geomorphology and palaeoclimatology of some upper cretaceous palaeosols in central india abstract the late cretaceous maastrichtian lameta formation exposed around jabalpur in central india comprises a pedogenically modified fluvial succession accumulation of early diagenetic calcite cement during penecontemporaneous pedogenesis has transformed the fluvial sandstones into calcareous sandstones the microscopic features of the palaeosol material include floating grain fabric root traces laminated micrite microspar micrite envelopes and corona structures illuvial clay envelopes poikilotopic spars and minor amounts of siliceous grain and void cutans the palaeosol profiles of the lameta formation can be classified into eleven different types nine of which are comparable to present day mature aridisols and two to inceptisols nine to thirteen palaeosol profiles are present in the individual sections of the lameta formation within the study area each profile comprises two to four distinct pedohorizons arranged in vertical succession there are six types of pedohorizons present in the area of study namely partly altered hosts light coloured and poorly cemented horizons horizons with root traces and root related alterations horizons of subspherical glaebules horizons of platy fused glaebules and subhorizontal root traces and horizons with solution features they show many characteristics similar to the present day soil horizons for example those termed c bk k a and e by pedologists the palaeosol characteristics show correspondence to the nature of fluvial channel migration and the depth of the groundwater table controlled by the geomorphic setting a shallow groundwater table favoured the development of rootmat bearing and gleyed horizons horizons with vertical root traces and glaebules occur where the groundwater table was low mature and thin palaeosols developed in those parts of the lameta fluvial tract that were characterized by stable channels and wide floodplains in contrast thin and comparatively immature soils occurred in the parts where laterally migrating channels were operative the characteristics of the lameta palaeosols suggest that a hot and semiarid climate prevailed for most of the time represented by the lameta formation but short term wet spells occurred during the initial and the terminal phases of the lameta accumulation the low latitudinal palaeoposition of india and the absence of the palaeo monsoon circulation might have produced the hot and semiarid climatic setting during this time

detection of miocene saucer shaped sills offshore senegal via integrated interpretation of seismic magnetic and gravity data the atlantic margin offshore senegal has been explored by seismic reflection and grav mag surveys high amplitude laterally transgressive seismic reflectors are found to coincide with gravimetric and magnetic highs once seismic data are integrated with potential fields modelling these reflectors can be safely interpreted as saucer shaped igneous sills up to some hundreds of metres thick some km wide the occurrence of hydrothermal vent complexes and forced folds in the stratigraphic sequence above the sills constrain the intrusion age to the miocene field observations and in situ magnetic susceptibility measurements of oligocene miocene and quaternary igneous rocks emplaced in coastal senegal support this interpretation

threshold dominated fluvial styles in an arid zone mud aggregate river the uplands of fowlers creek australia abstract fowlers creek is a mud aggregate fluvial system floodplain muds dominate the river s deposits and consist of silt fine to very fine quartzose sand and clay up to 80 of the silts and clays are bound into sand and silt sized aggregates and comprise a substantial component 42 of the floodplain muds mud aggregate sediments behave like sands during transport and as a result muds can be deposited under conditions of greater flow velocity than would otherwise be the case newly deposited floodplain muds are loose and easily entrained but older floodplain muds are cohesive and the distribution of modern and older floodplain muds influences erosion patterns across fowlers creek in the lower order streams of the fowlers creek uplands alternate reaches of shallow rectangular channels and unchannelled floodplains collectively form discontinuous ephemeral streams these landform sequences consist of gullies coalescing downstream to arroyos which terminate in distributary intermediate floodouts at fowlers creek floodouts are preferentially located at tributary junctions reflecting their origin during very large floods at floodouts low slope and high vegetation density promote sheetflow infiltration and landform stability their efficiency in retaining runoff make floodouts drought refugia they are an important ecological element in this arid area the higher order channel of the mid uplands is a mobile low sinuosity single thread arroyo incised into wide muddy unstable floodplains fluvial processes are dominated by episodic flood driven channel avulsion and variability in stream energy and boundary resistance contributes to a non equilibrium fluvial style frequent reach scale channel relocation is accompanied by the burial of the abandoned channel in floodplain muds and both erosion and aggradation in downstream floodplains

palaeoenvironmental significance of palustrine carbonates and calcretes in the geological record interest in palustrine carbonates and calcretes has increased over the last 20 years since they contain significant environmental information much of the work performed in this area has focused on either of two types of terrestrial carbonate palustrine carbonates or calcretes pedogenic and groundwater yet their simultaneous study shows there may be a gradual transition from one form to the other revealing the interplay between pedogenic sedimentary and diagenetic processes three main factors control the formation of these carbonates the position of the water table the host rock and the period of sub aerial exposure in pedogenic calcretes precipitation of carbonate takes places mostly in the vadose zone above the water table and within a previous host rock or sediment in groundwater calcretes the precipitation of carbonate also occurs within a previous host rock and around the groundwater table in palustrine carbonates however the precipitation of lime mud occurs in a lacustrine water body palustrine carbonates necessarily form on previous lacustrine mud whereas both types of calcretes may form on any type of sediment or soil the sub aerial exposure time needed to form palustrine carbonates may by relatively short even a season whereas pedogenic calcretes need more time several years to millions of years groundwater calcretes do not form on the topographic surfaces so there is no need of sub aerial exposure however stable surfaces favour the development of thick groundwater calcretes small fluctuations in the water table cause gradual transitions of these three types of terrestrial carbonates and the subsequent mixture of their characteristic features causing difficulties in the interpretation of these carbonates the formation of these carbonates is controlled by palaeoenvironmental factors both commonly form in semi arid climates arid climates are also suitable for calcretes but sub humid conditions are more suitable for palustrine carbonates more indications of climatic conditions may be obtained through the analysis of the d18o content of both calcretes and palustrine carbonates and from the depth of the horizon containing carbonate nodules in pedogenic calcretes vegetation is also important in the formation of these types of carbonates data on the prevailing vegetation can be obtained from the analysis of the micro and macrofabric as well as from the d13c signal of the primary carbonates which in pedogenic carbonates has also been used to estimate atmospheric pco2 during the phanerozoic these terrestrial carbonates are widely distributed on floodplains and distal areas of alluvial basins their presence and characteristics can be used as indicators of aggradation subsidence or accommodation rates and therefore as indicators of different tectonic regimes even though the study of these carbonates has notably increased in recent years much less is known about them than about marine carbonates presently there is much emphasis on obtaining a general model for sequence stratigraphy in terrestrial basins with a need to include the carbonates analysed in this paper

hydrographic biases in global coupled climate models and their relation to the meridional overturning circulation comparison of the volumetric s distribution of models participating in the climate model intercomparison project 3 cmip3 indicates that these models differ widely in their ability to represent the thermohaline properties of water masses relationships between features of the quasi equilibrium hydrographic mean state of these models and aspects of their overturning circulations are investigated this is achieved quantitatively with the help of seven diagnostic hydrographic stations these few stations were specifically selected to provide a minimalist schematic of the global water mass system relationships between hydrographic conditions in the north atlantic measured with a subset of these stations suggest that hydrographic properties in the subpolar north atlantic are set by the circulation field of each model pointing towards deficiencies in the models ability to resolve the gulf stream north atlantic current system as a major limitation since diapycnal mixing and viscosity parameterizations differ across cmip3 models and exert a strong control on the overturning it is likely that these architectural differences ultimately explain the main across model differences in overturning circulation temperature and salinity in the north atlantic the analysis of properties across the quasi equilibrium states of the cmip3 models agrees with previously reported relationships between meridional steric height gradients or horizontal density contrasts at depth and the strength of the deep water cell robust relationships are also found in the southern ocean linking measures of vertical stratification with the strength of the abyssal circulations across the cmip3 models consistent correlations between aspects of the quasi equilibrium hydrography in the southern ocean and the sensitivity of the abyssal cell to increasing radiative forcing by 2100 were found using these relations in conjunction with modern hydrographic observations to interpolate the fate of the abyssal cell suggests that the southern abyssal cell may decrease by roughly 20 by the end of the century similar systematic relationships between the quasi equilibrium hydrographic states of the models and the sensitivity of their atlantic deep water cell could not be found

comparison of lacustrine successions and their palaeohydrological implications in two sub basins of the triassic cuyana rift argentina fil benavente cecilia andrea consejo nacional de investigaciones cientificas y tecnicas centro cientifico tecnologico conicet mendoza instituto argentino de nivologia glaciologia y ciencias ambientales provincia de mendoza instituto argentino de nivologia glaciologia y ciencias ambientales universidad nacional de cuyo instituto argentino de nivologia glaciologia y ciencias ambientales argentina

flow of winter transformed pacific water into the western arctic abstract the dynamics of the flow of dense water through barrow canyon is investigated using data from a hydrographic survey in summer 2002 the focus is on the winter transformed bering water the highest volumetric mode of winter water in the chukchi sea which drains northward through the canyon in spring and summer the transport of this water mass during the time of the survey was 0 2 0 3 sv as the layer flowed from the head of the canyon to the mouth it sank decelerated and stretched strong cyclonic relative vorticity was generated on the seaward side of the jet which compensated for the stretching this adjustment was incomplete however in that it did not extend across the entire current possibly because of internal mixing due to shear instabilities the resulting vorticity structure of the flow at the canyon mouth was conducive for baroclinic instability and eddy formation multiple eddies of winter transformed bering water were observed along the chukchi beaufort shelfbreak those to the west of barrow canyon were in the process of being spawned by the eastward flowing shelfbreak current emanating from herald canyon while the single eddy observed to the east originated from the barrow canyon outflow it is argued that such an eddy formation is a major source of the ubiquitous cold core anti cyclones observed historically throughout the canada basin implications for the ventilation of the upper halocline of the western arctic are discussed

palaeoclimate reconstructions of the middle jurassic of kachchh western india an integrated approach based on palaeoecological oxygen isotopic and clay mineralogical data abstract the middle jurassic sedimentary succession of the kachchh basin shows significant changes in lithology faunal content and diversity across the bathonian callovian boundary the change in lithology from predominantly carbonates in the middle and late bathonian to siliciclastics in the callovian is accompanied by a drop in species diversity from high to moderate characteristic faunal elements of the bathonian corals sponges the bivalves eligmus and members of the opinae are absent or rare in the callovian in contrast callovian sediments contain comparatively high proportions of nuculid bivalves the clay mineral assemblage reveals prominent variations in the smectite vs kaolinite abundance especially during the bathonian to callovian transition from the bajocian to the middle bathonian smectite becomes the dominant clay mineral and is nearly exclusively present in the late bathonian towards the middle callovian its content gradually decreases and kaolinite becomes the dominant clay mineral illite generally is only a minor component throughout the middle jurassic based on these results the bajocian to middle bathonian time interval is interpreted to represent subtropical climatic conditions with seasonal droughts and a moderate supply of terrigenous clastics to the basin the late bathonian was a period with a semi arid climate hot seasonal droughts and a minor input of terrigenous clastics the increase in kaolinite contents in the callovian suggests a subtropical humid climate with less prominent seasonal droughts and a higher input of siliclastics into the basin accompanied by a higher nutrient influx oxygen isotope ratios measured on bathonian to early callovian brachiopod shells give palaeotemperatures of 19 24 c callovian to oxfordian paleotemperatures calculated from oxygen isotopes measured on belemnite rostra range from 11 to 21 c the lower palaeotemperatures estimated from the oxygen isotopic composition of belemnite calcite are explained by assuming that the belemnites spent most of their life span in colder regions of the malagassy gulf and migrated into the warmer subtropical waters of the kachchh basin combining these various lines of evidence the changes in lithology and faunal composition taking place in the kachchh basin during the middle jurassic appear to reflect largely a change towards increasing humidity rather than a distinct decrease in water temperature

unconfined flow deposits in distal sectors of fluvial distributary systems examples from the miocene luna and huesca systems northern spain abstract thin sheet sandstone beds in a continental succession may be the products of unconfined overbank flow or deposition in lacustrine environments in the distal parts of fluvial distributary systems these two settings may be intercalated recording fluctuations in lake level in response to climatic changes field studies of the luna and huesca fluvial distributary systems in the miocene of the ebro basin spain are here used to characterise sheet sandstone deposited in distal fluvial and lake margin environments in this study we document the facies and relationships between 322 individual sandbodies deposited in both alluvial settings as sheetfloods and in lacustrine settings as deltaic lobes the alluvial sandstone sheets were deposited from lateral and frontal sheetflood events when unconfined flow replaced channelised flow as the main transport mechanism the term sheetflood is used here to refer to sub aerial unconfined turbulent flow events that undergo expansion thinning and deceleration with increasing radial distance from source the depositional process is similar to modern floodouts and terminal splays of the channel country and lake eyre central australia such a process was thought to be the dominant depositional mechanism during lake lowstands the sheets formed in the lacustrine setting are interpreted as deltaic sediments deposited in a shallow gradient lake similar to the modern day volga delta caspian sea shallow water friction dominated deltaic processes are thought to have been the primary depositional mechanism in the distal sectors of the fluvial systems during lake highstands

cenozoic volcanism in western senegal and its relationship to the opening of the central atlantic ocean abstract the westernmost part of the senegalese basin in west africa is characterized by an extensive magmatic event exposed in the form of sills and dykes mainly during the quaternary but beginning in the tertiary 35 0 1 ma miocene to 0 8 0 1 ma pleistocene the main exposures are located in the cap vert peninsula and thies plateau to the east of dakar this magmatic event postdated the opening of the central atlantic and displays chemical similarities to the tertiary quaternary magmatic centres within the african plate two main facies are distinguished among the flows microporphyric and doleritic textured olivine rich alkaline basalts the former often containing spinel lherzolite mantle inclusions the alkaline sodic affinity of the volcanic series and the ti k na nb p2o5 and zr content of the lavas are typical of magmas generated in a within continent plate tectonic setting this volcanism appears to have resulted from an active mantle hotspot over which the african plate migrated as the central atlantic ocean opened using reactivated faults contemporaneous with central atlantic opening as magmatic pathways

triassic climates state of the art and perspectives abstract the climate of the triassic period was characterized by a non zonal pattern dictated by a strong global monsoon system with effects that are most evident in the tethys realm this strong monsoonal regime is related to the aggregation of the pangaean supercontinent which by triassic time was already completed climate oscillations existed within this framework the harsh hot house climatic conditions that characterized the late permian and perhaps precipitated the end permian mass extinction were probably maintained during the early triassic and may account for the impoverished but distinctive faunal and floral lower triassic associations although metazoan reef builders were probably the most affected group carbonate production remained high at least in the western tethys realm the middle triassic was characterized locally by humid episodes although their geographical distribution has yet to be clarified the carnian pluvial event marks an episode of increased rainfall documented worldwide was the most distinctive climate change within the triassic different hypotheses have been proposed for its causes changes of atmospheric or ocean circulation driven by plate tectonics a peak of the global monsoon due to maximum continent aggregation or triggering by the eruption of a large igneous province subsequently the late carnian and norian seem to have been climatically stable although minor climatic changes have recently been described even from this time period finally the end triassic extinction event is also associated with climate change specifically warming and increased rainfall but this evidence comes mostly from the northern parts of the central atlantic magmatic province and the global pattern of climate change at the triassic jurassic boundary has still to be resolved many facets of triassic climate are intriguing and deserve further research however palaeoclimate studies on the triassic have so far been carried out only locally with different proxies those proxies will require inter calibration in order to depict correctly the temporal and geographical patterns of triassic climate

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

a salt oscillator in the glacial atlantic 1 the concept as shown by the work of dansgaard and his colleagues climate oscillations of one or so millennia duration punctuate much of glacial section of the greenland ice cores these oscillations are characterized by 5 c air temperature changes severalfold dust content changes and 50 ppm co2 changes both the temperature and co2 change are best explained by changes in the mode of operation of the ocean in this paper we provide evidence which suggests that oscillations in surface water conditions of similar duration are present in the record from a deep sea core at 50 n based on this finding we suggest that the greenland climate changes are driven by oscillations in the salinity of the atlantic ocean which modulate the strength of the atlantic s conveyor circulation

the north pacific a global scale estuary abstract the atmospheric net flow of water from the atlantic to the pacific ocean is supposed to maintain the salinity difference between the two oceans assuming the existence of a subsurface level of no horizontal pressure gradient in the ocean the mean sea level in the northern pacific must be higher than in the arctic ocean this mean sea level difference is supposed to drive the observed mean flow through the bering strait the estimated flow of freshwater through the bering strait is approximately equal to the estimated atmospheric net flow of water from the north atlantic to the north pacific this justifies the formulation of a simple estuary model for the north pacific in which the brackish water exits through the bering strait the salinity difference between the two oceans is shown to be controlled by the topography of the bering strait the estuary model gives residence times for water in the upper layer 1000 m thick of approximately 1000 years and in the lower layer of approximately 400

tracing pacific water in the north atlantic ocean 1 in the arctic ocean pacific source water can be distinguished from atlantic source water by nitrate phosphate concentration relationships with pacific water having higher phosphate concentrations relative to those of nitrate furthermore pacific water originally from the inflow through bering strait is clearly recognizable in the outflows of low salinity waters from the arctic ocean to the northern north atlantic ocean through the canadian arctic archipelago and through fram strait in the canadian arctic archipelago we observe that almost all of the waters flowing through lancaster and jones sounds most of the water in the top 100 m in smith sound containing the flow through nares strait and possibly all waters in hudson bay contain no water of atlantic origin significant amounts of pacific water are also observed along the western coast of baffin bay along the coast of labrador and above the 200 m isobath of the grand banks there is a clear signal of pacific water flowing south through fram strait and along the east coast of greenland extending at least as far south as denmark strait pacific water signature can be seen near the east coast of greenland at 66 n but not in data at 60 n temporal variability in the concentrations of pacific water has been observed at several locations where multiple year observations are available

tectono sequence stratigraphy and u pb zircon ages of the rincon blanco depocenter northern cuyo rift argentina abstract extensional processes that followed the gondwanan orogeny rise to the development of a series of rift basins along the continental margin over older accreted eopaleozoic terranes stratigraphic structural paleontological and isotopic studies are presented in this work in order to constrain the ages of the sedimentary infilling and to analyze the tectosedimentary evolution of one of the cuyo basin depocenters known as rincon blanco this asymmetrical half graben was filled by continental sediments under a strong tectonic control the infilling was strongly controlled by tectonics which in term produced distinctive features along the whole sedimentary sequence using a combination of lithological and structural data the infilling was subdivided into packages of genetically linked units bounded by regional extended surfaces several tuffs and acid volcanic rocks have been collected across the whole section of the rincon blanco sub basin for shrimp and la mc icpms u pb zircon dating the ages obtained range from 246 4 1 1 ma to 230 3 1 5 ma which is the time elapsed for the deposition of three tectono sequence units separated by regional unconformities and mainly constrained to the middle triassic they are interpreted as a result of a reactivation of the extensional system that has evolved along strike as segments of faults that linked together and or as laterally propagating faults regional correlation with coeval rift basins permits to establish north south propagation in the extensional regime along the western margin of sw gondwana this trending started in the lowermost triassic and extended until the latest triassic two of them were precisely correlated with cerro puntudo and cacheuta half graben systems the new data indicate that the three sequences were mostly deposited during the middle triassic 246 to 230 ma with no evidence of sedimentation during norian and rhaetian which is in conflict with some previous biostratigraphic studies

a quantitative model for deposition of thin fluvial sand sheets sheetfloods are typically invoked as the mechanism responsible for the kilometre scale transport of sand sized sediment grains in shallow gradient fluvial systems this concept is based on the lateral extent of ancient thin sheet sandstone deposits rather than on fluid dynamics which has resulted in a loosely constrained model for sheetfloods this study tested the conceptual mechanism by developing a depth averaged 2d computational fluid dynamics model the model results compare well against observations from modern deposits at lake eyre to provide a quantitative physically sound basis for sheetfloods that can be applied in ancient and modern settings to constrain otherwise qualitative interpretations

integrated ichnofacies models for deserts recurrent patterns and megatrends abstract although it is commonly assumed that the trace fossil record of eolian dunes and associated environments is invariably poor a systematic review of the available information indicates that this is not necessarily the case a model involving five main phases of colonization of desert environments through the phanerozoic is proposed in this paper the first phase cambrian silurian involved animal incursions into coastal dune fields directly from the sea although it is unlikely that these animals would have remained for long periods of time in coastal deserts the second phase devonian reflects the activities of dune pioneers that left their fluvial habitat to enter temporary or permanently into inland deserts the third phase carboniferous permian involved the colonization of deserts by tetrapods the fourth phase triassic cretaceous involved a major exploitation of the infaunal ecospace as reflected by the appearance of more varied behavioral patterns in sub superficial structures the fifth phase paleogene recent reflects the appearance of the ecological dynamic that characterizes modern desert communities the invertebrate ichnofacies for eolian dunes is re named herein as the octopodichnus entradichnus ichnofacies honoring the seminal work of previous workers who simultaneously tackled the issue of eolian dune ichnofacies the chelichnus ichnofacies is retained for vertebrate trace fossil assemblages in eolian settings both ichnofacies occur in mobile and temporary stabilized sandy substrates subject to frequent erosion and deposition and to strong seasonality desert settings consist of complex mosaics of habitats or physical units associated with organism activity trace fossil distribution can be understood as reflecting the partitioning of desert settings in a mosaic of landscape units characterized by water content and its temporal fluctuations nutrient availability nature of the substrate and the dominant organisms present in turn desert systems are dynamic entities that change as a response to regional climate landscape units such as eolian sand seas salt flat and playa lake systems ephemeral rivers and alluvial fans interact in response to regional scale climate variations in hyper arid arid and semiarid climatic settings ancient deserts completely developed under hyper arid climatic conditions rarely preserve trace fossils due to the absence of moisture near the surface however the alternation of wet periods may represent windows for life development and thus preservation of biogenic structures arid deserts display complex patterns of dunes combined with dry wet and flooded interdunes dry desert elements e g dunes interdunes sand sheets typically record the entradichnus octopodichnus and chelichnus ichnofacies slight rises in regional precipitation produce elevation of the water table and increase of fluvial discharges that provide water and sediment to the system these processes may result in the local concentration of trace fossils in wet interdunes and ephemeral fluvial systems illustrating the scoyenia and chelichnus ichnofacies in semiarid systems playa lakes expand by the addition of freshwater evolving into freshwater lakes and fluvial systems may become more common lake margins and fluvial overbanks typically contain trace fossil assemblages that may be ascribed to the scoyenia ichnofacies

integrated analysis for constraining palaeoclimatic and volcanic influences on clay mineral assemblages in orogenic basins palaeogene andean foreland northwestern argentina abstract variations in clay mineral assemblages in ancient continental deposits are frequently used to reconstruct past climate changes in active settings volcanic events can supply highly labile volcaniclastic material which can easily be transformed into smectite via diagenesis which can produce a noticeable footprint in clay mineral assemblages southern central andean foreland deposits are appropriate case studies to ascertain whether the climatic signal was preserved in the clay assemblages of their fine grained sediments as tectonic uplift volcanism and sedimentation have been interacting since the cretaceous we have studied a 1400 m thick coarsening upward palaeogene succession of the tin tin basin northern calchaqui valley argentina applying x ray diffraction xrd electron microscopy and detailed sedimentary facies analysis with the aim of comparing tendencies in the vertical fluctuations of clay minerals with evidence from sedimentological facies illite muscovite plus smectite account for 78 to 100 of the clay minerals in the fine fraction with kaolinite and chlorite in subordinate amounts the vertical variation of sedimentary settings from an overbank lacustrine domain to fluvial braided plains and an aeolian dune field suggests a gradual increase in aridity upsection however smectite abundances do not show a gradual decreasing trend compatible with progressively lower hydrolyzing conditions their relative abundances vary widely throughout the section depicting pulse like abrupt fluctuations despite the absence of field evidence for volcanic influence several indications of volcanic and volcaniclastic material have been found under scanning electron microscopy sem in levels with high smectite abundances from the middle to the top of the succession they include quartz crystals showing embayments and skeletal forms with smectite filling the voids microcrystalline silica as well as heulandite crystals in close association with authigenic smectite the xrd analyses of these levels evidence well crystallized smectite which is characteristic of a volcaniclastic origin therefore the increase in smectite abundance in these beds reflects a significant volcaniclastic contribution which is also evidenced by a centimetre thick ash layer topward in the sequence the only smectite rich level near the base of the tin tin section also contains well crystallized smectite associated with heulandite thus probably evidencing volcaniclastic input we infer that most of the smectite in these sediments formed during early diagenesis probably through the dissolution of labile tuffaceous material textural and morphological analysis by sem is essential to determine whether clay mineral assemblages could be interpreted in terms of palaeoclimate

the role of the alaskan stream in modulating the bering sea climate 1 a numerical ocean circulation model with realistic topography but with an idealized forcing that includes only lateral transports is used to study the role of the alaskan stream as in modulating the bering sea bs variability sensitivity experiments each one with a different strength of the as transport reveal a nonlinear bs response an increase of as transport from 10 to 25 sv causes warming 0 25 c mean 0 5 c maximum and sea level rise in the bs shelf due to increased transports of warmer pacific waters through the eastern passages of the aleutian islands but an increase of as transport from 25 to 40 sv had an opposite impact on the bs shelf with a slight cooling 0 1 c mean 0 5 c maximum as the as transport increases flows through passages farther downstream in the western aleutian islands are affected and the variability in the entire bs is reduced transport variations of 0 1sv in the bering strait are found to be correlated with mesoscale variations of the as and associated transport variations in the aleutian islands passages these results have important implications for understanding the observed variations in the bering strait and potential future climate variations in the arctic ocean

sedimentary facies and environmental ichnology of a permian playa lake complex in western argentina abstract a moderately diverse arthropod ichnofauna occurs in permian ephemeral lacustrine deposits of the paganzo basin that crop out at bordo atravesado cuesta de miranda western argentina sedimentary successions are interpreted as having accumulated in a playa lake complex deposits include three sedimentary facies a laminated siltstone and mudstone b current rippled cross laminated very fine grained sandstone and c climbing and wave rippled cross laminated fine grained sandstone deposited by sheet floods under wave influence in the playa lake complex analysis of facies sequences suggests that repeated vertical facies associations result from transgressive regressive episodes of variable time spans the bordo atravesado ichnofauna includes cruziana problematica diplocraterion isp cf diplopodichnus biformis kouphichnium isp merostomichnites aicunai mirandaichnium famatinense monomorphichnus lineatus palaeophycus tubularis umfolozia sinuosa and umfolozia cf u longula the assemblage is largely dominated by arthropod trackways and represents an example of the scoyenia ichnofacies trace fossils are mostly preserved as hypichnial ridges on the soles of facies c beds being comparatively rare in facies a and b ichnofossil preservation was linked to rapid influx of sand via sheet floods entering into the lake four taphonomic variants types 1 4 are recognized each determined by substrate consistency and time averaging type 1 is recorded by the presence of low density assemblages consisting of poorly defined trackways which suggests that arthropods crawled in soft probably slightly subaqueous substrates type 2 is represented by low to moderate density suites that include sharply defined trackways commonly associated with mud cracks suggesting that the tracemakers inhabited a firm desiccated lacustrine substrate type 3 displays features of types 1 and 2 and represents palimpsestic bedding surfaces resulting from the overprint of terrestrial ichnocoenoses over previously formed softground suites type 4 differs from type 2 only in that assemblages display a high density of traces recorded by numerous superimposed trackways which suggests a major time gap of subaerial exposure before sheet flood entrance therefore type 4 surfaces are mostly interpreted as track imprinted omission surfaces

tetrapod tracks in a marginal lacustrine setting middle triassic argentina taphonomy and significance abstract fossil tetrapod footprints not only provide valuable information about trackmaker paleobiology but also to give insight into details of the depositional conditions of the substrate at the time of imprinting therefore in the present study the mode of formation and taphonomy of footprints in different substrates was used to investigate the gait and walking dynamics of the trackmakers as well as a source of additional information on the environmental conditions of the track bearing beds during imprinting the analyzed section corresponds to thick middle triassic lacustrine deltaic deposits of the ischichuca los rastros formation ischigualasto villa union basin that crops out at the quebrada de ischichuca in northwestern argentina part of the track bearing surfaces correspond to the top of sandy distributary channel mouth bars in a distal delta front setting that were exposed along the lake margin during a lake level fall cross cutting relationships observed among ripple marks the footprints and invertebrate traces of a softground suite of the scoyenia ichnofacies suggest an omission surface measured trackway orientations in the sandstones are perpendicular to the paleo shoreline with the animals coming and going along the exposed top of the bars probably for drinking laterally the distal delta front deposits interfinger with track bearing wackestone beds of palustrine origin deposited in a restricted local embayment lateral to the delta influenced environment trackway orientations in the wackestones are in contrast consistent with the animals moving nearly parallel to the lake border probably along a preferred route evidences of a relative high groundwater table at the time of imprinting in the track bearing surfaces are revealed by the well developed rims of extruded sediment and collapsed digits in the studied tracks and the nearly absence of associated desiccation cracks on the same surfaces nevertheless temporary emergence cannot be ruled out when paleosoil formation was probably promoted as can be observed in the microstructure of both sandstones and wackestones moreover footprint preservation in the wackestones might have been enhanced by partial hardening of the trampled surface during subaerial exposure combining ichnofossil content and taphonomy with facies analysis we identified in the lower part of the ischichuca los rastros succession a relatively rapid withdrawal of the water basinward that was probably due to a forced regression during early rifting of basin evolution footprints can also provide valuable information about locomotion dynamics and trackmaker behavior thus the sideways deformation observed in the studied footprints attributed to basal archosaurs and putative basal dinosaurs can be related to an outward rotation of the foot during the step cycle a condition that might allied to the development of the parasagittal posture in archosauria besides the densely trampled surface described herein constitutes the first documented evidence of putative social behavior among therapsid dicynodonts the most important group of herviborous animals in the early mesozoic terrestrial ecosystems throughout gondwana

sedimentology and architecture of the douglas creek terminal splay lake eyre central australia douglas creek terminal splay sited on the western shoreline of lake eyre north central australia covers a surface area of approximately 4 km2 with a down system length of 2 5 km from the distributary channels terminus to the splay fringe two distributary channels feed two sediment lobes which have amalgamated to form the terminal splay three primary facies associations have been identified sub dividing the creek terminus into distributary channel proximal and distal splay sections proximal splay sediments are characterized by erosionally based relatively thick 100 mm stacked sheets of coarse to medium sand which commonly display trough and planar cross bedding whereas the distal splay is characterized by thin generally 50 mm massive beds of very fine sand silt and clay the change in splay sedimentology is interpreted as reflecting the transition from bedload dominated deposition to suspended load dominated deposition from decelerating sheetfloods as they spread out from the channel onto the dry lake bed a proximal to distal splay transition zone is also noted where deposits of both facies associations interfinger laterally and vertically in scale geometry and facies associations the douglas creek terminal splay is very different to the often cited neales terminal splay complex located 70 km to the north it is suggested that these architectural differences reflect variations in discharge volume input sediment distribution and the degree of vegetation cover understanding the variation in terminal splay architecture has very significant implications for the modelling of analogous subsurface petroleum systems which at present relies on few modern day analogues

climate of the supercontinent pangea numerous climate models predict that the geography of the supercontinent pangea was conducive to the establishment of a megamonsoonal circulation in general geologic evidence supports the hypothesis of a megamonsoon that reached maximum strength in the triassic pangea in the late carboniferous had widespread peat formation in what is now central and eastern north america and europe and relatively dry conditions on the colorado plateau the equatorial region of the continent became drier through the end of the carboniferous by the permian the equatorial region of pangea was dry and indicators of aridity and rainfall seasonality became more widespread wind directions from colorado plateau eolian sandstones are consistent with an increasing influence of monsoonal circulation at this time in the triassic climate in the colorado plateau region became relatively wet though still seasonal and the few eolian sandstones indicate a major shift in wind direction at that time in addition sedimentation

a role for north pacific salinity in stabilizing north atlantic climate 1 a simple ocean atmosphere feedback may reduce the amplitude of climate variability in around the north atlantic during interglacial compared to glacial states when climate is warm in the north atlantic region the intertropical convergence zone has a relatively northward position and moisture is exported from the tropical atlantic to the tropical pacific at the same time the east asian summer monsoon is strong which helps maintain a positive balance of precipitation over evaporation in the subpolar north pacific this is thought to account for lower salinity in the north pacific relative to the north atlantic which in turn drives northward flow through the bering strait to the northern north atlantic freshening in the north atlantic by water of pacific origin suppresses the meridional overturning circulation and reduces the heat flux the opposite situation exists during cold climate thus the combination of atmospheric vapor transport and flow through bering strait tends to cool the north atlantic region when warm and warm the region when cool

structure of the southern senegalo mauritanian basin west africa from geoelectrical studies abstract the southern part of the senegalo mauritanian basin is geologically featureless and little is known about the division of the sedimentary sequence and regional geological structure because of the scarcity of deep drillholes a magnetotelluric survey was therefore carried out in an attempt to obtain details concerning the electrical characteristics thickness and structure of the mesozoic cainozoic senegalo mauritanian basin from the surface down the interpretation of magnetotelluric data shows several distinct electrical boundaries the general trend shows a decrease in near surface resistivity from east to west the basin is characterized by water saturated unconsolidated low resistivity sediments with a westward sloping monoclinal style the combination of borehole and geological data with a geoelectrical cross section provides a simplified model of the geological electrical units of the basin this shows the distribution of strata above and below the base of the senonian conductive material 10 30 ohm m at depth in the central part of the profile may represent a northern extension of the palaeozoic sediments of the bove sequence which lies immediately south of the magnetotelluric line significant electrical contrasts between the sediments and basement rocks allow mapping of the top of the high resistivity basement which is characterized by progressively deeper burial to the west

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

surface and deep ocean interactions during the cold climate event 8200 years ago evidence from a north atlantic deep sea sediment core reveals that the largest climatic perturbation in our present interglacial the 8200 year event is marked by two distinct cooling events in the subpolar north atlantic at 8490 and 8290 years ago an associated reduction in deep flow speed provides evidence of a significant change to a major downwelling limb of the atlantic meridional overturning circulation the existence of a distinct surface freshening signal during these events strongly suggests that the sequenced surface and deep ocean changes were forced by pulsed meltwater outbursts from a multistep final drainage of the proglacial lakes associated with the decaying laurentide ice sheet margin

amplitude and timing of temperature and salinity variability in the subpolar north atlantic over the past 10 k y paired planktic foraminiferal 18 o and mg ca data reveal trends of increasing temperatures 3 c and salinities in the subpolar north atlantic over the course of the holocene which were punctuated by abrupt events the trends likely refl ect an insolation forced northward retreat of the boundary between polar and north atlantic subsurface waters the superimposed variability does not appear to be periodic but tends to recur within a broad millennial band the records provide convincing evidence of open ocean cooling nearly 2 c and freshening during the 8 2 ka event and suggest similar conditions at 9 3 ka however the two largest temperature oscillations in our record 2 c occurred during the past 4 k y suggesting a recent increase in temperature variability relative to the mid holocene perhaps in response to neoglaciation which began at about this time

an integrative physical mineralogical and ichnological approach to characterize underfilled lake basins fil mancuso adriana cecilia consejo nacional de investigaciones cientificas y tecnicas centro cientifico tecnologico conicet mendoza instituto argentino de nivologia glaciologia y ciencias ambientales provincia de mendoza instituto argentino de nivologia glaciologia y ciencias ambientales universidad nacional de cuyo instituto argentino de nivologia glaciologia y ciencias ambientales argentina

the role of sea ice and other fresh water in the arctic circulation salinity stratification is critical to the vertical circulation of the high latitude ocean we here examine the control of the vertical circulation in the northern seas and the potential for altering it by considering the budgets and storage of fresh water in the arctic ocean and in the convective regions to the south we find that the present day greenland and iceland seas and probably also the labrador sea are rather delicately poised with respect to their ability to sustain convection small variations in the fresh water supplied to the convective gyres from the arctic ocean via the east greenland current can alter or stop the convection in what may be a modern analog to the halocline catastrophes proposed for the distant past the north atlantic salinity anomaly of the 1960s and 1970s is a recent example it must have had its origin in an increased fresh water discharge from the arctic ocean similarly the freshening and cooling of the deep north atlantic in recent years is a likely manifestation of the increased transfer of fresh water from the arctic ocean into the convective gyres finally we note that because of the temperature dependence of compressibility a slight salinity stratification in the convective gyres is required to efficiently ventilate the deep ocean

carbonate deposition in an ephemeral siliciclastic alluvial system jurassic shuttle meadow formation newark supergroup hartford basin usa abstract nonmarine carbonate accumulation from a sedimentologic perspective has multiple possible origins clastic load springs groundwater discharge soil development biochemical seasonal changes cave speleothems and eolian influx many of these carbonates contain diagnostic textures from which origin is easy to ascertain however differentiating between clastic and spring groundwater carbonates especially in depositional systems dominated by siliciclastics is not always simple here criteria for recognition are developed subsurface hydrology and tectonic origin of the depositional basin source rocks surrounding the basin sedimentologic packaging and extent of the carbonates in question as well as their sedimentary structures and fossil content and stable isotopic analyses of the carbonates can all aid in determining carbonate origin and correctly assessing sedimentary and basin evolution the present study focuses on a nonmarine carbonate deposit found within the jurassic shuttle meadow formation of the hartford basin one of many rift basins which formed as a result of the opening of the north atlantic ocean during the late triassic to early jurassic the upper 50 m of the formation at plainville connecticut is interpreted as continental in origin and is composed of five facies red ripple cross laminated muddy siltstones to sandstones fr sr massive red siliciclastic mudstones fm buff fine to medium grained horizontally laminated sandstones sh red to white disrupted calcareous shales fl3 and rare micritic carbonates pm facies fr sr sh and fm are interpreted as an alluvial plain system in an ephemeral setting with bedload sheetflooding as the dominant depositional mechanism similar to the channel country drainage system of central australia in addition preservation of lateral accretion sandstone sheets accretionary benches indicate that the shuttle meadow alluvial system was rapidly accreting the anomalous existence of carbonate beds in facies fl3 and pm in the smf bedload dominated siliciclastic sequence is attributed mainly to the unique nature of the clastic sediment load in the basin no direct evidence for a spring groundwater input is apparent the shallow carbonate lake margin or a pond with vegetation and periodic exposure is envisioned as the depositional environment the intraclastic texture of the micrites with rare primary relict lamination indicates early diagenetic alteration from root penetration desiccation cracks and burrowing

lacustrine system evolution during early rifting el castellar formation galve sub basin central iberian chain abstract a detailed sedimentological study of the first synrift deposits recorded in the galve sub basin of ne spain during the early cretaceous aided in the reconstruction of climatic and tectonic influences the el castellar formation is composed of siliciclastics and carbonates unit 1 claystones gypsum and carbonates unit 2 and marls and limestones unit 3 unit 1 facies formed in alluvial and palustrine plains a local alluvial fan and low energy shallow lake subenvironments low energy lacustrine facies characterised unit 2 in contrast palustrine low and high energy lake facies were identified for unit 3 the spatial and temporal distribution of lithofacies representing the several environments were used to propose a general lacustrine system evolution from isolated ponds and marshes to low energy shallow lakes and finally to an extensive high energy lake lake evolution is related to changes in the subsidence pattern from local to basin scale subsidence which was ultimately related to the transition from the initial rift to rift climax stage shallowing upwards lacustrine successions have been interpreted as climatically forced and linked to sediment and water supply both tectonics and climate determined the change from an overfilled to a balanced fill lake basin through time

glacial interglacial sediment transport to the meiji drift northwest pacific ocean evidence for timing of beringian outwashing abstract a large sediment deposit known as the meiji drift located in the northwestern pacific ocean is thought to have formed from deep water exiting the bering sea although no notable deep water forms there presently we determine the terrigenous sources since 140 ka to the drift using bulk sediment 40ar 39ar and nd isotopic analyses on the silt sized 20 63 m terrigenous fraction from ocean drilling program odp site 884 to reconstruct paleo circulation patterns there are large changes in both isotopic tracers varying on glacial interglacial cycles during glacial intervals bulk sediment 40ar 39ar ages range between 40 and 80 ma while nd isotopic values range from end 1 to 2 during interglacial intervals sediments become much younger and more radiogenic with bulk sediment ages falling to 2 15 ma and nd isotopic values ranging between end 5 and 9 these data and quantitative comparison to potential source rocks indicate that the young kamchatkan and aleutian arcs lying nw and ne of the meiji drift contribute the majority of sediment during interglacials conversely older source rocks such as those drained by the yukon river and northeast russia are the dominant origin of sediments during glacials mixing model calculations suggest that as much as 35 45 of the sediment deposited in the meiji drift during glacials is from the bering sea it remains unclear whether thermohaline type circulation or focussing of bering sea flow lead to the glacial interglacial sediment source changes observed here

the role of seasonality in abrupt climate change abstract a case is made that seasonality switches dominated by wintertime were instrumental in abrupt climate changes in the north atlantic region during the last glaciation and into the holocene the primary evidence comes from mismatches between mean annual temperatures from greenland ice cores in comparison with snowline changes in east greenland northern europe and north america the most likely explanation is a shutdown or reduction in strength of the conveyor this allows the spread of winter sea ice across the north atlantic thus causing the northern region to experience much colder winters because they mimic the greenland temperature rather than the snowline signal changes in the atlantic intertropical convergence zone and the asian monsoon may also share a winter linkage with greenland thus the paleoclimate record is consistent with the notion that a huge continental sector of the northern hemisphere stretching from greenland to asia was close to an extreme winter threshold during much of the last glaciation winter climate crossed this threshold repeatedly with marked changes in seasonality that may well have amplified and propagated a signal of abrupt change throughout the hemisphere and into the tropics

rapid freshening of the deep north atlantic ocean over the past four decades the overflow and descent of cold dense water from the sills of the denmark strait and the faroe shetland channel into the north atlantic ocean is the principal means of ventilating the deep oceans and is therefore a key element of the global thermohaline circulation most computer simulations of the ocean system in a climate with increasing atmospheric greenhouse gas concentrations predict a weakening thermohaline circulation in the north atlantic as the subpolar seas become fresher and warmer1 2 3 and it is assumed that this signal will be transferred to the deep ocean by the two overflows from observations it has not been possible to detect whether the ocean s overturning circulation is changing but recent evidence suggests that the transport over the sills may be slackening4 here we show through the analysis of long hydrographic records that the system of overflow and entrainment that ventilates the deep atlantic has steadily changed over the past four decades we find that these changes have already led to sustained and widespread freshening of the deep ocean

moisture transport across central america as a positive feedback on abrupt climatic changes reconstruction of sea surface salinities in the eastern equatorial pacific ocean over the past 90 000 years detected strong millennial scale fluctuations that are associated with the migration of the tropical atlantic intertropical convergence zone thus millennial scale fluctuations of moisture transport constitute an important feedback mechanism for abrupt climate changes modulating the north atlantic freshwater budget and hence north atlantic ocean circulation moisture transport from the atlantic to the pacific ocean across central america leads to relatively high salinities in the north atlantic ocean1 and contributes to the formation of north atlantic deep water2 this deep water formation varied strongly between dansgaard oeschger interstadials and heinrich events millennial scale abrupt warm and cold events respectively during the last glacial period3 increases in the moisture transport across central america have been proposed to coincide with northerly shifts of the intertropical convergence zone and with dansgaard oeschger interstadials with opposite changes for heinrich events4 here we reconstruct sea surface salinities in the eastern equatorial pacific ocean over the past 90 000 years by comparing palaeotemperature estimates from alkenones and mg ca ratios with foraminiferal oxygen isotope ratios that vary with both temperature and salinity we detect millennial scale fluctuations of sea surface salinities in the eastern equatorial pacific ocean of up to two to four practical salinity units high salinities are associated with the southward migration of the tropical atlantic intertropical convergence zone coinciding with heinrich events and with greenland stadials5 the amplitudes of these salinity variations are significantly larger on the pacific side of the panama isthmus as inferred from a comparison of our data with a palaeoclimate record from the caribbean basin6 we conclude that millennial scale fluctuations of moisture transport constitute an important feedback mechanism for abrupt climate changes modulating the north atlantic freshwater budget and hence north atlantic deep water formation

eastern pacific warm pool paleosalinity and climate variability 0 30 kyr multiproxy geologic records of 18o and mg ca in fossil foraminifera from sediments under the eastern pacific warm pool epwp region west of central america document variations in upper ocean temperature pycnocline strength and salinity i e net precipitation over the past 30 kyr although evident in the paleotemperature record there is no glacial interglacial difference in paleosalinity suggesting that tropical hydrologic changes do not respond passively to high latitude ice sheets and oceans millennial variations in paleosalinity with amplitudes as high as 4 practical salinity units occur with a dominant period of 3 5 ky during the glacial deglacial interval and 1 0 1 5 ky during the holocene the amplitude of the epwp paleosalinity changes greatly exceeds that of published caribbean and western tropical pacific paleosalinity records epwp paleosalinity changes correspond to millennial scale climate changes in the surface and deep atlantic and the high northern latitudes with generally higher lower paleosalinity during cold warm events in addition to intertropical convergence zone itcz dynamics which play an important role in tropical hydrologic variability changes in atlantic pacific moisture transport which is closely linked to itcz dynamics may also contribute to hydrologic variations in the epwp calculations of interbasin salinity average and interbasin salinity contrast between the epwp and the caribbean help differentiate long term changes in mean itcz position and atlantic pacific moisture transport respectively

combined stratigraphic and isotopic studies of triassic strata cuyo basin argentine precordillera the evolution of the triassic cretaceous cuyo basin located in the andean precordillera nw argentina was mainly controlled by extensional transtensional tectonics along nw trending structures inherited from paleozoic sutures stratigraphic and isotopic studies are herein presented in order to constrain the sedimentary infilling and source areas as well as the age of triassic strata related to the active faulted margin of the cacheuta sub basin the whole stratigraphic package of the cuyo basin is interpreted as a second order depositional sequence comprising lowstand alluvial fan deposits transgressive fluvial deltaic and facustrine deposits and highstand oxidized lacustrine and high sinuosity luvial deposits systems tracts this stacking pattern was mainly controlled by the interplay between sedimentary influx and accommodation space with important volcaniclastic contribution especially during the early stages of basin development sm nd depleted mantle model ages tdm for both basement and volcano sedimentary samples range from 867 to 1345 ma all samples have negative end t values 0 33 to 5 02 basal deposits present older tdm model ages compared to intermediate positioned deposits and upper deposits present a wider range of model ages this time integrated factor correlates with paleocurrent data which indicate changes in the source areas through time the nd isotopic signature also suggests that sources of sediments for the cacheuta sub basin during the triassic were restricted to units of mesoproterozoic grenvillian age probably related to the cuyania terrane the u pb sensitive high resolution ion microprobe shrimp zircon age of 243 5 ma obtained in juvenile magmatic zircons from a lithoclast free ignimbrite interlayered within the basal alluvial fan facies of the cacheuta sub basin places the lowstand deposits in the olenekian and establishes a potential chronostratigraphic horizon for future correlations

high resolution stratigraphy of an underfilled lake basin wilkins peak member eocene green river formation wyoming u s a abstract lakes tend to respond noticeably to minor changes in sediment and water balance driven by climatic tectonic or geomorphic processes this unique behavior of lacustrine basins can provide a high resolution record of geologic processes within the continental setting far from the globally averaged record of marine strata the wilkins peak member of the eocene green river formation in wyoming usa is dominated by aggradation of repetitive sedimentary facies successions recording distinct lacustrine expansions and contractions these lacustrine cycles consist of up to four successive facies associations littoral profundal sublittoral palustrine and salt pan because they comprise disparate facies that may never have been simultaneously deposited in the basin wilkins peak member cycles are non waltherian successions that do not readily equate to any established sequence stratigraphic unit the completeness of the stratigraphic record in the wilkins peak member varies continuously across the basin at least 126 cycles are present in the erda white mountain 1 core near the basin depocenter whereas only about one third as many are recognizable 53 km north nearer to the basin margin cycle boundaries terminate northward by gradual amalgamation into palustrine facies reflecting the interplay between varying lake levels and a south dipping deposition gradient evidence for complete desiccation and hiatuses is commonplace even near the basin center a truly complete record may not be present anywhere based on recent 40ar 39ar geochronology of tuffs the average cycle duration in the white mountain 1 core is approximately 10 000 years however the true average duration is shorter due to the presence of lacunae and the time required for deposition of the thinnest cycles may have been less than 1000 years no external driving mechanism is presently known for eocene cycles of such short duration raising the possibility that they are instead autogenic cycles related to geomorphic instability of the surrounding landscape evaporite deposition corresponded closely in time and space to maximum differential subsidence suggesting that tectonic influences on drainage patterns and basin accommodation exerted a primary control differential accommodation was most pronounced during deposition of the lower wilkins peak member and became progressively less so after that time based on structural and stratigraphic observations a period of increased tectonic uplift commenced concurrent with deposition of the lower wilkins peak member

mineralogy and origin of rhizoliths on the margins of saline alkaline lake bogoria kenya rift valley abstract a wide range of rhizoliths occurs around the margins of lake bogoria kenya these include root casts moulds tubules rhizocretions and permineralised root systems these rhizoliths are variably composed of opaline silica calcite zeolites mainly analcime fluorite and possibly fluorapatite either alone or in combinations some rhizoliths are infilled moulds with detrital silicate grains most rhizoliths are in situ showing both vertical and horizontal orientations reworked rhizoliths have been concentrated locally to form dense rhizolites hot spring fluids concentrated by evapotranspiration and capillary evaporation have provided most of the silica for the permineralisation of the plant tissues precipitation involved the growth of silica nanospheres and microspheres that coalesced into homogeneous masses calcite rhizoliths formed following evaporative concentration evapotranspiration and or co2 degassing of ca bearing runoff water that infiltrated the sediment or by mixing of runoff with saline alkaline groundwater fluorite precipitated in areas where mixing of hot spring and meteoric waters occurred or possibly where hot spring fluids came into contact with pre existing calcite zeolitic rhizoliths formed during a prolonged period of aridity when capillary rise and evaporative pumping brought saline alkaline waters into contact with detrital silicate minerals around roots

significance of different modes of rhizolith preservation to interpreting paleoenvironmental and paleohydrologic settings examples from paleogene paleosols bighorn basin wyoming u s a abstract different modes of preservation of root traces rhizoliths provide information on soil moisture regimes in alluvial paleosols in the paleogene fort union and willwood formations of the bighorn basin wyoming u s a this paper links different styles of rhizolith preservation to paleosols whose other pedogenic attributes provide information on ancient soil drainage consequently the paleodrainage significance of different rhizolith preservation patterns is established more confidently rhizoliths consisting of elongate gray mottles rhizohaloes with red rims are common in relatively well drained red paleosols the gray zones are depleted of iron fe whereas the color of the rims indicates hematite accumulation these features are typical of surface water gley processes that caused fe and manganese mn to move from the root channel outward to the soil matrix calcareous rhizocretions either calcareous tubular concretions or micro accumulations of carbonate within gray rhizotubules are also common in moderately well drained red paleosols more poorly drained purple paleosols also have rhizoliths consisting of fe depletion zones however the rhizohaloes are surrounded by yellow brown goethite rims rather than red hematitic rims in many paleosols the red and yellow brown accumulation rims were partly removed by continued depletion producing abundant irregular shaped mottles the nature of the rhizohalo and the color of the fe oxide rim provide information about the intensity of gleying that a paleosol underwent the poorly drained paleosols also show rhizoliths preserved as goethite accumulations contained within gray depletion zones indicating movement of reduced fe from the matrix towards the root channel probably as a result of groundwater gley processes the goethite accumulations are intermingled locally with lignite representing preservation of the root organic material some rhizoliths are preserved as tube shaped concentrations of small 0 1 0 2 mm diameter black spheres whose color indicates a mix of mn and fe oxides these are most common in paleosols with low chroma gray matrix colors indicating very poor drainage the very poorly drained paleosols also show rhizoliths preserved in jarosite which is an oxidation product of pyrite careful observational and geochemical analysis of rhizoliths which are common in continental deposits can help produce a clearer and more thorough interpretation of ancient drainage conditions information about degree of ancient soil wetness or moisture is important for understanding past climatic conditions and for reconstructing terrestrial paleolandscapes

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

a high resolution absolute dated late pleistocene monsoon record from hulu cave china oxygen isotope records of five stalagmites from hulu cave near nanjing bear a remarkable resemblance to oxygen isotope records from greenland ice cores suggesting that east asian monsoon intensity changed in concert with greenland temperature between 11 000 and 75 000 years before the present yr b p between 11 000 and 30 000 yr b p the timing of changes in the monsoon as established with 230th dates generally agrees with the timing of temperature changes from the greenland ice sheet project two gisp2 core which supports gisp2 s chronology in this interval our record links north atlantic climate with the meridional transport of heat and moisture from the warmest part of the ocean where the summer east asian monsoon originates

southward migration of the intertropical convergence zone through the holocene titanium and iron concentration data from the anoxic cariaco basin off the venezuelan coast can be used to infer variations in the hydrological cycle over northern south america during the past 14 000 years with subdecadal resolution following a dry younger dryas a period of increased precipitation and riverine discharge occurred during the holocene thermal maximum since approximately 5400 years ago a trend toward drier conditions is evident from the data with high amplitude fluctuations and precipitation minima during the time interval 3800 to 2800 years ago and during the little ice age these regional changes in precipitation are best explained by shifts in the mean latitude of the atlantic intertropical convergence zone itcz potentially driven by pacific based climate variability the cariaco basin record exhibits strong correlations with climate records from distant regions including the high latitude northern hemisphere providing evidence for global teleconnections among regional climates

stratigraphic classification of ancient lakes balancing tectonic and climatic controls lakes and lake deposits present two fundamental paradoxes 1 modern lakes are vastly complicated but the rock records of lakes are relatively simple extensive observations reveal three distinct facies associations of common and widespread occurrence these are referred to here as fluvial lacustrine fluctuating profundal and evaporative facies associations 2 most explanations of modern and ancient lakes attribute their nature to climate but neither modern lake parameters lake size depth and salinity nor the character of ancient lake strata thickness extent lithology correlate with measured or inferred climatic humidity we propose that it is the relative balance of rates of potential accommodation mostly tectonic with sediment water fill mostly a function of climate that controls lake occurrence distribution and character lake basins may be termed overfilled balanced fill or underfilled depending on the balance between these rates we conclude that climate and tectonics exert coequal influence on lake deposits at both mesoscales 1 m to hundreds of meters and macroscales hundreds to thousands of meters

taphonomic controls on animal tracks at saline alkaline lake bogoria kenya rift valley impact of salt efflorescence and clay mineralogy abstract the morphologies of subaerial vertebrate tracks and invertebrate traces in surficial sediments can be used in paleoenvironmental reconstructions by providing details about the initial substrate conditions e g water content and any changes experienced during trace preservation a qualitative field study was undertaken to identify the factors affecting track taphonomy in saline lake margin environments supported by experiments to isolate selected factors that modify track morphology and substrate characteristics flamingo tracks ubiquitous around saline alkaline lake bogoria in the kenya rift were the focus of field study although other vertebrate tracks and surficial invertebrate trails and tunnels were also considered a complex and interacting set of environmental factors were identified such as the effect of lake level fluctuations on substrate grain size and capillary evaporation several factors affect the early taphonomic modification destruction stabilization and or cementation of tracks and their associated substrates the laboratory experiments tested the effects of pore water salinity clay mineralogy e g swelling or non swelling clays drying method e g solar radiation or wind and wetting and drying cycles on tracks these data were supported by statistical analyses of impression depth and planimetric area the results show that track morphology in smectitic substrates is altered rapidly by wetting and drying particularly if the substrates contain saline pore waters but morphology is less affected in non swelling clays the degree of morphologic alteration and or resistance to wetting and drying varies with the rate of substrate drying and the type of the resulting salt efflorescence e g interstitial vs surficial crust these short term process related taphonomic effects help to explain the morphologic variants of tracks in lake marginal settings and provide insights into the preserved distribution patterns of subaerial ichnofossil assemblages

defining the morphological quality of fossil footprints problems and principles of preservation in tetrapod ichnology with examples from the palaeozoic to the present abstract the morphology of fossil footprints is the basis of vertebrate footprint ichnology however the processes acting during and after trace fossil registration which are responsible for the final morphology have never been precisely defined resulting in a dearth of nomenclature therefore we discuss the concepts of ichnotaphonomy ichnostratinomy taphonomy biostratinomy registration and diagenesis and describe the processes acting on footprint morphology in order to evaluate the morphological quality of tetrapod footprints we introduce the concept of morphological preservation which is related to the morphological quality of footprints m preservation acronym mp and distinguish it from physical preservation p preservation acronym pp which characterizes whether or not a track is eliminated by taphonomic and diagenetic processes m preservation includes all the morphological features produced during and after track registration prior to its study and may be divided into substages ichnostratinomic registrational taphonomic stratinomic diagenetic moreover we propose an updated numerical preservation scale for m preservation it ranges from 0 0 worst preservation to 3 0 best preservation intermediate values may be used and specific features may be indicated by letters in vertebrate footprint ichnotaxonomy we regard the anatomy consistent morphology and to a lesser extent the trackway pattern as the only acceptable ichnotaxobases only footprints showing a good morphological preservation grade 2 0 3 0 are useful in ichnotaxonomy whereas ichnotaxa based on poor morphological preservation grade 0 0 1 5 are considered ichnotaphotaxa nomina dubia characterized by extramorphologies we applied the preservation scale on examples from the palaeozoic to the present time including three ichnotaphotaxa and 18 anatomy consistent ichnotaxa morphotypes attributed to several vertebrate footprint producers results indicate the utility feasibility and suitability of this method for the entire vertebrate footprint record in any lithofacies strongly recommending its use in future ichnotaxonomic studies

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

rapid sea level rise and holocene climate in the chukchi sea three new sediment cores from the chukchi sea preserve a record of local paleoenvironment sedimentation and flooding of the chukchi shelf 50 m by glacial eustatic sea level rise radiocarbon dates on foraminifera provide the first marine evidence that the sea invaded hope valley southern chukchi sea 53 m as early as 12 ka the lack of significant sediment accumulation since ca 7 ka in hope valley southeastern chukchi shelf is consistent with decreased sediment supply and fluvial discharge to the shelf as deglaciation of alaska concluded abundant benthic foraminifera from a site west of barrow canyon indicate that surface waters were more productive 4 6 ka and this productivity varied on centennial time scales an offshore companion to this core contains a 20 m record of the holocene these results show that carefully selected core sites from the western arctic ocean can have a temporal resolution equal to the best cores from other regions and that these sites can be exploited for high resolution studies of the paleoenvironment

mesozoic climates general circulation models and the rock record abstract general circulation models gcms use the laws of physics and an understanding of past geography to simulate climatic responses they are objective in character however they tend to require powerful computers to handle vast numbers of calculations nevertheless it is now possible to compare results from different gcms for a range of times and over a wide range of parameterisations for the past present and future e g in terms of predictions of surface air temperature surface moisture precipitation etc gcms are currently producing simulated climate predictions for the mesozoic which compare favourably with the distributions of climatically sensitive facies e g coals evaporites and palaeosols they can be used effectively in the prediction of oceanic upwelling sites and the distribution of petroleum source rocks and phosphorites models also produce evaluations of other parameters that do not leave a geological record e g cloud cover snow cover and equivocal phenomena such as storminess parameterisation of sub grid scale processes is the main weakness in gcms e g land surfaces convection cloud behaviour and model output for continental interiors is still too cold in winter by comparison with palaeontological data the sedimentary and palaeontological record provides an important way that gcms may themselves be evaluated and this is important because the same gcms are being used currently to predict possible changes in future climate the mesozoic earth was by comparison with the present an alien world as we illustrate here by reference to late triassic late jurassic and late cretaceous simulations dense forests grew close to both poles but experienced months long daylight in warm summers and months long darkness in cold snowy winters ocean depths were warm 8 c or more to the ocean floor and reefs with corals grew 10 of latitude further north and south than at the present time the whole earth was warmer than now by 6 c or more giving more atmospheric humidity and a greatly enhanced hydrological cycle much of the rainfall was predominantly convective in character often focused over the oceans and leaving major desert expanses on the continental areas polar ice sheets are unlikely to have been present because of the high summer temperatures achieved the model indicates extensive sea ice in the nearly enclosed arctic seaway through a large portion of the year during the late cretaceous and the possibility of sea ice in adjacent parts of the midwest seaway over north america the triassic world was a predominantly warm world the model output for evaporation and precipitation conforming well with the known distributions of evaporites calcretes and other climatically sensitive facies for that time the message from the geological record is clear through the phanerozoic earth s climate has changed significantly both on a variety of time scales and over a range of climatic states usually baldly referred to as greenhouse and icehouse although these terms disguise more subtle states between these extremes any notion that the climate can remain constant for the convenience of one species of anthropoid is a delusion although the recent rate of climatic change is exceptional

a shape and compositional analysis of ice rafted debris in cores from iodp expedition 323 in the bering sea abstract sediment cores recovered during iodp expedition 323 in the bering sea northern pacific contained numerous ice rafted debris ird clasts up to 85 mm in length the physical properties including roundness and sphericity of 136 clasts from the working half of the cores a subsample of the total clast number were analysed and their composition determined using standard petrographic techniques after removal of pumice and possible fall in derived material from the clast population a total of 86 clasts from the original collection were considered to be ird while roundness and sphericity vary greatly in the clast population the ird are predominately discoid in shape with oblate prolate indices typically between 5 and 5 there are four time periods over the approximately 4 5 ma sample interval 0 36 0 67 ma 0 82 1 06 ma 1 54 1 77 ma and 3 28 ma where there are no ird in the sample set for sites of the bering slope suggesting that these times may have been ice free most clasts show some rounding and are likely to have spent time on beaches with wave action wave action on beaches suggests periods of no ice or only seasonal sea ice the low roundness values of other clasts however suggest they underwent little working and therefore the presence of glaciers or more permanent sea ice at times in those locations the abundance of rounded and unfaceted clasts as ird suggests a lack of large ice sheets in the area during cool periods clast composition of the ird is divided into four broad groups basalt and andesite granite and metamorphic sedimentary and felsic volcanic the granite and metamorphic and more mature sedimentary lithologies are most likely derived from the alaskan continental margin while the extrusive igneous clasts could be derived from a variety of volcanic sources surrounding the bering sea both emergent now or emergent at times of lower sea level there is only a poor correlation with ird abundance and marine isotope stages mis for the time period 2 mm transported by sea ice may not be a good indicator of past climate conditions

some controls on flow and salinity in bering strait 1 during 1993 1994 steric forcing of flow through bering strait represented a northward sea level drop of 0 7 m from the bering sea basin to the adjacent deep arctic ocean of which 2 3 was due to the salinity difference between the basins seasonal variability of steric forcing appears small 0 05 m in contrast to large seasonal wind effects interannual changes in steric forcing may exceed 20 however and warm inflow from the north atlantic accumulation of freshwater in the southwest canada basin and temperature and salinity changes in the upper bering sea have all contributed to recent changes the mean salinity balance in bering strait is primarily maintained by large runoff to the bering shelf dilute coastal inflow from the gulf of alaska and on shelf movement of saline and nutrient rich oceanic waters from the bering sea basin in bering strait therefore both the throughflow and its salinity are affected by remote events

the 8200 year b p event in the slope water system western subpolar north atlantic 1 stable isotope trace metal alkenone paleothermometry and radiocarbon methods have been applied to sediment cores in the western subpolar north atlantic between hudson strait and cape hatteras to reveal the history of climate in that region over the past 11 kyr we focus on cores from the laurentian fan which is known to have rapid and continuous accumulation of hemipelagic sediment although results among our various proxy data are not always in agreement the weight of the evidence alkenone sea surface temperature sst 18o and abundance of globigerinoides ruber indicates a continual cooling of surface waters over laurentian fan from about 18 c in the early holocene to about 8 c today alternatively mg ca data on planktonic foraminifera indicate no systematic change in holocene sst the inferred long term decrease in sst was probably driven by decreasing seasonality of northern hemisphere insolation two series of proxy data show the gradual cooling was interrupted by a two step cold pulse that began 8500 years ago and lasted about 700 years although this event is associated with the final deglaciation of hudson bay there is no 18o minimum anywhere in the labrador sea yet there is some evidence for it as far south as cape hatteras finally although the 8200 year b p event has been implicated in decreasing north atlantic ventilation and hence widespread temperature depression on land and at sea we find inconsistent evidence for a change at that time in deep ocean nutrient content at 4 km water depth