a mechanism for the low frequency variability of the mid latitude troposphere low frequency variability of the zonal mean zonal wind in long runs of a simplified global circulation model is dominated by bands of wind anomaly which originate in the tropics and propagate slowly poleward with speeds of 0 2 m s 1 such propagation could be the result of the interaction between the zonal wind anomaly and an equatorward flux of rossby wave activity originating in middle or high latitude a barotropic weakly nonlinear model of this process has been derived it suggests propagation of zonal wind anomalies similar to those produced in the global circulation model and leads to comparable propagation speeds

eddy zonal flow feedback in the southern hemisphere abstract the variability of the zonal mean zonal wind in the southern hemisphere is studied using eof analysis and momentum budget diagnostics of ncep reanalysis data 1978 97 the leading eof of the zonal mean zonal wind is well separated from the remaining eofs and represents the north south movement of the midlatitude jet analysis of the momentum budget shows that a positive feedback between the zonal mean wind anomalies and the eddy momentum fluxes accounts for the unusual persistence of eof1 and plays an important role in the selection of the leading eof of midlatitude variability further analysis also shows a propagating feedback common to both eof1 and eof2 which is responsible for the poleward drift of wind anomalies with time the observations support the following feedback mechanism anomalous baroclinic wave activity is generated at the latitude of anomalous temperature gradient that by thermal wind coincides with the latitude of the anomalous zonal jet the net propagation of baroclinic

a model of early calcite cementation in alluvial fans evidence from the burdigalian sandstones and limestones of the valles penedes half graben ne spain abstract the valles penedes half graben developed during a neogene extensive period as part of the catalan coastal ranges in the northwestern edge of the valencia trough the neogene deposits of the valles penedes half graben consist of from base to top three lithostratigraphic complexes i a lower continental complex of aquitanian early langhian age ii a continental to marine complex with reefal carbonate platforms of langhian age and iii an upper continental complex of middle serravallian tortonian age this study focuses on the calcite cements of the lower continental complex consisting of red beds mudstones sandstones and conglomerates and lacustrine limestones deposited in alluvial fan environments the studied materials are cemented by calcite precipitated from meteoric waters that circulated through the sediments during the early diagenesis at shallow burial depths the calcite cement was studied by means of petrographic cathodoluminescence microprobe and 18o and 13c isotopic analysis the petrographic and geochemical results show that these cements vary according to the sedimentary environment i sandstones deposited in proximal alluvial fan environments cemented by calcite with low contents of mg fe mn sr and na 13c values of 7 4 pdb and 18o values from 6 5 to 6 2 pdb ii sandstones sedimented in medium to distal alluvial fan environments cemented by mn rich and mg fe sr and na poor calcite with 13c values from 7 9 to 6 9 pdb and 18o values from 10 5 to 8 6 pdb and iii lacustrine limestones of distal alluvial fan environments cemented by fe rich calcite with variable contents of mn and low contents of mg sr and na the distribution of these calcite cements allows us to propose a model of fluid circulation and early calcite cementation within an alluvial fan indicating spatial and temporal variations in chemical composition of meteoric water during migration from proximal to distal alluvial fan environments residence time of waters eh ph conditions and the degree of sediment water interactions influenced by the mineral composition of the host sediments are the most important parameters controlling variations in calcite cement composition

the dynamics of intraseasonal atmospheric angular momentum oscillations abstract the global and zonal atmospheric angular momentum aam budget is computed from seven years of national centers for environmental prediction data and a composite budget of intraseasonal 30 70 day variations during northern winter is constructed regressions on the global aam tendency are used to produce maps of outgoing longwave radiation 200 hpa wind surface stress and sea level pressure during the composite aam cycle the primary synoptic features and surface torques that contribute to the aam changes are described in the global budget the friction and mountain torques contribute about equally to the aam tendency the friction torque peaks in phase with subtropical surface easterly wind anomalies in both hemispheres the mountain torque peaks when anomalies in the midlatitude northern hemisphere and subtropical southern hemisphere are weak but of the same sign the picture is different for the zonal mean budget in which the meridional convergence of the northward relative angular moment

freshwater carbonates within a late triassic siliciclastic fluvial system in a gondwana rift basin the maleri formation india abstract spring related carbonates are valuable repositories for information on physical and biotic environments of the past however for mesozoic and earlier records their use is limited due to their low preservation potential and difficulties in accurate identification of the carbonate type this study aims to characterise the source of carbonate clasts preserved as thin beds of cross bedded grainstones in late triassic siliciclastic fluvial deposits the maleri formation pranhita godavari rift basin india the microfabric and stable isotopic compositions of the grainstones and thin and discontinuous beds of impure limestones of this formation are presented here these features help in identifying the freshwater carbonate precipitating environments that existed in an ancient continental rift basin though such deposits are not primarily preserved in this case the microfabric of the carbonate grains provides evidence for carbonate precipitation in aquatic environment related to karstic springs taking together the microfabric stable isotopic compositions as well as the associated geological and palaeontological information it could be shown that the calcium carbonate precipitation took place in palustrine pools related to tufa like spring systems and in alluvial swamps under a pluvial climatic setting this research thus demonstrates that carbonate grains derived from spring related carbonate deposits formed mainly near the margins of rift basins can be incorporated into nearby siliciclastics if interpreted correctly they can be used in reconstructing ancient karstic carbonate environments even when in situ primary record is not preserved therefore making it possible to access the information stored in freshwater carbonates

petrographic and trace element analysis of uranium rich tufa calcite middle miocene barstow formation california usa an integrated petrographic and spectroscopic x ray diffraction phosphor imaging and synchrotron x ray absorption spectroscopy study of tufas from the miocene barstow formation california relates sample morphology mineralogy and geochemical composition the tufas composed mainly of calcite that formed at the interface between an ancient alkaline lake and ground or spring waters have textures similar to those of microbially mediated terrestrial stromatolites and travertines the tufas have elevated concentrations of a number of trace elements including mn fe sr and u synchrotron x ray fluorescence analyses show that u concentrations can exceed 500 p p m x ray absorption spectroscopy indicates that the u in these samples is incorporated as u iv it is suggested here that alkaline lake waters had a high u ca ratio and tufa calcite formed where groundwater or possibly epithermal springs brought in ca and trace elements such as sr the rapid and possibly microbially mediated precipitation of calcite allowed for incorporation of high concentrations of trace metals as either structural substitutes or extremely fine scale inclusions

climatic and tectonic controls on carbonate deposition in syn rift siliciclastic fluvial systems a case of microbialites and associated facies in the late jurassic this work provides new insights to assess the factors controlling carbonate deposition in the siliciclastic fluvial systems of rift basins sedimentological and stable isotope data of microbialites and associated carbonate facies along with regional geological information are shown to reveal the influence of climate and tectonics on the occurrence and attributes of carbonate deposits in these settings the vega formation a 150 m thick lower kimmeridgian siliciclastic fluvial sequence in asturias province northern spain constitutes a candidate for this approach this unit includes varied facies stromatolites rudstones packstones and wackestones containing oncoids intraclasts charophytes and shell bioclasts marlstones and polygenic calcareous conglomerates that formed in a low gradient fluvial lacustrine system consisting of shallow low sinuosity oncoid bearing channels and pools within marshy areas with sporadic coarse alluvial deposition the sedimentological attributes indicate common erosion by channel overflow and rapid lateral changes of subenvironments caused by water discharge variations the carbonate fluvial lacustrine system developed near uplifted marine jurassic rocks the occurrence of the system was conditioned by normal faults active during the deposition of the unit that favoured i springs of hco3 ca rich water from a rhaetian sinemurian carbonate rock aquifer and ii carbonate deposition in areas partially isolated from the adjacent siliciclastic fluvial system the microbialite 13c and 18o values support deposition in a hydrologically open system fed by ambient temperature meteoric water with riparian vegetation three types of lamination in the stromatolites and oncoids reflect distinct morphological types of cyanobacterial communities the textural pattern of lamination parallels 13c and 18o changes suggesting short term cycles of precipitation and temperature a moderately to strongly contrasted seasonal and or pluriannual precipitation regime is inferred from the cyclic 13c pattern of the lamination and from the discontinuous and asymmetrical growth of oncoids thus the isotopic and sedimentological attributes of the carbonate deposits were linked to short term climate changes associated with semi arid conditions consistent with the studied climatic zone

observational evidence for the mechanism of the poleward propagation of zonal wind anomalies over the north atlantic this study examines the physical mechanisms that drive the poleward propagation of zonal mean flow anomalies over the north atlantic during the boreal winter with ecmwf era40 reanalysis data that spans the years 1958 2001 an empirical orthogonal function eof analysis is performed on the zonal wind anomalies for the north atlantic domain based on time lag correlations between the first two eofs 14 poleward propagation events are identified a series of lagged composite calculations of the poleward propagation events reveals the following processes midlatitude synoptic scale rossby waves propagate equatorward and break when they encounter their critical latitudes in the tropics and subtropics a new critical latitude is generated slightly poleward of the previous critical latitude and the subsequent wave breaking occurs at this new critical latitude this interaction between the wave breaking and the critical latitude drives the poleward propagation of zonal mean zonal wind anomaly a comparison between the poleward propagation events and zonal index events indicates that the poleward propagation is associated with a relatively weak potential vorticity gradient over the subtropics these north atlantic poleward propagation events are found to be preceded by anomalously strong transient eddy activity over the subtropical north pacific and by anomalously weak tropical convection over the tropical atlantic ocean and the amazon copyright c 2012 royal meteorological society

morphologie des constructions microbiennes en contexte de fan delta oligocene exemple du rift rhenan europe occidentale abstract at the eocene oligocene boundary a first paroxysmal phase of subsidence created the rhine rift graben system a lacustrine basin with variable salinity was formed in the graben in the southern part of this basin a large fan delta developed extending northward along the graben axis microbial ecosystems settled on the fringe of the lake in the paludal zone they formed carbonate encrustations on living plants or detrital vegetal remains littoral carbonate facies occur as algal bioherms and stromatolitic reefs in the distal lacustrine zone a discrete nodular texture within the sediment reveals the microbial origin of micrite and gives the limestone a breccial pattern all these facies illustrate the lateral zonation of the different types of constructions from fluvio deltaic to lacustrine in certain cases the microbial coating on plants permits to reconstruct seasonal rhythms and the processes of substratum colonisation

palaeogene lacustrine record in mallorca nw mediterranean spain depositional palaeogeographic and palaeoclimatic implications for the ancient southeastern iberian margin abstract middle bartonian middl1e eocene and chattian upper oligocene lacustrine and alluvial lacustrine successions exist in mallorca northwestern mediterranean they yield a significant palaeobiological record comprising fossil land plants and vertebrates and constitute the best available depositional record for a preliminary palaeogeographic and palaeoclimatic analysis of the northeastern part of the palaeogene southern iberian margin western tethys both lacustrine systems developed under similar warm and humid climatic conditions increasingly affected by seasonal and or pluriannual rhythms marked by changing precipitation patterns these climatic conditions were mainly caused by the low palaeolatitude where the lacustrine systems evolved the still probable although decreasing influence of indo pacific warm surface currents and the occurrence of very extensive marine water masses adjacent to wide emerged continental zones the palaeogene lacustrine systems were shallow and characterised by low gradient depositional conditions they were perennial open freshwater oligosaline carbonate lakes where eutrophy thermal meromixis and anoxic bottom conditions could develop allowing the accumulation and preservation of macrophytic plant remains of the circum lacustrine marginal belt and neighbouring terrestrial mesophytic zones the higher plant algal bacterial organic contribution balance favoured the generation of mineable huminitic coal seams carbominerites and of gas prone kerogene type iii and iib source rocks the hydrogen bicarbonate rich waters led to high biogenic and inorganic bio induced carbonate sediment production biogenic muds cyanobacterial incrustations and travertine and tufa precipitation constituted the major carbonate products moreover the high ph conditions in the lacustrine water bodies and in the lacustrine sediment pore waters favoured bacterial sulphate reduction contributing to organic sulphur enrichment in coals these highly alkaline conditions also resulted in some cases in the exceptionally good preservation of chlorophyllinite a rare liptinite maceral recorded in the eocene coal deposits the middle eocene lacustrine system did not evolve in a very well defined tectonic setting characterised by the generation of low uplifts and gently subsiding basins terrigenous contributions from the source areas and clastic sedimentation were restricted to river mouths high autochthonous carbonate production and accumulation characterised most of the remaining lacustrine zones a significant spring recharge of the lacustrine zones is suggested by the well developed spring related travertine deposits by contrast the oligocene lacustrine zones developed both in fluvial related floodplains and in the distal zones of a major alluvial system which was fed from tectonically active source areas located to the nw of the island a more significant water runoff contribution is suggested for this system although underground recharge could also be substantial

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

circumglobal teleconnections and wave packets associated with israeli winter precipitation this investigation performs diagnostic analyses with ncep ncar reanalysis data and israeli precipitation data to examine the dynamical processes that drive the teleconnection pattern associated with israeli winter precipitation anomalies a dipole teleconnection pattern with one centre of action over western europe and another centre of action over the eastern mediterranean em is found to be associated with the winter precipitation anomalies in israel this pattern is referred to as the southern levant sl pattern the positive negative phase of the sl pattern is found to be closely associated with enhanced reduced precipitation in israel composites of the 300 hpa stream function field show that both phases of the sl pattern are associated with eastward propagating wave packets that traverse about three quarters of the distance across the globe these wave packets which have a phase speed that is close to zero originate over the northeast pacific the east coast of north america for the positive negative sl phase and propagate eastward across the north atlantic western europe the em and southern asia until they eventually decay over the northwest pacific as these wave packets pass over western europe and the em the sl pattern is triggered and anomalies in israeli winter precipitation occur the sl pattern is found to have an e folding time scale of 4 days which is determined by the group velocity of the wave packet the time averaged spatial structure of the circumglobal wave packet closely resembles the first empirical orthogonal function of the hemispheric 300 hpa meridional wind field this suggests that the circumglobal wave packet may be a fundamental pattern of variability which is relevant for the entire northern hemisphere copyright 2008 royal meteorological society

temporal and environmental significance of microbial lamination insights from recent fluvial stromatolites in the river piedra spain despite extensive research the environmental and temporal significance of microbial lamination is still ambiguous because of the complexity of the parameters that control its development a 13 year monitored record of modern fast accreting calcite stromatolites mean 14 mm year 1 from artificial substrates installed in rapid flow in the river piedra north east spain allows comparison of the sedimentological attributes of successive six month depositional packages with the known climatic hydrophysical and hydrochemical parameters of the depositional system the stromatolites are formed of dense porous and macrocrystalline composite laminae the dense and porous composite laminae which are composed of two to eight laminae consisting largely of calcified cyanobacteria are characterized by i dense composite laminae up to 15 mm thick mostly with successive dense laminae and minor alternating dense and porous laminae and ii porous composite laminae up to 12 mm thick consisting mainly of porous laminae alternating with thinner dense laminae most of the dense composite laminae formed during the warm periods april to september whereas most of the porous composite laminae developed in the cool periods october to march each dense and porous composite lamina represents up to or slightly longer than six months the alternation of these two types of composite laminae parallels seasonal changes in temperature the dense and porous laminae result from shorter for example intraseasonal variations in temperature insolation and hydrological conditions the macrocrystalline laminae with crystals 100 m long occur isolated and grouped into composite laminae up to 1 7 mm thick their occurrence suggests the absence or poor development of microbial mats over periods of weeks to several months thus stromatolite lamination can record different order periodic and non periodic changes in the magnitude of environmental parameters over a single year these results hold important implications for the temporal and environmental interpretation of lamination in microbial structures

osl dating of brazilian fluvial carbonates tufas using detrital quartz grains abstract robust chronologies for quaternary fluvial carbonate sediments tufas from brazil are still challenging we obtain optically stimulated luminescence osl ages of brazilian tufa deposits using the single aliquot regeneration sar dose protocol applied to single grains and multigrain aliquots of quartz extracted from tufa matrix additionally radiocarbon 14 c ages were obtained for the carbonate matrix and organic matter from tufas the methodology was applied in samples from tufa deposits in southwestern and southeastern regions with quaternary carbonate deposition equivalent doses distributions have overdispersion between 19 and 81 indicating that the studied samples comprise well bleached as well as poorly bleached sediments the quartz grains extracted from the studied tufas show an osl signal dominated by the fast component and reliable dose response curves with saturation doses 2 d 0 ranging from 35 to 210 gy most of the samples has low radiation dose rates 0 40 0 02 to 0 78 0 05 gy ka with variation in the water content due to compaction and matrix recrystallization being the major factors that could induce dose rate changes through time hard tufa samples with intense carbonate cementation show quartz osl ages from 51 9 4 8 to 150 3 35 9 years these ages are much older than the radiocarbon ages of the carbonate matrix and organic matter the osl ages of soft tufa samples related with the active fluvial system ranged from 0 8 0 1 to 5 5 0 4 years and are in the same interval of radiocarbon ages presented in the previous works we interpret that carbonate recrystallization or uptake of organic matter after tufa formation are responsible for the younger radiocarbon ages compared to quartz osl ages in the older tufa samples for these samples age differences vary up to tens of thousands years between deposition of quartz grains and the last episode of carbonate recrystallization or organic matter uptake consequently we believe that the obtained osl ages are the best estimates for the starting deposition time of the studied tufa samples

cosmic markers 40ar 39ar dating and paleomagnetism of the kt sections in the anjar area of the deccan large igneous province abstract bhandari et al bhandari et al geophys res lett 22 1995 433 436 bhandari et al geol soc am spec paper 307 1996 417 424 reported the discovery of iridium bearing sediments sandwiched between basalt flows in the anjar area kutch province india they concluded that the signature of the k t impact had been recorded and that onset of volcanism in the deccan traps preceded the k t boundary excluding the possibility of a causal connection this paper reports complementary analyses of anjar outcrops by a joint indo french team where we focused on cosmic markers iridium and spinels in the intertrappean sediments and 40ar 39ar dating and paleomagnetism of the lava flows anomalous ir concentrations up to 0 4 ng g are confirmed with up to three thin and patchy enriched layers which cannot be traced throughout the exposed sections despite careful search no ni rich spinels were found eight basalt samples provided 40ar 39ar results four on plagioclase bulk samples four on whole rocks spectra for whole rocks all indicate some amount of disturbance and ages based on plagioclase bulk samples seem to be consistently more reliable hofmann et al earth planet sci lett 180 2000 13 28 the three flows underlying the ir bearing sediments are dated at 66 5 ma and two overlying flows at 65 ma magnetic analyses both thermal and by alternating fields uncovered clear reversed primary components in the upper flows and more disturbed normal components in the lower flows with evidence for an additional reversed component there are reports bajpai geol soc india mem 37 1996 313 319 bajpai j geol soc london 157 2000 257 260 that the intertrappean sediments contain uppermost maastrichtian dinosaur and ostracod remains above the uppermost ir bearing level and may not be mechanically disturbed we propose the following scenario to interpret these multiple field and analytical observations deccan trap volcanism started within uppermost maastrichtian normal chron c30n at 66 5 67 ma in the anjar area volcanism then stopped at least locally and lacustrine sediments were deposited over a period that could be in the order of 1 2 ma the k t bolide impact was recorded as a deposit of ir and possibly though not necessarily spinels volcanism resumed shortly after the k t boundary within reversed chron c29r as witnessed by the three reversely magnetised overlying basalt flows dated 65 ma this was responsible for erosion and destruction of part of the uppermost sediments including spinels if there were any and heterogeneous and non uniform redeposition of ir at a number of underlying sedimentary levels this was also responsible for the partial remagnetisation of the underlying flows these findings generally confirm and complement those of bhandari et al bhandari et al geophys res lett 22 1995 433 436 bhandari et al geol soc am spec paper 307 1996 417 424 and are compatible with the occurrence of the k t impact at the paleontological k t boundary and of deccan trap volcanism straddling the boundary and starting before the impact anjar provides evidence for minor volcanism somewhat earlier than suggested by some authors though still within normal chron c30n there is no indication contradicting the view that the bulk of deccan trap volcanism occurred over only three chrons c30n c29r c29n courtillot evolutionary catastrophes the science of mass extinctions cambridge university press 1999 courtillot et al earth planet sci lett 80 1986 361 374 vandamme et al rev geophys 29 1991 159 190

syn rift geofluids in fractures related to the early middle miocene evolution of the valles penedes half graben ne spain abstract the valles penedes half graben located in the central part of the catalan coastal ranges is ne sw oriented 10 14 km wide and up to 100 km long and is filled mainly with continental to marine siliciclastic deposits ranging from late oligocene early miocene to late miocene the syn rift burdigalian continental deposits are well stratified s 0 approx 090 15n red beds affected by nnw sse trending nearly vertical extensive fractures middle burdigalian early langhian in age these extensive fractures which cut quasi perpendicularly the stratification are filled with calcite veins presenting two filling stages the first stage is formed by millimetric to centrimetric thick bands of crystalline calcite consisting of palisade calcite cement anhedral spar texture and rhombic calcite cement non to dull orange luminescent with 18 o values ranging from 9 0 to 5 2 pdb 13 c values varying from 9 3 to 6 7 pdb 87 sr 86 sr ratios ranging from 0 71088 to 0 71361 relatively high values of mg and variable values of mn sr and fe this stage is interpreted as subaerial speleothems rimstones which developed in meteoric vadose conditions the second stage is formed by euhedral spar megaspar cement and coconut meat cement with zoned bright yellow to orange luminescence with 18 o values ranging from 16 2 to 9 9 pdb 13 c values varying from 7 4 to 1 9 pdb 87 sr 86 sr ratios ranging from 0 71608 to 0 71800 and relatively low values of mg mn sr and fe this stage is interpreted as subaquatic cements developed in meteoric phreatic conditions the two stages of calcite fillings reflect successive circulation of two meteoric fluids with very different geochemical compositions and represent the change from vadose to phreatic environment during the evolution of the syn rift stage middle late burdigalian to late langhian the meteoric fluid producing the first filling stage was a superficial fluid proceeding directly from the surface influenced by soil co 2 the meteoric fluid generating the second stage was a more evolved fluid which had previously undergone interaction with sediments and reservoir effect the beginning of the thermal subsidence coupled with a change from a more arid to a hotter and perhaps wetter climate could had been the ultimate reasons for the observed evolution

in situ accretion mechanism of concavo convex lacustrine oncoids swallow nests from the oligocene of the mainz basin rhineland frg oligocene oncoids from the lacustrine su wasserschichten of the mainz basin exhibit a pustular surface and a characteristic cavity on their undersides which gave rise to their designation as swallow nests oncoids grew in situ under low energy conditions during a pause in sedimentation three features indicate that they were never overturned 1 oncoids can be placed in a stable position on a smooth surface 2 laminae which are mostly concentric around the entire oncoids display polar thickenings on the upper side during all growth stages of oncoids 3 the depression is always situated on the stable side of oncoids development of this cavity is favoured by unionid shells in the stable position serving as oncoid nuclei but occurs also under flat nuclei it is interpreted as a product of reduced growth of cyanobacteria due to oligophotic conditions on a fine grained substrate and is considered as diagnostic of the in situ growth of oncoids oncoid cortices are mainly formed by a scytonema like cyanobacterium whose carbonaceous organic remnants are occasionally preserved growth stages of individual oncoids were image processed and compared by means of shape parameters due to optimum growth conditions cyanobacteria try to occupy the entire available space hence oncoids tend to achieve a spherical form even during in situ growth renewed background sedimentation triggered the formation of pustular surfaces and finally caused the cessation of cyanobacterial growth

hotspots mantle plumes flood basalts and true polar wander persistent long lived stationary sites of excessive mantle melting are called hotspots hotspots leave volcanic trails on lithospheric plates passing across them the global constellation of fixed hotspots thus forms a convenient frame of reference for plate motions through the orientations and age distributions of volcanic trails left by these melting anomalies hotspots appear to be maintained by whole mantle convection in the form of upward flow through narrow plumes evidence suggests that plumes are deflected little by horizontal flow of the upper mantle mantle plumes are largely thermal features and arise from a thermal boundary layer most likely the mantle layer just above the core mantle boundary experiments and theory show that gravitational instability drives flow beginning with the formation of diapirs such a diapir will grow as it rises fed by flow through the trailing conduit and entrainment of surrounding mantle the structure thus develops a large spherical plume head and a long narrow tail on arrival at the base of the lithosphere the plume head flattens and melts by decompression producing enormous quantities of magma which erupt in a short period these are flood basalt events that have occurred on continents and in ocean basins and that signal the beginning of major hotspot tracks the plume supported hotspot reference frame is fixed in the steady state convective flow of the mantle and is independent of the core generated axial dipole paleomagnetic reference frame comparison of plate motions measured in the two frames reveals small but systematic differences that indicate whole mantle motion relative to the earth s spin axis this is termed true polar wander and has amounted to some 12 since early tertiary time the direction and magnitude of true polar wander have varied sporadically through the mesozoic probably in response to major changes in plate motions particularly subduction zone location that change the planet s moments of inertia

sedimentology and isotope geochemistry of lacustrine carbonates of the oligocene campins basin north east spain the non marine campins basin developed in the oligocene during a period of early rifting of the catalan coastal ranges lacustrine deposits interbedded between two alluvial units comprise shallow and deep lacustrine facies the lower shallow lacustrine facies are made up of microbialite buildups and thin limestone beds in the studied area these facies are overlain by deep lacustrine facies which consist of alternations of several metre thick carbonate and mudstone dominated intervals in addition to calcite which is characteristic of the shallow lacustrine facies aragonite and abundant dolomite are present in the deep lacustrine facies this mineralogical change in the sequence reflects an overall increase in the mg ca ratio of the lake waters the deep lacustrine sequences are interpreted as having formed in a hydrologically closed basin that was subject to changes in the mg ca ratio of the water probably related to variations in the evaporation precipitation rate the sedimentological mineralogical and isotopic characteristics of the campins basin dolomites suggest that in general they are primary in origin the stable isotope data show an approximate covariance between 13c and 18o in the lower shallow lacustrine carbonates calcite which suggests that they formed during the onset of closure of the lake the 13c and 18o values of the deep lacustrine carbonates display three different clusters that are roughly related to the carbonate mineralogy normalisation with respect to calcite of the isotopic compositions of dolomite and aragonite from the deep lacustrine carbonates allows the integration of all these isotope values into one covariant trend the sequential appearance of different carbonate minerals and the isotopic covariant trend may indicate an overall evaporative concentration of the lake waters the change in slope of the covariant trend for the isotope values between the shallow and the deep lacustrine carbonates might reflect the change in the waterbody morphology recorded in the basin fill sequence

lacustrine stromatolites as multi scale recorders of climate change insights from the miocene ebro basin sedimentological 13c and 18o data from stromatolites in a lower and middle miocene sequence from the ebro basin n spain are used to assess the potential of ancient lacustrine stromatolite lamination as an archive of palaeoenvironmental and palaeoclimatic change the isotopic evolution through the studied sequence supports a general trend toward less saline conditions with time stromatolites and muddy grainy laminated limestones developed in lake water that underwent little renewal compared with other carbonate facies the palaeoclimatic value of the stable isotope changes and concurrent textural variations in calcite stromatolite lamination is studied at different orders of cyclicity stromatolite lamination consists of simple laminae dark dense light dense and light porous grouped into alternating composite light and dark laminae 13c and 18o analyses in consecutive composite laminae bulk sampling yielded a cyclic pattern that mimics textural variations light laminae with lower 13c and 18o values reflect higher precipitation evaporation ratio p e and more influence of biogenic 12c dark laminae with higher 13c and 18o values reflect drier conditions more complete atmospheric co2 exchange with water and photosynthetic 12co2 uptake textural features of laminae support these results the dark laminae are related to higher calcite saturation in lake water during drier periods isotopic values from high resolution sampling through a 2 1 cm thick stromatolite reveal palaeoclimate variations at different temporal scales isotopic variation in 3rd order cyclicity of alternating light dark simple laminae is recording seasonal p e variations light and dark composite laminae 2nd order cyclicity correspond to pluriannual dominantly humid or dry conditions respectively a gradual succession from light to dark composite laminae forms the 1st order cycles driven by decreasing p e through longer pluriannual periods resulting in lake level lowering the stromatolites are thus recording lake level changes of centennial to millennial scale

annular variability and eddy zonal flow interactions in a simplified atmospheric gcm part i characterization of high and low frequency behavior abstract experiments have been performed using a simplified newtonian forced global circulation model to investigate how variability of the tropospheric jet can be characterized by examining the combined fluctuations of the two leading modes of annular variability eddy forcing of this variability is analyzed in the phase space of the leading modes using the vertically integrated momentum budget the nature of the annular variability and eddy forcing depends on the time scale at low frequencies the zonal flow and baroclinic eddies are in quasi equilibrium and anomalies propagate poleward the eddies are shown primarily to reinforce the anomalous state and are closely balanced by the linear damping leaving slow evolution as a residual at high frequencies the flow is strongly evolving and anomalies are initiated on the poleward side of the tropospheric jet and propagate equatorward the eddies are shown to drive this evolution strongly eddy location and amplitude reflect the past baroclinicity while

late cretaceous india madagascar fit and timing of break up related magmatism a u pb zircon age of 91 2 0 2 myr from western india st mary islands confidently links india with the late cretaceous magmatic province in madagascar 84 92 ma and the u pb age is within analytical error of the u pb age of the analalava gabbro pluton 91 6 0 3 myr in northeastern madagascar palaeomagnetic data from india and madagascar allow us to postulate a new india madagascar fit euler latitude 14 24 longitude 38 8 and rotation angle 69 2 this fit is applicable to the late cretaceous directly prior to and during the early phase of madagascar india separation in our late cretaceous reconstruction south west india runs roughly subparallel with the first known break up related magnetic anomaly a34 it maintains a close connection between mada gascar and india but places india slightly rotated compared to the eastern margin of madagascar and more northerly compared with some reconstructions st mary magmatism is linked to the initial break up between india and madagascar and magmatism probably resulted from rift related extensional processes initially induced by the marion hotspot underlying southern madagascar during the late cretaceous

the karnataka late cretaceous dykes as products of the marion hot spot at the madagascar india breakup event evidence from 40ar 39ar geochronology and geochemistry two late cretaceous mafic dykes with an ene strike that is orthogonal to the west coast of india and located nearly 200 km inland around huliyardurga karnataka state yield 40ar 39ar plateau ages of 90 0 1 0 and 87 5 0 9 ma these fe ti enriched tholeiites are essentially co eval with at least four other igneous suites widely scattered in southern india namely the south and north kerala dykes the agali anaikatti dykes of central kerala tamil nadu and lavas of st mary islands off the west coast of india the karnataka cretaceous dykes are also co eval and compositionally very similar to the fe ti enriched tholeiitic lavas and dykes around mananjary a major phase of late cretaceous magmatism along the eastern rifted margin of madagascar which are believed to be products of the marion hot spot that extruded at 88 ma synchronous with the india madagascar break up event the age and compositional similarities between these late cretaceous magmatic rocks from india and madagascar constitute a clear evidence for extension of the thermal manifestations deep into the indian peninsula

annular modes in the extratropical circulation part i month to month variability the leading modes of variability of the extratropical circulation in both hemispheres are characterized by deep zonally symmetric or annular structures with geopotential height perturbations of opposing signs in the polar cap region and in the surrounding zonal ring centered near 458 latitude the structure and dynamics of the southern hemisphere sh annular mode have been extensively documented whereas the existence of a northern hemisphere nh mode herein referred to as the arctic oscillation ao has only recently been recognized like the sh mode the ao can be defined as the leading empirical orthogonal function of the sea level pressure field or of the zonally symmetric geopotential height or zonal wind fields in this paper the structure and seasonality of the nh and sh modes are compared based on data from the national centers for environmental prediction national center for atmospheric research reanalysis and supplementary datasets the structures of the nh and sh annular modes are shown to be remarkably similar not only in the zonally averaged geopotential height and zonal wind fields but in the mean meridional circulations as well both exist year round in the troposphere but they amplify with height upward into the stratosphere during those seasons in which the strength of the zonal flow is conducive to strong planetary wave mean flow interaction midwinter in the nh and late spring in the sh during these active seasons the annular modes modulate the strength of the lagrangian mean circulation in the lower stratosphere total column ozone and tropopause height over mid and high latitudes and the strength of the trade winds of their respective hemispheres the nh mode also contains an embedded planetary wave signature with expressions in surface air temperature precipitation total column ozone and tropopause height it is argued that the horizontal temperature advection by the perturbed zonal mean zonal wind field in the lower troposphere is instrumental in forcing this pattern a companion paper documents the striking resemblance between the structure of the annular modes and observed climate trends over the past few decades

wave driven zonal flow vacillation in the southern hemisphere the variability of the zonal mean flow in the southern hemisphere during the period 1985 94 is studied using european centre for medium range forecasts analyses the dominant mode of variability has approximately equivalent barotropic variations of opposite signs centered at 40 8 and 608s this structure is dominant in all seasons and has similar variance in all seasons the temporal variance of the amplitude of this mode is well modeled as gaussian red noise with a correlation e folding time of about 10 days zonal wind anomalies are maintained against frictional drag by variations in the zonal flow accelerations driven by transient eddies and associated mean meridional circulations the eddy structures suggest that equatorward propagation is favored when the jet is displaced poleward and zonal propagation is favored when the jet is displaced equatorward

stable isotope record of palaeoclimatic change in a british holocene tufa a 9 5 m long continuous core through holocene tufa deposits in derbyshire uk was analysed for oxygen and carbon stable isotope variations with a view to palaeoclimatic and palaeoenvir onmental interpretation the depositional environment of the tufa is well understood and dated using 14c and biostratigraphy it is argued that smooth variations of up to 1 in 18o of the tufa calcites are caused by changes in the isotopic composition of regional rainfall which in turn were caused mainly by changes in air temperature palaeoclimate in general lower 13c values for calcite correspond with higher 18o values suggesting a stronger component of soil derived carbon during warmer climatic phases a stable isotope approach to reading terrestrial palaeoclimatic records is of widespread application to holocene and older tufa deposits which are common in carbonate bedrock regions

indices of the southern hemisphere zonal wind abstract the time and spatial variation of the southern hemisphere 500 hpa zonal wind has been examined using spectral analysis bandpass filtering and empirical orthogonal function eof analysis applied to a 15 year sample of australian hemispheric analyses spectral analysis of the entire 15 year record of the 500 hpa zonal mean westerly wind shows significant peaks only on the seasonal and interannual time scales the mean segmented spectra for each season failed to show any additional significant peaks at higher frequencies or any significant differences between seasons the primary pattern of variation revealed by eof analysis shows similar behavior on synoptic to seasonal time scales with compensating departures occurring at intervals of 20 latitude the first eof has opposing peaks near 40 and 60 s and shows an equivalent barotropic pattern when correlated with individual grid point values at 500 hpa and mean sea level pressure correlations of the zonal pattern with regional anomalies reveal

late pleistocene holocene palaeoenvironmental evolution of the anamaza river valley iberian range ne spain multidisciplinary approach on the study of carbonate fluvial systems abstract the uppermost pleistocene and holocene palaeoenvironmental evolution of the anamaza river valley iberian range ne spain is deduced using multidisciplinary approach including stratigraphical mineralogical palynological geochemical geophysical methods and drilling main changes were registered in distinct subenvironments of a carbonate fluvial system including the channelled zone and wetlands in the floodplain tufa barrages dominated although pools also existed geophysical survey and coring reveal tufa build ups and pool facies also in the subsoil lower water temperature and scarce evaporation are deduced for the pleistocene fluvial system that progressively changed through the holocene with more hydrologically closed areas and higher evaporation influence a general aggrading evolution during warm stages related with increasing base level and damming due to fast carbonate precipitation characterised the holocene detrital tufa indicates erosive high energy floods or colder stages when water level would decrease favouring erosion 14 c and 230 th 234 u dating reveal high sedimentation rates and three main discontinuities related with cold episodes younger dryas middle part of the holocene climate optimum and iron age epoch during the uppermost pleistocene tufa growth would be enhanced during warmer episodes as the bolling allerod in the younger dryas scarce vegetation favoured erosion of both slopes and tufa constructions subsequent warmer temperatures during the first part of the holocene favoured vegetated slopes enhanced tufa growing although interrupted in the middle part of the holocene climate optimum and development of wetlands with riparian vegetation in the floodplain where either siliciclastics or detrital tufa incoming alternated with low energy waters stages and mud settling progressive decline in tufa is deduced for the upper holocene but it is not possible to determine whether this and other palaeoenvironmental changes were related either to climate or increasing human activities during the roman and medieval warm periods more oxidizing conditions in the wetlands and increasing erosion prevailed probably conditioned by human activities the pollen record shows for the early holocene development of pinus forest with betula and expansion of deciduous quercus xerophilous and heliophilous grassland subsequent increasing moisture supported open forests with deciduous quercus ulmus corylus and evergreen quercus ilex pistacia species from ca 4000 yrbp a dominant deciduous quercus forest with groves of corylus ulmus acer fagus and taxus expanded and human activities grazing occurred from 1200 yrbp dry grassland expanded due to intensive land use agropastoral activities almost completely deforested plateaus surround the site today with slopes covered by patchy grass with junipers groves and screeds with little soil

sedimentology of tufa facies and continental microbialites from the palaeogene of mallorca island spain abstract the middle eocene and oligocene basins in northwestern half of mallorca are filled by detrital rocks conglomerates calcarenites and lutites and carbonates deposited in alluvial fluvial palustrine and lacustrine settings the sediment supplied to these continental basins was transported by alkaline bicarbonate rich waters as a consequence these non marine carbonate deposits display extensive developments of diverse kinds of organosedimentary facies the sedimentary record from three localities peguera alaro and sineu enabled one to characterize well preserved eocene tufa deposits as well as eocene and oligocene stromatolites and oncolites a total of ten carbonate facies are described and interpreted the facies related to tufa deposits are diverse types of boundstones rudstones and packstones of bryophytes in situ stems s l phytoclasts etc and associated oncolite rudstones and bioclastic mudstones to packstones these facies deposited in littoral lacustrine and fluvio lacustrine environments oncolites and stromatolites present a great variety of shapes and sizes oncolites from centimetres to several meters in diameter that somehow were conditioned by environmental conditions within shallow lacustrine and fluvial settings three types of lamination apparently dependent on the type of microorganisms responsible for calcite precipitation are distinguished fan shaped and bush shaped grouped filaments and isolated to loosely grouped filaments the 13c and 18o composition of these deposits agrees with carbonate precipitation in freshwater hydrologically open systems with similar carbon sources throughout the eocene and oligocene however eocene and oligocene samples are set apart by the 18o oligocene facies present lower 18o values which might indicate deposition from isotopically less evolved waters perhaps under slightly greater precipitation evaporation conditions tufa facies have the highest 13c composition and show some positive covariation of 13c and 18o primarily reflecting the effects of physical and biological co2 outgassing and maybe also minor evaporation

correlation between isotope records in marine and continental carbon reservoirs near the palaeocene eocene boundary changes in the isotope content of the large marine carbon reservoir can force shifts in that of the smaller carbon pools in the atmosphere and on land the carbon isotope compositions of marine carbonate sediments from the late palaeocene vary considerably exhibiting a sudden decrease close to the palaeocene eocene boundary which coincides with deep sea benthic extinctions1 and with changes in ocean circulation here we report that these fluctuations in the marine carbon isotope record are closely tracked by the terrestrial records provided by palaeosol carbonates and mammalian tooth enamel in using palaeosol carbonates to reconstruct the co2 content of the ancient atmosphere2 isotope shifts of this sort will have to be taken into account the sharp decrease in 13c 12c ratios in the late palaeocene provides a datum for precise correlation of marine and continental records and suggests that abrupt climate warming at this time may have played an important role in the evolution of land mammals

preferred modes of variability and their relationship with climate change spatial structure of annular modes shows a remarkable resemblance to that of the recent trend in the observed circulation thompson et al this study performs a series of multilevel primitive equation model simulations to examine the extent to which the annular mode is capable of predicting changes in the zonal mean flow response to external heat perturbations each of these simulations represents a statistically steady state and differs from each other in the values of the imposed tropical heating h and high latitude cooling c defining the annular mode as the first empirical orthogonal function eof1 of zonal mean tropospheric zonal wind it is found that the climate predictability is generally high in the small c large h region of the parameter space but is markedly low in the large c small h region in the former region eof1 represents meridional meandering of the midlatitude jet while in the latter region eof1 and eof2 combine to represent coherent poleward propagation of zonal mean flow anomalies it is also found that the climate predictability tends to be higher with respect to changes in c than to changes in h the implications of these findings for the southern hemisphere climate predictability are also presented

the azuaje travertine an example of aragonite deposition in a recent volcanic setting n gran canaria island spain the azuaje travertines in the north of gran canary island crop out in the azuaje gorge which is incised into miocene volcanic breccias of the roque nublo formation this travertine is interesting because of the scarcity of travertines in the canary islands and its close relationship with recent volcanic events part of the travertine overlies a lava flow dated at 2420 40 years bp the travertine is composed mainly of aragonite in both perched systems on the gorge walls and in gorge bottom deposits parallel to the present stream perched systems include deposits from the feeder conduits within the roque nublo formation that spilled out through waterfalls and over barriers the gorge valley bottom system includes barrier and pool deposits the main laminated facies are composed mostly of large fibrous aragonite crystals while there are also common micritic and porous microfabrics shrubs coated grains coated bubbles and rafts are present in many of the deposits detailed study of the microfacies shows the presence of sparsely distributed biogenic features the 13c values are between 4 0 and 11 0 and the 18o values range between 11 0 and 2 0 vpdb these 13c values lie within the range of those of thermogene travertines fed by thermal waters that cooled downstream as indicated by the increase of the carbon and oxygen isotope values in that direction the common occurrence of fibrous radial fabrics is interpreted to be due to disequilibrium conditions during aragonite precipitation disequilibrium was likely caused by rapid co2 degassing of the thermalwaters that led to a rapid increase in the degree of aragonite saturation thus themorphology of the aragonite crystals and the isotopic composition indicate that the formation of the azuaje travertine was mostly due to abiogenic processes induced by rapid degassing of thermalwaters linked to the presence of a recent lava flow within the gorge in short the azuaje travertine shows the important role of volcanic activity in travertine deposition in the facies distribution as well as in the geochemical signatures of the deposits and so can be used as a recent analogue for ancient travertine deposits in volcanic settings

an observational study of the intraseasonal poleward propagation of zonal mean flow anomalies abstract the poleward propagation of zonal mean relative angular momentum mr anomalies is examined using ncep ncar reanalysis data for both the winter and summer seasons of the northern and southern hemisphere this analysis is performed with a regression analysis using base latitudes in the subtropics midlatitudes and high latitudes it is found that the poleward mr anomaly propagation occurs at all latitudes with the propagation speed being greater in the subtropics and high latitudes compared to midlatitudes other fields such as eddy angular momentum flux convergence eddy heat flux friction torque and 300 mb streamfunction are regressed for the northern hemisphere winter and the southern hemisphere summer the main finding is that in the subtropics and midlatitudes the poleward mr anomaly propagation is primarily due to high frequency 10 day transient eddy angular momentum flux convergence and in high latitudes the propagation is mostly due to the summation of cross frequency and low fr

relations between annular modes and the mean state southern hemisphere winter in a zonally symmetric climatology with a single eddy driven jet such as prevails in the southern hemisphere summer the midlatitude variability is dominated by fluctuations of the jet around its mean position as described by the southern hemisphere annular mode sam to study whether this result holds for a zonally asymmetric climatology the observed variability of the southern hemisphere winter is analyzed the mean state in this case is characterized by relatively weak stationary waves yet there exist significant zonal variations in the mean strength and meridional structure of the subtropical jet stream as in summer the winter sam signature is annular in shape and the corresponding wind anomalies are dipolar but it is associated with two different behaviors of the eddy driven jet in different longitudinal ranges over the indian ocean the sam is associated primarily with a latitudinal shift of the jet around its mean position over the pacific sector it is instead characterized by a seesaw in the wind speed between two distinct latitudes corresponding to the positions of the midlatitude and subtropical jets composites of eddy forcing and baroclinicity over both sectors appear consistent with the two different behaviors as in the zonal mean case high frequency eddies both force and maintain the low frequency wind anomalies associated with the sam the positive feedback by eddies is however not local changes in the eddy forcing are influenced most strongly by zonal wind anomalies located upstream

oxygen isotopes of marine mollusc shells record eocene elevation change in the pyrenees abstract constraining paleoaltimetry of collisional orogens is critical to understand the dynamics of topographic evolution and climate tectonics retroactions here we use oxygen stable isotope record on oyster shells preserved in marine foreland deposits to examine the past elevation of the pyrenees during the eocene our approach is based on the comparison with the paris basin an intracratonic basin not influenced by orogenic growth the finding of a shift of 1 5 between 49 and 41 ma indicating more negative 18oc in the south pyrenean foreland is interpreted to reflect the inflow of river water sourced from higher elevation in the pyrenees to test this and provide paleoelevation estimate we adopt a morphologic hydrological model accounting for the hypsometry of drainage basin our best fitting model shows that the pyrenees rose up to 2000 m this indicates that the pyrenees reached high elevation in the eocene thus providing new critical constraints on their long term orogenic development 18o of marine mollusc shells are proved potentially attractive for paleoelevation studies especially for mountain belts where elevated continental surfaces have not been preserved

wave zonal flow interaction and ultra low frequency variability in a simplified global circulation model the interactions between baroclinic waves and the zonal mean mid latitude flow are investigated using a multilevel primitive equation model of moderate resolution with idealized surface friction and diabatic heating terms integrations corresponding to 100 years were performed under perpetual solstice conditions empirical orthogonal function eof analyses were carried out on the zonal mean zonal wind u adjustments to the winter hemisphere zonal jet due to the growth and decay of baroclinic waves can be described in terms of the first two eof patterns which exhibit substantial interannual variability for a single lifecycle such adjustments take the form of a circuit of a phase space whose axes are defined by the amplitudes of the two eofs the cumulative effects of multiple lifecycles lead to a low frequency circulation in the phase space with a timescale of about 150 days we suggest that this effect may be intrinsically related to the internal generation of ultra low frequency variability in the model normal mode lifecycle calculations including dissipation reveal similar phase space properties the growth rates of normal mode disturbances are shown to be highly sensitive to the phase of the first eof

palaeomagnetism and 36ar40ar vs 39ar40ar isotope correlation ages of dyke swarms in central kerala india tectonic implications palaeomagnetic results and whole rock 36ar40ar vs 39ar40ar isotope correlation ages are presented for dyke swarms occurring within the granulite terrain in central kerala west coast of india palaeomagnetic data show that the nnw sse leucogabbro dykes sampled are normally magnetised d 307 and i 57 whereas most nw se dolerite dykes are reversely magnetised d 163 and i 61 one dolerite dyke shows normal polarity the present 40ar39ar isotope ages for dolerite dykes and previously reported k ar ages for leucogabbros suggest 81 2 ma for acquisition of the leucogabbro directions and 69 1 ma for acquisition of the reverse magnetic directions within the dolerites based on palaeomagnetic and isotopic age data the dolerite dykes appears to represent feeder systems for the deccan volcanic sequences we suggest that the deccan eruptions were widespread and probably extended southward at least into central kerala the entire magmatic episode may have occurred between ca 70 and 65 ma during the 31r period probably corresponding to the cretaceous tertiary boundary the leucogabbros may correspond to the 34n superchron and represent a plume related magmatic episode precursory to the main volcanic event these data suggest that an early intraplate plume tectonic setting since ca 80 ma preceded lithospheric thinning asthenospheric upwelling and consequent massive flood volcanism during the period 70 65 ma this culminated in ocean floor spreading between the seychelles mascarene and continental india

stable isotopic compositions of recent freshwater cyanobacterial carbonates from the british isles local and regional environmental controls recent 50 years old freshwater cyanobacterial carbonates from diverse environments streams lakes waterfalls throughout britain and ireland were analysed for their stable carbon and oxygen isotope compositions the mean 18o value of 5 9 pdb for river and stream data represents calcite precipitation in equilibrium with the mean oxygen isotopic composition of precipitation in central britain 7 5 smow assuming a mean water temperature of 9 c the mean 18o of lake data 4 5 pdb is statistically different reflecting the effects of residence time and or variations in the oxygen isotopic composition of rainfall carbon isotopes have wide variations in both fluviatile and lake data sets 3 to 12 pdb these variations are principally controlled in the fluviatile samples by contribution of isotopically light soil zone carbon relative to isotopically heavier carbon from limestone aquifer rock dissolution lake samples have the heaviest carbon isotope values reflecting a trend toward isotopic equilibrium between atmospheric co2 and aqueous hco 3 we infer that isotopic compositions of ancient cyanobacterial carbonates should also record environmental information although the effects of stabilization and diagenesis on primary 18o values will need careful consideration primary carbon isotope compositions should be well preserved although in marine samples values will be buffered by the isotopic composition of aqueous marine bicarbonate

spatial variability of sea level pressure and 500 mb height anomalies over the southern hemisphere abstract the spatial variability of mean sea level pressure slp and 500 mb height anomalies over the southern hemisphere during summer djf and winter jja is determined using eigenvector analysis based on daily synoptic maps from 1972 to 1979 the patterns of spatial distribution of pressure and height anomalies are further verified and examined by means of station data and the eigenvectors are compared between the seasons and to those found for the northern hemisphere the first eigenvector shows that midlatitude anomalies of slp and 500 mb height are of an opposite sign to those found over and around antarctica the pattern is highly barotropic and suggests strengthening and weakening of the zonal wind in alternating latitude belts the 500 mb height differences are calculated for five midlatitude to antarctic station pairs using data from the late 1950 s onward these latitudinal height differences are also used to describe the association between the hemispheric westerlies and the southern oscil

tectonic and climatic controls on the development of foreland fan deltas montserrat and sant llorenc del munt systems middle eocene ebro basin ne spain abstract along an early cenozoic foreland hinterland boundary in northeastern spain we synthesize the depositional environment climatic conditions structural evolution age catchment geometry and altitudinal characteristics from sedimentologic palynologic stratigraphic structural and paleomagnetic data as the transpressional catalan coastal ranges rose during the paleogene two large fan deltas prograded into the ebro foreland basin adjacent to the northeastern part of the range the apices of the fans likely were localized by lateral ramps or tear faults along which rivers from hinterland catchments debouched into the foreland beginning in the late lutetian proximal debris flow sheetflood and distal fluvial deposits maintained the fan surface at or above sea level despite rapid basin subsidence during the succeeding 4 4 my palynological data suggest that a warm humid climate prevailed throughout this interval the mapped extent of the two fans permits an estimation of their volumes whereas the spatial distribution of distinctive lithologies within the ancestral catalan coastal ranges serves to delimit the approximate catchment areas for each of the fans we estimate mean hinterland denudation rates to range from 100 to 180 m my and mean catchment elevation to range from 700 to 1250 m the steep gradients between these catchments and the low lying fan deltas is attributed to the tectonic style of the ancestral catalan coastal ranges which are characterized by an uplifted basement block along a steep frontal thrust accompanied by folding of cover rocks the considerable topographic relief of these catchments is inferred to have combined with co seismic shaking to produce landslides and rockfalls which were reworked as debris and fluid gravity deposits on the fan surfaces

paleohydrology of the late miocene ridge basin lake california uppermost miocene lacustrine deposits of the ridge basin california contain benthic microbial carbonates as fragmentary crusts on clasts in resedimented conglomerates of the violin breccia deposited adjacent to the fault controlled southwestern margin of the basin and as fragments and more extensive in situ stromatolitic units within the ridge route formation around the northeastern part of the basin ridge route microbialites formed in the open waters of the ridge basin paleolake and have 18 o values that are significantly higher than those from the violin breccia the latter may have formed in warm littoral pools or in streams maintained by local runoff their 18 o compositions are consistent with carbonate precipitation from meteoric waters supplied by rainfall dominated by 67 75 winter precipitation at temperatures of 16 17 c ridge route formation microbialites have a wide range of 13 c and 18 o values and indicate that the lake fluctuated between hydrologically open and closed conditions light carbon isotopic compositions for carbonate that precipitated during periods of open basin conditions suggest that inflow waters came from a fluvial system with a well vegetated flood or delta plain relatively high 18 o values imply that the lake was not maintained by local rainfall but depended on inflow that came either from an upstream lake or from a region to the east or northeast where the rainfall regime was dominated by isotopically heavy summer precipitation

a review of indian phanerozoic palaeomagnetism implications for the india asia collision abstract an overview is presented of the indian apparent polar wander path apwp for the phanerozoic and in particular for post late palaeozoic times this apwp is compiled on basis of data available at october 1981 from peninsular and extrapeninsular indo pakistan and from dsdp cores from the indian plate one of the more important and newly recognized features of this apwp is a large scale triassic jurassic loop this loop indicates a changeover from a late palaeozoic early mesozoic northwards and counter clockwise rotational movement with greater india reaching moderately low southern latitudes into a southwards and clockwise rotational movement during the early to middle jurassic recognizable likewise in apwp s from other gondwana continents this loop reflects the opening of the neotethys studies of extrapeninsular regions up to and north of the indus tsangpo suture zone have shown wide spread presence of magnetic overprints which delineate two regionally confined age groups younger overprints 20 40 m y predominate in the more external thrust zones older overprints 50 60 m y in contrast are found in the more internal zones both north and south of the indus tsangpo suture zone the latter are interpreted to reflect a late phase of relaxation in the early tertiary collision of greater india with south central asia or off shore island arcs which occurred at equatorial to low northern palaeolatitudes 0 10 n subsequent northwards movement over 2500 3000 km or more and impingement of greater india into southern asia resulted into large scale underthrusting of greater india along the main central thrust beneath southern tibet and to clockwise rotation of thrust units in the western himalaya a discrepancy between indian palaeomagnetic data and results available todate from southern tibet is discussed

sedimentology and isotopic composition of upper palaeocene to eocene non marine stromatolites eastern ebro basin ne spain stromatolites are abundant in upper palaeocene to eocene fluvial deposits from the eastern ebro basin whereas they are scarce in the lacustrine facies the fluvial stromatolites display a variety of growth forms oncoids domes laminated crusts and locally irregular bioherms these morphologies are related to the hydrological behaviour of the fluvial systems elongate domes dome heads and ovoid to subspherical oncoids accreted in flowing water laminated crusts and small sized cylindrical to lenticular oncoids formed during low discharge and ponding phases in the lacustrine environments only small sized ovoid oncoids occur the main characteristics of the ebro basin non marine stromatolites are 1 laminations and concentric banding exhibited by all morphologies 2 the occurrence of well preserved cyanobacterial filaments in several laminae suggesting that these microbiotas are the major contributors in growth 3 the asymmetrical growth of cortices in domes and oncoids with polar thickening on the upper side suggesting in situ growth under low energy conditions 13c values of carbonate stromatolites range from 11 3 o late palaeocene to 4 4 o middle late eocene during the late palaeocene under tectonically quiescent conditions the 13c values mainly reflect the influence of soil derived co2 during the eocene the progressive uplift of the surrounding catchment areas led to an increase of exposures of mesozoic rocks consequently the 13c values of the eocene stromatolites reflect the influence of the co2 derived from the dissolution of mesozoic marine carbonates there is a 2 5 o shift in 18o values of stromatolites from the late palaeocene to middle late eocene which is consistent with the variation in 18o of precipitation due to changes in altitude of the catchment area during this time the isotopic values of all middle upper eocene laminated crust samples show distinct covariant trends suggesting that these stromatolites were formed either in ponding zones of fluvial channels or in disconnected pools developed during low discharge episodes

relation between annular modes and the mean state southern hemisphere summer abstract the annular modes emerge as the leading mode of extratropical month to month climate variability in both hemispheres here the influence of the background state on the structure and dynamics of the southern hemisphere annular mode sam during the austral summer when the climatology is characterized by a single well defined eddy driven jet is studied subsets of the climatology are constructed for early and late summer and for contrasting polarities of the enso cycle the analysis is based both on observations and on perpetual state gcm experiments the main differences between the subsets involve variations of the latitude of the mean jet it is found that in all the cases the sam is characterized by latitudinal shifts of the jet about its mean position reinforced by a positive momentum flux feedback from baroclinic waves this result is consistent with previous studies of the dynamics of the zonally averaged circulation but is found here to hold over all longitudes and for different positi

effect of upper and lower level baroclinicity on the persistence of the leading mode of midlatitude jet variability abstractthe sensitivity of the variability of an eddy driven jet to the upper and lower level baroclinicity of the mean state is analyzed using a three level quasigeostrophic model on the sphere

time scale and feedback of zonal mean flow variability the physical processes that determine the time scale of zonal mean flow variability are examined with an idealized numerical model that has a zonally symmetric lower boundary in the part of the parameter space where the time mean zonal flow is characterized by a single double jet the dominant form of zonalmean flow variability is the zonal index poleward propagation and the time mean potential vorticity gradient is found to be strong and sharp weak and broad the e folding time scale of the zonal index is found to be close to 55 days much longer than the observed 10 day time scale the e folding time scale of the poleward propagation is about 40 days the long e folding time scales for the zonal index are found to be consistent with an unrealistically strong and persistent eddy zonal mean flow feedback a calculation of the refractive index indicates that the background flow supports eddies that are trapped within midlatitudes undergoing relatively little meridional propagation additional model runs are performed with an idealized mountain to investigate whether zonal asymmetry can disrupt the eddy feedback for single jet states the time scale is reduced to about 30 days if the mountain height is 4 km or less the reduction in the time scale occurs because the stationary eddies excited by the mountain alter the background flow in a manner that leads to the replacement of zonal index events by shorter time scale poleward propagation with a 5 km mountain the time scale reverts and increases to 105 days this threshold behavior is again attributed to a sharpening of the background zonal jet which arises from an extremely strong stationary wave momentum flux convergence in contrast for double jet states the time scale changes only slightly and the poleward propagation is maintained in all mountain runs

seasonal wetland carbonate sequences and dynamic catenas a re appraisal of palustrine limestones abstract palustrine carbonate cyclothems are compared directly to well documented peritidal carbonate cyclothems in the case of the latter the trend of increasing degree of exposure seen vertically through a cyclothem has been interpreted as due to the aggradation of the sediment surface relative to the semi diurnal tidal regime palustrine cyclothems also show similar increases in the degree of exposure and this is interpreted as the effect of sediment aggradation in a seasonal wetland regime the resulting changes in the drainage characteristic revealed in a palustrinc cyclothem reflect a type of seasonally variable catenary relationship referred to here as a dynamic catena this model explains the key features of palustrine cyclothems and suggests the importance of seasonal wetland environments in ancient continental settings

petrology geochemistry and magnetic properties of sadara sill evidence of rift related magmatism from kutch basin northwest india abstract mafic volcanic rocks of the mesozoic kutch basin represent the earliest phase of deccan volcanic activity an olivine clinopyroxene plagioclase phyric undersaturated basalt occurs as a sill near sadara in the pachham upland northern kutch the sadara sill is deformed and emplaced along faults the sill is alkaline in character and is transitional between basalt and basanite compared to primitive mantle the sadara sill is enriched in sr ba pb and lree but depleted in nb cr y cs and lu fractional crystallization of olivine and clinopyroxene from an alkaline mafic melt generated by low degree partial melting of mantle peridotite can explain the observed chemical variation in the sill irm and l f test experiments and mineral analyses show titano magnetite as the major remanence carrying magnetic mineral af and thermal demagnetizations of the sadara sill yielded a mean chrm direction as d 315 6 i 43 0 95 9 78 k 25 38 and the corresponding vgp at 25 s 114 6 e dp dm 6 58 11 6 the sadara sill pole is significantly different from those of the deccan 65 ma and the rajmahal traps 118 ma but is close to the cretaceous poles of 85 91 ma rock units from southern india this suggests a pre deccan age for the sill

a baroclinic mechanism for the eddy feedback on the zonal index a baroclinic mechanism for the positive eddy feedback on the zonal index is proposed the author considers the quasigeostrophic response of the zonally averaged flow to forcing by baroclinic eddies the transient response to eddy forcing is a largely barotropic acceleration of the westerly flow and a reduction of the baroclinicity at the latitudes of eddy generation but over time the action of surface drag leads to enhanced baroclinicity at these latitudes the steady state response has positive baroclinicity at the latitude of eddy generation if the eddies propagate away from this latitude before they dissipate this reinforcement of the low level baroclinicity provides a positive feedback if it is assumed that baroclinic eddies are generated more vigorously in regions of stronger low level baroclinicity the proposed mechanism explains observed and modeled features of zonal index variations the frequency and drag dependence of eddy feedback the bandedness in latitude of zonal wind variations and the tendency for anomalies in the zonally averaged zonal wind to drift poleward over time

characterization of the causative fault system for the 2001 bhuj earthquake of mw 7 7 abstract precise hypocenters erh m w 2 0 5 3 delineate a east west trending blind thrust dipping 45 towards south named as north wagad fault nwf about 25 km north of kachchh main land fault kmf as the causative fault for the 2001 bhuj earthquake of m w 7 7 the aftershock zone involves a crustal volume of 60 40 35 km 3 lying between kmf and nwf the waveform inversion of 45 aftershocks of m 3 0 5 3 suggest that the e w trending south dipping reverse faulting mainly characterizes the aftershock zone however some reverse faulting along nw se as well as ne sw planes and some strike slip faulting along nw se trending vertical plane are also noticed the estimated p axes point on an average towards n s while t axes orient in e w agreeing well with the prevailing regional plate tectonic stress directions the estimated velocity model from one dimensional inversion of 8000 p and 5000 s travel times from 600 aftershocks deciphers a detailed crustal structure of the region upper 0 6 km on an average suggests a low velocity zone characterizing the jurassic and younger sediments the depth range 6 42 km is characterized on an average by larger values of v p 6 31 to 6 98 km s with an average of 6 71 km s v s 3 64 to 4 05 km s with an average of 3 85 km s and v p v s 1 69 to 1 81 with an average of 1 75 the high crustal velocities can be attributed to the existence of a high velocity mafic intrusive rift pillow structure beneath the region probably emplaced during the rifting time this brittle competent high velocity zone beneath the epicentral area in response to the compression due to the northward movement of the indian plate could induce sufficient local stress perturbation for generating large intraplate earthquakes of m w 7 5 in the lower crust further within the overall higher velocity depth range a layer characterized by large values of v p 6 98 km s v s 3 854 km s and v p v s 1 81 is found in the depth range of 20 5 30 km which is inferred to be fractured and saturated high crack density and probably fluid filled presence of numerous fractures in this layer is confirmed by a high b value 0 75 1 1 in this layer this layer might have facilitated the nucleation process of the 2001 bhuj earthquake the major rupture and 47 of the aftershocks have occurred within this layer

quaternary fluvial tufas of sarikavak area southwestern turkey facies and depositional systems abstract in the quaternary sarikavak fluvial tufa deposits situated in south western turkey different facies have been identified and their mutual relationships have been unravelled this tufa formation is located at the northern part of acigol graben the development of the sarikavak tufas represents typical fluvial and fluvio palustrine environments in total eight tufa lithofacies have been differentiated namely moss tufa facies oncolitic tufa facies stromatolitic tufa facies phytoclastic tufa facies intraclastic tufa facies extra formational clastic tufa facies sapropelitic tufa facies and palaeosol preliminary u th datings indicate that these deposits formed prior to 219 ka and continued until 80 ka mis 7 till 5 13c signatures 0 78 to 1 63 v pdb best can be explained by limestone dissolution processes or respiration from c4 plant vegetation into the soil or both for that reason the sarikavak tufa deposits can be classified as travitufa which precipitated in a distal facies system of travertine occurrences 18o values 7 49 to 10 83 v pdb of the whole data set reflect some significant changes which is interpreted in terms of temperature variations reflecting warm and cold periods among mis 7 interglacial mis 6 glacial and mis 5 interglacial the development of the sarikavak tufa deposits are related to the maymundagi fault that borders the acigol graben in the north this fault and its synthetic fault segments play a pivotal role with regard to the palaeo flow direction of water and control the basin topography these structural structures are likely responsible for all active and ancient springs in the study area as well as their respective carbonate precipitates

zonal flow vacillation and eddy forcing in a simple gcm of the atmosphere abstract zonal flow vacillation with very long time scales is observed in a 3070 day simple gcm simulation with zonally symmetric forcing the long lasting zonal wind anomalies suggest that zonal flow vacillation is self maintained wave mean flow interactions are investigated by composite analysis and transform eulerian momentum budget analysis nonlinear life cycle simulations are conducted to demonstrate that each extreme phase of the zonal flow vacillation is a quasi stable state and is self maintained by the embedded synoptic eddies the firm eof mode of zonal mean wind shows an out of phase relation between anomalies at 60 s and at 40 s with a barotropic structure this structure is similar to the dominant vacillation pattern observed in the southern hemisphere the composite jet stream in the high low index phase of zonal flow vacillation shifts poleward equatorward from the time mean location and becomes broader narrower and weaker stronger composite eddies in the high index phase tilt nw

the arctic oscillation signature in the wintertime geopotential height and temperature fields the leading empirical orthogonal function of the wintertime sea level pressure field is more strongly coupled to surface air temperature fluctuations over the eurasian continent than the north atlantic oscillation nao it resembles the nao in many respects but its primary center of action covers more of the arctic giving it a more zonally symmetric appearance coupled to strong fluctuations at the 50 hpa level on the intraseasonal interannual and interdecadal time scales this arctic oscillation ao can be interpreted as the surface signature of modulations in the strength of the polar vortex aloft it is proposed that the zonally asymmetric surface air temperature and mid tropospheric circulation anomalies observed in association with the ao may be secondary baroclinic features induced by the land sea contrasts the same modal structure is mirrored in the pronounced trends in winter and springtime surface air temperature sea level pressure and 50 hpa height over the past 30 years parts of eurasia have warmed by as much as several k sea level pressure over parts of the arctic has fallen by 4 hpa and the core of the lower stratospheric polar vortex has cooled by several k these trends can be interpreted as the development of a systematic bias in one of the atmosphere s dominant naturally occurring modes of variability

dinosaur eggshell isotope geochemistry as tools of palaeoenvironmental reconstruction for the upper cretaceous from the tremp formation southern pyrenees abstract the isotopic compositions 13 c and 18 o of dinosaur eggshells have been widely used in palaeoenvironmental studies although the geochemical signatures of eggshells are not usually contrasted with other proxies in this work the isotopic signatures of eggshells from a large maastrichtian succession from the tremp formation southern pyrenees spain are compared to those of carbonate pedogenic nodules occurring in the same levels the isotopic signatures of eggshells vary according to the stratigraphic unit and geographical location a group of samples from several localities corresponding to eggshells without significant diagenetic imprints has isotopic values differing from the associated nodules the late cretaceous isotopic composition record from the tremp fm is consistent that is the eggshells have distinct primary signatures preserved however the eggshells from another locality which exhibit neomorphed textures display isotopic signatures similar to the associated pedogenic carbonate which suggests a diagenetic isotopic signature and confirms alteration in the eggshells both microscopic and geochemical data suggest that an early meteoric diagenesis pedogenesis is responsible for the secondary signatures the 13 c values in the carbonate pedogenic nodules indicate a carbon isotopic composition typical of c3 plants although the slight difference in 13 c between the palaeosol carbonate of coeval successions may be due to slightly different palaeoenvironmental conditions the small discrepancy in the 13 c calculated for c3 plants from carbonate nodules and from eggshells may be because the palaeosol carbonate gives the isotopic composition of the vegetation grown at a local site whereas the 13 c from eggshells is a proxy for the ingested food in the area in which the dinosaurs lived the oxygen isotopic compositions from palaeosol carbonate nodules have been used for calculation of the air temperature and we may conclude that the mean air temperature in the studied area during the late cretaceous was approximately 21 c the use of this temperature gives rational results in the calculation of the oxygen isotopic composition of palaeosol carbonate by means of the isotopic composition of the eggshell carbonate additionally a relatively continuous isotopic record of nodules shows an overall vertical trend towards negative 13 c values and a high relative and steady 18 o content throughout the maastrichtian the k pg boundary was recorded by a negative excursion in 13 c in the carbonate nodules

is the atmospheric zonal index driven by an eddy feedback abstract the authors address the question of whether or not eddy feedback plays an important role in driving the anomalous relative angular momentum associated with the zonal index zi in the atmosphere for this purpose composites of anomalous relative angular momentum and anomalous eddy angular momentum flux convergence eddy forcing are examined with national centers for environmental prediction national center for atmospheric research reanalysis data by using an empirical orthogonal function analysis it is found that zi behavior dominates the summer season of both hemispheres and also the winter season of the southern hemisphere for the summer season the zi is characterized by meridional displacements of the midlatitude eddy driven jet and for the southern hemisphere winter it is characterized by a simultaneous movement of the subtropical and eddy driven jets in the opposite direction for the zi of each of the above seasons unfiltered eddy forcing did not exhibit a prominent eddy feedback h

on the midlatitude thermal response to tropical warmth 1 observations reveal a global scale thermal response to el nino and la nina featuring a band of tropospheric anomalies opposite in sign to those in the tropics when an idealized dynamical model is forced by increased tropical heating a similar response with an enhanced subtropical thermal gradient results in the idealized model this response is produced by changes in transient eddy fluxes of heat and momentum and consequent changes in the mean meridional circulation the similarity between the two level model results and those from observations as well as the zonally averaged behavior of a gcm suggests that the same dynamics operate in nature this represents a tropical midlatitude teleconnection distinct from and in addition to that mediated by stationary rossby wave trains

a mechanism and simple dynamical model of the north atlantic oscillation and annular modes a simple dynamical model is presented for the basic spatial and temporal structure of the large scale modes of intraseasonal variability and associated variations in the zonal index such variability in the extratropical atmosphere is known to be represented by fairly well defined patterns and among the most prominent are the north atlantic oscillation nao and a more zonally symmetric pattern known as an annular mode which is most pronounced in the southern hemisphere these patterns may be produced by the momentum fluxes associated with large scale midlatitude stirring such as that provided by baroclinic eddies it is shown how such stirring as represented by a simple stochastic forcing in a barotropic model leads to a variability in the zonal flow via a variability in the eddy momentum flux convergence and to patterns similar to those observed typically the leading modes of variability may be characterized as a mixture of wobbles in the zonal jet position and pulses in the zonal jet strength if the stochastic forcing is statistically zonally uniform then the resulting patterns of variability as represented by empirical orthogonal functions are almost zonally uniform and the pressure pattern is dipolar in the meridional direction resembling an annular mode if the forcing is enhanced in a zonally localized region thus mimicking the effects of a storm track over the ocean then the resulting variability pattern is zonally localized resembling the north atlantic oscillation this suggests that the north atlantic oscillation and annular modes are produced by the same mechanism and are manifestations of the same phenomenon the time scale of variability of the patterns is longer than the decorrelation time scale of the stochastic forcing because of the temporal integration of the forcing by the equations of motion limited by the effects of nonlinear dynamics and friction for reasonable parameters these produce a decorrelation time of the order of 5 10 days the model also produces some long term 100 days or longer variability without imposing such variability via the external parameters except insofar as it is contained in the nearly white stochastic forcing

a mechanism for the poleward propagation of zonal mean flow anomalies abstract observational studies have shown that tropospheric zonal mean flow anomalies frequently undergo quasi periodic poleward propagation a set of idealized numerical model runs is examined to investigate the physical mechanism behind this poleward propagation this study finds that the initiation of the poleward propagation is marked by the formation of negative zonal wind anomalies in the tropics these negative anomalies arise from meridional overturning breaking of waves that originate in midlatitudes this wave breaking homogenizes the potential vorticity pv within the region of negative zonal wind anomalies and also leads to the formation of positive zonal wind anomalies in the subtropics subsequent equatorward radiation of midlatitude waves is halted which results in wave breaking at the poleward end of the homogenized pv region this in turn generates new positive and negative zonal wind anomalies which enables a continuation of the poleward propagation the shielding of the homogenized

mechanisms of hemispherically symmetric climate variability inspired by paleoclimate evidence that much past climate change has been symmetric about the equator the causes of hemispherically symmetric variability in the recent observational record are examined using the national centers for environmental prediction national center for atmospheric research reanalysis dataset and numerical models it was found that the dominant cause of hemispherically symmetric variability is the el nino southern oscillation during an el nino event the tropics warm at all longitudes and the subtropical jets in both hemi spheres strengthen on their equatorward flanks poleward of the tropical warming there are latitude belts of marked cooling extending from the surface to the tropopause in both hemispheres at all longitudes and in all seasons the midlatitude cooling is caused by changes in the eddy driven mean meridional circulation changes in the transient eddy momentum fluxes during an el nino event force upper tropospheric ascent in midlatitudes through a balance between the eddy fluxes and the coriolis torque the eddy driven ascent causes anomalous adiabatic cooling which is primarily balanced by anomalous diabatic heating using a quasigeostrophic spherical model forced by an imposed surface eddy disturbance of chosen wave number and frequency it is shown that the anomalous eddy momentum fluxes are caused by the impact that the changes in the tropically forced subtropical jets have on the propagation in the latitude height plane of transient eddies changes in zonal winds and associated changes in the meridional gradient of potential vorticity create an anomalous region of low meridional wavenumber in the midlatitudes that refracts waves away both poleward and equatorward tropical forcing of variability in the eddy driven mean meridional circulation is another way in addition to rossby wave teleconnections whereby the tropics can influence extratropical climate unlike teleconnections this mechanism causes climate variability that has strong zonally and hemispherically symmetric components and operates throughout the seasonal cycle

late jurassic biogeochemical microenvironments associated with microbialite coated unionids bivalvia asturias n spain abstract the vega formation upper jurassic asturias spain contains two kimmeridgian outcrops the abeu and huerres sections with bivalves coated with carbonate microbialites oncoids the bivalve shells in the abeu section were replaced early by calcite while those of the huerres section were dissolved and the moulds filled by late diagenetic cements for the abeu shells there was a loss of sr and a gain of mg fe and mn during the replacement process but much of the original organic matter was retained part of the s present can be linked to this organic matter and part to the sulphate that replaced the carbonate groups in the calcite the thin microbialites inside the shells of some of the huerres bivalves have allowed us to estimate the influence of the growth rate on the incorporation of trace elements in calcite with an approximately six times lower growth rate the internal microbialite incorporates less mg and s and more fe and mn than the external one while maintaining the sr content the trace element content mg s sr fe and mn of the huerres and abeu microbialites are similar these coatings may therefore have been formed in the same type of freshwater but in different microenvironments given the substantial sulphate content of the microbialites from both sections this freshwater probably came from hot springs however the high fe mn ratio of the abeu microbialites suggests a reducing environment during their formation indeed 18o and 13c data confirm that the abeu and huerres microbialites were formed in different microenvironments the positive covariance for 18o and 13c of the abeu microbialites and replaced shells r2 0 83 and the wide range of variation for 18o 2 80 together suggest that the microbialite crusts grew in isolated ponds where evaporation was dominant in the huerres microbialites the positive covariance for 18o and 13c r2 0 59 and the narrow range of variation for 18o 0 74 indicate that the huerres section was formed in a more open environmental system where degassing predominated over evaporation

geology of the saucer shaped sill near mahad western deccan traps india and its significance to the flood basalt model an 22 m thick saucer shaped sill occurs near mahad and is exposed as a curvilinear miniature ridge within the deccan traps the sill has variable dips 42 55 it has a 7 1 km long axis and 5 3 km short axis aspect ratio of 1 4 and is larger than the mv sill of the golden valley sill complex south africa and the panton sill australia the sill has distinct glassy upper and lower chilled margins with a coarse grained highly jointed core the samples from the margin are invariably fractured and iron stained because of deuteric alteration the rock from the sill is plagioclase phyric basalt at least three thick sill like apophyses emanate from the base of the main sill the apophyses change direction because of bending and thinning from a horizontal concordant sheet at the top to a discordant inclined form that bends again to pass into a lower horizontal concordant sheet we interpret such features as nascent saucer shaped sills that did not inflate to form nested sills geochemically the sill consists of poorly differentiated tholeiitic basalt that has a restricted geochemical range critical trace element ratios and primitive mantle normalised trace and ree patterns indicate that the sills have geochemical affinities to the poladpur chemical type and that the pahoehoe flow they intrude belongs to the bushe formation calculated magmatic overpressures during sill emplacement range from 8 4 to 11 3 mpa for young s modulus e 5 gpa and 16 7 to 22 5 mpa for e 10 gpa and depth to magma chamber ranges from 8 5 to 11 5 km e 5 gpa and 17 1 to 22 9 km e 10 gpa consistent with petrological and gravity modelling the volume of the mahad sill is approximately 276 km3 and is constant irrespective of the variations in the values of host rock young s modulus in 1980 cox j petrol 21 629 650 1980 proposed a conceptual model of the crust mantle section beneath the karoo cfb which is considered as the fundamental model for flood basalt volcanism our paper confirms the presence of a sill plus the inferred substructure beneath mahad that are compatible with predictions of that model in lips saucer shaped sills are formed in areas experiencing extensional tectonics where processes such as the cook gordon delamination and dundurs elastic extensional mismatch between layered sedimentary rocks or lava flows are responsible for the deflection of dykes into sills a similar process is envisaged for the formation of the mahad sill