seismic detection of a subglacial lake near the south pole antarctica 1 seismic reflection data are analyzed to verify radar identification of a subglaical lake near the geographic south pole the seismic amplitude variation with offset avo technique is applied to confirm the presence of extensive free water and seismic imaging of the subsurface constrains lake depth and deeper subglacial structure in the region this lake is at least 4 2 km wide and likely as much as 10 km in diameter is up to 32 10 m deep and occupies a basin of thick sedimentary strata these results imply that extensive water storage is occurring in the south pole region the proximity of this lake to the amundsen scott south pole station makes research drilling to sample the lake and underlying sediments feasible and supportable

the permian triassic transition in ocean island setting environmental disturbances and new high resolution carbon isotope record from the qiangtang basin nw china abstract the permian triassic boundary ptb events such as carbon isotope excursion volcanism and environmental disturbances are not well understood in an ocean island setting here we develop a new case study for the permian triassic transition in the qiangtang basin tibet by combining existing biostratigraphy new carbon isotope data sedimentological data u pb zircon age mineralogical and geochemical data these new data defined the permian triassic transition age as about 252 3 0 9 ma corresponding to the lowermost sea level a long term negative carbonate carbon isotope trend is present at the same stratigraphic level in different depositional environments strongly suggesting its global nature however our new ocean island setting is characterized by an abrupt shift in carbon isotope values across the level which is different from many chemostrtigraphic studies that show a gradual shift in carbon isotope values across the event horizon the most likely explanation for the abrupt shift in 13c values is a sedimentary hiatus at this level in the study section three stage climatic models through the ptb in the ocean island setting are identified stage 1 is characterized by a warm and humid climate with moderate chemical weathering while a hot and humid climate with intense chemical weathering dominates stage 2 the climate in stage 3 is a relatively hot and arid climatic condition with weak chemical weathering

intrabasin variability of the carbon isotope record across the permian triassic transition sverdrup basin arctic canada abstract a major late paleozoic depocentre the sverdrup basin canadian high arctic has been largely left out of the latest permian extinction debate as early workers presumed middle to late permian strata were absent basin scale sequence stratigraphic and chemostratigraphic correlations indicate late permian strata are only missing on the basin margins where they were removed by sub triassic erosion whereas continuous deposition is recorded in the basin centre the varying degree of sub triassic erosion has significant impact on the carbon isotope record across the latest permian extinction event where both the apparent rate and magnitude of carbon isotope shift vary as a function of basin position the intrabasin variability in apparent 13corg shift across the event is equivalent to that observed globally in contrast to the abrupt isotope shifts recorded on the basin margin similar to many records reported globally the basin centre section shows a systematic shift associated with the latest permian extinction the earth likely underwent a prolonged period of increasing environmental stress leading up to the event

changes in the global carbon cycle occurred as two episodes during the permian triassic crisis coeval records of ocean atmosphere and terrestrial change are crucial to understanding the pattern and causes of global mass extinction across the permian triassic boundary ptb however relationships among changes in different settings remain largely unclear primarily due to the challenges associated with the correlation among disparate records here we compare marine carbon isotopic records with marine and terrestrial environmental and biotic events recorded in sediments from the meishan ptb section of south china time scaled carbonate carbon isotopes exhibit two gradual major shifts across the ptb at meishan and these are duplicable elsewhere around the tethys ocean the two shifts are associated with two episodes of enhanced terrestrial weathering indicated by an increased abundance of 13 c enriched moretanes relative to hopanes and an elevated abundance of black carbon fragments key marine events previously reported for the ptb including photic zone euxinia faunal mass extinction and cyanobacterial expansion also occur as two episodes coinciding with both of the progressive shifts to negative 13 c values and enhanced weathering the temporal sequence of the duplicable events suggests that the biotic crisis was a consequence of prolonged and episodic changes in the marine and continental systems and argues against an extraterrestrial impact as the main cause

a viscoelastic flowline model applied to tidal forcing of bindschadler ice stream west antarctica abstract the motion of bindschadler ice stream west antarctica is dominated by sliding over a nearly plastic bed according to analysis of kinematic gps data using a new viscoelastic flowline model inversions of time averaged velocity data with viscous ice flow models can be consistent with multiple sliding laws but propagation of velocity perturbations in a viscoelastic model can distinguish between sliding laws with different exponents we develop such a model and apply it to a time series of velocity for the tidally modulated flow of bindschadler ice stream formerly ice stream d observed velocity perturbations are found to be consistent with a flow law exponent m 8 which indicates basal motion with a relatively weak till bed lower exponents consistent with motion dominated by deformation within the ice over a hard or frozen bed are found to be unlikely this result suggests that bindschadler ice stream would respond rapidly and significantly to any future loss of buttressing from the ross ice shelf

timing of the permian triassic biotic crisis implications from new zircon u pb age data and their limitations abstract the most profound biotic crisis in the earth s history causing the near extinction of both terrestrial and marine life occurred at the end of the permian period about 253 myr ago and marks the paleozoic mesozoic era boundary the cause of this event is still a matter of vigorous debate with both brief and catastrophic as well as gradual mechanisms having been proposed similar to a recent landmark study this study uses the u pb method on zircons from the uppermost permian lowermost triassic ash fall deposits at meishan zhejiang province se china in order to examine time and rate constraints for these events the results of both this study and previous work show that for these ash layers the effects of pb loss are combined with varying amounts and sources of inheritance resulting in an age scatter which prohibits the extraction of a statistically robust age in many cases though the effects of pb loss on the zircons analyzed in this study were reduced by leaching the grains in hydrofluoric acid as opposed to commonly applied air abrasion prior to analysis the presence within a single ash layer of multiple generations of older xenocrysts in many cases only slightly older than the depositional age has made quantitative interpretation even more difficult when these combined phenomena bias individual zircon ages by less than a percent they are extremely difficult to deconvolute and if multi grain analyses are used can become impossible to recognize because of the resulting age averaging monte carlo simulations using actual measurements of individual zircon crystals show that age excursions due to pb loss and xenocrystic contamination for the meishan bentonites are easily homogenized to the point of undetectability when replicate analyses of multi grain zircon samples are compared thus this study uses only high precision analyses of single crystals whether from our work or that of previous studies three main conclusions have emerged first our data require a significant increase in the age of the permian triassic boundary by more than 2 myr compared to the previous study which shifts the age to a value older than 253 ma second neither our data nor those from previous work can confirm or negate the possibility of a very abrupt biotic crisis third even large suites of very high quality single zircon u pb analyses for these tuffs cannot in most cases yield objective reliable and robust dates with accuracies at the sub myr level though the temptation to perform arbitrary selection of subsets of the analyses for that purpose is almost irresistible the last conclusion is not an indictment of zircon u pb dating in general other rocks and other zircon populations can and do behave very differently and further technical advances will likely improve our ability to prepare grains or sub grains of adequately enhanced quality for analysis consequently the results of the present study strongly suggest that for problems requiring time scale accuracy inferences from zircon u pb dating must be based on sufficiently large suites of single crystal or crystal domain high precision analyses

large scale ice flow over a viscous basal sediment theory and application to ice stream b antarctica recent seismic studies of ice stream b antarctica and field analysis of mid latitude glacial deposits suggest that deformable basal sediments e g water saturated till are important in determining ice sheet flow if the ratio of till viscosity to effective ice viscosity is small vertical shear associated with horizontal flow is confined to the deforming bed alone ice flow over a deformable bed is thus akin to that of floating ice shelves because ice shelves flow over inviscid seawater for some antarctic ice streams and possibly for portions of the late wisconsin ice sheets in north america and eurasia basal drag associated with deforming basal sediment does not induce significant vertical gradients of horizontal velocity instead basal drag affects the flow as if it were a horizontal body force balanced by longitudinal and transverse deviatoric stress gradients comparison of the observed flow of ice stream b to finite element simulations incorporating a viscous basal till suggest that a simple till theology is sufficient to explain the current velocity profile these simulations also highlight the importance of horizontal deviatoric stress in regions where driving stress and basal stress do not locally balance while bed deformation is critical to ice stream existence sensitivity tests suggest that ice shelf back pressure is still a crucial control affecting ice stream response to atmospheric and oceanic climate

association of 34s depleted pyrite layers with negative carbonate δ13c excursions at the permian triassic boundary evidence for upwelling of sulfidic deep ocean water masses 1 a marine permian triassic boundary ptb section at nhi tao vietnam contains a series of at least 9 pyritic horizons characterized by concurrent decreases in pyrite s 34spy and carbonate c isotopic compositions 13ccarb the first and largest of the events that precipitated these pyritic horizons was coincident with the late permian mass extinction while subsequent events were generally smaller and occurred at quasiperiodic intervals of 20 to 30 ka a near complete lack of organic carbon to drive bacterial sulfate reduction in sediment pore waters among other considerations argues against a diagenetic control for these relationships rather the covariant patterns documented herein are most easily explained as the product of recurrent upwelling of anoxic deep ocean waters containing 34s depleted hydrogen sulfide and 13c depleted dissolved inorganic carbon the sulfide 34s record of the study section represents a mixture of a small amount of isotopically heavy authigenic pyrite formed via in situ bacterial sulfate reduction with a generally larger quantity of isotopically light syngenetic pyrite precipitated within the water column during upwelling episodes although upwelling of toxic deepwaters has been invoked in earlier studies as a mechanism for the late permian marine mass extinction this is the first study to 1 document patterns of pyrite 13ccarb covariation that strongly support upwelling as a major process at the ptb and 2 provide evidence of multiple quasiperiodic upwelling events that may reflect reinvigoration of global ocean overturn following a prolonged interval of late permian deep ocean stagnation

georods the development of a four element geophone for improved seismic imaging of glaciers and ice sheets active seismic imaging of glaciers and ice sheets is important for constraining inputs to climate models such as englacial ice fabric and the nature of the basal interface however acquiring high quality seismic data is time consuming and resource intensive using traditional single element geophones requires ideal weather conditions e g light winds and excellent source coupling in addition deploying and retrieving these geophones is slow and cumbersome we have developed a four element georod that enhances signal levels by 20 30 db in a variety of conditions including blowing snow and poorly coupled source detonations the long slender design of these georods makes them easy to deploy and retrieve allowing researchers to acquire greater line kilometers of seismic data during field campaigns that are commonly time constrained

theoretical modeling of seismic noise propagation in firn at the south pole antarctica the problem of interfering noise produced by ground vehicles on teleseismic arrivals recorded by global seismic network sensors at south pole station is addressed using the wavenumber integration method theoretically calculated seismograms show that installing the gsn sensors in a borehole 200 to 300 m deep 10 km away from the station will significantly reduce the vehicle generated noise and improve signal quality because the intrinsic attenuation of seismic waves propagating in the polar firn is low most of the predicted noise reduction results from wavefront spreading rayleigh wave amplitude decay with depth and from placing the sensors below the refractive waveguide that traps much of the seismic energy in the near surface layers

complex fabric development revealed by englacial seismic reflectivity jakobshavn isbrae greenland this is the published version copyright 2008 american geophysical union all rights reserved

bed conditions of pine island glacier west antarctica although 90 of antarctica s discharge occurs via its fast flowing ice streams our ability to project future ice sheet response has been limited by poor observational constraints on the ice bed conditions used in numerical models to determine basal slip we have helped address this observational deficit by acquiring and analyzing a series of seismic reflection profiles to determine basal conditions beneath the main trunk and tributaries of pine island glacier pig west antarctica seismic profiles indicate large scale sedimentary deposits combined with seismic reflection images measured acoustic impedance values indicate relatively uniform bed conditions directly beneath the main trunk and tributaries comprising a widespread reworked sediment layer with a dilated sediment lid of minimum thickness 1 5 0 4 m beneath a slow moving intertributary region a discrete low porosity sediment layer of 7 3 m thickness is imaged despite considerable basal topography seismic observations indicate that a till layer at the ice base is ubiquitous beneath pig which requires a highly mobile sediment body to maintain an abundant supply these results are compatible with existing ice sheet models used to invert for basal shear stress existing basal conditions upstream will not inhibit further rapid retreat of pig if the high friction region currently restraining flow directly upstream of the grounding line is breached however small changes in the pressure regime at the bed as a result of stress reorganization following retreat may result in a less readily deformable bed and conditions which are less likely to maintain high ice flow rates

englacial seismic reflectivity imaging crystal orientation fabric in west antarctica abrupt changes in crystal orientation fabric cof and therefore viscosity are observed near the base of the ice sheet throughout west antarctica we report on active source seismic observations from wais divide mid stream and downstream on thwaites glacier and the onset region of bindschadler ice stream these data reveal a prevalence of englacial seismic reflectivity in the bottom quarter of the ice sheet the observed seismic reflectivity is complex but largely bed conformable with long spatial wavelength features observed in the flow direction and short wavelength features observed across flow a correspondence of englacial structures with bed features is also observed we determine the origin of the reflectivity to be abrupt changes in the cof of ice based on the following 1 observations of englacial reflectivity are consistent with current knowledge of cof within ice sheets 2 englacial reflectivity caused by cof contrasts requires the simplest genesis especially at ice divides and 3 amplitude analysis shows that the observed englacial reflectivity can be explained by contrasts in seismic velocity due to cof changes we note that the downstream increase in the quantity and complexity of observations indicates that direct observations of cof at ice divides likely underestimate the role that fabric plays in ice sheet dynamics

subglacial lake whillans seismic observations of a shallow active reservoir beneath a west antarctic ice stream abstract active subglacial lakes concentrate the distribution of water beneath ice sheets in both space and time seismic and surface observations from subglacial lake whillans slw west antarctica reveal that this active lake forms a persistent albeit fluctuating reservoir beneath whillans ice stream imaging and phase observations using active source seismic data show that slw is a perpetually shallow feature when surveyed near its low stand a water column was resolvable by seismic techniques along only 5 km of the 45 km profiled with a maximum depth of less than 8 m satellite altimetry shows that the high stand adds no more than 3 4 m to this this water column presents a suitable drill site at s 84 240 w 153 694 elsewhere the majority of the bed appears wet with soft sediment or water thicknesses of less than the imaging resolution of our data of approximately 2 m the surface expression of the active lake previously revealed by icesat elevation data and image differencing generally corresponds to the seismic estimate of soft sediment or water with notable exceptions occurring at the upstream and downstream ends of the lake these exceptions indicate that slw s water column is very shallow or absent in places at low stands or has disconnected or transiently active and inactive portions

anisotropy and crystalline fabric of whillans ice stream west antarctica inferred from multicomponent seismic data crystal orientation fabric cof describes the intrinsic anisotropic nature of ice and is an important parameter for modeling glacier flow we present the results of three component active source seismic observations from the whillans ice stream wis a fast flowing ice stream in west antarctica surface wave dispersion analysis ray tracing and traveltime inversion of compressional p and shear s waves reveal the presence of transversely isotropic ice with a vertical axis of symmetry vti beneath approximately 65 m of isotropic firn the ice stream is characterized by weak anisotropy involving an average ice thickness of approximately 780 m the analysis indicates that about 95 of the ice mass is anisotropic and the crystalline c axes span within an average broad cone angle of 73 10 with respect to the vertical axis moreover the mean temperature t below the firn estimated from seismic data is 15 5 c these data do not show evidence of englacial seismic reflectivity which indicates the lack of abrupt changes in the cof the presence of azimuthal anisotropy due to transversely compressive flow or fractures aligned along a preferential direction is also excluded we suggest that the observed vti ice structure is typical of large ice streams in regions where basal sliding and bed deformation dominate over internal glacial deformation

till beneath ice stream b 1 properties derived from seismic travel times seismic experiments conducted on ice stream b part of the marine ice sheet of west antarctica show a meters thick layer immediately beneath the 1000 m thick ice a seismic experiment consisting of wide angle reflection profiling along a line parallel to ice stream flow was conducted to determine the properties of this layer inversion of seismic travel times yields a compressional wave speed of less than 1700 m s 1 and a shear wave speed less than 160 m s 1 for the layer these very low wave speeds imply that the material in the layer is highly porous and is saturated with water at a high pore pressure based on wave speeds in other saturated unconsolidated sediments we believe that a porosity substantially greater than 0 32 probably around 0 4 and an excess of overburden pressure over pore pressure of only 50 kpa 0 5 bar characterize the layer at this location

40ar 39ar age of shear zones in the southwest meguma zone between yarmouth and meteghan nova scotia the cape st marys cranberry point and chebogue point shear zones lie within a zone of d2 deformation that overprinted d1 fabrics and reactivated f1 folds between the shear zones d2 was accompanied by greenschist facies metamorphism that variably overprinted earlier metamorphism of varying grade a collection of single muscovite grains from pressure shadows defining l2 in a sample from the cape st marys shear zone yields an average apparent age of 316 5 ma the age obtained for these grains from an isotope correlation diagram is 320 5 ma ages for muscovite separates for two samples from the cranberry point shear zone are 325 4 and 319 4 ma an amphibolite from this zone yields a discordant spectrum interpreted to reflect the combined effects of acadian metamorphism and excess argon the muscovite ages approximate the time of syntectonic deformation because muscovite closure temperatures approximate the temperature of metamorphism during d2 thus together with published data from the chebogue

extensive storage of basal meltwater in the onset region of a major west antarctic ice stream a major meltwater body exists beneath a tributary of bindschadler ice stream west antarctica in a region where subglacial lakes have not been mapped but near where rapid vertical motion of the ice sheet surface has suggested shifting of a subglacial water body the water is trapped by a local reversal in ice air surface slope arising from ice flow over variable basal topography and from the positive feedback of basal lubrication from the trapped water strong variations in the water content of the sediments upglacier of the water body arise from a similar process these results are revealed by a novel application of the amplitude variation with offset avo seismic technique the existence of such water bodies and of the strong spatial variation in subglacial sediment properties is not captured in current models of subglacial hydrology lubrication of ice stream motion and sediment transport

basal conditions on rutford ice stream west antarctica from seismic observations a seismic reflection profile perpendicular to the ice flow direction has been acquired on rutford ice stream antarctica an interpretation of both the amplitude and phase of the seismic reflections from the ice bed interface has been made to investigate the properties of the sub ice material the interpretation assumes that acoustic impedance can be used to imply subglacial sediment porosity multiple reflections from the ice bed interface on a number of seismic wide angle lines allowed a calibration of the reflection coefficient at the bed this enabled the acoustic impedance of the bed material to be calculated from the seismic reflection data mean acoustic impedance in the different sections of the bed ranges from 2 70 106 kg m 2 s 1 to 4 31 106 kg m 2 s 1 almost three quarters of the ice stream bed at this site appears to be saturated deforming sediments the rest of the bed is probably also saturated sediments but they are not deforming to any significant degree ice flow could include a combination of subglacial deformation and basal sliding localized regions which support disproportionately high amounts of basal shear stress may also occur from the seismic data it is not possible to apportion relative amounts of restraint to these different processes there does not appear to be any correlation between the different sections identified on the ice stream bed with either satellite images or nearby surface velocity data a feature which is interpreted as a subglacial drumlin is seen on the seismic section

interpretation of topography and bed properties beneath thwaites glacier west antarctica using seismic reflection methods abstract thwaites glacier tg west antarctica is losing mass in response to oceanic forcing future evolution could lead to deglaciation of the marine basins of the west antarctic ice sheet depending on ongoing and future climate forcings but also on basal topography bathymetry basal properties and physical processes operating within the grounding zone hence it is important to know the distribution of bed types of tg s interior and grounding zone and to incorporate them accurately in models in order to improve estimates of retreat rates and stability here we estimate properties of the bed by determining its acoustic impedance from amplitude analysis of reflection seismic data we report on the results from two lines a longitudinal l line and a transverse n line on a central flowline of tg 100 km inland from the grounding zone the data show considerable spatial variability in bed forms and properties similar to results from a comparable survey farther inland notably we find the same pattern here of hard presumed bedrock material on the stoss side of bumps and soft presumed till on the lee side physical understanding indicates the basal flow law describing motion over different regions of tg s bed likely varies from nearly viscous over the bedrock regions to nearly plastic over till regions providing guidance for modeling

seismic attenuation in glacial ice a proxy for englacial temperature 1 seismic attenuation or internal friction q 1 in glacial ice is highly sensitive to temperature particularly near the melting point here we detail a technique to estimate q and apply it to active source seismic data from jakobshavn isbrae greenland we compare our results to measured and modeled temperature profiles of the ice in the region we find an excellent match with differences between seismically estimated and modeled temperatures of less than 2 c mapping variations in seismic q through glacial ice thus is shown to allow detailed estimation of englacial temperature profiles which may be of special value in regions where in situ measurements are logistically difficult