus possible explanations for the productions of these traces are egg laying or predation behaviors that are both related to maturity further research in the paleo and modern ecology of glacial and nearctic lakes may shed more light on the maker of s bifurcauda

slow senonian and fast palaeocene early eocene uplift of the granitoids in the central eastern pontides turkey apatite fission track results abstract dating and forward modelling of the fission track data of apatite samples from the dereli ebinkarahisar region south of giresun in the eastern turkish pontides provides quantitative data on the regional tectonics resulting from the closure of neo thetys and the collision of eurasia and gondwana the age vs elevation profiles identified senonian 80 7 3 2 to 62 4 2 5 ma slow uplift and denudation interpreted as the result of the diapiric ascent of subduction related plutons above the neo tethyan subduction zone beneath the eurasian continent this was followed by rapid differential uplift during the palaeocene early eocene 57 4 2 4 to 47 8 2 4 ma which juxtaposed granitoid units of different ages compositions and emplacement levels in the crust and is thought to be related to the collision between the pontide eurasian and anatolide gondwana basements the modelling results must be interpreted with caution but appear to indicate a period of mio pliocene ca 5 ma reheating related to volcanism associated with the westward escape of the anatolian plate and uplift from the pliocene ca 3 5 ma up to the present

trace fossils from late pleistocene varved lacustrine sediments in eastern lithuania abstract trace fossils cochlichnus anguineus glaciichnium liebegastensis gordia carickensis new combination youngest occurrence gordia isp warvichnium ulbrichi cf warvichnium isp have been found in varved sediments of a small proglacial lake 17 16 kyr bp in east lithuania they occur mostly in summer and transitional laminae along bunches of discrete microlaminae vertical reworking is absent horizontal distribution of trace fossils along microlaminae studied in one varved package points to a patchy distribution of trace fossils resulting from a patchy distribution of food locally c anguineus dominates and commonly this is the only trace fossil its co occurrence with g liebegastensis is distinctly rarer than with other ichnotaxa probably due to a competition between their tracemakers the trace fossil assemblage can be ascribed to the mermia ichnofacies in which the glaciichnium ichnocoenosis arthropod trackways dominate and cochlichnus ichnocoenosis grazing traces dominate can be distinguished as the end members replacement of the glaciichnium ichnocoenosis by the cochlichnus ichnocoenosis can be interpreted as shallowing of hypolimnion and degrading ecological conditions

trace fossils from carboniferous floodplain deposits in western argentina implications for ichnofacies models of continental environments abstract floodplain deposits from the carboniferous tupe formation of western argentina contain a low diversity ichnofauna of invertebrates and plants the assemblage consists of archaeonassa fossulata didymaulichnus lyelli helminthoidichnites tenuis palaeophycus tubularis planolites isp and root traces this ichnofauna is characterized by the dominance of very simple forms superficial or very shallow trace fossils combination of locomotion grazing and dwelling structures and production by arthropods and vermiform organisms primary sedimentary fabrics are disturbed only by plant trace fossils while animal trace fossils are mostly restricted to bedding surfaces resulting in the virtual absence of bioturbation the poorly defined morphology of the ichnofossils and the absence of backfilled striated trace fossils and structures indicative of subaerial exposure suggest subaqueous production the envisaged depositional environment is a freshwater body developed in a floodplain and overfilled by overbank deposits a review of floodplain ichnofaunas documented from the stratigraphic record indicates that two recurrent assemblages may be recognized the first assemblage is characterized by low to rarely moderate diversity of invertebrate trace fossils moderate to high diversity of vertebrate structures and common presence of meniscate backfilled structures bilobate trace fossils with scratch marks arthropod trackways and tetrapod trackways it is regarded as an example of the scoyenia ichnofacies developed in desiccated floodplains the second type of floodplain assemblage illustrated by the example documented in this paper is characterized by grazing trails locomotion trails and dwelling burrows low to rarely moderate ichnodiversity simple trace fossils and superficial to very shallow structures although formed in floodplain environments also this type of assemblage is remarkably different from the scoyenia ichnofacies it is here regarded as an example of the mermia ichnofacies basic features of these ichnofaunas reflect subaqueous production of the assemblage the lower ichnodiversity of these floodplain assemblages in comparison with their equivalents from lacustrine basins is probably an expression of the less stable conditions and temporary nature of floodplain water bodies our study supports the view that ichnofacies are not indicators of sedimentary environments but reflect sets of environmental factors

constraints on southern hemisphere tropical climate change during the little ice age and younger dryas based on glacier modeling of the quelccaya ice cap peru 2015 the authors improving the late quaternary paleoclimate record through climate interpretations of low latitude glacier length changes advances our understanding of past climate change events and the mechanisms for past present and future climate change paleotemperature reconstructions at low latitude glaciers are uniquely fruitful because they can provide both site specific information and enhanced understanding of regional scale variations due to the structure of the tropical atmosphere we produce little ice age lia and younger dryas yd paleoclimate reconstructions for the huancane outlet glacier of the quelccaya ice cap qic and low latitude southern hemisphere regional sea surface temperatures ssts using a coupled ice flow and energy balance model we also model the effects of long term changes in the summit temperature and precipitiation rate and the effects of interannual climate variability on the huancane glacier length we find temperature to be the dominant climate driver of glacier length change also we find that interannual climate variability cannot adequately explain glacier advances inferred from the geomorphic record necessitating that these features were formed during past colder climates to constrain our lia reconstruction we incorporate the qic ice core record finding a lia air temperature cooling at the ice cap of between 0 7 c and 1 1 c and 0 4 c and regional ssts cooling of 0 6 c for the yd paleoclimate reconstructions we propose two limits on the precipitation rate since the ice core record does not extend into the pleistocene 1 the precipitation rate scales with the clausius clapeyron relationship upper limit on cooling and 2 the precipitation rate increases by 40 lower limit on cooling which is an increase about twice as great as the regional increases realized in gcm simulations for the period the first limit requires 1 6 c cooling in ice cap air temperatures and 0 9 c cooling in ssts and the second limit requires 1 0 c cooling in ice cap air temperatures and 0 5 c cooling in ssts our temperature reconstructions are in good agreement with the magnitude and trend of gcm simulations that incorporate the forcing mechanisms hypothesized to have caused these climate change events

ichnological signatures of shallow freshwater lakes in the glacial itarare group mafra formation upper carboniferous lower permian of parana basin s brazil abstract the upper carboniferous lower permian glacigenic deposits of the itarare group parana basin are widely known and cover an extensive area in the southern brazil these deposits consist of different arrangements of massive shales diamictites rhythmites and fine to medium grained sandstones with trough cross stratification five genetically distinct thickening locally coarsening upward successions of regular rhythmites can be distinguished in the mafra rio negro region santa catarina and parana states southern brazil representing the landward portion of an incised valley filled during deglaciation a low diversity invertebrate ichnofauna composed of a freshwater shallow burrow dominated suite and trackway dominated suite occurs in these rhythmites the freshwater shallow burrow dominated suite is composed of cochlichnus anguineus cruziana cf problematica gordia arcuata gordia marina hormosiroidea meandrica rusophycus cf carbonarius and treptichnus pollardi as well as undichnia consulca and intermittent rusophiciform traces it represents a mermia ichnocoenosis and occurs in the rhythmites of the upper sets commonly overlapped by d gouldi and d biformis in palimpsest preservation d gouldi and d biformis compose the trackway dominated suite which occur in all ichnofossiliferous paired mudstone siltstone of the basal sets of the rhythmite succession and in almost all of the upper sets it represents an atypical scoyenia ichnocoenosis and is generally preserved over microbially induced wrinkle structures the rhythmic deposition suggests distal underwater gravity flow deposits filling shallow lakes or ponds in depressed areas the siltstones deposited by melting currents and the mudstones by sediment fall out after the flux ceased the abundant wrinkle structures suggest quiet waters the overlying deposits bearing a brackish water ichnofauna and marine fossils suggest a coastal setting connected with the sea although atypical the occurrence of a scoyenia ichnocoenosis in almost all ichnofossiliferous rhythmite couplets preserved over wrinkle structures or overlapping mermia ichnocoenosis suggests that these shallow lakes were episodically dried up at least partially the present data allow interpreting the regular ichnofossiliferous rhythmites of the mafra formation as deposited in shallow lakes or ponds developed in outwash plains at the landward side of a fjord embayment during deglaciation

reconstruction of a late cisuralian early permian floodplain lake environment palaeontology and sedimentology of the tregiovo basin trentino alto adige northern italy abstract the late cisuralian is known as a time of increasing aridity compared to the late pennsylvanian early cisuralian although several studies highlighted this trend at low latitudes of western pangaea little is known from central pangaea environments the discovery of new fossiliferous horizons in the late cisuralian tregiovo formation northern italy allowed a new palaeoenvironmental study based on facies analysis ichnology palaeobotany plant insect interaction and palynology three facies associations were identified a c and correspond to floodplain lake ephemeral lacustrine and distal alluvial fan environments respectively the tetrapod ichnoassociation is more diverse than previously known including abundant diapsid and non diapsid reptile tracks and rarer temnospondyl amphibian tracks plant fossils are characterized by a predominance of hinterland taxa conifers hygrophytic plants are present as well the sporomorph association is dominated by miospores of cordaitales voltziales and peltaspermales while trilete lycopsid and fern miospores are rare which corresponds well with the macroflora invertebrate trace fossils and feeding traces on plant fossils are described for the first time from the tregiovo basin suggesting transitional low energy environments and a relatively low level of herbivory respectively this study evidences the development in the tregiovo basin of a wet and dry probably seasonal climate which became drier between facies associations a b this environment constitutes an important reference since few data are known to infer the late cisuralian climatic conditions of central pangaea

cool tropical temperatures shift the global δ18o t relationship an explanation for the ice core δ18o borehole thermometry conflict the discrepancy between central greenland borehole temperatures and the isotopic composition of last glacial maximum ice can be explained by a shift in the 5180 t relationship for the hydrological cycle linked to cooler tropical temperatures this concept is illustrated using a simple rayleigh distillation model an estimate for o a 5180 at lgm holocene of 0 37 o oc is determined with a simple graphical technique

mechanisms of abrupt climate change of the last glacial period 1 more than a decade ago ice core records from greenland revealed that the last glacial period was characterized by abrupt climate changes that recurred on millennial time scales since their discovery there has been a large effort to determine whether these climate events were a global phenomenon or were just confined to the north atlantic region and also to reveal the mechanisms that were responsible for them in this paper we review the available paleoclimate observations of abrupt change during the last glacial period in order to place constraints on possible mechanisms three different mechanisms are then reviewed ocean thermohaline circulation sea ice feedbacks and tropical processes each mechanism is tested for its ability to explain the key features of the observations particularly with regard to the abruptness millennial recurrence and geographical extent of the observed changes it is found that each of these mechanisms has explanatory strengths and weaknesses and key areas in which progress could be made in improving the understanding of their long term behavior both from observational and modeling approaches are suggested finally it is proposed that a complete understanding of the mechanisms of abrupt change requires inclusion of processes at both low and high latitudes as well as the potential for feedbacks between them some suggestions for experimental approaches to test for such feedbacks with coupled climate models are given

holocene hydrologic balance of tropical south america from oxygen isotopes of lake sediment opal venezuelan andes precipitation in the south american andes is derived from atlantic ocean evaporation which is modified by passage over lowland south america the isotopic composition of andean precipitation reflects evaporation conditions over the atlantic ocean moisture recycling during advection across the south american lowlands and uplift to the andes records of the oxygen isotope composition of precipitation in the venezuelan andes derived from lake sediment diatom 18 o measurements show a 2 4 decrease during the past 10 000 yr a simple model of the evaporation advection and uplift processes is used to understand the cause of the isotope shift the data and model suggest that the decreasing 18 o reflects a decrease in the fraction of moisture entering south america that reaches the andes ice cores from peru and bolivia exhibit similar isotope trends indicating that the shift occurred in both hemispheres an isotopic record of amazon river discharge is consistent with the andean records indicating increasing continental runoff was associated with the decreasing export of water vapor orbital changes in solar insolation cannot explain the synchronous trends in both hemispheres changing climate in the tropical pacific is an attractive explanation for the trends because modern interannual variability in this region has similar effects in both hemispheres

an abrupt decrease in southern hemisphere terrestrial temperature during the eocene oligocene transition abstract earth s climate transitioned from the greenhouse conditions of the eocene to the icehouse of the oligocene 34 million years ago global cooling was triggered by a decrease in atmospheric pco2 and is associated with the first appearance of large permanent ice sheets on antarctica the eocene oligocene transition eot is marked by a large two step increase in the 18 o of benthic foraminifera reflecting a combination of cooler global temperatures and continental ice growth we measured the hydrogen isotope composition of hydrated volcanic glasses deposited in patagonia during the eot to reconstruct stable isotope hydrology and associated changes in temperature during this critical climate transition these data show changes in stable hydrogen isotope ratios that are consistent with patagonian terrestrial mean annual temperature decreasing by 5 c during the eot before returning to 2 c below pre eot values by the early oligocene this is approximately synchronous with northern hemisphere cooling during the eot and indicates close coupling of temperature and atmospheric pco2

applications of ichnology in lacustrine sequence stratigraphy potential and limitations abstract in comparison with their marine counterparts lacustrine ichnofaunas have not been extensively used in sequence stratigraphy application of trace fossils in continental sequence stratigraphy cannot be based simply on the extrapolation of marine sequence stratigraphy marine substrate controlled ichnofacies in particular the firmground glossifungites ichnofacies develop in stable and cohesive substrates reflecting erosive exhumation of the sediment in contrast in lacustrine sequences substrate controlled ichnofossils only rarely indicate erosional exhumation because they are commonly related to desiccation of water bodies the tripartite division of overfilled balanced fill and underfilled lakes provides a comprehensive framework to evaluate the potential and limitations of lacustrine ichnofaunas in sequence stratigraphy based on a number of case studies spanning the three lake basin types a model of trace fossil distribution in overfilled balanced fill and underfilled basins is proposed overfilled lake basins contain well developed softground trace fossils mermia and skolithos ichnofacies and the softground suite of the scoyenia ichnofacies that are useful to delineate parasequences and parasequence sets fluvial discharge into overfilled lakes usually generates density currents that oxygenate lake bottoms allowing the establishment of epifaunal and infaunal communities additionally these are freshwater lakes where no stress due to hypersalinity occurs leading to the development of a relatively diverse benthos land plant derived organic matter is the prime source of nutrients favoring the development of a deposit feeding benthic fauna in permanently subaqueous low energy zones firmground suites are rare because such large lakes usually do not experience desiccation abundant firmground trace fossil suites of the scoyenia ichnofacies occur in balanced fill lakes but softground assemblages are usually depauperate lowstand deposits contain abundant and widespread ichnofaunas commonly meniscate trace fossils and arthropod trackways of the scoyenia ichnofacies lake hydrology is closed during lowstands and salinity usually increases imposing a stress factor on the lake biota and therefore softground ichnofaunas are depauperate ichnofaunas from turbidite systems in balanced fill lakes are less abundant and diverse than those from overfilled lacustrine turbidites freshwater conditions are common during transgression but dysaerobic conditions may prevail imparting a stress factor on the benthic biota in hydrologically closed lakes the scoyenia ichnofacies is widespread but the mermia ichnofacies is commonly absent or impoverished the scoyenia ichnofacies is associated with lowstand desiccated substrates in underfilled lakes the density of arthropod trackways may be high forming tracked omission surfaces some of these omission surfaces may represent sequence boundaries expressed by co planar surfaces of lowstand and subsequent flooding rapid changes in depositional conditions reflecting desiccation during vertical aggradation can lead to the formation of composite ichnofabrics that reflect successive bioturbation events

glacial interglacial dynamics of the eastern equatorial pacific cold tongue intertropical convergence zone system reconstructed from oxygen isotope records record mean late holocene lh last glacial maximum lgm d 18 o amplitudes ranging between 1 0 and 1 3 we estimate that mean sea surface temperatures sst in this region during the lgm were on average 1 5 0 5 c lower than the lh larger d 18 o amplitudes are observed in sites north of the equator indicating a spatial pattern of reduced meridional sst gradient across the equator during the lgm this result is supported by comparison of mg ca sst reconstructions from two sites straddling the equator we interpret the reduction of this gradient during the lgm as evidence for a less intense cold tongue intertropical convergence zone itcz frontal system a more southerly position of the itcz and weaker southeast equatorial trades in the eep indexterms 1620 global change climate dynamics 3309 3339 meteorology and atmospheric dynamics ocean atmosphere interactions 0312 4504 3344 meteorology and atmospheric dynamics paleoclimatology 4231 oceanography general equatorial oceanography 4267 oceanography general paleoceanography keywords cold tongue itcz oxygen isotopes tropical pacific

altitudinal variation of the stable isotopes of snow in regions of high relief a major discontinuity in the variation of 18 o d with altitude in high mountains was first seen in data from mount logan yukon territory canada holdsworth and others 1991 the profile of vs altitude revealed three well defined regions 1 a lower monotonic fractionation sequence below 3 km 2 a middle layer typically 1 2 km thick within which values are nearly constant or stepped with altitude and 3 part of another fractionation sequence in the quasi geostrophic flow region above 5 3 km the middle region was inferred to be a mixed layer combining moisture from regions 1 and 3 this type of structure is now seen to occur on other high altitude mountains including cerro aconcagua argentina where observations reach almost 7 km the new observations confirm the general occurrence of a multi layered atmosphere during precipitation at high altitude glacier sites this structure is linked to synoptic scale polar cyclones where the middle layer is identified as being the signature of the warm front zone these results have implications for the common practice of using a specific spatially derived isotopic thermometer in the time domain for the paleoclimatic interpretation of high altitude ice core records

last glacial maximum equilibrium line altitude and paleo temperature reconstructions for the cordillera de merida venezuelan andes abstract the pattern and magnitude of glacier equilibrium line altitude ela lowerings in the tropics during the last glacial maximum lgm are topics of current debate in the northern tropics paleo ela data are particularly limited inhibiting the ability to make regional and large scale paleoclimatic inferences to improve these records nine paleo glaciers in the venezuelan andes were reconstructed based on field observations aerial photographs satellite imagery and high resolution digital topographic data paleo glacier equilibrium line altitudes elas were estimated using the accumulation area ratio aar and the area altitude balance ratio aabr methods during the local lgm in venezuela 22 750 to 19 960 cal yr bp elas were 850 to 1420 m lower than present local lgm temperatures were are at least 8 8 2 c cooler than today based on a combined energy and mass balance equation to account for an ela lowering this is greater than estimates using an atmospheric lapse rate calculation which yields a value of 6 4 1 c cooler the paleo glacial data from the venezuelan andes support other published records that indicate the northern tropics experienced a greater ela lowering and possibly a greater cooling than the southern hemisphere tropics during the lgm

a 60 000 year record of hydrologic variability in the central andes from the hydrogen isotopic composition of leaf waxes in lake titicaca sediments abstract a record of the hydrogen isotopic composition of terrestrial leaf waxes d wax in sediment cores from lake titicaca provides new insight into the precipitation history of the central andes and controls of south american summer monsoon sasm variability since the last glacial period comparison of the d wax record with a 19 kyr d record from the nearby illimani ice core supports the interpretation that precipitation d is the primary control on d wax with a lesser but significant role for local evapotranspiration and other secondary influences on d wax the titicaca d wax record confirms overall wetter conditions in the central andes during the last glacial period relative to a drier holocene during the last deglaciation abrupt d wax shifts correspond to millennial scale events observed in the high latitude north atlantic with dry conditions corresponding to the bolling allerod and early holocene periods and wetter conditions during late glacial and younger dryas intervals we observe a trend of increasing monsoonal precipitation from the early to the late holocene consistent with summer insolation forcing of the sasm but similar hydrologic variability on precessional timescales is not apparent during the last glacial period overall this study demonstrates the relative importance of high latitude versus tropical forcing as a dominant control on glacial sasm precipitation variability

is the tropical atmosphere conditionally unstable abstract we examine the mean and standard deviation of the bouyancy of lifted boundary layer parcels from a large sample of soundings from the western equatorial pacific the inclusion of condensate loading in the definition of buoyancy is emphasized as is the precise originating level of parcels in the subcloud layer we confirm the observation of betts that those parts of the tropical atmosphere experiencing deep convection are nearly neutral to adiabatic parcel ascent from the subcloud layer when adiabatic condensate loading is included in the definition of buoyancy parcels lifted from the top of the subcloud layer are more nearly neutral than those originating near the surface and atmospheres with higher levels of free convection lfcs are closer to neutral than those with low lfcs we explore possible reasons for these observations and discuss their implications for tropical meteorology

hydroclimate variability of high arctic svalbard during the holocene inferred from hydrogen isotopes of leaf waxes abstract the response of the arctic hydrologic cycle to global warming includes changes in precipitation patterns and moisture availability associated with variable sea ice extent and modes of atmospheric circulation reconstructions of past hydroclimate changes help constrain the natural range of these systems identify the manners in which they respond to different forcing mechanisms and reveal their connections to other components of the climate system all of which lead to a better understanding of present and future changes here we examine hydroclimate changes during the holocene in the high arctic archipelago of svalbard by reconstructing the isotopic composition of precipitation we measured the hydrogen isotopic composition d values of leaf wax compounds n alkanes c 25 c 31 in a sediment core from lake hakluytvatnet on the island of amsterdamoya northwest spitsbergen we interpret d values of mid chain c 25 and long chain c 29 c 31 length n alkanes to represent changes in the isotopic composition of lake water and precipitation over the last 12 9 ka after deglaciation of the catchment water supply became restricted and the lake experienced significant evaporative isotopic enrichment indicating warmer conditions from 12 8 to 7 5 ka the isotope values suggest an increase in the delivery of moisture from warmer sub polar air masses between 12 8 and 9 5 ka followed by generally warm but unstable conditions between 9 5 and 7 5 ka possibly indicating a response to meltwater forcing sedimentary evidence indicates a hiatus in deposition c 7 5 5 0 ka likely as a result of desiccation of the lake at c 5 0 ka lacustrine sedimentation resumed and over the last 5 ka there was a progressive increase in the influence of polar air masses and colder conditions which culminated in an abrupt shift to colder conditions at c 1 8 ka this late holocene cooling ended c 0 18 ka when isotopic data indicate warmer conditions and greater influence of moisture derived from lower latitudes

low latitude ice cores record pacific sea surface temperatures matic records suggested that a more conventional temperature interpretation might be more appropriate with lower isotopic values during the presumed to be cooler little ice age and increasingly enriched precipitation during the 20th century warming phase thompson 2001 3 both of these conflicting interpretations suffer from inadequate calibration of the observed isotopic record in relation to climatic conditions during periods of snow accumulation on the ice caps here we use the results of on site meteorological measurements to determine the seasonality of the ice core record and then assess how the isotopic records relate to temperature precipitation and the larger scale circulation associated with snowfall at these high elevation sites we then extend the analysis to ice core records from the asian monsoon region this empirical approach to the interpretation of the isotopic record in low latitude ice cores complements theoretical studies by broecker 1997 and pierrehumbert 1999

a coupled climate model simulation of the last glacial maximum part 2 approach to equilibrium the climate of the last glacial maximum lgm is simulated with a coupled climate model the simulated climate undergoes a rapid adjustment during the first several decades after imposition of lgm boundary conditions as described in part 1 and then evolves toward equilibrium over 900 model years the climate simulated by the coupled model at this period is compared with observationally based lgm reconstructions and with lgm results obtained with an atmosphere mixed layer slab ocean version of the model in order to investigate the role of ocean dynamics in the lgm climate global mean surface air temperature and sea surface temperature sst decrease by about 10 c and 5 6 c in the coupled model which includes ocean dynamics compared to decreases of 6 3 and 3 8 c in slab ocean case the coupled model simulates a cooling of about 6 5 c over the tropics which is larger than that of the climap reconstruction 1 7 c and larger than that of the slab ocean simulation 3 3 c but which is in reasonable agreement with some recent proxy estimates the ocean dynamics of the coupled model captures features found in the climap reconstructions such as a relative maximum of ocean cooling over the tropical pacific associated with a mean la nina like response and lead to a more realistic sst pattern than in the slab model case the reduction in global mean precipitation simulated in the coupled model is larger 15 than that simulated with the slab ocean model 10 in conjunction with the enhanced cooling some regions such as the usa and the mediterranean region experience increased precipitation in accord with proxy paleoclimate evidence the overall much drier climate over the ocean leads to higher sea surface salinity sss in most ocean basins except for the north atlantic where sss is considerably lower due to an increase in the supply of fresh water from the mississippi and amazon rivers and presumably a decrease in salt transport by the weakened north atlantic overturning circulation the north atlantic overturning stream function weakens to less than half of the control run value the overturning is limited to a shallower depth less than 1000 m and its outflow is confined to the northern hemisphere in the southern ocean convection is much stronger than in the control run leading to a stronger overturning stream function associated with enhanced antarctic bottom water formation as a result southern ocean water masses fill the entire deep ocean the antarctic circumpolar current acc transport through the drake passage increases by about 25 the acc transport despite weaker zonal winds is enhanced due to changes in bottom pressure torque the weakening of the overturning circulation in the north atlantic and the accompanying 30 decrease in the poleward ocean heat transport contrasts with the strengthening of the overturning circulation in the southern ocean and a 40 increase in heat transport as a result sea ice coverage and thickness are affected in opposite senses in the two hemispheres the lgm climate simulated by the coupled model is in reasonable agreement with paleoclimate proxy evidence the dynamical response of the ocean in the coupled model plays an important role in determining the simulated and undoubtedly the actual lgm climate

huascaran δ18o as an indicator of tropical climate during the last glacial maximum the interpretation of the isotopic composition of andes tropical mountain glaciers is reconsidered using a simple rayleigh distillation model it is shown that andes snowline depression and huascaran ice core 18o are consistent with a 3 c drop in sea surface temperature during the last glacial maximum accompanied by an increase from 40 at present to 60 during the lgm in the annual mean proportion of atmospheric water lost to continental runoff broecker s interpretation of huascaran 18o as a paleo hygrometer is discussed

ice core evidence of abrupt climate changes ice core records show that climate changes in the past have been large rapid and synchronous over broad areas extending into low latitudes with less variability over historical times these ice core records come from high mountain glaciers and the polar regions including small ice caps and the large ice sheets of greenland and antarctica

coherent isotope history of andean ice cores over the last century 1 isotope records from andean ice cores provide detailed and high resolution climate information on various time scales however the relationship between these valuable isotope records and local or regional climate remains poorly understood here we present results from two new drillings in bolivia from the illimani and the sajama ice caps all four high altitude isotope signals in the andes now available huascaran quelccaya illimani and sajama show near identical decadal variability in the 20th century comparison with general circulation model results and meteorological data suggest that the andean high altitude records are primarily controlled by precipitation variability over the amazon basin

tropical climate changes at millennial and orbital timescales on the bolivian altiplano tropical south america is one of the three main centres of the global zonal overturning circulation of the equatorial atmosphere generally termed the walker circulation1 although this area plays a key role in global climate cycles little is known about south american climate history here we describe sediment cores and down hole logging results of deep drilling in the salar de uyuni on the bolivian altiplano located in the tropical andes we demonstrate that during the past 50 000 years the altiplano underwent important changes in effective moisture at both orbital 20 000 year and millennial timescales long duration wet periods such as the last glacial maximum marked in the drill core by continuous deposition of lacustrine sediments appear to have occurred in phase with summer insolation maxima produced by the earth s precessional cycle short duration millennial events correlate well with north atlantic cold events including heinrich events 1 and 2 as well as the younger dryas episode at both millennial and orbital timescales cold sea surface temperatures in the high latitude north atlantic were coeval with wet conditions in tropical south america suggesting a common forcing

paleohydrological significance of trace fossil distribution in oligocene fluvial fan to lacustrine systems of the ebro basin spain the ichnological study of fluvial fan and lacustrine oligocene deposits in the ne ebro basin has allowed the recognition of three ichnoassemblages having a strong correlation with depositional facies associations lacustrine deposits lack bioturbation due to the hydrologically closed character of the lake that caused hypersalinity and made it inhospitable for benthic life terminal lobe deposits of the fluvial fan which were sedimented in low wetland areas around the lake bear a moderately diverse ichnoassemblage dominated by horizontal taenidium barretti cochlichnus anguineus and bird footprints gruipeda isp this assemblage can be assigned to the shorebird ichnofacies a subset of the scoyenia ichnofacies that characterizes the subaerial part of low energy shore areas crevasse splay deposits in medial fluvial fan areas present a different assemblage also belonging to the scoyenia ichnofacies it includes most of the trace fossils found in the terminal lobes except for the bird tracks plus very abundant vertical burrows taenidium barretti and unnamed ornamented burrows the presence of deep tier traces of terrestrial invertebrates is consistent with the more proximal fan setting having greater relief and consequently a lower mean groundwater position than lowland areas fluvial channels only bear irregularly clustered tunnel structures of unknown affinity formed after abandonment of the channel thus trace fossil assemblages are mostly controlled by the position and fluctuations of water tables which are the cause for their composite nature

application of neoichnological studies to behavioural and taphonomic interpretation of fossil bird like tracks from lacustrine settings the late triassic early jurassic santo domingo formation argentina abstract the purpose of this study is to apply neoichnological observations to the behavioural and taphonomic interpretation of a late triassic early jurassic track surface from the santo domingo formation argentina containing hundreds of bird like tracks and trackways in addition the factors affecting the formation and preservation of bird tracks in lacustrine settings are particularly addressed the 5 5 m2 fossil track surface contains different types of trace fossils the avian ichnotaxa gruipeda dominguensis the most abundant bird like tracks with elongated hallux impressions small epichnial rounded pits and invertebrate traces skolithos taenidium the modern environmental analogue chosen for the neoichnological studies was a coastal freshwater pond the bajo de los huesos chubut argentina seasonally occupied by sandpipers calidris bairdii and calidris fuscicollis charadriiformes the comparison between the fossil succession and the modern example suggests that they share lithology and sedimentary structures and that sedimentary processes and local palaeoenvironment were fairly similar and do not bias ichnological comparisons field observations allowed to distinguish twenty one behaviours that produced distinct traces and four modern footprint types 1 to 4 related to specific substrate conditions in particular the preferential formation of bird tracks parallel to the waterline also confirmed by studies on droppings and invertebrate fauna of the pond and other associated sedimentary features ripple marks wrinkle marks mud drape thickness and trace fossils were important for recognition of the shoreline in the fossil example these observations also allowed us to distinguish the adjacent deeper and shallower parts of the fossil pond and can be applicable to other similar case studies contrasting sediment properties and footprint types some relationships and constraints on the formation and preservation of modern footprints are proposed although these are very complex processes that will require further studies five of the behaviours recognised in the modern pond were inferred from the sixteen trackways distinguished on the fossil track surface including walking walking with a zig zag path short runs probing and landing with legs directed forward possible trace of flight the recognition of traces of flight volichnia probing marks and tracks showing morphology similar to modern shorebirds g dominguensis strongly suggest an avian affinity for the producers of the fossil tracks and in consequence the santo domingo track site would be younger than supposed

biogenic structures in exhumed surfaces around saline lakes an example from lake bogoria kenya rift valley abstract five overprinted suites of animal and plant traces around saline alkaline lake bogoria kenya are described from a set of interfingering late pleistocene and holocene exhumed surfaces that reveal a complex story of changes in lake level and environmental controls on trace assemblages through time these stratigraphic surfaces are amalgamated and form a co planar surface the sediments were first deposited during intermediate lake levels then reworked and cemented during low lake levels and were subsequently overlain by transgressive lacustrine sediments during higher lake levels ongoing exhumation of the surface near the present shoreline has reactivated the sediments as a substrate for the activities of epifaunal and infaunal animals as well as plant growth the modern environmental setting at lake bogoria is very similar to that of the preserved exhumed surfaces which allows comparisons between observed sets of environmental factors affecting animal behaviour and their representation in the fossilized examples pronounced lateral environmental heterogeneity around saline and hypersaline lakes may lead to concentrations of life around sites with vital resources such as freshwater at lake bogoria groups of fault controlled hot springs represent these oases and have been a relatively consistent feature in and around the lake during the development of the exhumed surfaces together the diverse assemblage of animal trace suites from lake bogoria represent the mermia and scoyenia ichnofacies but can be divided into five suites that reveal details of laterally heterogeneous sets of environmental factors and are useful indicators of lake level substrate moisture content substrate cohesion etc suite 1 comprises the traces of chironomid larvae diptera chironomidae formed in subaqueous lacustrine settings suite 2 includes the traces of flamingos phoeniconaias minor and phoenicopterus ruber formed at the shoreline suite 3 compares well with the mermia ichnofacies and comprises trails and burrow systems associated with relatively fresh saturated to extremely shallow subaqueous substrates suite 4 is comparable to both the pre and post desiccation suites of the scoyenia ichnofacies is the most diverse suite at lake bogoria and mainly comprises the burrows of beetles earwigs and their larvae as well as a diverse set of rhizoliths and vertebrate tracks suite 5 contains a moderately diverse set of traces attributed to termites and ants