geochemical evidence for a catastrophic biotic event at the frasnian famennian boundary in south china a strong delta sup 13 c anomaly coincides with a weak ir anomaly at the frasnian famennian f f boundary exposed at xiangtian guangxi south china the maximum whole rock ir abundance is 0 23 ppb 0 35 ppb on a carbonate free basis compared with averages of 0 016 and 0 044 ppb above and below the boundary interval the delta sup 13 c in carbonate abruptly shifts from a late frasnian level of about 1 per thousand to minus 2 49 per thousand in the boundary interval and then abruptly returns to pre boundary levels suggesting a temporary reduction of surface water biomass a widely distributed over at least several hundred kilometers limestone breccia unit of possible wave deposit origin is present directly below the boundary in south china enrichments of al v cr as and u and a dip in the mn abundance in the boundary interval indicate that reducing conditions might have been associated with the element enrichment however an alternative hypothesis is that oceanic impact s at the f f boundary near south china provided the excess ir and caused the biotic crisis and the formation of breccia deposits observed in the late devonian south china sea

a low order model of the heinrich event cycle if heinrich events result from free oscillations in the size and basal melting conditions of the laurentide ice sheet a rigorous quantitative analysis of ice sheet physics should describe their dynamics to explore this possibility i exploit two characteristic timescales which arise from ice sheet physics to construct a relaxation oscillator model of the north atlantic s heinrich events the numerical implementation of this model confirms the notion that the periodicity of heinrich events approximately 7 000 years is determined by the gross properties of a steady glacial climate e g an annual average sea level temperature of 10 c and an adiabatic lapse rate

a multiproxy analysis of the frasnian famennian transition in western new york state u s a the late devonian frasnian famennian transition is well exposed along walnut creek in the western new york state region of the appalachian basin the studied stratigraphic interval encompasses approximately 29 m of section from the organic rich pipe creek formation the equivalent of the lower kellwasser carbonaceous horizon of europe to a thin black shale layer at the top of the overlying hanover formation that correlates with the upper kellwasser horizon the walnut creek section comprises five depositional cycles that are likely eustatic in nature perhaps modulated by tectonic dynamics of the basin clastic proxy trends ti al zr al suggest that deposits spanning the frasnian famennian boundary ffb accumulated in association with rising sea level a trend documented from elsewhere in the basin increasing si al across the ffb may reflect elevated post extinction biosiliceous productivity documented from lower famennian deposits elsewhere the walnut creek section includes two positive 13corg excursions of 3 the stratigraphically lower event associated with the pipe creek formation and the highest excursion approximately coincident with the upper kellwasser equivalent black shale layer these isotopic events are interpreted to record episodes of increased global productivity and organic carbon sequestration at the end of the frasnian stage redox proxy trends mo and u enrichment and pyrite framboid size distributions documented from black shale associated with the positive excursions suggest sediment accumulation under suboxic to intermittently anoxic or euxinic bottom water conditions in an unrestricted open marine setting the paleoclimate history of the end of the frasnian based on th k data is one of fluctuating atmospheric conditions including a brief cooling trend at the ffb though the walnut creek section sheds no light on the triggering mechanism or mechanisms of the late devonian extinction inferred climate fluctuations associated with the ffb reflected in the th k data may have contributed to the severity of the biological crisis

till beneath ice stream b 4 a coupled ice till flow model a nonsteady model of an ice stream flowing on deforming till shows that the system responds rapidly and in a stable manner to reasonable marginal perturbations for the model we require one dimensional flow and continuity of ice and till and we assume linear viscous till rheology and balance between the driving stress for ice flow and the resistive stress at the bed this allows us to write coupled equations for the time rate of change of the ice thickness and the till thickness in terms of these thicknesses and the till viscosity an analytic steady state solution for ice stream b west antarctica shows that till viscosity decreases slowly downstream probably in response to decreasing effective pressure downstream nonsteady numerical experiments with fixed ice thicknesses at the ends show that a marginal perturbation causes a wave of adjustment to travel the length of the ice stream in about 50 years with a new steady state in about 200 years changes in till thickness tend to moderate perturbations in the ice and thus stabilize the system

global iridium anomaly mass extinction and redox change at the devonian carboniferous boundary iridium abundance anomalies have been found on a global scale in the devonian carboniferous d c boundary interval which records one of the largest phanerozoic mass extinction events an event that devastated many groups of living organisms such as plants ammonoids trilobites conodonts fish foraminiferans brachiopods and ostracodes at or very close to the d c boundary there exists a geographically widespread black shale interval and ir abundances reach anomalous maxima of 0 148 ppb montagne noire france 0 138 ppb alberta canada 0 140 ppb carnic alps austria 0 156 ppb guangxi china 0 258 ppb guizhou china and 0 250 ppb oklahoma the discovery of global d c ir anomalies argues for an impact extinction model however nonchondritic ratios of ir to other important elements and a lack of physical evidence shocked quartz microtektites do not support such a scenario the fact that all ir abundance maxima are at sharp redox boundaries in these sections leads us to conclude that the ir anomalies likely resulted from a sudden change in paleo redox conditions during deposition and or early diagenesis 36 refs 2 figs 1 tab

pt group metal anomalies in the lower mississippian of southern oklahoma four iridium pt group elements abundance anomalies have been found within a stratigraphic span of 3 m in the lower mississippian of oklahoma in ascending order the first anomalies occur at the top of the woodford shale ir 0 25 ppb pt 48 ppb os 7 5 ppb and au 18 ppb the anomalies occur just below a redox boundary and we suspect that the enriched elements were precipitated from sea water that contacted the organic and sulfide rick black shale two more anomalies occur in the welden limestone the lower one weak and the upper one strong ir 0 42 ppb pt 50 ppb os 0 075 ppb and au 0 14 ppb the excess ir and pt also co as and ni might have been enriched from sea water by bacteria at these two horizons a 70 cm thick interval of excess heavy siderophiles occurs in the overlying caney shale the interval contains the following peak concentrations ir 0 56 ppb pt 150 ppb os 0 51 ppb co 725 ppm and ni 1450 ppm these elements vary in proportion to the al clay content and we suspect that they were carried in with detrital material from erosion of ultramafic source rocks we found no evidence of microspherules or shocked mineral grains in any of these anomaly zones

petroleum source rock identification of united kingdom atlantic margin oil fields and the western canadian oil sands using platinum palladium osmium and rhenium implications for global petroleum systems this study demonstrates that petroleum and source rocks are enriched in pt and pd to the ppb level and that the 187os 188os composition coupled with the pt pd value permits the fingerprinting of petroleum to its source oils from the united kingdom atlantic margin sourced from the upper jurassic kimmeridge clay fm as well as source rock samples have been analysed for pt and pd when the pt pd value is compared with 187os 188os calculated at the time of oil generation osg the values from both the known source and the oils are similar demonstrating that they can be used as an oil to source fingerprinting tool this inorganic petroleum fingerprinting tool is particularly important in heavily biodegraded petroleum systems where traditional fingerprinting techniques e g biomarkers are severely hampered e g the world s largest oil sand deposit the west canadian oil sands wcos this has caused the source of the wcos to be hotly debated with no present day consensus between inputs from potential source units e g exshaw and gordondale fms 187os 188os and pt pd fingerprinting of the oil sands shows that the majority of the petroleum have similar 187os 188os and pt pd values supporting the hypothesis of one principal source analysis of the potential source rocks establishes that the principal source of the oil sands to be from the jurassic gordondale fm with a minor exshaw fm input thus the combination of previously pioneered re os petroleum geochronology with 187os 188os and pt pd values of petroleum permits both a temporal and spatial understanding of petroleum systems

deforming beds viscous and plastic scales of deformation small scale field and laboratory observations of sub glacial sediment deformation suggest that failure is the predominant method of sediment deformation while large scale modelling studies using a viscous model of till deformation have been reasonably successful in predicting the geological consequences of ice sheet action this suggests that the cross over scale between viscous and plastic deformation occurs at a much larger scale than the grain grain interaction scale previously thought to be appropriate to deforming till sediment deformation and the drainage of sub glacial areas are considered at different scales and how these interact to produce failure and viscous behaviour is discussed a fluctuation length scale is proposed whose horizontal dimensions are of the order of the depth of the base of deformation 1 10 m on this length scale the glacier deforming bed system is highly variable in time as well as in space till deformation occurring at this length scale is plastic happening though failure events three other specific scales are proposed whose different properties depend on the relative spatial variability of static and seepage pressures and topography the key theoretical problem which has yet to be solved is how multiple small scale failure events combine into a viscous type flow this is discussed in the context of self organised criticality it is argued that the length scales appropriate for some drainage features are not appreciably larger than the scale for plastic failure events implying that modelling sub glacial drainage channels by balancing erosion rates against viscous closure of the channel is misconceived the outlines of an alternative model involving percolation theory are presented certain key questions which can only be settled through observations by glaciologists and glacial geologists are discussed it is suggested that the difference between lodgement tills and deformation tills reflects the rate of motion of till by deformation and that lodgement from the ice is unimportant

ice stream surface texture sticky spots waves and breathers the coupled flow of ice till and water this paper addresses the coupling of flows of ice till and water and the issue of whether such coupling provides mechanisms for meso scale kilometres to tens of kilometres variability in ice sheet flow and texture the question of whether effective pressures at the ice bed interface are statically or hydraulically controlled is examined in this paper the answer is scale dependent and has a significant effect on the relationship between ice surface and basal topography the consequences of these considerations on till flow coupled ice till flow and coupled ice till and water flow are examined an analysis of till flow kinematics and shock formation is carried out the linear stability of coupled long wavelength ice till flow is analysed and regions in parameter space where this flow is unstable with rather small rate constants are found upstream moving ice surface waves are predicted the linear stablity of coupled ice till water flow is examined where water flow is modelled using a basal flow system with effective pressure dependent properties again regions in parameter space where the system is linearly unstable are found this time with relatively rapid rate constants the water pressure exhibits breather modes these analyses assume that there is a substantial basal traction a problem with models of ice streams wholly restrained at the side is identified they seem to predict erosion rates which are unfeasibly large there appears to be sufficient variability in the ice till water system to potentially explain texture in ice stream surfaces variations in ice stream thickness of tens of metres not directly relatable to topography and waves moving upstream or downstream most importantly the ice stream bed system is shown to exhibit meso scale variability simply by coupling ice flow according to the shallow ice approximation till flow according to the hydrostatic thin till approximation and water flow according to an effective pressure dependent hydraulics

ru ir ratios at the cretaceous tertiary boundary implications for pge source and fractionation within the ejecta cloud abstract ruthenium and ir are the least mobile platinum group elements pges within the cretaceous tertiary k t boundary clay the ru ir ratio is therefore the most useful pge interelement parameter for distinguishing terrestrial and extraterrestrial contributions to the boundary clay the ru ir ratio of european marine k t sections 1 67 0 38 is statistically different from that of the north american continental sections 0 76 0 26 it is unlikely that this difference is due to secondary pge remobilization pge input to the boundary clay during multiple impacts or volcanic emissions the global difference in ru ir ratios in the boundary clay may therefore be primary the positive correlation between distance from the chicxulub impact structure yucatan mexico and ru ir ratio and the more than 1000 c difference in the condensation temperatures of these elements lead us to propose that fractionation of ru from ir during condensation from the ejecta cloud may have occurred resulting in the global difference in ru ir ratios

chaos on europa the characteristics of chaos regions on europa suggest they may be sites of melt through from below they are wide ranging in size location and age the largest are hundreds of kilometers across most are similar to conamara with a matrix reminiscent of frozen slush and often rafts of preexisting crust edges are of two types ramps perhaps the tapering of crustal thickness to zero or cliffs where rafts appear to have broken clear from the shore the small features called lenticulae generally appear to be small chaoses with textured matrix and occasional rafts and many domes may be small chaoses raised by isostatic compensation following refreezing of the crust the extent of chaoses often appears to be limited by ridge systems with the coastline parallel and set back by a distance comparable to the width of the ridge system preexisting ridges often survive as causeways or chains of rafts boundaries of chaoses are apparently not controlled by preexisting cracks consistent with formation by a thermal rather than mechanical process ridges may thicken the crust such that melt through is more likely but not always between ridge systems subsequent cracks and ridges form across preexisting chaoses ranging from fresh cases with few cracks or ridges across them with paths somewhat jagged as they meander among rafts to heavily dissected examples isolated tilted raft like blocks surrounded by densely ridged terrain may be relics of former chaotic terrain thus two fundamental resurfacing processes have alternated over europa s geological history melt through at various places and times forming chaos terrain and tectonic cracking and dilation building densely ridged and banded terrain mapping of chaos features based on morphology at 200 m shows that they correlate albeit imperfectly with dark regions in global 2 km resolution mosaics except dark regions due to ridge margins or craters extrapolating from our mapping of the 5 of europa covered by appropriate images at least 18 of the surface of europa is fresh appearing chaos an additional 4 is slightly modified chaos and much more older chaotic terrain has been overprinted by tectonic structures considerable area has been available globally to accommodate the expansion of crust that occurs along extensional ridges and bands chaos ubiquity suggests that europan geology has been dominated by the effects of having liquid water under a very thin ice shell with chaos regions being widespread indicators of occasional zero shell thickness

time specific facies and biological crises the kellwasser event interval near the frasnian famennian boundary late devonian abstract features which may be called time specific facies tsf can be linked to global bioevents or better biological crises the late devonian kellwasser kw event is a classic example of such a crisis this geologically short lived event shows recurring patterns within the critical time interval including patterns that are often minute not only do the well known lower and upper kw horizons yield specific features but also the adjacent sedimentary rocks show characteristic markers such observations were first made in facies settings of submarine rises with preferably carbonate rocks mainly cephalopod limestones but these features can now also be traced into basinal sections characterized by fine grained pelitic rocks these beds and markers are crossing traditional facies borders at exactly the same time making them perfect examples of tsf

search for impact remains at the frasnian famennian boundary in the stratotype area southern france abstract in order to detect whether the end frasnian worldwide biotic crisis is related to an extraterrestrial impact the global stratotype section of the frasnian famennian boundary and auxiliary sections within the stratotype area have been examined for impact indicators iridium ni rich spinel bearing spherules and glassy microtektites this area is particularly well suited to the search for discrete events because it exhibits biostratigraphically continuous sections of sedimentologically homogenous off shore deposits different environmental settings on oxygenated deep water seamounts such as the stratotype section at coumiac and in oxygen depleted depressions la serre section are available the latter is investigated in more detail because it is the least condensed across the boundary which is determined by the first occurrence of the typical morphotype of palmatolepis triangularis the indicator of the first famennian conodont biozone samples from the biostratigraphically defined boundary and adjacent levels failed to provide significantly high ir values and no ni rich spinel or microtektite has been recovered this is in contradiction with the results of earlier investigations carried out by h geldsetzer on the same section in contrast the values of ir concentrations that we measured are always very low or not detectable the small overabundances observed in some samples which are about two orders of magnitude lower than what is currently observed at the cretaceous tertiary boundary are probably due to the accumulation of the normal flux of cosmic dust during periods of relatively low depositional rates or to a terrestrial origin at present we have no evidence that an extraterrestrial impact occurred at the f f transition

origin and consequences of cyclic ice rafting in the northeast atlantic ocean during the past 130 000 years deep sea sediment cores recovered from the northeast atlantic ocean were examined in order to elucidate the influence of the earth s orbital parameters on major ice rafting analyses of coarse grained ice rafted debris and planktonic foraminifers revealed a strong reaction to the precession signal since 130 000 yr b p dropstone layers have been deposited each half period of a precessional cycle 11 000 1000 yr ice rafting occurs during times of winter minimum summer maximum insolation and summer minimum winter maximum insolation in the first case high summer insolation forces meltwater discharge from the ice sheets into the polar seas which subsequently enhances formation of sea ice during the winter in the second case growth of continental ice enhances iceberg production which also leads to a salinity reduction of surface seawater both situations result in a southward penetration of polar water thus the marine record of dropstones documents ice rafting not only during weichselian stades but also during cold events within interstades the regularity of ice rafting yields a useful framework to calibrate and elucidate climatic changes not only in the region of the north atlantic ocean but also in remote areas such as the pacific ocean and the antarctic

acraman impact ejecta and host shales evidence for low temperature mobilization of iridium and other platinoids geochemical investigations of a widely dispersed late proterozoic impact ejecta horizon and its host shales bunyeroo formation australia have provided strong evidence for low temperature mobilization of the platinum group elements pge including ir pge anomalies are associated with both the ejecta horizon ir up to 2 0 ppb pt up to 270 ppb and the green shales that envelop it several pge anomalies 0 073 0 45 ppb ir 3 11 314 ppb pt also occur in thin green shale beds at other stratigraphic levels within a predominantly red shale sequence average red shale background of 0 019 ppb ir and 0 89 ppb pt all green shale horizons analyzed regardless of their stratigraphic position have relatively high levels of ir and other pge the nonextraterrestrial postdepositional origin of the pge at other stratigraphic levels away from the impact ejecta horizon is indicated by their occurrence in thin permeable green beds in a predominantly red shale sequence their association with enrichments in cu v zn ni and their nonchondritic pge interelement ratios the impact ejecta horizon has a more chondritic pge geochemistry consistent with a meteoritic origin a redox precipitation model similar to that invoked for redbed cu u v deposits is proposed to explain the pge anomalies in the green shales

microtektite like impact glass associated with the frasnian famennian boundary mass extinction abstract glass spherules of probable impact origin occur in the late devonian hony section in belgium they are found at the stratigraphic level of the frasnian famennian boundary which is marked by one of the largest mass extinctions in the phanerozoic the hony glass spherules are chemically identical to those previously described from the stratigraphically equivalent senzeille section located 90 km to the west southwest in contrast to the senzeille layer the age of the hony spherule layer is well constrained by conodont biostratigraphy the spherules have morphological and chemical characteristics similar to tektites and microtektites they have a broad range of chemical composition with generally high k2o na2o and al2o3 na2o k2o their water content is in the range measured for tektites and impact glasses small inclusions of lechatelierite may be present in the spherules these glass spherules are likely to be the product of an impact event that occurred at or very near the frasnian famennian boundary the target rock was probably a clay rich pelitic sediment intermixed with carbonates the 368 1 ma siljan ring crater sweden is the most likely impact site candidate the hony and senzeille glass spherules represent the first substantial evidence that an impact event may be linked to the frasnian famennian mass extinction

binge purge oscillations of the laurentide ice sheet as a cause of the north atlantic s heinrich events ice rafted debris in sediment cores from the north atlantic suggests that the laurentide ice sheet lis periodically disgorged icebergs in brief but violent episodes which occurred approximately every 7 000 years here i propose that heinrich events i e what these episodes are called were caused by free oscillations in the flow of the laurentide ice sheet which arose because the floor of hudson bay and hudson strait is covered with soft unconsolidated sediment that forms a slippery lubricant when thawed the proposed heinrich event cycle has two phases the growth phase occurs when the sediment is frozen and the lis is stranded immobile on a rigid bed the volume of the lis slowly grows during this phase at a rate dictated by snow accumulation the purge phase occurs when the basal sediment thaws and a basally lubricated discharge pathway i e an ice stream such as those which occur in west antarctica today developes through hudson strait the volume of the lis rapidly equilibrates to the reduced basal friction during this phase by dumping icebergs into the labrador sea the periodicity t k sl2g 2 7000 years of the proposed heinrich event cycle is a function of the thermal conductivity and diffusivity of ice k and respectively the atmospheric sea level temperature sl in degrees celsius and the excess geothermal heat flux defined by g g k where is the atmospheric lapse rate and g is the geothermal heat flux agreement between the predicted t and the apparent periodicity implied by the marine record is the main virtue of the free oscillation mechanism i propose an alternative mechanism in which heinrich events are forced by periodic variations in external climate is implausible because periodic atmospheric temperature perturbations are strongly attenuated with depth in an ice sheet

realistic yet simple bottom boundary conditions for glaciers and ice sheets over a nondeformable and impermeable bed the sliding law has to be a relationship between the bottom shear stress b the sliding velocity u and the effective pressure n pi pw where pi is the ice load and pw is the pressure in the interconnected water cavities three horizontal scales are recognized 0 1 m 10 m and 10 km classical theories consider the smallest scale they are of dubious value because temperate ice is neither dry nor totally impermeable the intermediate scale yields sliding laws to be used in glacier modeling melting refreezing processes may then be ignored for modeling large cold ice sheets the sliding law at the largest scale is needed it is suggested that sliding occurs only when a bottom temperate layer exists how to tackle non newtonian rheology is discussed at the intermediate scale a microrelief model consisting of two superimposed bumpy profiles is favored if the smaller one had bumps of equal height b would be double valued but this model is unrealistic the ice avalanche at the allalingletscher in 1965 may be explained otherwise therefore a model with the smaller bumps of unequal height is adopted it yields an asymptotic sliding law at large sliding velocities b n cun1 n with and c as adjustable parameters a third one has to be introduced at low velocities although the correct interpolation function between both extreme cases remains unknown some qualitative results about kinematic waves are obtained

sulfur isotope anomaly associated with the frasnian famennian extinction medicine lake alberta canada a sharp positive 34s anomaly occurs in a 0 5 cm siltstone that contains syndepositional pyrite and was deposited below wave base as part of a basinal off reef succession at medicine lake alberta the bed rests on bioturbated oxygenated dolomitic siltstones that are positively correlated with biostromal banks containing the last frasnian shelly benthos over a wide region above laminated anoxic argillaceous lime mudstones grade upward into beds containing famennian conodonts and brachiopods geochemical and sedimentological data suggest flooding of the craton by basin derived anoxic water at the extinction horizon as a result of a sudden deep basin or ocean turnover comparable positive 34s values are found in sedimentary pyrite in frasnian black shales from a passive margin basin in the yukon to the north values decrease in the famennian

interaction and variability of ice streams under a triple valued sliding law and non newtonian rheology 1 ice streams are regions of fast flowing glacier ice that transport a significant portion of the total ice flux from present ice sheets the flow pattern of ice streams can vary both temporally and spatially in particular ice streams can become stagnant and change their path we study the dynamics of ice streams using an idealized model of an isothermal and power law viscous ice flow that includes horizontal lateral shear stresses the basal sliding law is assumed to be triple valued we investigate the spatiotemporal patterns formed because of the flow over a flat bed fed from an upstream mass source the ice flows from the mass source region through one or two gaps in a prescribed upstream topographic ridge which restricts the flow leading to the formation of one or two ice streams we find a relation between the parameters of the ice rheology and the width of the ice stream shear margins and show how these parameters can affect the minimum width of an ice stream we also find that complex asymmetric spatiotemporal patterns can result from the interaction of two ice streams sharing a common mass source the rich spatiotemporal variability is found to mostly be a result of the triple valued sliding law but non newtonian effects are found to play a significant role in setting a more realistic shear margin width and allowing for relevant time scales of the variability

iridium sulfur isotopes and rare earth elements in the cretaceous tertiary boundary clay at stevns klint denmark microbial activity and redox controlled precipitation have been of major importance in the process of metal accumulation in the strongly ir enriched cretaceous tertiary k t boundary clay the fish clay at stevns klint in denmark two important findings support this view 1 1 kerogen recovered by leaching the fish clay in hcl and hf shows an ir concentration of 1100 ppb this represents about 50 of the ir present in the bulk sample fish clay strong organometallic complexes is the most probable carrier phase for this fraction of ir kerogen separated from the k t boundary clay at caravaca spain similarly exhibits enhanced ir concentrations 2 2 sulfur isotope analyses of metal rich pyrite spherules which occur in extreme abundance about 10 by weight in the basal fish clay give a 34s value of 32 this very low value shows that sulfide formation by anaerobic bacteria was intensive in the fish clay during early diagenesis since the pyrite spherules are major carriers of elements such as ni co as sb and zn microbial activity may have played an important role for concentrating these elements in the fish clay large amounts of rare earth elements have precipitated from sea water on fish scales analyses reveal that compared with sea water the fish clay is only about four times less enriched in sea water derived lanthanides than in ir this shows that a sea water origin is plausible for elements that are strongly enriched in the clay but whose origin cannot be accounted for by a lithogenic precursor

spontaneous generation of pure ice streams via flow instability role of longitudinal shear stresses and subglacial till 1 a significant portion of the ice discharge in ice sheets is drained through ice streams with subglacial sediment till acting as a lubricant the known importance of horizontal friction in shear margins to ice stream dynamics suggests a critical role of longitudinal stresses the effects of subglacial till and longitudinal stresses on the stability of an ice sheet flow are studied by linear stability analysis of an idealized ice till model in two horizontal dimensions a power law viscous constitutive relation is used explicitly including longitudinal shear stresses the till which has compressible viscous rheology affects the ice flow through bottom friction we examine the possibility that pure ice streams develop via a spontaneous instability of ice flow we demonstrate that this model can be made intrinsically unstable for a seemingly relevant range of parameters and that the wavelengths and growth rates that correspond to the most unstable modes are in rough agreement with observed pure ice streams instabilities occur owing to basal friction and meltwater production at the ice till interface the most unstable wavelength arise because of selective dissipation of both short and long perturbation scales longitudinal stress gradients stabilize short transverse wavelengths while nye diffusion stabilizes long transverse wavelengths the selection of an intermediate unstable wavelength occurs however only for certain parameter and perturbation structure choices these results do not change qualitatively for a newtonian ice flow law indicating no significant role to shear thinning although this may very well be due to the restrictive assumptions of the model and analysis

sediment deformation beneath glaciers rheology and geological consequences experiments beneath breidamerkurjokull in iceland have led to development of flow laws for the subglacial till relating strain rate to shear stress and effective pressure and assuming either bingham fluid or nonlinearly viscous fluid behavior water pressures in the till are less than ice pressures and it is suggested that this may lead to infiltration of ice into the sediment which inhibits sliding at the ice sediment interface where water pressures are equal or near to ice pressures infiltration does not occur and sliding may result a one dimensional theory of subglacial deformation is developed in which the empirical flow law is coupled with a model of subglacial hydrology and consolidation this predicts stable states in which subglacial sediment either does not deform or a dilatant deforming horizon forms with positive effective pressures at the ice bed interface or unstable states where zero or negative effective pressures are predicted time dependent analyses show that response times following perturbations of the system may be of the order of 103 years and thus that unsteady behavior may be normal on glaciers flowing over unlithified sediment beds it is suggested that the natural variability of material properties in subglacial sediment beds leads to the development of drumlins on the glacier bed it is suggested that unstable deformation at zero or negative effective stress leads to piping in subglacial sediments at the glacier terminus and the growth of sediment floored subglacial tunnels their frequency is that which is sufficient to draw down subglacial water pressures so as to prevent unstable deformation where they discharge large water volumes subglacial sediments flow laterally toward them producing tunnel valleys this sediment is then removed by water flowing along the axial tunnel tunnel valleys can be regarded as the equivalent in soft sediment areas of eskers in bedrock areas

the frasnian famennian mid late devonian boundary in the type section of the long rapids formation james bay lowlands northern ontario canada utilizing a combination of conodont based biostratigraphy and geochemistry the frasnian famennian f f mid late devonian boundary was located in the long rapids formation northern ontario it is present in a black shale sequence 21 46 m above the base of the long rapids formation just above a carbonate bed that has similarities to the upper kellwasser limestone recognized as a lithological marker for the f f boundary in germany belgium and france there is no evidence of an impact event and sedimentation across the boundary is continuous although there is a discontinuity layer just below the boundary and the early part of the linguiformis zone of the standard late devonian conodont zonation is missing strong positive 13 c organic and carbonate values were identified and are interpreted as evidence of mass mortality in anoxic waters at the f f extinction positive 18 o values suggest the extinction was also accompanied by a temperature drop negative 34 s values also indicate anoxia and negative 15 n values provide evidence of reduced surface water productivity no iridium anomaly was found at the boundary although higher than usual values were found about 85 cm below the f f boundary sedimentological and biological evidence in the long rapids formation indicates that the f f extinction was coincident with major sea level fluctuations in the late frasnian including a transgression beginning at the end of the linguiformis zone and continuing through the early triangularis zone and beyond marine oscillations were accompanied by anoxia which appears to be the killing mechanism for the conodont and other end frasnian extinctions

anoxic events in the late frasnian causes of the frasnian famennian faunal crisis carbon isotope data collected from five frasnian famennian boundary sections in central europe show two positive 13 c excursions in the late frasnian 367 ma both anomalies coincided with the deposition of the bituminous lower and upper kellwasser horizons the carbon isotope patterns indicate two phases of enhanced burial and subsequent recycling of organic carbon a maximum formation of warm saline waters on the subtropical to tropical epicontinental shelves during late frasnian transgressive episodes may have induced oceanic oxygen deficits variations in the c org burial rates may have resulted in changes in the co 2 concentrations in the oceans and in the atmosphere culminating in global climatic changes we propose that the repeated co occurrences of sea level fluctuations anoxic conditions and global climatic changes during the late frasnian would affect especially the subtropical to tropical shallow water communities and reef ecosystems which were severely affected during the frasnian famennian faunal crisis

platinum group element enrichments and possible chondritic ru ir across the frasnian famennian boundary western new york state the frasnian famennian boundary is recognized as the culmination of a global mass extinction in the late devonian in western new york state the boundary is a distinct horizon within a pyritic black shale bed of the upper hanover shale defined by the first occurrence of palmatolepis triangularis in the absence of frasnian conodonts the boundary is characterized by a minor disconformity marked by a lag concentration of conodonts iridium at the boundary is 0 11 0 24 ng g two to five times background levels of 0 05 ng g other ir enrichments of 0 38 ng g and 0 49 ng g occur within 50 cm of the conodont constrained boundary numerous ir enrichments in the boundary interval suggest extraterrestrial accretion and platinum group element pge concentration at disconformities or mobilization and concentration in organic rich pyritic rich laminations from cosmic or terrestrial sources pge ratios of pt pd and ru ir at the boundary horizon approximate chondritic ratios and are suggestive of an unaltered extraterrestrial source these values do not conclusively establish a single extraterrestrial impact as the ultimate cause of the frasnian famennian mass extinction especially given the presence of similar ir enrichments elsewhere in the section and the absence at the boundary of microtektites and shocked mineral grains

post depositional mobility of platinum iridium and rhenium in marine sediments the discovery of high concentrations of iridium in cretaceous tertiary boundary sediments engendered the hypothesis1 that a meteorite collided with the earth 65 million years ago coincident with the mass extinction that occurred at that time iridium spikes of various magnitudes have subsequently been reported at more than 10 other extinction horizons2 11 it has been suggested on the other hand that geochemical processes might create or modify many of these spikes5 11 4 but a critical evaluation of these suggestions has been hindered by incomplete understanding of low temperature iridium geochemistry other platinum group elements ru rh pd re os pt au are often found to be associated with ir spikes and inter element ratios have been used to assess the cosmic or terrestrial nature of the enrichments5 7 9 28 29 but the geochemical influences on these relative abundances are also poorly constrained here we describe analyses of recent abyssal marine sediments which allow us to characterize the behaviour of pt re and ir during early diagenesis these elements are redistributed by changes in sedimentary redox conditions such changes can probably account for many of the small platinum group element spikes found in the geological record and may render ambiguous attempts to interpret inter element ratios

glacial sedimentary evidence supporting stick slip basal ice flow last glacial late devensian subglacial sediments in the southern sperrin mountains north central ireland comprise tabular bedrock rafts which are interbedded with diamicton till and brecciated bedrock and separated by glaciotectonic shears bedrock rafts and diamicton beds alternate laterally and vertically in the profile suggesting the ice bed interface was chequered with both high strength rock rafts and low strength diamicton patches during accumulation of the sediment pile the overall facies arrangement is interpreted qualitatively as reflecting stick slip basal ice flow and is related to the redistribution of free subglacial meltwater at the ice bed interface it is inferred that stick phases occur over bedrock rafts as meltwater migrates to lower pressure regions infilling shallow cavities on the surrounding diamicton surface slip occurs when porewater pressure increases and the meltwater layer backfills across bedrock raft surfaces causing ice bed uncoupling the presented field evidence suggests a stick slip rather than pervasive subglacial deformation model may be a better approximation for the flow of parts of the late devensian mid latitude ice sheets

trace element patterns at a non marine cretaceous tertiary boundary at the fossil pollen defined cretaceous tertiary boundary in the raton basin of new mexico and colorado an iridium abundance anomaly and excess scandium titanium and chromium are associated with a thin ash or dust fallout bed now kaolinitic clay that was preserved in freshwater coal swamps

the importance of pressurized subglacial water in separation and sliding at the glacier bed l effet de l eau sous glaciaire sous pression sur le processus de glissement est examine a partir d un parametre appele indice de decollement du lit qui exprime les contraintes perpendiculaires au lit et la pression d eau les donnees proviennent de trois glaciers de formes differentes une loi puissance du type de weertman avec un exposant voisin de 3 modifiee pour tenir compte de l effet de la pression des eaux sous glaciaires apparait comme etant la meilleure

flow thermal structure and subglacial conditions of a surge type glacier temperature measurements in a subpolar surge type glacier reveal a distinctive thermal structure associated with the boundary between the ice reservoir and receiving areas in the receiving area the glacier is cold based but bottom temperature has increased as much as 0 5 c between 1981 and 1982 and the basal heat flux is roughly 10 times the expected geothermal flux water percolation through permeable subglacial material is the probable energy source deformation of the substrate could destroy this drainage system and trigger a surge

event stratigraphic markers within the kellwasser crisis near the frasnian famennian boundary upper devonian in germany abstract short time intervals of anomalous conditions often referred to as global events yield excellent tools for detailed stratigraphic correlations in addition to pronounced effects minor features may be applied for fine scale comparisons of rock sequences these include alternations within a given portion of rock or distinct layers at exactly the same position such marker horizons are often traceable over long distances recognition of these specific markers allows for a more detailed search for possible causes of anomalous events

sliding with cavity formation we present a model for the determination of a sliding law in the presence of sub glacial cavitation this law determines the basal stress at a clean ice bedrock interf ace in terms of the velocity and effective pressure the method is based on an exact solut ion of the nye kamb linearly viscous sliding problem with cavities and uses ideas of lliboutry 979 to construct via re norma lization methods an approximate law for gene ral bedrock form we show that for a bedrock whose spectrum has a power law behaviour one obtains a sliding law which gives the basal shear stress proportional to a power of the velocity and to a power of the effective pressure the effect of subgl acial cavitation on the drainage system is exam ined using recent ideas of kamb for sufficiently high velocities drainage through a r6thlisberger tunnel system is unstable and drai nage takes place through the linked system of cavities this leads to a reduction of the effective pressure and by taking account of this one can rewrite the sliding law in terms of st ress and velocity only this sliding law can be multi v alued and it is suggested that this unde rlies the dynamic phenomenon of surges

frasnian famennian mass extinction and cold water oceans suturing of laurussia and africa elimination of easterly tropical currents and consequent equatorial diversion of cold and possibly dysaerobic waters are suggested as prime causes of the frasnian famennian late devonian extinction these are explained by using a new paleogeographic model evidence for stepdown extinctions that took place from givetian through frasnian time is taken particularly from atrypoid brachiopods one of the most abundant representatives of the global tropical shelly marine benthos during the middle and late devonian only 5 of 17 frasnian genera or subgenera of atrypoids are known to be present in late frasnian sediments and possibly only a single genus spinatrypa was present at the frasnian famennian boundary evidence for stepdown extinction from other invertebrate and vertebrate groups in general corroborates that provided by brachiopods

patterns of ice rafted detritus in the glacial north atlantic 40 55 n the observation by heinrich 1988 that during the last glacial period much of the input of ice rafted detritus to the north atlantic sediments may have occurred as a succession of catastrophic events rekindled interest on the history of the northern ice sheets over the last glacial period in this paper we present a rapid method to study the distribution of these events both in space and time using whole core low field magnetic susceptibility we report on approximately 20 cores covering the last 150 to 250 kyr well defined patterns of ice rafted detritus appear during periods of large continental ice sheet extent although these are not always associated within their maxima most of the events may be traced across the north atlantic ocean for the six most recent heinrich layers hl two distinct patterns exist hl1 hl2 hl4 hl5 are distributed along the northern boundary of the glacial polar front over most of the north atlantic between 40 and 50 n hl3 is more restricted to the central and eastern part of the northern atlantic the nd sr isotopic composition of the material constituting different heinrich events indicates the different provenance of the two patterns hl3 has a typical scandinavia arctic icelandic young crust signature and the others have a large component of northern quebec and northern west greenland old crust material these isotopic results obtained on core su 9008 from the north american basin are in agreement with the study by jantschik and huon 1992 who used k ar dating of silt and clay size fractions of an eastern basin core me 68 89 these data confirm the large spatial scale of these events and the enormous amount of ice rafted detritus they represent

stress balances of ice streams in a vertically integrated higher order formulation one challenge in improving our understanding of ice stream dynamics is to develop models of the spatial and temporal transition from ice sheet to ice stream flow we address this with a new vertically integrated higher order formulation for ice sheet dynamics that captures the leading order physics of low aspect ratio viscous fluid flow regardless of the amount of slip at the bed the theory introduces a parameter which approximates the ratio of the basal stress to the shear stress scale providing a measure of the relative importance of sliding and internal deformation our model is able to simultaneously describe the dynamics of both a slow moving sheet and rapidly flowing ice streams to test the formulation we apply a triple valued sliding law as the basal boundary condition and obtain numerical solutions that can be compared with previous work we investigate the sensitivity of flow regimes and shear margin width to parameter variation deriving a scaling for the latter we also consider a double valued sliding law which enforces a constant low basal stress beneath the ice stream comparisons of the resultant stress fields illustrate the different stress balances that can maintain

thermally controlled glacier surging bakaninbreen in svalbard and trapridge glacier in yukon territory canada are two prominent examples of surging glaciers which are thought to be controlled by their thermal regime both glaciers have developed large bulges which have propagated forward as travelling wave fronts and which are thought to divide relatively stagnant downstream cold based ice from faster moving warm based upstream ice additionally both glaciers are underlain by a wet metres thick layer of deforming till we develop a simple model for the cyclic surging behaviour of these glaciers which interrelates the motion of the ice and till through a description of the subglacial hydrology we find that oscillations surges can occur if the subglacial hydrological transmissivity is sufficiently low and the till layer is sufficiently thin and we suggest that these oscillations are associated with the development and propagation of a travelling wave front down the glacier we therefore interpret the travelling wave fronts on both trapridge glacier and bakaninbreen as manifestations of surges in addition we find that the violence of the surge in the model is associated with the resistance to ice flow offered by undulations in the bed and the efficiency with which occasional hydrological events can release water accumulated at the glacier sole

osmium isotopic evidence against an impact at the frasnian famennian boundary two sections across the frasnian famennian boundary were analyzed for re and os concentrations and 187os 188os ratios to evaluate evidence for a meteoritic input coincident with this boundary and its associated mass extinction these sections are from a siltstone and shale sequence at irish gulf in new york us and a calcareous shale and ferromanganese oxide sequence at la serre in france the irish gulf section with an initial 187os 188os of about 0 49 does not show the characteristic meteoritic os imprint with an 187os 188os of about 0 13 the integrity of the section with regard to re and os retention is indicated by the construction of an isochron with an age of 388 41 ma consistent with independently determined ages for the frasnian famennian boundary although the la serre section with os concentrations as high as 33 ppb and re concentrations ranging from 1 4 to 7 4 ppb might be expected to show excellent evidence for a meteoritic contribution the highly radiogenic isotopic composition 187os 188os 2 42 3 61 instead suggests recent massive re loss or addition of radiogenic os these possibilities prevent the reconstruction of an initial 187os 188os value for the boundary for this site assuming reasonable re concentrations prior to loss the os isotopic composition is inconsistent with a large meteoritic component this inconsistency emphasizes that platinum group element pge concentrations are not reliable indicators of extraterrestrial addition

a continuum mixture model of ice stream thermomechanics in the laurentide ice sheet 2 application to the hudson strait ice stream episodic exportations of ice rafted debris to the north atlantic in the late pleistocene suggest quasiperiodic ice streaming or surging activity on the northeastern margin of the laurentide ice sheet much of this efflux of ice may originate from an ice stream issuing from hudson strait and tapping into core regions of the laurentide in hudson bay labrador and the foxe basin applying the continuum mixture theory outlined by marshall and clarke this issue we model the thermomechanical evolution of the hudson strait ice stream in a three dimensional finite difference model of the laurentide ice sheet our simulations focus on internal dynamics of the ice stream under thermal regulation of basal flow we find surge cycles of stream activity interspersed with quiescent periods where the ice stream is frozen to the bed modeled surge durations vary from 105 to 3260 years while surge periodicities range from 585 to 22 410 years with pervasively warm or cold internal temperature distributions in the ice ice streams can also establish modes of permanent activity or inactivity under thermal regulation our most vigorous ice streams produce peak values of approximately 0 03 sv of freshwater flux to the north atlantic from basal meltwater and iceberg production associated ice stream velocities in this maximum case approach 6700 m yr 1 the total ice volume mobilized in a single surge event is equivalent to a global sea level rise of 0 04m in the most tranquil surge and almost 0 6m in the most extreme case these velocities and sea level impacts are an order of magnitude less than those predicted by macayeal 1993a b and only our most exuberant streams approach the iceberg flux estimates of dowdeswell et al 1995 we propose that the sediment load of icebergs emanating from hudson strait in a surge event may exceed expectations from contemporary icebergs

late devonian kellwasser event mass extinction horizon in germany no geochemical evidence for a large body impact the hypothesis that the late devonian frasnian famennian mass extinction was triggered by an asteroidal impact has received renewed attention with the discovery of a late devonian ir anomaly in australia in europe the mass extinction event corresponds stratigraphically to the geographically widespread kellwasser black shale and bituminous limestone units and the biological crisis itself has been alternatively designated the kellwasser event the authors report here the results of an extensive geochemical analysis of the kellwasser stratigraphic interval in a section with exceptional conodont zonal control in the federal republic of germany no ir anomaly was found neither at the biological crisis horizon recognized in europe nor at the conodont horizon that corresponds to the ir anomaly zone reported in australia no shock metamorphosed quartz sanidine spherules or siderophile rich magnetic spherules were found which might have been indicative of a cometary impact oxygen isotope ratios show little variation across the mass extinction horizon though carbon isotope data suggest a sudden increase in phytoplankton activity they further note that the australian ir anomaly 1 is most likely not associated with a large body impact because no equivalent ir signature occurs in europe and 2 is stratigraphically above the european biological crisis horizon thus postdating the kellwasser mass extinction event

laser argon dating of melt breccias from the siljan impact structure sweden implications for a possible relationship to late devonian extinction events in earlier studies the 6575 km diameter siljan impact structure in sweden has been linked to the late devonian mass extinction event the siljan impact event has previously been dated by kar and ar ar chronology at 342368 ma with the commonly quoted age being 362 7 2 2 ma 2 recalculated using currently accepted decay constants until recently the accepted age for the frasnian famennian boundary and associated extinction event was 364 ma which is within error limits of this earlier siljan age here we report new ar ar ages extracted by laser spot and laser step heating techniques for several melt breccia samples from siljan interpreted to be impact melt breccia the analytical results show some scatter which is greater in samples with more extensive alteration these samples generally yield younger ages the two samples with the least alteration yield the most reproducible weighted mean ages one yielded a laser spot age of 377 2 2 5 ma 95 confidence limits and the other yielded both a laser spot age of 376 1 2 8 ma 95 confidence limits and a laser stepped heating plateau age over 70 6 39ar release of 377 5 2 4 ma 2 our conservative estimate for the age of siljan is 377 2 ma 95 confidence limits which is significantly different from both the previously accepted age for the frasnian famennian f f boundary and the previously quoted age of siljan however the age of the f f boundary has recently been revised to 374 5 2 6 ma by the international commission for stratigraphy which is within error the same as our new age however the currently available age data are not proof that there was a connection between the siljan impact event and the f f boundary extinction this new result highlights the dual problems of dating meteorite impacts where fine grained melt rocks are often all that can be isotopically dated and constraining the absolute age of biostratigraphic boundaries which can only be constrained by age extrapolation further work is required to develop and improve the terrestrial impact age record and test whether or not the terrestrial impact flux increased significantly at certain times perhaps resulting in major extinction events in earths biostratigraphic record

a model computation of moisture content in polythermal glaciers measurements in boreholes and internal radar horizons confirm the existence of polythermal structures in polar and subpolar glaciers and ice sheets the occurrence of an intraglacial phase boundary makes it necessary to consider two layers and transition conditions between cold and temperate ice the governing field equations and constitutive relations for a two layer model are stated with emphasis on boundary and transition conditions a reduced plane flow model is introduced by using dimensionless variables and performing a scale analysis according to the stated aim of this paper a parallel sided slab model is suitable for rough analyses of borehole measurements its governing equations are deduced and their analytical and numerical treatment is discussed sensitivity tests reveal a strong dependence of moisture content profiles on the numerical value of the water diffusivity coefficient and especially on strain heating

detrital carbonate rich sediments northwestern labrador sea implications for ice sheet dynamics and iceberg rafting heinrich events in the north atlantic much of the bed of the central and eastern sectors of the laurentide ice sheet was underlain by paleozoic carbonates we propose that pulses of detrital carbonate rich sediments in two cores from the northwestern labrador sea reflect episodes when an ice stream from the hudson strait extended to the shelf break and delivered sediment onto the slope and deep sea plain atomic mass spectroscopy 14 c dating of planktonic foraminifera in cores hu75 009 iv 55 2 4 km water depth and hu87 033 009 1 4 km water depth and 150 km north northwest indicates that the youngest event occurred between ca 14 and 15 ka and another occurred between 19 and 21 ka the two carbonate intervals in northwestern labrador sea cores are coeval with heinrich events 1 and 2 in the eastern north atlantic lat 45 50 n long 20 w where they are associated with an increase in lithic fragments and a drastic reduction in the numbers of foraminifera these changes have been linked with the massive production of icebergs associated with ice sheet surges our evidence indicates that heinrich events 1 and 2 are associated with the dynamics of the hudson strait ice stream and denote considerable glaciological instability

carbon and sulfur isotope anomalies across the frasnian famennian extinction boundary alberta canada stable isotopes of carbonate and organic carbon from two upper devonian sections in alberta canada indicate a heavy carbon event across the frasnian famennian f f boundary and a negative 13 c shift at the boundary at cinquefoil mountain 13 c values in carbonate are anomalously high 3 5 to 4 5 pdb peedee belemnite within an interval 10 m below to 23 m above the f f boundary representing about 1 m y two standard conodont zones compared to background values of 1 above and below this interval superimposed on this positive excursion is a marked 13 c decrease of about 1 at precisely the f f boundary at medicine lake a similar 13 c excursion in organic carbon from a background of 29 to a high of 26 5 is recorded across the f f boundary the 34 s data for iron sulfide in these sections show unusually high values as much as 15 to 20 cdt canon diablo troilite in the f f boundary interval these data together with those from other areas indicate a period of major global changes e g greenhouse and marine anoxia that took place well before and after the sudden biomass killing event at the f f boundary existing evidence suggests that the f f mass extinction was probably caused by a large extraterrestrial impact on earth at a time when the global ecosystem was already severely stressed

a theory of glacier surges we propose a model of glacier flow that is capable of explaining temperate glacier surges the laws of conservation of mass and momentum are supplemented by the prescription of a sliding law that gives the basal shear stress as a function of the basal velocity u and the effective pressure n the effective drainage pressure n is determined by a simple study of the subglacial hydraulic system following rothlisberger we determine n nr for the case of drainage through a single subglacial tunnel alternatively following kamb we find that the corresponding theory for a linked cavity drainage system yields n nk nr furthermore the stability of each drainage system depends on the velocity u such that for large enough u there is a transition from tunnel to cavity drainage consequently one can write n n u we then find that the sliding law u is multivalued and hence so also is the flux depth relation q q h an analysis of the resulting system of equations is sketched for large enough accumulation rates a glacier will undergo regular relaxation oscillations resembling a surge the surge is triggered at the point of maximum stress from this point two hydraulic transition fronts travel up and down glacier to calculable boundary points the speed of propagation is the order of 50 metres an hour at these fronts the tunnel drainage system collapses and a high water pressure cavity drainage system is installed this activated zone has high velocities and quickly relaxes surges to a quasi equilibrium state this relaxation is much like opening a sluice gate in that a large wave front propagates forward behind this wave front the velocity can decay oscillatorily and thus the flow can be compressive we conclude with some discussion of the effects of seasonal variation and of prospects for the current theory s applicability to soft bedded glaciers

late devonian rugose corals and the frasnian famennian crsis all well dated occurrences of upper devonian septate corals subclass rugosa are analyzed at species and higher taxonomic levels platform dwelling corals are differentiated from the less numerous basin dwelling corals through early and middle frasnian time evolution outpaced the extinction of rugose corals so that late frasnian faunas were more diverse than the earliest frasnian faunas this gradual increase of genera was terminated abruptly by an almost complete extinction of late frasnian platform dwelling rugosa in uppermost gigas to lower palmatolepis triangularis zone time none of the 151 species and probably as few as two or three at most five of the 47 genera of late frasnian shallow water corals survived this event the loss of coral biomass was equally devasting very few early famennian platform dwelling corals are known although platform carbonates of this age were deposited in several regions platform faunas comprising 66 species and 27 genera reappeared in late famennian time the

flow variability in the scandinavian ice sheet modelling the coupling between ice sheet flow and hydrology abstract there is increasing geologic evidence for periodic flow variability within large ice sheets manifested as spatially and temporally variable areas of fast ice flow and resulting in the very complex patterns of lineations observed in formerly glaciated areas however many ice sheet models do not replicate this behaviour a possible reason for this is that such models do not include a detailed treatment of basal hydrology changes in the character of sub glacial drainage systems are believed to cause surges in valley glaciers recent ice sheet models which have included basal hydrology or at least a link between basal velocity and the presence of water at the bed often show flow variability however these models have typically assumed a deformable bed or have made no assumptions about the nature of the bed whilst these assumptions seem applicable to areas close to the former margins of quaternary ice sheets they are less applicable to interior areas these areas typically show thin or scanty till cover over eroded bedrock and the presence of eskers which are indicative of drainage in sub glacial tunnels we have developed a two dimensional time dependent ice sheet model that includes hard bed basal hydrology this allows calculation of sub glacial water pressures and the use of a water pressure dependent sliding law to calculate ice sheet velocities when used to simulate the weichselian scandinavian ice sheet with late quaternary climate and sea level as forcing functions this model develops localised areas of fast flowing ice which vary in extent and in distance of penetration into the interior of the ice sheet both spatially and temporally the behaviour of these lobes depends crucially on the influence of the evolving ice sheet topography on the routing of subglacial water flow due to the resulting variations in the subglacial hydraulic potential which drive the water flow bedrock topography also has some influence but fast flow areas are not confined to obvious topographic troughs a relatively thin ice sheet with low surface slopes is produced in areas experiencing fast ice flow generally two to four separate areas of fast flow can be recognised and these are similar in size and shape to the lobes identified in some geologically based reconstructions of the scandinavian ice sheet within the fast flowing areas sub glacial drainage is typically in a cavity based system however tunnel based drainage is predicted to have extended up to 150 km from the ice sheet margin particularly during deglaciation because of the changes in ice sheet topography associated with fast flow and the resulting changes in the pattern of sub glacial water flow the model predicts that these fast flowing lobes interacted in complex ways and exhibited quasi periodic switching between fast and slow flow

high geothermal heat flow basal melt and the origin of rapid ice flow in central greenland age depth relations from internal layering reveal a large region of rapid basal melting in greenland melt is localized at the onset of rapid ice flow in the large ice stream that drains north off the summit dome and other areas in the northeast quadrant of the ice sheet locally high melt rates indicate geothermal fluxes 15 to 30 times continental background the southern limit of melt coincides with magnetic anomalies and topography that suggest a volcanic origin

no geochemical evidence for an asteroidal impact at late devonian mass extinction horizon one way to test the proposed causal relationship between mass biotic extinction and asteroidal impact is to search for the geochemical signature of such an event at additional mass extinction horizons other than the cretaceous tertiary k t boundary we report here the results of an extensive search for an indium ir anomaly across the late devonian frasnian famennian f f extinction horizon 367 myr bp over a decade ago mclaren1 proposed that this extinction was caused by an astrophysical catastrophe we have examined three sedimentary sequences in new york state the standard reference section for eastern north america and one in belgium the type locality which cross the f f boundary no ir anomaly was found in any of the analysed stratigraphie sections the highest values of ir detected ranged from only 0 2 to 2 of those reported for the marine and terrestrial ir analyses at the k t boundary further devonian pyrite rich sediments similar to those at the k t boundary do not contain high ir concentrations

a forest fire and soil erosion event during the late devonian mass extinction abstract the late devonian mass extinction occurred in a stepwise manner and culminated close to the frasnian famennian f f boundary 372 million years ago organic molecular indices from marine sedimentary rocks at the sinsin section belgium indicate that the sequence of combustion of land vegetation soil erosion and anoxia euxinia occurred close to this boundary the increased concentrations of biomarkers indicating forest fire and soil erosion measured in the sinsin section suggest that fire became widespread at this time leading to various damaging consequences increased runoff and oceanic anoxia that caused marine extinctions magnetic susceptibility data in the sinsin section indicate a relatively dry climate spanning the f f boundary which would have encouraged forest fires the study of organic biomarkers presents several lines of evidence to link forest fire and soil erosion to the late devonian mass extinction

origin of the middle pleistocene transition by ice sheet erosion of regolith the transition in the middle pleistocene 0 9 ma seen in 18o deep sea core records from relatively low amplitude high frequency 41 kyr to high amplitude low frequency 100 kyr ice volume variations under essentially the same orbital forcing can be attributed to a change from an all soft bedded to a mixed hard soft bedded laurentide ice sheet through glacial erosion of a thick regolith and resulting exposure of unweathered crystalline bedrock a one dimensional ice sheet and bedrock model which includes transport of sediment and ice by subglacial sediment deformation demonstrates that a widespread deforming sediment layer maintains thin ice sheets before the transition which respond linearly to the dominant 23 and 41 kyr orbital forcing progressive removal of the sediment layer eventually causes a transition to thicker ice sheets whose dominant timescale of change 100 kyr reflects nonlinear deglaciation processes in model simulations over the last 3 ma initialized with no ice and a uniform 50 m sediment layer the time series of ice volume and extent agree in several important aspects with the observed records

basinward nitrogen limitation demonstrates role of terrestrial nitrogen and redox control of δ15n in a late devonian black shale the depleted nitrogen isotope values observed in devonian black shales have been ascribed to the dominant role of marine diazotrophic n fixation in providing new reactive n for the productivity that generated the organic matter preserved in these organic rich facies however the emergence of substantial terrestrial ecosystems in the middle to late devonian introduced an additional source of reactive n to shallow epicontinental seas we examined three sites along a deepening transect of the late devonian appalachian basin eastern united states employing element ratios of c n and p degree of pyritization and stable isotope analyses of c n and s in order to determine whether basinward changes in n biogeochemistry and 15 n reflect the dominant source of new reactive n high degrees of pyritization and depleted 34 s indicate a persistently dysoxic to euxinic water column sediment n p rises basinward indicating that p was relatively more abundant than n in the water column with increasing distance from the shore nitrogen isotope values however are very consistently 0 at all three sites despite the evidence of distally increasing n limitation suggesting that terrestrially derived reactive n was an important source of reactive n and that marine n fixation was not sufficient to address the stoichiometric deficit depleted 15 n values in these black shales do not provide a reliable record of diazotrophy rather they are a product of the reducing state of the water column and underlying sediments

a general theory of glacier surges we present the first general theory of glacier surging that includes both temperate and polythermal glacier surges based on coupled mass and enthalpy budgets enthalpy in the form of thermal energy and water is gained at the glacier bed from geothermal heating plus frictional heating expenditure of potential energy as a consequence of ice flow enthalpy losses occur by conduction and loss of meltwater from the system because enthalpy directly impacts flow speeds mass and enthalpy budgets must simultaneously balance if a glacier is to maintain a steady flow if not glaciers undergo out of phase mass and enthalpy cycles manifest as quiescent and surge phases we illustrate the theory using a lumped element model which parameterizes key thermodynamic and hydrological processes including surface to bed drainage and distributed and channelized drainage systems model output exhibits many of the observed characteristics of polythermal and temperate glacier surges including the association of surging behaviour with particular combinations of climate precipitation temperature geometry length slope and bed properties hydraulic conductivity enthalpy balance theory explains a broad spectrum of observed surging behaviour in a single framework and offers an answer to the wider question of why the majority of glaciers do not surge

the frasnian famennian boundary in central and eastern united states abstract the frasnian famennian f f boundary in the northern appalachian basin is found in the hanover shale and equivalent strata where it is locally associated with a black shale interval equivalent to the upper kellwasser interval in europe the boundary is immediately below the dunkirk huron black shale that represents the iie t r cycle transgression in the southern appalachian basin the southern edge of the cincinnati arch and the southeastern illinois basin the f f boundary is at or near the base of the gassaway member of the chattanooga shale or the morgan trail member of the new albany shale or was eroded during the iie t r cycle beginning in the middle triangularis zone in the western illinois basin grassy creek shale michigan basin lachine member of antrim shale and southern continental margin woodford shale the f f boundary is within black shale strata indicative of an earlier development of the iie t r transgression inboard of the appalachian basin or the juxtaposition of the iie t r transgression over the upper kellwasser black shale interval without significant or recognizable lithologic change

trace element anomalies at the mississippian pennsylvanian boundary in oklahoma and texas trace element abundance anomalies have been found at the mississippian pennsylvania boundary at sites in oklahoma and texas where the boundary has been precisely located on the basis of an abrupt change in conodont diversity and species composition enriched elements include osmium indium platinum chromium most chalcophiles rare earths and uranium the anomalies are more intense e g os 4 ppb ir 0 38 ppb pt 6 ppb cr 12000 ppm u 380 ppm and peisist through a thicker interval at the south central texas locality than in oklahoma and in bolh locations the anomalies are associated with an increase in phosphate content of the rocks there is no tangible evidence of an asteroid or comet impact source for the excess pt group elements and fauna crisis the cause of the elemental enrichments and the biological disturbance may possibly be related to a change in the ocean chemistry of the paleozoic seaway such as increased upwelling stagnation or nearby submarine volcanism

a theory of ice sheet surges a simplified model of a two dimensional ice sheet is described it includes basal ice sliding dependent on the basal water pressure which itself is described by a simple theory of basal drainage we show that this simple but sophisticated model predicts surges of the ice mass in realistic circumstances and we describe these surges by solving the problem numerically we also are able to describe some parts of the surge analytically the numerical solution of the model is a delicate matter and highlights pitfalls to be avoided if more complicated models are to be solved successfully

unstable behavior of the laurentide ice sheet over deforming sediment and its implications for climate change geologic records of fluctuations of the laurentide ice sheet margin following the most recent glacial maximum ca 20 000 14c yr b p identify fundamental differences in ice sheet behavior depending on subglacial bed conditions rapid and irregular icemargin fluctuations occurred only over areas of deforming sediment indicating nonclimatic forcing controlled by the inherent instability of coupled ice sheet deforming sediment dynamics in contrast largely uninterrupted ice margin retreat with no evidence of significant readvance occurred over rigid bed areas indicating stable behavior unstable ice sheet behavior was most pronounced from 15 000 until 10 000 14c yr b p by which time most of the ice margin had retreated onto a rigid bed unstable ice sheet behavior would have been an integral component in controlling variable fluxes of icebergs and meltwater as well as meltwater routing to the north atlantic thus affecting thermohaline circulation the abrupt climate oscillations in the north atlantic region that ended at 10 000 14c yr b p may thus have their origin in the inherently unstable behavior of the laurentide ice sheet overriding deforming sediment

the frasnian famennian extinction event in a stable cratonic shelf setting trout river northwest territories canada abstract the frasnian famennian f f extinction event occurs in platformal carbonates along the trout river n w t paleogeographically the site was about 150 km east of the western cratonic margin faunal evidence based primarily on corals and sedimentation rates of passive cratonic margins suggest that the late frasnian redknife and kakisa formations include both the upper rhenana and the linguiformis conodont zones the high biodiversity of the benthic fauna during the late frasnian decreases sharply at the f f boundary and recovers only gradually during the early famennian this trend is also expressed by the profile of 13c values microfacies analysis has identified two minor hiatuses at the f f boundary the lower break is represented by an unconformity at the upper contact of the latest frasnian kakisa formation and is associated with surficial karstification and brecciation of the underlying biostromal carbonates anomalous trace element concentrations at the unconformity are interpreted as possible fallout material related to a bolide impact this hiatus probably corresponds to the lower triangularis zone because no diagnostic conodonts of that time interval are present the lower and upper hiatuses are separated by the deposition of a wackestone sandstone facies with an impoverished fauna of sponges calcareous algae and some foraminifera preserved only in neptunian dykes in the upper kakisa and as fragments in the overlying famennian trout river formation the depauperate fauna suggests hostile conditions and or a habitat ravaged by a catastrophic event common characteristics after a global extinction event conodont evidence suggests that this brief depositional interval corresponds to the middle triangularis zone the upper hiatus probably straddles the contact of the middle and upper triangularis zones evidence for this break are angular fragments of the wackestone sandstone facies in the basal trout river formation conodonts from the basal siliciclastics of the trout river formation assign the lower part of this unit to the upper triangularis zone in alberta 900 km to the south similar platformal carbonates show a longer hiatus the entire triangularis zone at the f f contact whereas slope sediments indicate continuous sedimentation with a sharp drop in biodiversity and a temporary proliferation of primitive organisms at the f f boundary

the frasnian famennian biotic crisis how many if any bolide impacts the prime causation of the mid late devonian mass extinction near the frasnian famennian f f boundary remains uncertain nevertheless geochemical evidence has been presented recently as decisive evidence of a giant bolide impact occurring precisely at the f f boundary which promoted the global mortality episode palaeobiological data however imply a gradual global change which is otherwise seen as a record of either multiple extraterrestrial catastrophes or of impact triggered earth bound mechanisms sedimentological mega tsunami physical craters microtektites and geochemical records remain either elusive in many aspects or incompatible with the predicted impact crisis pattern biotic succession across the f f horizon is still poorly known especially in continental domains to evidence a synchronous bedding plane killing event at the close of the crisis instead the commonly documented stepwise loss of biomass and an unproved distinctive dead zone are hard to explain simply as sampling artifacts the assumed mass mortality precisely at the f f boundary may be limited mainly to the pelagic realm the underestimated role of early variscan tectonism and associated volcanic hydrothermal processes resulting in thermal and nutrient pulses as possible prime controls of the f f crisis is suggested as well as resemblances to the superplume conditioned eventful mid cretaceous interval exemplified in the cenomanian turonian mass extinction additional shocks generated by minor cometary strikes are not excluded but may have affected some f f biotas or areas

a simple holistic hypothesis for the self destruction of ice sheets abstract ice sheets are the only components of earth s climate system that can self destruct this paper presents the quantitative force balance for bottom up modeling of ice sheets as first presented qualitatively in this journal as a way to quantify ice bed uncoupling leading to self destruction of ice sheets hughes 2009a rapid changes in sea level and climate can result if a large ice sheet self destructs quickly as did the former laurentide ice sheet of north america between 8100 and 7900 bp thereby terminating the last cycle of quaternary glaciation ice streams discharge up to 90 percent of ice from past and present ice sheets a hypothesis is presented in which self destruction of an ice sheet begins when ubiquitous ice bed decoupling quantified as a floating fraction of ice proceeds along ice streams this causes ice streams to surge and reduce thickness by some 90 percent and height above sea level by up to 99 percent for floating ice so the ice sheet undergoes gravitational collapse ice collapsing over marine embayments becomes floating ice shelves that may then disintegrate rapidly this floods the world ocean with icebergs that reduce the ocean to atmosphere heat exchange thereby triggering climate change calving bays migrate up low stagnating ice streams and carve out the accumulation zone of the collapsed ice sheet which prevents its recovery decreases earth s albedo and terminates the glaciation cycle this sequence of events may coincide with a proposed life cycle of ice streams that drain the ice sheet a first order treatment of these life cycles is presented that depends on the longitudinal force balance along the flowbands of ice streams and gives a first approximation to ice bed uncoupling at snapshots during gravitational collapse into ice shelves that disintegrate thereby removing the ice sheet the stability of the antarctic ice sheet is assessed using this bottom up approach

hydraulic run away a mechanism for thermally regulated surges of ice sheets by using a simple parameterized model of thermomechanically coupled flow in cold ice sheets together with a physically based sliding law which includes a description of basal drainage we show that relationships between ice flux and ice thickness can realistically be multi valued and hence that hydraulically induced surges can occur we term this mechanism hydraulic run away as it relies on the positive feed back between sliding velocity and basal melt production for this feed back to operate it is essential that water pressure increases with water storage this is consistent with various recent ideas concerning drainage under ice sheets be it through a system of canals a distributed film or a subglacial aquifer for confined flows such as valley glaciers e g trapridge glacier or topographically constrained ice streams e g hudson strait in the laurentide ice sheet which are underlain by sufficiently deformable sediment we can expect thermally regulated surges to occur while in a laterally unconfined drainage basin such as that which flows into the ross ice shelf we might expect ice streams to develop

iridium and other platinum group elements as geochemical markers in sedimentary environments iridium anomalies are commonly used as indicators of extraterrestrial processes which caused global extinctions however platinum group element pge anomalies may result from several different processes both syn and postdepositional acting separately or jointly the sources and processes of enrichment involved are extraterrestrial asteroid comet cosmic dust impact ejecta volcanogenic pge rich condensates precipitation from seawater low sedimentation rate anoxic conditions microbial concentration dissolution reprecipitation exhalative hydrothermal leaching transport and precipitation at a redox boundary geochemical characteristics of pge and accompanying elements in sediments can be used to identify the contributions of some specific sources and processes 1 extraterrestrial chondritic ru ir and os ir ratios low 187os 186os ratio high ir compared to pt pd and au 2 volcanogenic high iridium and se as sb and in contents 3 exhalative hydrothermal high au pd and pt relatively low ir contents 4 precipitation from seawater high pt and low pd contents also high cd and ag contents under anoxic conditions and 5 reduction from intermediate and low temperature solutions high pge and cu or mo u ni contents

galileo magnetometer measurements a stronger case for a subsurface ocean at europa on 3 january 2000 the galileo spacecraft passed close to europa when it was located far south of jupiter s magnetic equator in a region where the radial component of the magnetospheric magnetic field points inward toward jupiter this pass with a previously unexamined orientation of the external forcing field distinguished between an induced and a permanent magnetic dipole moment model of europa s internal field the galileo magnetometer measured changes in the magnetic field predicted if a current carrying outer shell such as a planet scale liquid ocean is present beneath the icy surface the evidence that europa s field varies temporally strengthens the argument that a liquid ocean exists beneath the present day surface

evidence for massive discharges of icebergs into the north atlantic ocean during the last glacial period sediments in the north atlantic ocean contain a series of layers that are rich in ice rafted debris and unusually poor in foraminifera1 here we present evidence that the most recent six of these heinrich layers deposited between 14 000 and 70 000 years ago record marked decreases in sea surface temperature and salinity decreases in the flux of planktonic foraminifera to the sediments and short lived massive discharges of icebergs originating in eastern canada the path of the icebergs clearly marked by the presence of ice rafted detrital carbonate can be traced for more than 3 000 km a remarkable distance attesting to extreme cooling of surface waters and enormous amounts of drifting ice the cause of these extreme events is puzzling they may reflect repeated rapid advances of the laurentide ice sheet perhaps associated with reductions in air temperatures yet temperature records from greenland ice cores appear to exhibit only a weak corresponding signal moreover the 5 10 000 yr intervals between the events are inconsistent with milankovitch orbital periodicities raising the question of what the ultimate cause of the postulated cooling may have been

formation of cycloidal features on europa cycloidal patterns are widely distributed on the surface of jupiter s moon europa tensile cracks may have developed such a pattern in response to diurnal variations in tidal stress in europa s outer ice shell when the tensile strength of the ice is reached a crack may occur propagating cracks would move across an ever changing stress field following a curving path to a place and time where the tensile stress was insufficient to continue the propagation a few hours later when the stress at the end of the crack again exceeded the strength propagation would continue in a new direction thus one arcuate segment of the cycloidal chain would be produced during each day on europa for this model to work the tensile strength of europa s ice crust must be less than 40 kilopascals and there must be a thick fluid layer below the ice to allow sufficient tidal amplitude