late pleistocene and holocene subtropical vegetation dynamics recorded in perched lake deposits on fraser island queensland australia abstract the dune system of fraser island in subtropical queensland australia contains numerous perched lakes with organic rich sediments these lakes are located in the subtropics and their water levels are strongly influenced by precipitation the lakes act as natural rainfall gauges which make them highly sensitive to environmental change paleoecological and paleoclimatological investigations are performed on sediment cores from lake allom a small perched lake on central fraser island a detailed chronology is based on a series of closely spaced ams radiocarbon dates supplemented with sedimentological information based on extrapolation the chronology indicates an age range from 56 14c kyr bp to present with a major hiatus occurring during the last glacial maximum pollen analysis of the lake allom sediment record reveals strong changes between rainforest and open woodland vegetation the holocene portion of the record shows a stepwise vegetation development from dry conditions in the early holocene to high lake levels and increasing forest between 5 5 and 3 cal kyr bp at 3 2 cal kyr bp a large diversification occurred towards the present day heterogeneous sub tropical rainforest vegetation followed by a small rainforest decline at 0 45 cal kyr bp additionally charcoal analysis indicates increases in fire occurrence contemporaneously with periods of drier vegetation and or low lake levels part of the reconstructed vegetation changes can be related to local and regional factors including forest succession dune formation sea level rise and human impact comparison with paleoclimate records from tropical and temperate regions indicates that the temporal and spatial dynamics of vegetation changes in eastern australia are primarily controlled by climate variability particularly the increasing activity of the el nino southern oscillation enso during the late holocene caused greater climate variability in eastern australia resulting in more heterogenous vegetation cover

structure of the california coast ranges and san andreas fault at safod from seismic waveform inversion and reflection imaging 1 a seismic reflection and refraction survey across the san andreas fault saf near parkfield provides a detailed characterization of crustal structure across the location of the san andreas fault observatory at depth safod steep dip prestack migration and frequency domain acoustic waveform tomography were applied to obtain highly resolved images of the upper 5 km of the crust for 15 km on either side of the saf the resulting velocity model constrains the top of the salinian granite with great detail steep dip reflection seismic images show several strong amplitude vertical reflectors in the uppermost crust near safod that define an 2 km wide zone comprising the main saf and two or more local faults another prominent subvertical reflector at 2 4 km depth 9 km to the northeast of the saf marks the boundary between the franciscan terrane and the great valley sequence a deep seismic section of low resolution shows several reflectors in the salinian crust west of the saf two horizontal reflectors around 10 km depth correlate with strains of seismicity observed along strike of the saf they represent midcrustal shear zones partially decoupling the ductile lower crust from the brittle upper crust the deepest reflections from 25 km depth are interpreted as crust mantle boundary

an 1800 year archeological experiment in remagnetization limestone masonry from monuments dating from 300ad to 1940 remagnetized partly and progressively with age parallel to the present earth s magnetic field the remagnetization was probably due to viscous remanent magnetization vrm neel s theory predicts that natural vrm acquired at low temperature ta over a long time ta disappears at a higher temperature td in a short period td in the laboratory i calibrated td for the postconstruction remagnetization against the ages td of masonry used in well dated historic buildings in eastern england empirically ta m td b relates acquisition age tayears to demagnetization temperature td c lincoln limestone remagnetizes more quickly m 1 298 10 6 and b 3 598 over 1800 years than speeton chalk m 4298 10 6 and b 1 973 over 350 years magnetite with an effective pseudo single domain or small multidomain size carries the remanence

australasian evidence for mid holocene climate change implies precessional control of walker circulation in the pacific abstract at its simplest the pacific cell of the walker circulation may be regarded as a thermal circulation driven by the temperature contrast between the west and east pacific evidence from australasia is summarized indicating that this circulation was greatly weakened in the early holocene but was enhanced at about 5000 bp the initial effect of this enhancement was the intensification of the northern australian monsoon within the australian tropics but this was reversed abruptly after 3700 bp the enhancement is attributed to an increased polar equator pressure gradient after 5000 bp with consequent intensification of the overall circulation pattern in the southern hemisphere including the mid latitude westerlies and the trade winds this in turn appears to be a result of increased southern hemisphere seasonality driven by the precessional cycle it is proposed that increased wind speeds off south america after 5000 bp in both mid and low latitudes enhanced upwelling and reduced sea surface temperatures ssts this created a greater east west surface temperature contrast across the pacific compounding a pre existing trend towards enhanced circulation and flipping climates in the southern hemisphere pacific basin from an early holocene mode to a late holocene mode under reduced circulation conditions early holocene climates at low latitudes in the southern hemisphere may have been dominated by monsoonal heat transfers

remagnetization dating of roman and mediaeval masonry abstract the earth s magnetic field may partially remagnetize rocks often by the process of viscous remanent magnetization vrm thus when masonry is turned and placed in a monument it magnetizes parallel to the earth s field partly overprinting any older geological magnetizations the intensity of this new overprint will increase with time if the masonry is undisturbed older magnetizations require higher demagnetization temperatures thus the unblocking temperature tub relates directly to the age of the remanence and therefore the age of stabilization of the masonry for suitable rocks we can calibrate the remagnetization time scale for the rock using historically dated structures the scale then permits us to estimate the age of more enigmatic structures by interpolation or modest extrapolation some samples reveal multiple episodes of remagnetization showing that masonry was recycled acquiring a differently oriented remanence each time that it stabilized in a new architectural configuration masonry from the bishop s palace lincoln u k includes roman c ad 300 mediaeval 1160 1450 post civil war 1650 1720 and victorian phases that have characteristic tub consistent with their ages some mediaeval masonry recycled roman building materials

rapid disintegration of the wordie ice shelf in response to atmospheric warming the breaking up of ice shelves around the antarctic peninsula has been cited1 as a sign that a dangerous warming is beginning in antarctica here we present satellite images showing the disintegration of the wordie ice shelf which lies off the west coast of the antarctic peninsula fig 1 fracture either in the form of surface crevasses or rifts extending to the bottom of the ice shelf has been responsible for iceberg calving and weakening the central region of the ice shelf these fracture processes which led to retreat of the ice front were apparently enhanced by the presence of increased amounts of melt water resulting from a warming trend recorded in mean annual air temperatures in marguerite bay if this warming trend continues other nearby ice shelves on the antarctic peninsula may be at risk but substantial additional warming would be required before similar processes could initiate breakup of the ross and filchner ronne ice shelves which help stabilize the west antarctic ice sheet

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

three positive feedback mechanisms for ice sheet melting in a warming climate three positive feedback mechanisms that accelerate ice sheet melting are assessed in a warming climate using a numerical ice model driven by atmospheric climate models the greenland ice sheet gris is the modeling test bed under accelerated melting conditions the first feedback is the interaction of sea water with ice it is positive because fresh water melts ice faster than salty water owing primarily to the reduction in water heat capacity by solutes it is shown to be limited for the gris which has only a small ocean interface and the grounding line of some fast glaciers becomes land terminating during the 21st century the second positive feedback strain heating is positive because it produces further ice heating inside the ice sheet the third positive feedback granular basal sliding applies to all ice sheets and becomes the dominant feedback during the 21st century a numerical simulation of jakobshavn isbrae over the 21st century reveals that all three feedback processes are active for this glacier compared with the year 2000 level annual ice discharge into the ocean could increase by 1 4 km3 a 1 5 of the present annual rate by 2100 granular basal sliding contributes 40 of this increase

a stalagmite record of holocene indonesian australian summer monsoon variability from the australian tropics oxygen isotopic data from a suite of calcite and aragonite stalagmites from cave kni 51 located in the eastern kimberley region of tropical western australia represent the first absolute dated high resolution speleothem record of the holocene indonesianeaustralian summer monsoon iasm from the australian tropics stalagmite oxygen isotopic values track monsoon intensity via amount effects in precipitation and reveal a dynamic holocene iasm which strengthened in the early holocene decreased in strength by 4 ka with a further decrease from w2 to 1 ka before strengthening again at 1 ka to years to levels similar to those between 4 and 2 ka the relationships between the kni 51 iasm reconstruction and those from published speleothem time series from flores and borneo in combination with other data sets appear largely inconsistent with changes in the position and or organization of the intertropical convergence zone itcz instead we argue that the el nino southern oscillation enso may have played a dominant role in driving iasm variability since at least the middle holocene given the muted modern monsoon rainfall responses to most el nino events in the kimberley an impact of enso on regional monsoon precipitation over northwestern australia would suggest non stationarity in the long term relationship between enso forcing and iasm rainfall possibly due to changes in the mean state of the tropical pacific over the holocene

mid holocene palaeolimnological record of a southern hemisphere subtropical lake spanning the last 6000 years lake jennings fraser island australia the study of climate mechanisms in the southern hemisphere during the holocene remains an area of active research that lacks the spatial and temporal connectivity characteristic of northern hemisphere research the subtropical temperate climate transition zone of eastern australia provides a unique location to investigate long term environmental changes during the holocene lake jennings on fraser island was used to investigate climate change in this transition zone using palaeolimnological techniques the beginning of the lake jennings record 6000 to 3500 cal ybp is characterised by fluctuations in geochemical signals an abundance of sponge spicules and a lack of diatom species results suggest gradually increasing precipitation and water depth which affect nutrient cycling however the biological functioning of the lake is difficult to interpret due to the absence of diatoms possibly a result of turbidity dissolution and or predation by freshwater sponges during the late holocene 3500 cal

shallow structure characterization by 2d elastic full waveform inversion assessing the effectiveness of elastic full waveform inversion fwi algorithms when applied to shallow 2d structures in the presence of a complex topography is critically important by using fwi we overcome inherent limitations of conventional seismic methods used for near surface prospecting acoustic tomography and multichannel spectral analysis of surface waves the elastic forward problem formulated in the frequency domain is based on a mixed finite element p0p1 discontinuous galerkin method to ensure accurate modeling of complex topography effects at a reasonable computing cost the inversion problem uses an fwi algorithm to minimize the misfit between observed and calculated data based on results from a numerical experiment performed on a realistic landslide model inspired from the morphostructure of the super sauze earthflow we analyzed the effect of using a hierarchical preconditioning strategy based on a simultaneous multifrequency inversion of damped data to mitigate the strong nonlinearities coming from the surface waves this strategy is a key point in alleviating the strong near surface effects and avoiding convergence toward a local minimum using a limited memory quasi newton method improved the convergence level these findings are analogous to recent applications on large scale domains although limited sourcereceiver offset ranges low frequency content of the source and domination of surface waves on the signal led to some difficulties regarding the impact of data decimation on the inversion results we have learned that an inversion restricted to the vertical data component can be successful without significant loss in terms of parameter imagery resolution in our investigations of the effect of increased source spacing we found that a sampling of 4 m less than three times the theoretical maximum of one half wavelength led to severe aliasing

waveform tomography in 2 5d parameterization for crooked line acquisition geometry 1 a method for 2 5d viscoacoustic waveform tomography that can be applied to generate 2d models of velocity and attenuation from inversion of refraction waveforms on land seismic reflection data acquired along crooked roads is developed it is particularly useful for typical crustal reflection surveys first arrival travel time tomography is applied using a 3d method but with constraints on the intermediate 3d velocity model the result is the starting model for the next step a 2 5d frequency domain full waveform inversion stage parameterizes 3d geometry in the seismic source and receiver arrays with the assumption that the velocity and attenuation models are homogeneous in the out of plane direction this approach results in superior results compared to a strictly 2d approach when the acquisition line is crooked with a moderate increase in computational cost a case study using data acquired in the nechako basin in south central british columbia canada exemplifies and validates the procedure the velocity model derived from 2 5d waveform tomography is compared with that from a previous study in which 2d waveform tomography was applied to the same data set and with results from 3d travel time tomography the resolution and accuracy of the velocity model from 2 5d waveform tomography are demonstrated to be greater than those from travel time or 2d waveform tomography a model of viscoacoustic attenuation which was not possible in the 2d case is also generated these models are interpreted jointly to highlight features of geological interest such as a sedimentary basin basement rocks and faults from surface to about 3 km depth

physical conditions at the base of a fast moving antarctic ice stream boreholes drilled to the bottom of ice stream b in the west antarctic ice sheet reveal that the base of the ice stream is at the melting point and the basal water pressure is within about 1 6 bars of the ice overburden pressure these conditions allow the rapid ice streaming motion to occur by basal sliding or by shear deformation of unconsolidated sediments that underlie the ice in a layer at least 2 meters thick the mechanics of ice streaming plays a role in the response of the ice sheet to climatic change

palaeoclimate reconstruction based on pollen records from the tangke and riganqiao peat sections in the zoige plateau china abstract palynological analysis was performed to characterize the holocene paleoflora of china s zoige plateau and the implications for the plateau s palaeoclimate using the tangke and riganqiao peat sections atomic mass spectrometry 14 c dates and modern surface soil pollen materials were used to reconstruct the palaeoclimate parameters from 7940 to 6420 cal bp the climate under which the tangke section formed was slightly warm and humid from 6420 to 6030 cal bp the climate was warm and humid from 6030 to 3790 cal bp the climate was slightly cold and dry from 3790 to 1580 cal bp the climate was cold and dry and from 1580 cal bp to the present the climate was very warm and humid from 7320 to 6955 cal bp the climate under which the riganqiao section formed was warm and humid from 6955 to 5467 cal bp the climate was very cold and dry from 3641 to 5467 cal bp the climate was slightly cold and dry from 2337 to 3641 cal bp the climate was warm and slightly humid and from 2337 to 615 cal bp the climate was warm and slightly humid several cold and dry events have therefore occurred since 8000 cal bp in both sections there is a close relationship between peat mire development and paleoenvironmental change during the five periods recorded in the two sections

ice ocean processes over the continental shelf of the southern weddell sea antarctica a review 1 interactions between the southern ocean and the weddell sea ice shelves are important both to the antarctic ice sheet and to the production of globally significant water masses here we review the interaction between the filchner ronne ice shelf and the shelf sea in which it floats the continental shelf processes leading to the production of weddell sea deep and bottom waters from the original off shelf source waters are discussed and a new view is offered of the initial production of high salinity shelf water data from ship based measurements at the ice front from glaciological methods and from measurements made within the sub ice shelf cavity itself are used to describe the pattern of flows beneath the ice shelf we also consider the variability observed within the cavity from tidal to interannual time scales and finish with a discussion of future research priorities in the region

a low velocity zone with weak reflectivity along the nankai subduction zone three dimensional seismic reflection data reveal the presence of a low seismic velocity zone lvz with weak reflectivity character along the nankai accretionary prism this lvz is intercalated between an upper offscraped layer and a lower underthrusting layer in the outer accretionary wedge wide angle ocean bottom seismograph data also support the presence of the lvz which is estimated to be a maximum of 2 km thick 15 km wide and 120 km long the lvz could be an underthrust package underplated in response to the lateral growth of the nankai accretionary prism underplating of the underthrusting layer beneath the overlying offscraped layer would maintain a critical taper of the accretionary prism so that the offscraped layer can continue to grow seaward the lvz could have elevated fluid pressure leading to rigidity reduction of the entire outer accretionary wedge the rigidity lowered outer wedge containing the lvz may be more easily uplifted and thus eventually foster tsunami generation during a nankai megathrust earthquake if the fluid rich lvz supplies a significant amount of the fluid to the megasplay fault zone at depth it may affect stick slip behavior of the fault

crustal seismic imaging from multifold ocean bottom seismometer data by frequency domain full waveform tomography application to the eastern nankai trough 1 in offshore surveys the deep crust is generally investigated by traveltime tomography applied to sparse ocean bottom seismometer data the inferred velocity models are of limited resolution precluding a quantitative analysis of deep tectonic discontinuities if dense arrays of ocean bottom seismometers can be deployed the resulting data sets become amenable to full waveform processing which should lead to a significant improvement in the resolution of structures such acquisition and processing were achieved on the eastern nankai subduction zone full waveform inversion is entirely implemented in the frequency domain enabling efficient finite difference wave modeling and allowing to limit the inversion to a few discrete frequencies of increasing value such a hierarchical procedure defines a multiresolution approach to seismic imaging the data set was recorded by 93 instruments deployed along a 105 km long profile thirteen frequencies ranging from 3 to 15 hz were inverted sequentially wavelengths imaged by the full waveform inversion typically range between 0 5 and 8 km full waveform tomography reveals an intense compression in the upper prism and underlying backstop as evidenced by several thrusts the frontal ones still active corresponding to negative velocity anomalies the subducting paleo zenisu ridge is also reconstructed above which stands the decollement velocities in the upper mantle beneath the ridge are rather low 7 5 km s 1 suggesting partially serpentinized mantle beneath the ridge this paper demonstrates the feasibility of full waveform tomography based on dense ocean bottom seismometer data sets and its ability to quantitatively image the entire crust with a significantly improved resolution compared to what is usually achieved through traveltime tomography

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

effects of surface scattering in full waveform inversion in full waveform inversion of seismic body waves often the free surface is ignored on grounds of computational efficiency asyntheticstudywasperformedtoinvestigatetheeffectsofthissimplification intermsofsizeandfrequency the testmodelanddataconformtoareallong offsetsurveyofthe upper crust across the san andreas fault random fractal variations are superimposed on a background model with strong lateral and vertical velocity variations ranging from 1200to6800m s syntheticdatawerecomputedandinverted forthismodelanddifferenttopographies afullyviscoelastic time domain code was used to synthesize the seismograms and a viscoacoustic frequency domain code was utilized to invert them the inversion was focused on early arrivals which are dominated by p waves but also contain strong p rayleigh wave conversions from the near field of the receiver resulting waveform models show artifacts and a loss of resolution from neglecting the free surface in the inversion but the inversions are stable and they still improve the resolution of kinematic models the extent of deterioration dependsmoreonthesubsurfacethanonthesurfacestructure inversion results were improved at no additional expense by introducing a weak contrast along a staircase function above shotsandreceivers

thermoviscous remagnetization in some appalachian limestones experimental evidence shows that blocking temperature relaxation time theory for single domain magnetite can grossly underestimate laboratory thermal demagnetization temperatures of a present day viscous remanence in devonian limestones of new york state thermoviscous remagnetization at moderately elevated burial temperatures during the late paleozoic cannot therefore be readily excluded for the origin of the stable secondary component in these rocks more generally these results emphasize that single domain thermal activation theory cannot be assumed a priori to correctly predict temperature of thermoviscous remanence acquisition in all magnetite bearing rocks the prob leto

quantitative imaging of the permo mesozoic complex and its basement by frequency domain waveform tomography of wide aperture seismic data from the polish basin in this study we present the workflow and results of 2d frequency domain waveform tomography applied to the global offset seismic data acquired in central poland along a 50 km long profile during the grundy 2003 experiment the waveform tomography method allows full exploitation of the wide aperture content of these data and produces in a semi automatic way both the detailed p wave velocity model and the structural image i e perturbations in respect to the starting model thirteen frequencies ranging from 4 to 16 hz were inverted sequentially gradually introducing higher wavenumbers and more details into the velocity models although the data were characterised by relatively large shot spacings 1 5 km we obtained clear images both of the mesozoic and permian sedimentary cover velocity patterns indicated facies changes within the jurassic and zechstein strata a high velocity layer ca 5500 m s was found near the base of triassic scythian which made the imaging of a deeper layer difficult nevertheless we were able to delineate the base of the permian i e base of the rotliegend which was not possible to derive from conventional common depth point processing as well as some deeper events attributed to the carboniferous the sub permian events formed a syn form which favoured our previous interpretation of a depression filled with upper carboniferous molasse the validity of the waveform tomography derived model was confirmed by well log data forward ray tracing modelling and synthetic seismograms calculations provided another justification for the key structures present in the waveform tomography model

recent antarctic ice mass loss from radar interferometry and regional climate modelling observed estimates of ice losses in antarctica combined with regional modelling of ice accumulation in the interior suggest that east antarctica is close to a balanced mass budget but large losses of ice occur in the narrow outlet channels of west antarctic glaciers and at the northern tip of the antarctic peninsula

climate an exceptionally long interglacial ahead today9s comparatively warm climate has been the exception more than the rule during the last 500 000 years or more if recent warm periods or interglacials are a guide then we may soon slip into another glacial period but berger and loutre argue in their perspective that with or without human perturbations the current warm climate may last another 50 000 years the reason is a minimum in the eccentricity of earth9s orbit around the sun

toward a robust workflow for deep crustal imaging by fwi of obs data the eastern nankai trough revisited crustal scale imaging by the full waveform inversion fwi of long offset seismic data is inherently difficult because the large number of wavelengths propagating through the crust makes the inversion prone to cycle skipping therefore efficient crustal scale fwi requires an accurate starting model and a stable workflow minimizing the nonlinearity of the inversion here we attempt to reprocess a challenging 2d ocean bottom seismometer obs dataset from the eastern nankai trough the starting model is built by first arrival traveltime tomography fat which is fwi assisted for tracking cycle skipping we iteratively refine the picked traveltimes and then reiterate the fat until the traveltime residuals remain below the cycle skipping limit subsequently we apply laplace fourier fwi in which progressive relaxation of time damping is nested within frequency continuation to hierarchically inject more data into the inversion these two multiscale levels are complemented by a layer stripping approach implemented through offset continuation the reliability of the fwi velocity model is assessed by means of source wavelet estimation synthetic seismogram modeling ray tracing modeling dynamic warping and checkerboard tests although the visco acoustic approximation is used for wave modeling the synthetic seismograms reproduce most of the complexity of the data with a high traveltime accuracy the revised fwi scheme produces a high resolution velocity model of the entire crust that can be jointly interpreted with migrated images derived from multichannel seismic data this study opens a new perspective on the design of obs crustal scale experiments amenable to fwi however a further assessment of the optimal obs spacing is required for reliable fwi

melting of ice shelves and the mass balance of antarctica we calculate the present ice budget for antarctica from measurements of accumulation minus iceberg calving run off and in situ melting beneath the floating ice shelves the resulting negative mass balance of 469 gt year 1 differs substantially from other recent estimates but some components are subject to high temporal variability and budget uncertainties of 20 50 annual accumulation from an earlier review is adjusted to include the antarctic peninsula for a total of 2144 gt year 1 an iceberg production rate of 2016 gt year 1 is obtained from the volume of large icebergs calculated from satellite images since 1978 and from the results of an international iceberg census project ice shelf melting of 544 gt year 1 is derived from physical and geochemical observations of meltwater outflow glaciological field studies and modeling of the sub ice ocean circulation the highest melt rates occur near ice fronts and deep within sub ice cavities run off from the ice sheet surface and from beneath the grounded ice is taken to be 53 gt year 1 less than half of the negative mass balance need come from the grounded ice to account for the unattributed 0 45 mm year 1 in the ipcc best estimate of the recent global sea level rise

till beneath ice stream b 3 till deformation evidence and implications most of the velocity of ice stream b near the upstream b camp upb west antarctica appears to arise from deformation of a seismically detected subglacial till layer that averages 6 m thick available evidence indicates that the entire thickness of this till layer is deforming and is eroding subjacent bedrock into flutes parallel to ice flow and hundreds of meters across the resulting till flux beneath upb is equivalent to an average erosion rate of about 0 4 mm yr 1 in the catchment area and suggests that till deltas tens of kilometers long have been deposited at the grounding line during the holocene such deltas should be characterized by partial ice till decoupling across a water film and by a small ice air surface slope they may have been discovered by recent geophysical work

snowfall in coastal west antarctica much greater than previously assumed 1 a new antarctic accumulation distribution based on regional model output calibrated with 1900 in situ observations is used to re assess accumulation in 24 antarctic ice drainage basins when compared to the previous compilation good agreement is found for 19 of the 24 basins representing 93 of the ice sheet that is reasonably well covered with observations in contrast the amundsen sea sector of west antarctica and the western antarctic peninsula both data sparse regions are found to receive 80 96 more accumulation than previously assumed for the pine island and thwaites glaciers west antarctica which have recently undergone rapid acceleration and thinning this means a downward adjustment of their contribution to global sea level rise from 0 24 to 0 14 mm per year model time series do not show a significant change in antarctic accumulation over the period 1980 2004

application of waveform tomography to a crooked line 2d land seismic data set abstractonshore hydrocarbon exploration in the back arc region of japan suffers from seismic imaging challenges seismic data are typically acquired in a difficult environment along a crooked line over a rough surface topography underlain by severe near surface weathering layers the area is geologically complex and prestack depth processing is desirable however the data quality is suboptimal at near offsets and it hinders the migration velocity analysis required for depth processing thus a geologic interpretation needs to rely on time processing results nevertheless some data sets are rich in low frequency components and contain clear refracted and wide angle reflected waves both of which are favorable conditions for the application of waveform tomography we have used one such data set and we determined the applicability of waveform tomography to estimate the p wave velocity distribution to mitigate difficulties in the data we carefully optimized waveform tomography strategies and diligentl

late quaternary cycles of mangrove development and decline on the north australian continental shelf mangrove communities in the australian tropics presently occur as narrow belts of vegetation in estuaries and on sheltered muddy coasts palynological data from continental shelf and deep sea cores indicate a long term cyclical component of mangrove development and decline at a regional scale which can be linked to specific phases of late quaternary sealevel change extensive mangrove development relative to today occurs during periods of marine transgression whereas very diminished mangrove occurs during marine regressions and during rarer periods of relative sea level stability episodes of flourishing mangrove cannot be linked to phases of humid climate as has been suggested in studies elsewhere rather the cycle of expansion and decline of mangrove communities on a grand scale is explained in terms of contrasting physiographic settings characteristic of continental shelf coasts during transgressive and regressive phases in particular by the existence or lack of well developed tidal estuaries copyright c 1999 john wiley sons ltd

receiver coupling effects in seismic waveform inversions seismic waveform inversion offers opportunities for detailed characterization of the subsurface however its full potential can only be exploited when any systematic source and receiver effects are either carefully avoided or appropriately accounted for during the inversions repeated crosshole measurements in the mont terri switzerland underground laboratory have revealed that receiver coupling may significantly affect the seismic waveforms more seriously coupling conditions may vary during the course of a monitoring experiment to address this problem we have developed a novel scheme that estimates medium properties frequency dependent source functions and frequency dependent receiver coupling factors we demonstrate the efficacy of the new scheme via a synthetic 2d crosshole experiment in which realistic receiver coupling factors are incorporated because determination of medium parameters and estimation of source functions and receiver coupling factors are largely separated the method can be

multiscale imaging of complex structures from multifold wide aperture seismic data by frequency domain full waveform tomography application to a thrust belt summary an application of full waveform tomography to dense onshore wide aperture seismic data recorded in a complex geological setting thrust belt is presented the waveform modelling and tomography are implemented in the frequency domain the modelling part is solved with a finite difference method applied to the visco acoustic wave equation the inversion is based on a local gradient method only the p wave velocity is involved in the inversion the inversion is applied iteratively to discrete frequency components by proceeding from low to high frequencies this defines a multiscale imaging in the sense that high wavenumbers are progressively incorporated in images the linearized waveform tomography requires an accurate starting velocity model that has been developed by first arrival traveltime tomography after specific pre processing of the data 16 frequency components ranging between 5 4 and 20 hz were inverted ten iterations were computed per frequency component leading to 160 tomographic models the waveform tomography has successfully imaged southwest dipping structures previously identified from other geophysical data as being associated with high resistivity bodies the relevance of the tomographic images is locally demonstrated by comparison of a velocity depth function extracted from the waveform tomography models with a coincident vertical seismic profiling vsp log available on the profile moreover comparison between observed and synthetic seismograms computed in the starting traveltime and waveform tomography models demonstrates unambiguously that the waveform tomography successfully predicts for wide angle reflections from southwest dipping geological structures this study demonstrates that the combination of first arrival traveltime and frequency domain full waveform tomographies applied to dense wide aperture seismic data is a promising approach to quantitative imaging of complex geological structures indeed wide aperture acquisition geometries offer the opportunity to develop an accurate background velocity model for the subsequent waveform tomography this is critical because the building of the macromodel remains an open question when only near vertical reflection data are considered

high resolution near surface velocity model building using full waveform inversion a case study from southwest sweden full waveform inversion fwi is an iterative optimization technique that provides high resolution models of subsurface properties frequency domain acoustic fwi was applied to seismic data acquired over a known quick clay landslide scar in southwest sweden we inverted data from three 2 d seismic profiles 261 572 m long two of them shot with small charges of dynamite and one with a sledgehammer to our best knowledge this is the first published application of fwi to sledgehammer data both sources provided data suitable for waveform inversion the sledgehammer data containing even wider frequency spectrum inversion was performed for frequency groups between 27 5 and 43 1 hz for the explosive data and 27 5 51 0 hz for the sledgehammer the lowest inverted frequency was limited by the resonance frequency of the standard 28 hz geophones used in the survey high velocity granitic bedrock in the area is undulated and very shallow 15 100 m below the surface and exhibits a large p wave velocity contrast to the overlying normally consolidated sediments in order to mitigate the non linearity of the inverse problem we designed a multiscale layer stripping inversion strategy obtained p wave velocity models allowed to delineate the top of the bedrock and revealed distinct layers within the overlying sediments of clays and coarse grained materials models were verified in an extensive set of validating procedures and used for pre stack depth migration which confirmed their robustness

pollen evidence for the transition of the eastern australian climate system from the post glacial to the present day enso mode abstract a review of holocene climate patterns in eastern australia is presented on the basis of a series of high resolution pollen records across a north to south transect previously published radiocarbon data are calibrated into calendar years and fitted with an age depth model the resulting chronologies are used to compare past environmental changes and describe patterns of climate change on a calendar age scale based on the present day australian climate patterns and impact of the el nino southern oscillation enso the palynological data are interpreted and the prevalent climate mode throughout the holocene reconstructed results show that early holocene changes are strongly divergent and asynchronous between sites while middle to late holocene conditions are characterized by more arid and variable conditions and greater coupling between northern and southern sites which is in agreement with increasing influence of enso

attempts to date salt making activity in iron age britain using magnetic inclinations briquetage describes all the ceramic equipment used in extracting sea salt in prehistoric and roman europe it includes semi cylindrical desiccating troughs and the stands supports and clips used to stabilize them over hearths in many cases the baked hearth linings or superstructures are included under briquetage upon first inspection archeomagnetic dating seems inappropriate because the briquetage is broken and disoriented material although the ancient magnetic declination cannot be referred to the present north direction the desiccating troughs acted as water level gauges so that a tilt correction permits one to estimate the archaeoinclination the range of inclinations brackets the age of the saltern use within the last two centuries bc this is compatible with ceramic styles and two radiocarbon ages of charcoal from a supposedly contemporaneous hearth the archeodeclination of 7 95 10 from a contemporary in situ hearth confirms this age range thus we attribute the lincolnshire salterns to late iron age activity the paleosecular variation for southern britain over the last two millennia is sufficiently characteristic in many time periods to permit the use of inclinations alone to bracket ages this requires material with some original horizontal reference e g masonry fragments disrupted sedimentary layers or pot bases

ice sheet grounding line dynamics steady states stability and hysteresis 1 the ice sheet ice shelf transition zone plays an important role in controlling marine ice sheet dynamics as it determines the rate at which ice flows out of the grounded part of the ice sheet together with accumulation this outflow is the main control on the mass balance of the grounded sheet in this paper we verify the results of a boundary layer theory for ice flux in the transition zone against numerical solutions that are able to resolve the transition zone very close agreement is obtained and grid refinement in the transition zone is identified as a critical component in obtaining reliable numerical results the boundary layer theory confirms that ice flux through the grounding line in a two dimensional sheet shelf system increases sharply with ice thickness at the grounding line this result is then applied to the large scale dynamics of a marine ice sheet our principal results are that 1 marine ice sheets do not exhibit neutral equilibrium but have well defined discrete equilibrium profiles 2 steady grounding lines cannot be stable on reverse bed slopes and 3 marine ice sheets with overdeepened beds can undergo hysteresis under variations in sea level accumulation rate basal slipperiness and ice viscosity this hysteretic behavior can in principle explain the retreat of the west antarctic ice sheet following the last glacial maximum and may play a role in the dynamics of heinrich events

analysis of velocity field mass balance and basal melt of the lambert glacier amery ice shelf system by incorporating radarsat sar interferometry and icesat laser altimetry measurements 1 by incorporating recently available remote sensing data we investigated the mass balance for all individual tributary glacial basins of the lambert glacier amery ice shelf system east antarctica on the basis of the ice flow information derived from sar interferometry and icesat laser altimetry we have determined the spatial configuration of eight tributary drainage basins of the lambert amery glacial system by combining the coherence information from sar interferometry and the texture information from sar and modis images we have interpreted and refined the grounding line position we calculated ice volume flux of each tributary glacial basin based on the ice velocity field derived from radarsat three pass interferometry together with ice thickness data interpolated from australian and russian airborne radio echo sounding res surveys and inferred from icesat laser altimetry data our analysis reveals that three tributary basins have a significant net positive imbalance while five other subbasins are slightly positive or close to zero balance overall in contrast to previous studies we find that the grounded ice in lambert glacier amery ice shelf system has a positive mass imbalance of 22 9 4 4 gt a 1 the net basal melting for the entire amery ice shelf is estimated to be 27 0 7 0 gt a 1 the melting rate decreases rapidly from the grounding zone to the ice shelf front significant basal refreezing is detected in the downstream section of the ice shelf the mass balance estimates for both the grounded ice sheet and the ice shelf mass differ substantially from other recent estimates

antarctic ice sheet mass loss estimates using modified antarctic mapping mission surface flow observations 1 the long residence time of ice and the relatively gentle slopes of the antarctica ice sheet make basal sliding a unique positive feedback mechanism in enhancing ice discharge along preferred routes the highly organized ice stream channels extending to the interior from the lower reach of the outlets are a manifestation of the role of basal granular material in enhancing the ice flow in this study constraining the model simulated year 2000 ice flow fields with surface velocities obtained from insar measurements permits retrieval of the basal sliding parameters forward integrations of the ice model driven by atmospheric and oceanic parameters from coupled general circulation models under different emission scenarios provide a range of estimates of total ice mass loss during the 21st century the total mass loss rate has a small intermodel and interscenario spread rising from approximately 160 km3 yr at present to approximately 220 km3 yr by 2100 the accelerated mass loss rate of the antarctica ice sheet in a warming climate is due primarily to a dynamic response in the form of an increase in ice flow speed ice shelves contribute to this feedback through a reduced buttressing effect due to more frequent systematic tabular calving events for example by 2100 the ross ice shelf is projected to shed 40 km3 during each systematic tabular calving after the frontal section s attrition the remaining shelf will rebound consequently the submerged cross sectional area will reduce as will the buttressing stress longitudinal differential warming of ocean temperature contributes to tabular calving because of the prevalence of fringe ice shelves oceanic effects likely will play a very important role in the future mass balance of the antarctica ice sheet under a possible future warming climate

traveltime tomographic inversion with simultaneous static corrections well worth the effort we have developed a first arrival traveltime inversion scheme that jointly solves for seismic velocities and source and receiver static time terms the static time terms are included to compensate for varying time delays introduced by the near surface low velocity layer that is too thin to be resolved by tomography results on a real data set consisting of picked first arrival times from a seismic reflection 2d 3d experiment in a crystalline environment show that the tomography static time terms are very similar in values and distribution to refraction static corrections computed using standard refraction statics software when applied to 3d seismic reflection data tomography static time terms produce similar or more coherent seismic reflection images compared to the images using corrections from standard refraction static software furthermore the method provides a much more detailed model of the near surface bedrock velocity than standard software when the static time terms are included in the inversion low velocity zones in this model correlate with other geologic and geophysical data suggesting that our method results in a reliable model in addition to generally being required in seismic reflection imaging static corrections are also necessary in traveltime tomography to obtain high fidelity velocity images of the subsurface

new evidence for episodic post glacial sea level rise central great barrier reef australia abstract we present an extensive database of 364 radiocarbon dates from coastal and marine sediments of the central great barrier reef gbr shelf of which 110 are previously unpublished the elevation data have been reduced to a common datum australian height datum ahd and the various sources of error have been assessed using modern lithological and biological relationships with sea level the elements of the radiocarbon database have been converted into sea level indicators the upper bound of the assembled dataset corresponds to a best estimate sea level curve and the full dataset provides a narrow envelope for sea level rise on the gbr shelf for the last 11 12 kyr not including hydro isostatic crustal flexing the envelope is consistent with episodic rise in post glacial sea levels the rising post glacial sea level probably included stillstands or minor falls at ca 45 m ahd at ca 10 5 kyr b p 5 m 7 8 kyr b p 2 m ca 6 kyr b p and 1 7 m 5 5 kyr b p there is evidence for a significant fall in sea level between stillstands at 11 m 8 5 kyr b p and 17 m 8 2 kyr b p stillstand durations apparently ranged between the vertical spread in the derived sea level data is very wide the use of shell material for dating seems unreliable and prone to large and unpredictable errors data from bulk mangrove muds appear reliable for determination of ancient sea level but may at times result in sea level being placed up to 4 m below the true level in situ biogenic carbonates such as preserved oyster beds and coral micro atolls are the most reliable indicators of sea level position while deposits of mangrove mud give a useful first order approximation of ancient sea levels caution should be used in drawing sea level curves from few data points we conclude that the post 11 12 kyr b p relative rise in sea level was episodic on the central gbr continental margin more data are required to define clearly sea level change up to ca 20 m at 9 kyr b p

three dimensional seismic refraction tomography a comparison of two methods applied to data from the faeroe basin this paper presents a comparison of two tomographic methods for determining three dimensional 3 d velocity structure from first arrival travel time data the first method is backprojection in which travel time residuals are distributed along their ray paths independently of all other rays the second method is regularized inversion in which a combination of data misfit and model roughness is minimized to provide the smoothest model appropriate for the data errors both methods are nonlinear in that a starting model is required and new ray paths are calculated at each iteration travel times are calculated using an efficient implementation of an existing method for solving the eikonal equation by finite differencing both inverse methods are applied to 3 d ocean bottom seismometer obs data collected in 1993 over the faeroe basin consisting of 53 479 travel times recorded at 29 obss this is one of the most densely spaced large scale 3 d seismic refraction experiments to date different starting models and values for the free parameters of each tomographic method are tested a new form of backprojection that converges more rapidly than similar methods compares favorably with regularized inversion but the latter method provides a simpler model for little additional computational expense when applied to the faeroe basin data bounds on two model features are assessed using regularized inversion with combined smoothness and flatness constraints an inversion of synthetic data corresponding to 100 data recovery from the real experiment shows a marked improvement in lateral resolution at deeper depths and demonstrates the potential of currently feasible 3 d refraction experiments to provide well resolved long wavelength velocity models the similarity of the final models derived from the two tomographic methods suggests that the results from the new form of backprojection can be relied on when limited computational resources rule out regularized inversion

hitting the ice sheets where it hurts in both hemispheres glacier discharge to the sea has increased markedly in recent years as warm water from intermediate depths is melting the floating ends of glaciers from below accelerating them

full waveform inversion and distance separated simultaneous sweeping a study with a land seismic data set abstract dense wide aperture and broad frequency band acquisition improves seismic imaging and potentially allows the use of full waveform inversion for velocity model building the cost of dense acquisitions however limits its applicability blended or simultaneous shooting could lead to a good compromise between cost and dense acquisition although the cross talk between simultaneous sweeps may reduce imaging capabilities onshore a compromise is achieved with distance separated simultaneous sweeping acquisition because the shots are easily separated when the processing focuses on pre critical reflected events full waveform inversion for velocity model building however relies on post critical reflected refracted and diving events these events can interfere in a distance separated simultaneous sweeping acquisition by using a single vibrator single receiver data set recorded in inner mongolia china a distance separated simultaneous sweeping data set is created to study the robustness of full waveform inversion in this acquisition context this data set is well suited for full waveform inversion since it contains frequencies down to 1 5 hz and offsets up to 25 km full waveform inversion after a crude deblending of the distance separated simultaneous sweeping data set leads to a result very similar to the one obtained from the single vibrator single receiver data set the inversion of the blended data set gives a slightly less good result because of the cross talk but it is still quite satisfactory

the search for a topographic signature of life landscapes are shaped by the uplift deformation and breakdown of bedrock and the erosion transport and deposition of sediment life is important in all of these processes over short timescales the impact of life is quite apparent rock weathering soil formation and erosion slope stability and river dynamics are directly influenced by biotic processes that mediate chemical reactions dilate soil disrupt the ground surface and add strength with a weave of roots over geologic time biotic effects are less obvious but equally important biota affect climate and climatic conditions dictate the mechanisms and rates of erosion that control topographic evolution apart from the obvious influence of humans does the resulting landscape bear an unmistakable stamp of life the influence of life on topography is a topic that has remained largely unexplored erosion laws that explicitly include biotic effects are needed to explore how intrinsically small scale biotic processes can influence the form of entire landscapes and to determine whether these processes create a distinctive topography

theoretical calving rates from glaciers along ice walls grounded in water of variable depths calving has been studied for glaciers ranging from slow polar glaciers that calve on dry land such as on deception island 63 0 s 60 6 w in antarctica through temperate alaskan tide water glaciers to fast outlet glaciers that float in fiords and calve in deep water such as jakobshavns isbrae 69 2 n 49 9 w in greenland calving from grounded ice walls and floating ice shelves is the main ablation mechanism for the antarctic and greenland ice sheets as it was along marine and lacustrine margins of former pleistocene ice sheets and is for tide water and polar glaciers

ice ocean interaction on ronne ice shelf antarctica detailed glaciological studies have been completed at 28 sites lying on an approximate flow line extending 760 km across ronne ice shelf parameters measured at each location include ice velocity thickness principal strain rates surface elevation temperature and accumulation rate the data have been used in a steady state model to derive the basal mass flux and the temperature profile with depth at each site from the principles of mass and energy conservation these calculations indicate basal melting in excess of 1 m yr 1 over the first 100 km of the flow line downstream of the grounding line where the ice shelf is between 1200 m and 1600 m thick the maximum melt rates in this region occur near the inland margin and exceed 4 m yr 1 melting continues at a lesser rate over the next 200 km before freezing commences freezing then dominates up to the final 100 km before the ice front resulting in the accumulation of a layer of basal sea ice up to 50 m thick this is rapidly removed as melt rates increase to over 6 m yr 1 at the ice front the cumulative effect of this pattern of basal accumulation and ablation is the wastage by melting of 85 of the mass discharged across the grounding line before it reaches the ice front supporting evidence for a layer of saline ice underlying the ice shelf is provided by the strength of basal radar reflections observed during radio echo sounding of ice thickness in the region where mass balance calculations suggest an accretion of basal sea ice reflection coefficients are consistently low ranging from 6 db to 38 db most of these weak reflections are believed to originate from the true base of the ice shelf the additional energy loss being the result of increased attenuation of the radar signal within the saline layer the derived pattern of basal melting and freezing is consistent with a simple model of sub ice shelf oceanic circulation involving a deep thermohaline convection cell dense saline water which is formed over the continental shelf during winter when the sea surface freezes drains into the deepest parts of the sub ice shelf cavity at the inland margin where this water mass comes into contact with the ice shelf its temperature is 1 c above the local pressure freezing point melting of ice results producing a buoyant outflow of cold relatively fresh water along the ice shelf base basal freezing occurs towards the ice front where the ascending water becomes supercooled this circulation has important implications for the production of antarctic bottom water and for the response of the ice shelf to driving stresses through the temperature dependent viscosity of ice

inverse theory applied to multi source cross hole tomography part 1 acoustic wave equation method1 frequency domain methods are well suited to the imaging of wide aperture cross hole data however although the combination of the frequency domain with the wavenumber domain has facilitated the development of rapid algorithms such as diffraction tomography this has also required linearization with respect to homogeneous reference media this restriction and association restrictions on source receiver geometries are overcome by applying inverse techniques that operate in the frequency space domain in order to incorporate the rigorous modelling technique of finite differences into the inverse procedure a nonlinear approach is used to reduce computational costs the method of finite differences is applied directly to the frequency domain wave equation the use of high speed high capacity vector computers allow the resultant finite difference equations to be factored in place in this way wavefields can be computed for additional source positions at minimal extra cost allowing inversions to be generated using data from a very large number of source positions synthetic studies show that where weak scatter approximations are valid diffraction tomography performs slightly better than a single iteration of non linear inversion however if the background velocities increase systematically with depth diffraction tomography is ineffective whereas non linear inversion yields useful images from one frequency component of the data after a single iteration further synthetic studies indicate the efficacy of the method in the time lapse monitoring of injection fluids in tertiary hydrocarbon recovery projects

a holocene pollen and diatom record from vanderlin island gulf of carpentaria lowland tropical australia sedimentary palynological and diatom data from a dunefield lake deposit in the interior of vanderlin island in the gulf of carpentaria are presented prior to the formation of present perennial lake conditions the intensified australian monsoon associated with the early holocene marine transgression allowed cyperaceae sedges to colonise the alluvial margins of an expansive salt flat surrounded by an open eucalyptus woodland as sea level stabilised between 7500 and 4500 cal yr b p coastal dunes ceased to develop allowing dense melaleuca forest to establish in a restionaceae swamp dune sand input into the swamp was diminished further as the increasingly dense vegetation prevented fluvial and aeolian transported sand arriving from coastal sources this same process impounded the drainage basin allowing a perennial lake to form between 5500 and 4000 cal yr b p myriophyllum and other aquatic taxa colonised the lake periphery under the most extensive woodland recorded for the holocene the palynological data support an effective precipitation model proposed for northern australia that suggests more variable conditions in the late holocene a more precise measure of effective precipitation change is provided by diatom based inferences that indicate few changes in lake hydrology such interpretations are explained in terms of palynological sensitivity to adjustments in local fire regimes where regional precipitation change may only be recorded indirectly through fire promoting mechanisms including intensified enso periodicity and human impact

detecting near surface objects with seismic waveform tomography three shallow high velocity rubble filled targets are imaged using waveform tomography in an engineering scale clay embankment at seven sisters falls manitoba canada to locate targets buried at approximately 7 m as a blind test of geophysical imaging methods previous studies use near offset reflection methods to image the targets however this test uses waveform tomography of the long offset refracted arrivals to image p velocity and seismic attenuation the targets are invisible to standard traveltime tomography using weight drop data with frequencies of 20 150 hz the subwavelength targets are resolved in the velocity images and complementary images of seismic q are produced the interpreted target locations are consistent with limited survey information from the embankment construction multiple quality control efforts paired with a very good fit between model and observed data indicate the reliability of the results

surface elevation changes at the front of the ross ice shelf implications for basal melting 1 rapid melting beneath the ross ice shelf ris occurs near the ice front likely in response to a secondary buoyant plume with tidal mixing and is sensitive to seasonal water temperatures in front of the ice shelf the front of the ris is examined using glas icesat laser altimetry data spatial and temporal changes in surface elevations are attributed to enhanced basal melting of the ice shelf near the ice front melt rates b increase exponentially as the front is approached from approximately zero at 40 km from the front to an average of 2 8 1 0 m a 1 within the front kilometer melt estimates within the front 60 km are best fit by the relationship b 2 0 exp x 11900 m a 1 where x denotes distance from the front frontal melt totals approximately 16 km3 a 1 in the front 40 km which accounts for between 10 and 40 of current published estimates of total melt beneath the ris spatial averaging along the front reveals a different pattern of melt in regions that have recently calved as compared to regions that have not calved frontal melt is modeled as a one dimensional buoyant plume with tidal currents included these model results imply that modest increases in sea surface temperatures will lead to considerable increases in melting beneath the ice shelf front

salt production in pre funan vietnam archaeomagnetic reorientation of briquetage fragments bronze to iron age briquetage found in the northern mekong delta has an appearance similar to analogous material from europe and asia however the orientation in which the briquetage was employed during the production of salt is still under debate as a consequence of the heating and subsequent cooling of the briquetage during the evaporative recovery of salt the magnetic mineral particles within the ceramic formed a stable thermoremanent magnetization in alignment with the earth s magnetic field it thus becomes possible to find the orientation in which the ceramics were last fired by aligning their recorded archaeomagnetic signal with estimates of the earth s ancient field direction in vietnam the archaeomagnetic directions obtained from 22 samples taken from five different briquetage artefacts are somewhat scattered but they reveal a consistent orientation and thus the mode in which the briquetage was employed can be reconstructed

vegetation change following mid holocene marine transgression of the torres strait shelf a record from the island of mua northern australia the pollen and sedimentary record from a coastal backdune swamp on the island of mua torres strait australia is presented a 4 55 m core collected from the swamp centre provides a record of vegetation and landscape change spanning the postglacial marine transgression to present prior to 6000 radiocarbon years before present yr bp results show mangrove vegetation encroaching on the core site periodically displacing non mangrove taxa until the establishment of an extensive mangrove forest between 6000 yr bp and 3000 yr bp within the mangrove community a transition from lower tidal rhizophora forest to an upper intertidal ceriops community is evident this is followed by the development of the current herbaceous freshwater swamp in the late holocene the dryland vegetation record is dominated by sclerophyll and rainforest elements with strongest forest representation occurring around the mid holocene before a decline in tree density and the establishment of open woodlands in the late holocene the data suggest vegetation change accompanied marine transgression and a humid mid holocene climate before stabilization of sea levels and the initiation of dominant on shore catchment processes signalling drier climatic conditions and possible human activity

detailed structural image around splay fault branching in the nankai subduction seismogenic zone results from a high density ocean bottom seismic survey 1 to investigate megathrust earthquake and tsunami generation in a subduction seismogenic zone it is important to know the detailed structure around the plate boundary and active splay fault system the nankai trough southwestern japan is among the best studied subduction zones with splay faults in the world this paper presents a detailed structural image around the splay fault in the coseismic rupture zone of the 1944 tonankai earthquake in the central nankai trough based on results from a wide angle high density ocean bottom seismograph obs survey our seismic image clearly shows for the first time the subduction structure along with the splay fault reflections imaged on a previously obtained seismic reflection profile the correspondence between the splay fault reflections imaged on the seismic reflection profile and our reflectivity image was confirmed by converting the time migrated section to a depth section by using a velocity model estimated from the wide angle obs data in this study the high resolution seismic image obtained by the high density obs survey demonstrates a zone of low velocity gradient just above and along the splay fault we focused specifically on the detailed structure around the splay fault a simple amplitude analysis of the wide angle reflections showed that it is likely that a thin low velocity layer with velocities 0 5 1 5 km s slower than those in the surrounding rock exists above the splay fault the presence of this layer suggests elevated fluid pressure in the fault zone which cut through the relatively homogeneous rock of the accretionary prism and generated past megathrust earthquakes

lowstand rivers need not incise the shelf an example from the great barrier reef australia with implications for sequence stratigraphic models a key tenet of many sequence stratigraphic models is that sea level lowering causes widespread fluvial incision of the subaerially exposed continental shelf and that river borne terrigenoclastic sediments bypass the lowstand shoreline via canyons to the continental slope and basin floor consequently the occurrence of incised channels is considered a fundamental criterion for the recognition of sequence boundaries in ancient shelf successions contrary to this we argue that rivers may not necessarily incise during glacio eustatic lowstands if they flow out onto a coastal plain flanked by a broad low angle shelf on the great barrier reef shelf fluvial incision did not occur during the last glacial maximum lgm but instead subaerial accommodation was created and infilled as contemporary rivers graded to the lgm bayline incision was restricted to the lowstand shelf break where canyons of limited extent formed by nickpoint retreat

evidence of episodic coastal change during the late holocene the dungeness barrier complex se england plater a j stupples p roberts h m 2009 evidence of episodic coastal change during the late holocene the dungeness barrier complex se england geomorphology 104 1 2 47 58

misfit functionals in laplace fourier domain waveform inversion with application to wide angle ocean bottom seismograph data in seismic waveform inversion non linearity and non uniqueness require appropriate strategies we formulate four types of l2 normed misfit functionals for laplace fourier domain waveform inversion i subtraction of complex valued observed data from complex valued predicted data the conventional phase amplitude residual ii a conventional phase only residual in which amplitude variations are normalized iii a logarithmic phase amplitude residual and finally iv a logarithmic phase only residual in which the only imaginary part of the logarithmic residual is used we evaluate these misfit functionals by using a wide angle field ocean bottom seismograph obs data set with a maximum offset of 55 km the conventional phase amplitude approach is restricted in illumination and delineates only shallow velocity structures in contrast the other three misfit functionals retrieve detailed velocity structures with clear lithological boundaries down to the deeper part of the model we also test the performance of additional phase amplitude inversions starting from the logarithmic phase only inversion result the resulting velocity updates are prominent only in the high wavenumber components sharpening the lithological boundaries we argue that the discrepancies in the behaviours of the misfit functionals are primarily caused by the sensitivities of the model gradient to strong amplitude variations in the data as the observed data amplitudes are dominated by the near offset traces the conventional phase amplitude inversion primarily updates the shallow structures as a result in contrast the other three misfit functionals eliminate the strong dependence on amplitude variation naturally and enhance the depth of illumination we further suggest that the phase only inversions are sufficient to obtain robust and reliable velocity structures and the amplitude information is of secondary importance in constraining subsurface velocity models

holocene palaeoenvironments and change at three quarter mile lake silver plains station cape york peninsula australia pollen and diatom analyses of organic sediments from three quarter mile lake a perched lake on cape york peninsula north queensland indicate that significant changes in vegetation and hydrology occurred during the holocene early holocene grass dominated landscapes were replaced in mid holocene times by increasingly woody vegetation comprising tropical heathlands savanna and rainforest early holocene lake levels fluctuated widely from mid holocene times lake levels stabilized and water became increasingly acidic as a mature swamp forest developed adjacent to the lake and contributed tannins to the lake water the timing and character of changes are consistent with those described from the atherton tableland in wet tropical queensland holocene dry phases described from the northern territory and the western shores of cape york cannot be identified from three quarter mile lake rainforest is currently close to its greatest holocene extent suggesting that the rainforest dependent endemic fauna of no

paleothermometry of the sydney basin evidence from overprinting of magnetizations of late permian and mesozoic rocks and from the rank of permian coals and mesozoic phytoclasts coal particles suggests that surface rocks in the sydney basin eastern australia have been raised to temperatures of the order of 200 sup 0 c or higher as vitrinite reflectance an index of coal rank or coalification is postulated to vary predictably with temperature and time estimates of the paleotemperatures in the sydney basin based on observed vitrinite reflectance measurements can be made in conjunction with reasonable assumptions about the tectonic and thermal histories of the basin these estimates give maximum paleotemperatures of present day surface rocks in the range 60 249 sup 0 c depending on factors such as location in the basin the thickness of the sediment eroded and the maximum paleogeothermal gradient higher coal rank and consequently larger eroded thicknesses and paleogeothermal gradients occur along the eastern edge of the basin and may be related to seafloor spreading in the tasman sea on the basin s eastern margin a theory of thermal activation of magnetization entailing the dependence of magnetic viscosity on the size distribution of the magnetic grains is used to obtain an independent estimate of the maximum paleotemperaturesmore in the sydney basin this estimate places the maximum paleotemperature in the range 250 300 sup 0 c along the coastal region both coalification and thermal activation of magnetization models provide strong evidence of elevated paleotemperatures which in places exceed 200 sup 0 c and the loss of sediment thicknesses in excess of 1 km due to erosion less

holocene evolution of coastal wetlands in wet tropical northeastern australia pollen in sediments drilled from the innisfail coastal plain northeast queensland australia was examined to reconstruct the evolution of holocene wetlands in a wet tropical environment in contrast to monsoonal australia stable environmental conditions created by year round rainfall and low tidal range caused abrupt unidirectional transitions in wetland zonation as marine influence changed this has enabled a detailed reconstruction of the marine transgression and subsequent progradation mangroves colonized in response to marine transgression at c 7400 bp as riverine mangroves of low salt tolerance migrated up the mulgrave river these were replaced around 7000 bp by extensive rhizophora dominated mangroves coinciding with the development of a rhizophora dominated community in wyvuri embayment mangroves reached their greatest extent as sea level rise slowed towards 6000 bp with a stillstand indicated by a brief return to terrestrial conditions at one site high freshwater input depressed salt i

a palynological investigation of holocene vegetation change in torres strait seasonal tropics of northern australia the islands of torres strait occupy a shallow area of submerged continental shelf narrowly separating cape york peninsula australia from new guinea the human history of torres strait is unique with respect to mainland northern australia island vegetation however exhibits a strong affinity with the environments of the western lowlands regions of cape york peninsula and with the vegetation of seasonal tropical australia in general cape york peninsula is both climatically and biologically diverse yet few pollen studies have been carried out in its seasonally tropical environments a summary presentation of palynological results tracing the nature of vegetation change in torres strait offers a possible framework for vegetation changes in similar environments on mainland australia and also provides an opportunity to explore the relationship between quaternary change in humid tropical australian environments and their seasonal tropical counterparts six pollen records from torres strait provide evidence of vegetation change and fire history over approximately the last 8000 years near shore sediments reveal a holocene succession in vegetation incorporating lower tidal mangrove upper tidal mangrove saltmarsh and freshwater swamp communities extensive stable mangrove communities dominated coastal torres strait between approximately 6000 and 3000 radiocarbon years before present yr bp inland the strongest myrtaceae forest and rainforest representation occurs around the mid holocene only to be replaced by open sclerophyll woodlands as tree density and diversity decline in the last 3000 years the development of continuous island freshwater swamp conditions at the coast and inland is similarly restricted to the late holocene c 2600 yr bp and fire as a prominent feature in the torres strait environment is also a relatively recent phenomenon comparisons with regional mainland australian palynological records reveal a degree of consistency in results from torres strait suggesting a similarity in late quaternary trends through australian humid and seasonally tropical environments a number of differences however are also apparent highlighting a degree of diversity which warrants further attention

holocene palynology of whitehaven swamp whitsunday island queensland and implications for the regional archaeological record palynological study of whitehaven swamp whitsunday island provides the first holocene palaeoenvironmental record for the whitsunday region on the central queensland coast sediment stratigraphy and radiocarbon dating indicate continuous freshwater swamp conditions since around 7000 radiocarbon years before present bp pollen and charcoal analyses provide local and regional vegetation and fire histories for the site and surrounding area varying representation of swamp elements particularly leptocarpus and cyperaceae provides evidence for phases of permanent and ephemeral swamp conditions the regional vegetation record is dominated by rainforest sclerophyll and beach strand elements strongest rainforest representation occurs around the mid holocene while sclerophyll elements increase from the late holocene to present charcoal analyses indicate that fire has been a constant component of the whitsunday environment throughout the period represented negative correlation between high charcoal and leptocarpus pollen concentrations suggests a strong local component to the charcoal record and a history of on site burning during ephemeral swamp phases the vegetation reconstruction suggests moister than present conditions at whitehaven between approximately 7000 to 4500 bp this complies with claims for a mid holocene climatic optimum based on pollen records from the atherton tableland to the north but contrasts with suggested mid holocene aridity based on a surrogate lake water level record from fraser island to the south comparisons with the regional archaeological record provide no evidence for direct links between major environmental change and archaeologically identified cultural change in particular claims for late holocene population intensification are not matched by changes in the charcoal record this may suggest that widespread vegetation burning was not a predominant feature of hunter gatherer strategies that were focused towards marine resources and or that human induced fire regimes were already well entrenched prior to intensification

full waveform tomography for lithospheric imaging results from a blind test in a realistic crustal model summary a comprehensive validation of 2 d frequency domain acoustic wave equation tomography was undertaken in a blind test using third party realistic elastic wave equation data the synthetic 2 d wide angle seismic data were provided prior to a recent workshop on the methods of controlled source seismology the true model was not revealed to the authors until after the presentation of our waveform tomography results the original model was specified on a detailed grid with variable p wave velocity s wave velocity density and viscoelastic q factor structure designed to simulate a section of continental crust 250 km long and 40 km deep synthetic vertical and horizontal component data were available for 51 shot locations spaced every 5 km recorded at 2779 receivers spaced every 90 m evenly spread along the surface of the model the data contained energy from 0 2 to 15 hz waveform tomography a combination of traveltime tomography and 2 d waveform inversion of the early arrivals of the seismic waveforms was used to recover crustal p velocity structure from the vertical component data using data from 51 sources 1390 receivers and frequencies between 0 8 and 7 0 hz the waveform tomography result contained apparent structure at wavelength scale resolution that was not evident on the traveltime tomography result the predicted acoustic waveforms in the final result matched the original elastic data to a high degree of accuracy during the workshop the exact model was revealed over much of the model the waveform tomography results provided a good correspondence with the true model from large to intermediate wavelength scales with a resolution limit on the order of 1 km a significant near surface low velocity zone invisible to traveltime methods was correctly recovered the results also provided a high resolution image of the complex structure of the entire crust and the depth and nature of the crust mantle transition some inaccuracies were observed near the edges of the images due to the limited ray coverage inherent to the footprint of the survey geometry several aspects of the waveform tomography strategy were critical to the success of the acoustic method with realistic synthetic viscoelastic data i the accuracy of the starting model from traveltime tomography ii implementation in the frequency domain iii the use of complex valued frequencies to effect time damping of the data residuals iv the selection of a suitable subset of data and data frequencies v progressive inversion of low to high frequency components of the data vi initial pre inversion matching of the amplitudes between observed and modelled data and vii sufficient preconditioning of both the data and the update images combined these strategies were effectively equivalent to a multiscale approach that mitigated the non linearity of the seismic inverse problem during the inversion we carried out repeated forward modelling to ensure our modelled waveforms matched the observed data as closely as possible in both frequency and time domains the synthetic data set used in this paper provides a benchmark for future testing of modelling inversion and imaging algorithms for wide angle lithospheric imaging

pleistocene vegetation of the humid tropics of northeastern queensland australia abstract the humid tropics area of northeastern queensland has provided a substantial and detailed history of rainforest and rainforest savanna interactions from the pollen analysis of a number of sites on the volcanic atherton tableland through the latter part of the quaternary period a recent extension of the record to close to the base of the quaternary from examination of an offshore core provides a broad regional and temporal context for evaluation of the true significance of late quaternary changes it is apparent that the major sustained change in the vegetation of the region occurred relatively recently probably within the last 140 000 yr and that this change was time transgressive it involved the replacement of extensive moist rainforest by open eucalypt woodland and is considered to have been most likely caused by the burning activities of aboriginal people some aspects of the record from this region can contribute to the history of rainforest on a global scale but other features of it re inforce the unique nature of the development of australian vegetation patterns

waveform tomography of field vibroseis data using an approximate 2d geometry leads to improved velocity models waveform tomography a combination of traveltime tomography or inversion and waveform inversion is applied to vibroseis first arrival data to generate an interpretable model of p wave velocity for a site in the nechako basin south central british columbia canada we use constrained 3d traveltime inversion followed by 2d full waveform inversion to process long offset 14 4 km first arrival refraction waveforms resulting in a velocity model of significantly higher detail than a conventional refraction statics model generated for a processing workflow the crooked line acquisition of the data set makes 2d full waveform inversion difficult thus a procedure that improves the tractability of waveform tomography processing of vibroseis data recorded on crooked roads is developed to generate a near surface 2 km velocity model for the study area the data waveforms are first static corrected using a time shift determined by 3d raytracing which accounts for the crossline offsets produced by the crooke

cross well seismic waveform tomography for monitoring co2 injection a case study from the ketzin site germany seismic full waveform inversion waveform tomography is a method to reconstruct the underground velocity field in high resolution using seismic data the method was first introduced during the 1980 s and became computationally feasible during the late 1990 s when the method was implemented in the frequency domain this work presents three case studies and one synthetic benchmark of full waveform inversion applications two of the case studies are focused on time lapse cross well and 2d reflection seismic data sets acquired at the ketzin co2 geological storage site these studies are parts of the co2sink and co2man projects the results show that waveform tomography is more effective than traveltime tomography for the co2 injection monitoring at the ketzin site for the cross well geometry for the surface data sets we find it is difficult to recover the true value of the velocity anomaly due to the injection using the waveform inversion method but it is possible to qualitatively locate the distribution of the injected co2 the results agree well with expectations based upon conventional 2d cdp processing methods and more extensive 3d cdp processing methods in the area a further investigation was done to study the feasibility and efficiency of seismic full waveform inversion for time lapse monitoring of onshore co2 geological storage sites using a reflection seismic geometry with synthetic data sets the results show that waveform inversion may be a good complement to standard cdp processing when monitoring co2 injection the choice of method and strategy for waveform inversion is quite dependent on the goals of the time lapse monitoring of the co2 injection the last case study is an application of the full waveform inversion method to two crooked profiles at the forsmark site in eastern central sweden the main goal of this study was to help determine if the observed reflections are mainly due to fluid filled fracture zones or mafic sills one main difficulty here is that the profiles have a crooked line geometry which corresponds to 3d seismic geometry but a 2d based inversion method is being used this is partly handled by a 3d to 2d coordinate projection method from traveltime inversion the results show that these reflections are primarily due to zones of lower velocity consistent with them being generated at water filled fracture zones

on acoustic waveform tomography of wide angle obs data strategies for pre conditioning and inversion we successfully apply the acoustic laplace fourier waveform tomography method to delineate p wave velocity structures of the mega splay fault system in the central part of the seismogenic nankai subduction zone offshore japan using densely sampled wide angle ocean bottom seismograph obs data originally acquired in 2004 our success is due to new and carefully designed data preconditioning and inversion strategies to mitigate i the well known non linearity of waveform inversion ii the challenges arising from crustal scale survey designs e g undersampling of the obss and iii modelling errors due to the use of the acoustic assumption we identify a sixfold set of key components that together lead to the success of the high resolution waveform tomography image i availability of low frequency components starting at 2 25 hz reducing the non linearity and access to large offset data up to 55 km increasing the depth of illumination and the recovery of low wavenumber components ii a highly accurate traveltime tomography result with an rms error of approximately 60 ms that further mitigates the non linearity iii a hierarchical inversion approach in which phase spectra are inverted first to reduce artefacts from the acoustic assumption and amplitude information is only incorporated in the final stages iv a laplace fourier domain approach that facilitates a multiscale approach to mitigate non linearity by restricting the inversion to the low frequency components and early arrivals first and sequentially including higher frequencies and later arrivals v a pre conditioning strategy for eliminating undesirable high wavenumber components from the the gradient vi a strategy for source estimation that reduce the influence of the instrumental design in the obs case study used for illustration purposes laplace fourier waveform tomography retrieves velocity anomalies as small as 700 m horizontally and 350 m vertically above the top of the philippine sea plate the resulting velocity structures include low velocity zones and thrust structures which have not been previously identified clearly the velocity models are validated by scrutiny of synthetic and observed waveforms by evaluating the coherency of source estimates and by comparison with 3 d pre stack migrated presdm images chequerboard tests and point scatter tests demonstrate both the reliability and the limitations of the acoustic implementation

stability of the west antarctic ice sheet in a warming world equivalent to 0 28 to 0 56 mm yr 1 sea level rise with the rate growing over at least the past two decades 1 4 these observations intensify concerns about the ice sheet s stability that were first raised more than four decades ago 5

diffraction tomography and multisource holography applied to seismic imaging seismic tomography is emerging as an imaging method for determining subsurface structure when the view angle coverage is limited and the scale of the medium inhomogeneities is comparable with the wavelength as is often true in geophysical applications the performance of ordinary ray tomography becomes poor other tomographic methods are needed to improve the imaging process here we study diffraction tomography and multisource holography and evaluate their performances for surface reflection profiling srp vertical seismic profiling vsp and cross hole measurements theoretical formulations are derived for two dimensional geometry in terms of line sources along a source line and line receivers along a receiver line the theory for diffraction tomography is based on the born or rytov approximation the performances of diffraction tomography and multisource holography are evaluated by examining the information coverage in the spatial frequency domain and by numerical examples multisource holography

paleomagnetism of a lateritic paleoweathering horizon and overlying paleoproterozoic red beds from south africa implications for the kaapvaal apparent polar wander path and a confirmation of atmospheric oxygen enrichment the olifantshoek group in southern africa contains paleoproterozoic red beds that are exceptionally well preserved lying unconformably atop a regionally extensive lateritic paleoweathering profile we studied the basal unit of this succession known as the gamagara or mapedi formation and the lateritized substrate so called drakenstein or wolhaarkop paleosol on which it developed two ancient magnetic components are observed one int usually with a distributed unblocking spectrum between 300 and 600 c but occasionally persisting to 675 c is directed shallowly southward or northward a mesoscale fold test at south sishen mine indicates that this component was acquired during deformation similarity of the direction to previous results suggests that it was acquired at 1240 ma during early namaqua orogenesis combining our int results with existing data from the namaqua eastern zone nez we calculate the nez pole at 44 9 n 021 5 e k 23 2 a 95 12 8 q 5 the most stable component from our data set hig always persisting as a nonzero endpoint to demagnetization at 665 680 c is observed in 32 samples from south sishen and beeshoek mines directed moderately east downward sishen or west upward beeshoek this component predates the mesoscale fold at sishen more importantly a conglomerate test at beeshoek indicates that hig was acquired prior to paleoproterozoic deposition of the gamagara mapedi formation the concordance between directions from the paleoweathering zone and immediately overlying red beds indicates that hig is a primary magnetic remanence for the basal gamagara mapedi bgm formation the dual polarity bgm paleomagnetic pole 02 2 n 081 9 e dp 7 2 dm 11 5 q 6 lies neatly between previous kaapvaal poles with ages of 2220 ongeluk lavas and 2060 ma bushveld complex our data thus support recent correlations of the gamagara mapedi formation with pre bushveld sediments of the pretoria group a pre bushveld age for bgm is also consistent with its substantial distance from a new albeit less reliable paleomagnetic pole from the 1930 ma hartley lavas higher within the olifantshoek succession 12 5 n 332 8 e k 18 6 a95 16 0 q 3 our conglomerate test at beeshoek confirms previous allegations that the intense hematitization observed in the drakenstein wolhaarkop paleosol occurred during paleoproterozoic weathering under a highly oxygenated atmosphere

reassessment of the potential sea level rise from a collapse of the west antarctic ice sheet theory has suggested that the west antarctic ice sheet may be inherently unstable recent observations lend weight to this hypothesis we reassess the potential contribution to eustatic and regional sea level from a rapid collapse of the ice sheet and find that previous assessments have substantially overestimated its likely primary contribution we obtain a value for the global eustatic sea level rise contribution of about 3 3 meters with important regional variations the maximum increase is concentrated along the pacific and atlantic seaboard of the united states where the value is about 25 greater than the global mean even for the case of a partial collapse

variational estimation of the wind stress drag coefficient and the oceanic eddy viscosity profile abstract a variational optimal control technique is used to assimilate both meteorological and oceanographic observations into an oceanic ekman layer model an identical twin experiment is discussed first in which the observations are created by the dynamic model the field measurements from the lotus 3 long term upper ocean study 3 buoy are then analysed by fitting the model results to the data the unknown boundary condition the wind stress drag coefficient and the unknown vertical eddy viscosity distribution are deduced simultaneously from the data and an optimal estimate of the current field is obtained though the model is simple the results show that the variational assimilation technique is capable of extracting from the available observations a reasonable wind stress drag coefficient and vertical eddy viscosity distribution

secular variation of archaeomagnetic direction in the american southwest a d 750 1425 oriented archaeomagnetic samples were collected from 158 in situ features at 33 archaeological sites in the southwestern united states seventy three independently dated features were used for analysis of secular variation a moving window technique with outlier rejection was developed to compute a smoothed secular variation curve this technique incorporates weighted fisher statistics to account for imprecision in both the age of remanence acquisition and the direction of magnetization for each feature a mean direction and measure of dispersion is generated for each window of time the secular variation record covers the period a d 750 1425 during which time the virtual geomagnetic pole vgp moves in a generally counterclockwise fashion from 86 n 83 e at a d 750 to the lowest latitude position of 74 n 192 e at a d 1075 and then to 85 n 236 e at a d 1425 the median rate of vgp movement is 0 036 yr 1 similar to present day rates the southwestern record shows close correspondence to other north american archaeomagnetic limnomagnetic and speleomagnetic records the archaeomagnetic record can serve as the master curve for the derivation of archaeomagnetic dates

the link between climate warming and break up of ice shelves in the antarctic peninsula a review of in situ and remote sensing data covering the ice shelves of the antarctic peninsula provides a series of characteristics closely associated with rapid shelf retreat deeply embayed ice fronts calving of myriad small elongate bergs in punctuated events increasing flow speed and the presence of melt ponds on the ice shelf surface in the vicinity of the break ups as climate has warmed in the antarctic peninsula region melt season duration and the extent of ponding have increased most break up events have occurred during longer melt seasons suggesting that meltwater itself not just warming is responsible regions that show melting without pond formation are relatively unchanged melt ponds thus appear to be a robust harbinger of ice shelf retreat we use these observations to guide a model of ice shelf flow and the effects of meltwater crevasses present in a region of surface ponding will likely fill to the brim with water we hypothesize building on weertman 1973 hughes 1983 and van der veen 1998 that crevasse propagation by meltwater is the main mechanism by which ice shelves weaken and retreat a thermodynamic finite element model is used to evaluate ice flow and the strain field and simple extensions of this model are used to investigate crack propagation by meltwater the model results support the hypothesis