twenty first century warming of a large antarctic ice shelf cavity by a redirected coastal current the redirection of warm water under the filchner ronne ice shelf during the second half of this century could cause the ice shelf base to melt at a rate 20 times higher than at present warm ocean currents are known to erode ice shelves from below but changes in currents can be forced by many different mechanisms leading to uncertain outcomes this study highlights the vulnerability to climate change of a small antarctic coastal region which has potentially severe consequences for the mass balance of a large antarctic ice shelf hellmer et al use climate modelling to show that the projected loss of sea ice in the weddell sea east of the antarctic peninsula leads to an increase in wind stress which in turn accelerates a warm ocean current far underneath the vast filchner ronne ice shelf the authors predict that the increased warmth could increase melt by a factor of 20 with possible consequences for ice stream dynamics in the east antarctic ice sheet the antarctic ice sheet loses mass at its fringes bordering the southern ocean at this boundary warm circumpolar water can override the continental slope front reaching the grounding line1 2 through submarine glacial troughs and causing high rates of melting at the deep ice shelf bases3 4 the interplay between ocean currents and continental bathymetry is therefore likely to influence future rates of ice mass loss here we show that a redirection of the coastal current into the filchner trough and underneath the filchner ronne ice shelf during the second half of the twenty first century would lead to increased movement of warm waters into the deep southern ice shelf cavity water temperatures in the cavity would increase by more than 2 degrees celsius and boost average basal melting from 0 2 metres or 82 billion tonnes per year to almost 4 metres or 1 600 billion tonnes per year our results which are based on the output of a coupled ice ocean model forced by a range of atmospheric outputs from the hadcm35 climate model suggest that the changes would be caused primarily by an increase in ocean surface stress in the southeastern weddell sea due to thinning of the formerly consolidated sea ice cover the projected ice loss at the base of the filchner ronne ice shelf represents 80 per cent of the present antarctic surface mass balance6 thus the quantification of basal mass loss under changing climate conditions is important for projections regarding the dynamics of antarctic ice streams and ice shelves and global sea level rise

a low velocity zone within the layer 3 region of 118 myr old oceanic crust in the western north atlantic summary during the north atlantic transect nat experiment 11 expanding spread profiles esps were acquired using a 3300 cubic inch 30 element tuned airgun array s v prospekta and a 48 channel seismic streamer r v moore extremely high density seismograms were obtained that have allowed us to transform the observed data to the domain of intercept time and ray parameter tau p with high precision velocity structures have been obtained by tau sum inversion ray tracing and synthetic wkbj and reflectivity seismogram methods both in tau p and x t domains a detailed analysis of esp 5 suggests the presence of a low velocity zone within the lower oceanic crust at the location of magnetic anomaly mo 118 ma it is apparently 2 86km thick has a p wave velocity of 6 5 km s 1 s wave velocity of 3 6 km s 1 and occupies much of the depth interval normally associated with oceanic layer 3 converted shear wave arrivals have been used to obtain the shear velocity structure we find that the vp vs ratio in the uppermost part of the crust is about 1 9 and much of the crustal section has a vp vsratio greater than 3 attenuation of p and s waves has been studied by amplitude analysis and we find that in most of the lower crust qs is equal to qp comparison with solutions from esps in younger and older parts of the basin suggests that esp 5 is unique in displaying a low velocity zone despite this the total thickness at esp 5 is not significantly different than at other locations we suggest that this provides strong evidence that the low velocity zone occurs within the lower crust and does not result from serpentinization of the upper mantle beneath thin oceanic crust

a minimum sample size required from schmidt hammer measurements the schmidt hammer is a useful tool applied by geomorphologists to measure rock strength in field conditions the essence of field application is to obtain a sufficiently large dataset of individual rebound values which yields a meaningful numerical value of mean strength although there is general agreement that a certain minimum sample size is required to proceed with the statistics the choice of size i e number of individual impacts was usually intuitive and arbitrary in this paper we show a simple statistical method based on the two sample student s t test to objectively estimate the minimum number of rebound measurements we present the results as 1 the mean and median solutions each providing a single estimate value and 2 the empirical probability distribution of such estimates based on many field samples schmidt hammer data for 14 lithologies 13 81 samples for each with each sample consisting of 40 individual readings have been evaluated assuming different significance levels the principal recommendations are 1 the recommended minimum sample size for weak and moderately strong rock is 25 2 a sample size of 15 is sufficient for sandstones and shales 3 strong and coarse rocks require 30 readings at a site 4 the minimum sample size may be reduced by one third if the context of research allows for higher significance level for test statistics interpretations based on less than 10 readings from a site should definitely be avoided copyright 2009 john wiley sons ltd

a multichannel seismic study of lithospheric flexure across the hawaiian emperor seamount chain existing models assume that the thickened crust beneath seamounts is the result of a surface volcanic load flexing an elastic plate new results suggest that flexed oceanic crust beneath the hawaiian emperor seamount chain is underlain by a 4 km thick deep crustal body we intepret the body as a deep crustal sill complex associated with the tholeiitic stage of volcano building along the chain

potential sea level rise from antarctic ice sheet instability constrained by observations large parts of the antarctic ice sheet lying on bedrock below sea level may be vulnerable to marine ice sheet instability misi a self sustaining retreat of the grounding line triggered by oceanic or atmospheric changes there is growing evidence that misi may be underway throughout the amundsen sea embayment ase which contains ice equivalent to more than a metre of global sea level rise if triggered in other regions the centennial to millennial contribution could be several metres physically plausible projections are challenging numerical models with sufficient spatial resolution to simulate grounding line processes have been too computationally expensive to generate large ensembles for uncertainty assessment and lower resolution model projections rely on parameterizations that are only loosely constrained by present day changes here we project that the antarctic ice sheet will contribute up to 30 cm sea level equivalent by 2100 and 72 cm by 2200 95 quantiles where the ase dominates our process based statistical approach gives skewed and complex probability distributions single mode 10 cm at 2100 two modes 49 cm and 6 cm at 2200 the dependence of sliding on basal friction is a key unknown nonlinear relationships favour higher contributions results are conditional on assessments of misi risk on the basis of projected triggers under the climate scenario a1b ref 9 although sensitivity to these is limited by theoretical and topographical constraints on the rate and extent of ice loss we find that contributions are restricted by a combination of these constraints calibration with success in simulating observed ase losses and low assessed risk in some basins our assessment suggests that upper bound estimates from low resolution models and physical arguments up to a metre by 2100 and around one and a half by 2200 are implausible under current understanding of physical mechanisms and potential triggers

attenuation of p waves in oceanic crust multiple scattering from observed heterogeneities we extend a model for seismic attenuation that includes scattering and intrinsic components q 1 qs 1 qi 1 over five decades in experimental wavelength core images and downhole logs from ocean drilling program hole 735b near the atlantis ii fracture zone in the indian ocean were used to construct vertical profiles of heterogeneity and to compute a spectrum of qs 1 using a multiple scattering model measurements of velocity dispersion suggest that qi 1 is small compared to the measured attenuation q 1 measurements of q 1 made at this site by laboratory sonic log and vsp experiments are compared to the computed qs 1 spectrum peaks in the qs 1 spectrum occur at six major correlation lengths scales 0 006 m 0 02 m 0 16 m 0 63 m 8 0 m and 25 0 m which can be attributed to formation heterogeneity compared to typical seismic estimates of q 1 in oceanic layer 3 the measurements in hole 735b are high but can be explained by large qs 1 values in shallow fractured ocean crust based on this comparison we conclude that the proximity of the experimental wavelength in band limited studies to the length scale of heterogeneity is critical in the measurement of q 1

rock cities and ruiniform relief forms processes terminology abstract spectacular rock cut landscapes consisting of closely spaced residual rock blocks separated by narrow intersecting corridors are variously termed as rock cities and ruiniform relief the former term has apparently never been formally defined except in the local czech literature whereas the latter was applied in various contexts making the terminology confusing this paper provides a global overview of forms processes and controls behind such rock city ruiniform landscapes and develops geomorphological terminology for such rock cut landscapes including the proposal that rock city becomes a legitimate scientific term rock cities and ruiniform relief are two different types of rock erosional morphology typified by the presence of a street pattern in the former and highly weathered surfaces of rock residuals in the latter although there is some overlap between the two examples of rock cities or ruiniform relief may be found in different lithologies and in a wide range of climates from hyper arid to humid classic localities include northern czechia and adjacent parts of poland and germany australia central sahara and the gran sabana in venezuela lithologies particularly suitable to the development of this type of erosional morphology include mainly sandstones followed by limestones dolomites conglomerates tuffs but igneous rocks such as granites or rhyolites are also known to support this type of relief the combination of sufficient rock strength to support steep rock faces and the presence of discontinuities which preferentially eroded turn to streets and squares in the rock cities appears as the basic prerequisite a wide range of processes are involved in their formation including surface and subsurface weathering mass movements salt tectonics karst processes in both carbonate and silicate rocks piping fluvial erosion and aeolian abrasion despite being spectacular landscapes very few models of long term evolution of rock city and ruiniform relief have been presented in the literature and it is unlikely that any one scenario would apply to all situations rather rock cities are examples of geomorphological convergence

solution weathering rate and origin of karst landforms and caves in the quartzite of auyan tepui gran sabana venezuela abstract the paper reports the results of sio 2 analyses in the aonda cave system located on the auyan tepui one of the widest table mountains of the gran sabana south venezuela characterised by karst landforms developed in siliceous rock chemical analyses underline the very low concentration of sio 2 of the surface water percolation and cave drip waters have a sio 2 concentration of about 1 mg l the mean silica load of the cave stream is 184 mg s mainly derived from surface solution removal in the allogenic recharge area in the aonda cave system the mean sio 2 dissolved load is 40 mg s in part from surface solution 15 and mainly from underground processes 85 the low solubility of sio 2 in slightly acidic water implies the importance of the time factor in the formation of cave systems with the present dissolution rate about 10 ma would be necessary to form the known karst system this estimation can be significant only if we assume that climate has been stable in the last few tens of millions of years furthermore this age can be taken as a minimum estimate while according to the geomorphic evolution of the area the origin of the aonda cave system could be reasonably dated back to at least 20 30 ma that is to the oligocene

seismic attenuation in upper ocean crust at hole 504b seismic attenuation and its relationship to borehole stratigraphy in the upper 1 8 km of ocean basement at hole 504b are determined from analysis of vertical seismic profile vsp data vsp data provide unambiguous measurements of seismic amplitude decay along a vertical propagation path through the crust and ancillary borehole measurements enable detailed modeling of the relative contributions from geometrical spreading scattering and intrinsic loss mechanisms to this decay about 60 of the total observed amplitude decay occurs in the pillow basalt section and is due mostly to geometrical spreading and scattering from impedance contrasts the remaining amplitude decrease is concentrated in two layers at 500 650 and 800 900 meters below seafloor mbsf 225 375 and 525 625 m below basement across which amplitude rapidly decays and the frequency characteristics of the downgoing wave field are significantly and permanently modified attenuation in these layers is not due to scattering but rather to an intrinsic mechanism that can be characterized by q of 10 and 8 respectively it is likely that the q structure of both of these intervals is formed with the crust near the ridge and thus related to fundamental ocean crust forming processes the shallow interval coincides with a change in alteration mineralogy deposited by late stage fluid flow and may separate lower lavas that were emplaced within the rift zone from upper lavas that were emplaced by off axis flow through large lava tubes intrinsic attenuation in the deeper horizon is probably due to an increase in porosity and cracking associated with either intracrustal deformation or subhorizontal faulting there is negligible attenuation of seismic frequencies in the dikes below 1000 mbsf 725 m subbasement

integrating satellite observations with modelling basal shear stress of the filcher ronne ice streams antarctica using inverse methods constrained by recent satellite observations we have produced a comprehensive estimate of the basal shear stress beneath the filchner ronne ice streams the inversions indicate that a weak bed approx 4 20 kpa underlies much of these ice streams compared to the ross ice streams the distribution of weak subglacial till is more heterogeneous with sticky spots providing much of the resistance to flow a weak bed beneath recovery ice stream extends several hundred kilometres inland with flow along this ice stream discrepancies between thickness measurements and flux estimates suggest the existence of a deep 1400 m trough not resolved by existing maps of subglacial topography we hypothesize that the presence of this and other deep troughs is a major influence on this sector of the ice sheet that is not fully incorporated in current models of ice sheet evolution

a deep subglacial embayment adjacent to the grounding line of institute ice stream west antarctica the institute ice stream iis in west antarctica may be increasingly vulnerable to melting at the grounding line through modifications in ocean circulation understanding such change requires knowledge of grounding line boundary conditions including the topography on which it rests here we discuss evidence from new radio echo sounding res data on the subglacial topography adjacent to the grounding line of the iis in doing so we reveal a previously unknown subglacial embayment immediately inland of the iis grounding zone which is not represented in the bedmap2 compilation we discuss whether there is an open water connection between the embayment and the ice shelf cavity the exact location of the grounding line over the embayment has been the subject of considerable uncertainty with several positions being proposed recently from our compilation of data we are able to explain which of these grounding lines is most likely and in doing so highlight the need for accurate bed topography in conjunction with satellite observations to fully comprehend ice sheet processes in this region and other vulnerable locations at the grounded margin of antarctica

pravcice rock arch bohemian switzerland national park czech republic deterioration due to natural and anthropogenic weathering the dominant factors affecting the development of weathering forms on the pravcice rock arch pra bohemian switzerland national park czech republic are discussed based on the in situ monitoring of weathering forms as well as on laboratory studies of materials taken from this site the in situ monitoring shows the progressive development of distinct weathering forms delamination of case hardened rock surfaces cyclic efflorescence and granular disintegration of exposed weakly cemented rock mass in specific portions of the pra whose location is controlled by lithology jointing local hydrodynamics and exposure to climatic factors from the analysis of efflorescence the dominance of sulphates is evident particularly of gypsum and alums however the synergistic effects of the several salt species that also include minor nitrates and chlorides upon the mechanical disintegration of sandstone can be expected

the effect of thermal cycling on the seismic quality factor q of some sedimentary rocks changes in q in rocks in response to thermal cycling can result from the removal of physisorbed water an increase in the crack density and therfore the internal surface area at temperatures greater than 400 sup 0 c the removal of chemisorbed oh groups which are the adsorption sites for physisorbed water and the dehydration of clays when rocks are heated to temperatures of 100 sup 0 200 sup 0 c the change is q is reversible and is due to the removal of physisorbed water sioux quartzite and berea sandstone were thermally cycled to 657 sup 0 c and 385 sup 0 c respectively the q of both rocks increased when measured over a broad range of relative humidities but decreased at low humidities 10 25 measurements of adsorption isotherms for the berea sandstone showed no loss of adsorptive capacity of the clays although the surface area of thermally cracked rocks increased the number of sites where adsorbed water molecules contribute to attenuation decreased

steep reverse bed slope at the grounding line of the weddell sea sector in west antarctica the bed of the west antarctic ice sheet is in places more than 1 5 km below sea level1 2 it has been suggested that a positive ice loss feedback may occur when an ice sheet s grounding line retreats across a deepening bed1 2 3 applied to the west antarctic ice sheet this process could potentially raise global sea level4 by more than 3 m hitherto attention has focussed on changes at the siple coast5 6 7 and amundsen sea embayment8 9 10 sectors of west antarctica here we present radio echo sounding information from the ice sheet s third sector the weddell sea embayment that reveals a large subglacial basin immediately upstream of the grounding line the reverse bed slope is steep with about 400 m of decline over 40 km the basin floor is smooth and flat with little small scale topography that would delay retreat indicating that it has been covered with marine sediment5 11 and was previously deglaciated upstream of the basin well defined glacially carved fjords with bars at their mouths testify to the position of a former ice margin about 200 km inland from the present margin evidence so far suggests that the weddell sea sector of the west antarctic ice sheet has been stable but in the light of our data we propose that the region could be near a physical threshold of substantial change

bedrock channel incision along piccaninny creek australia piccaninny creek in northwestern australia drains approximately 60 km 2 of high relief sandstone and conglomerate terrain the drainage basin lies within the seasonal tropics and is characterized by ephemeral high magnitude flows flood flows of 50 to 100 m 3 s 1 are probably primarily responsible for the spectacular erosional features along the bedrock channel segments of the creek these erosional features occur in two basic forms as parallel longitudinal grooves and as a downstream sequence that begins with shallow linear depressions parallel to flow and ends in a deep narrow inner channel the longitudinal grooves are assumed to result from longitudinal vortices while the erosional sequences which closely resemble von karman vortex streets are interpreted to result from turbulent vortices shed off irregularities of the channel bed the longitudinal grooves and the inner channel sequence alternate with each other and with gravel floored depositional reaches downstream along the channe

subglacial geology and geomorphology of the pensacola pole basin east antarctica the east antarctic ice sheet eais is underlain by a series of low lying subglacial sedimentary basins the extent geology and basal topography of these sedimentary basins are important boundary conditions governing the dynamics of the overlying ice sheet this is particularly pertinent for basins close to the grounding line wherein the eais is grounded below sea level and therefore potentially vulnerable to rapid retreat here we analyze newly acquired airborne geophysical data over the pensacola pole basin ppb a previously unexplored sector of the eais using a combination of gravity magnetic and ice penetrating radar data we present the first detailed subglacial sedimentary basin model for the ppb radar data reveal that the ppb is defined by a topographic depression situated 500 m below sea level gravity and magnetic depth to source modeling indicate that the southern part of the basin is underlain by a sedimentary succession 2 3 km thick this is interpreted as an equivalent of the beacon supergroup and associated ferrar dolerites that are exposed along the margin of east antarctica however we find that similar rocks appear to be largely absent from the northern part of the basin close to the present day grounding line in addition the eastern margin of the basin is characterized by a major geological boundary and a system of overdeepened subglacial troughs we suggest that these characteristics of the basin may reflect the behavior of past ice sheets and or exert an influence on the present day dynamics of the overlying eais

influence of subglacial conditions on ice stream dynamics seismic and potential field data from pine island glacier west antarctica 1 we interpret seismic reflection and airborne potential field data acquired on pine island glacier west antarctica and find variations in the subglacial geology which correlate with variations in ice dynamics immediately beneath the glacier is a mixture of soft deforming sediments and harder non deforming sediments beneath this a sedimentary basin lies under part of the main glacier with another under one of its slower moving tributaries a tectonic boundary underlies the main trunk of the glacier separating these sedimentary basins to the north from crystalline rocks to the south which also include a thick rift related magmatic intrusion the boundary correlates with changes in the basal roughness ice flow speed and basal drag smoother bed faster flow and lower basal drag characterize the thicker sedimentary sequences to the north but there is no corresponding lateral change in the acoustic properties of the bed changes in the sub bed i e deeper than the ice bed interface lithology appear to account for the contrasting basal drag and ice velocity patterns over the glacier subglacial erosion could remove a thin layer of soft sediments to the south of the geological boundary leading to increased basal drag and reduced ice flow in the future we conclude that the subglacial geology plays a significant role in controlling the spatial pattern of present day ice flow and that the consequences of subglacial erosion may be reflected in temporal changes to the ice dynamics in the past and perhaps also in the near future

combined 40 ar 39 ar and u th he geochronological constraints on long term landscape evolution of the second parana plateau and its ruiniform surface features parana brazil regional correlation of dated weathered land surfaces provides the necessary constraints to test long term continental landscape evolution models but major challenges remain in properly dating these surfaces the geomorphological province of second parana plateau parana state brazil is a high elevation ca 800 m land surface characterized by widely distributed deep saprolites and scattered lateritic profiles e g vila velha and serra das almas prolonged exposure to weathering and erosion has promoted the pseudo karstic and ruiniform features that are characteristic of this landscape in this study 40 ar 39 ar laser incremental heating geochronology on 22 grains of supergene mn oxyhydroxides from lateritic profiles at vila velha yielded results ranging from 17 2 0 7 to 9 1 0 7 ma u th he geochronology on 28 goethite grains from the same profile yielded results ranging from 36 4 3 6 to 1 0 0 1 ma with an age cluster lying within the 17 2 0 7 to 7 9 0 8 ma interval u th he geochronology on 17 goethite grains from the serra das almas lateritic profile located 20 km from vila velha yield results ranging from 35 1 3 5 to 14 1 1 4 ma the combined results for the two sites reveal a common weathering history that started ca 35 ma suggesting that the second parana plateau results from regional fluvial incision and denudation before 35 ma followed by a decline in denudation rates and proportionally more intense weathering consistent with the laterite profile central ages weathering was particularly intense during the miocene 17 8 ma denudation intensified after the pliocene

quantifying subglacial bed roughness in antarctica implications for ice sheet dynamics and history abstract glaciated landscapes consist of complex assemblages of landforms resulting from ice flow dynamic regimes and ice sheet history superimposed over and in turn modifying preglacial topography lithology and geological structure insights into the formation of glaciated landscapes can in principle be obtained by analysing modern ice sheet beds but terrain analyses beneath modern ice sheets are restricted by the inaccessibility of the bed it is however possible to quantify roughness the vertical variation of the subglacial interface with horizontal distance along two dimensional images of the bed obtained from radio echo sounding res here we collate several case studies from antarctica where roughness calculations have been used as a glaciological tool to infer basal processes and ice sheet history over large 500 km 2 areas we present two examples from west antarctica which demonstrate the utility of bed roughness in determining the presence and extent of subglacial sediments glacial dynamics and former ice sheet size we also present two examples from east antarctica which illustrate how roughness provides knowledge of ice sheet dynamics in the interior and pre quaternary ice sheet histories in modern ice sheet settings characterising bed roughness along res tracks has the twin advantages of being relatively simple to calculate while producing informative subsurface data and is especially powerful at furthering understanding when coupled with knowledge of ice flow from field satellite and modelling investigations the technique also offers significant potential for the comparison of modern and former ice sheet terrains contributing to an improved understanding of the formation and evolution of glaciated landscapes

a global review of solutional weathering forms on quartz sandstones abstract solutional landforms in limestone have been described for over a hundred years but landforms of similar morphology on highly siliceous sandstones and quartzites have also been identified in a wide variety of environments and generally termed pseudokarst these include large bedrock pinnacles and towers caves corridors grikes solution basins and runnels and even silica speleothems quartzites and quartz sandstones have been held to be amongst the most chemically resistant of rocks but the similarity both in morphology and genetic process of many landforms developed from them to features of known solutional origin on limestone has prompted some authors to refer to these quartzose landforms as true karst the most detailed studies of quartzose karst landforms have been in present day tropical regions or areas believed to have been tropical in the geologically recent past this concentration of research in hot wet areas allied with the long held assertion of the insolubility of silica especially quartz has led to a belief that tropical climatic conditions are necessary for karstic solution of these rocks however the existence of quartzose karst landforms in temperate and even sub polar latitudes especially where there is no evidence of prior tropical conditions suggests that the requirement of tropical weathering is no longer tenable the reports of these quartzose solutional landforms are widely scattered through the geomorphological and geological literature but a comprehensive world wide review of the range of solutional landforms on quartzose rocks has not previously been published because of the increasing awareness in this karst type such a summary is sorely overdue

salt as an agent in the development of cavernous weathering a well recognized spatial association exists between cavernous weathering and the presence of soluble salts disaggregation of rock in caverns is attributed to mechanical pressures exerted by crystallization and or hydration of salts however scanning electron micrographic analysis demonstrates that solutional etching is the major weathering process in caverns in near coastal locations in western australia and new south wales australia it is suggested here that the breakdown of the caverned sandstone is caused by sodium chloride which increases the rate of quartz dissolution

1 ma east pacific rise oceanic crust and uppermost mantle exposed by rifting in hess deep equatorial pacific ocean abstract a series of dives with the french submersible nautile has been conducted in the region of the galapagos triple junction at the tip of the cocos nazca propagator most of the dives were devoted to the study of hess deep a rift valley produced by rifting of oceanic crust created at the axis of the very fast spreading east pacific rise the dives have enabled the structures and lithologies to be assessed and confirmed earlier dredging evidence for abundant basal crust and mantle rocks on an intra rift ridge located to the north of the axis of hess deep this shows that this situation is not specific not slow spreading ridges a nearly complete and unique log of young 1 ma crust and uppermost mantle representative of very fast accretion can be reconstructed even though the section is discontinuous and highly dismembered we propose two models that can explain the exposure on the ocean floor of the deep foundation of the oceanic crust one model stresses symmetry and vertical tectonics through isostasy diapirism of serpentinized mantle another model requires a larger amount of extension and emphasizes asymmetry with denudation of the upper crust through low angle detachment faulting in both models which differ mainly in the structure at depth amagmatic extension of east pacific rise crust by the cocos nazca propagator tip results in uplift and denudation of lower crust and upper mantle

sandstone landforms of the tropical east kimberley region northwestern australia extensive karst in quartzose rocks has been reported from several tropical cratons but has been overlooked in widely accepted models of tropical landscape evolution the siliceous sedimentary rocks in the east kimberley region of northwestern australia display many large scale karst features like those found in limestones the sandstone terrain of the east kimberleys has been eroded under a tropical savanna climate from highly quartzose sediments which suffered intense antecedent leaching of silica the leaching can be attributed to long lasting tropical climates acting on a highly stable cratonic surface diverse topography on the sandstones reflects marked contrasts in mass strength and modes of failure that were determined primarily by porosities at the onset of weathering in the late mesozoic or early tertiary

future sea level rise due to projected ocean warming beneath the filchner ronne ice shelf a coupled model study a general ocean circulation model is coupled with a 3d thermodynamical ice sheet shelf model to simulate the response of the filchner ronne ice shelf fris antarctica and coastal parts of its catchment basin to a postulated inflow of warm deep water into the ice shelf cavity on a 1000 yr timescale prescribed ocean warming based on climate projections enters the ice shelf cavity in the up to 1500 m deep filchner trough and penetrates deep into the sub ice cavity increasing basal melt rates induce geometry changes of the cavity which in turn have an impact on the ocean circulation and therefore the modelled melt rates highest melt rates of about 20 m yr 1 follow the up to 180 km retreating grounding line basal mass loss reaches about 250 km3 yr 1 doubling the present day value the most vulnerable areas below the fris are the bailey ice stream and the area between the institute and moeller ice streams where the increased melting accounts for about 80 km of the modelled grounding line retreat on the backward sloping bedrock the potential additional contribution to the eustatic sea level rise due to the grounded ice loss simulated in an ensemble approach against a transient control experiment is about 0 05 mm yr 1 during the first 500 yr and about 0 17 mm yr 1 thereafter

macro scale bed roughness of the siple coast ice streams in west antarctica subglacial bed conditions are key to understanding ice stream behaviour and evolution with bed roughness re ecting substrate composition and ow resistance here we present an analysis of bed roughness in the siple coast region of west antarctica from airborne radio echo sounding data the ice streams are associated generally with low bed roughness values which decrease downstream the bed of the slow owing ice stream c 10 000 km2 is also characterized by being smooth at all scales wavelengths ranging from 5 km to in excess of 40 km furthermore the bed is smooth either side of ice stream c this suggests the location of the ice stream is controlled by internal ice sheet dynamics rather than by bed morphology if the ice stream were encouraged to migrate laterally when active there would be little resistance offered by the subglacial morphology other inter ice stream regions are rough however indicating a subglacial topographic in uence on ice stream position bed roughness increases up ow of ice streams which unless the bed is modi ed may limit the inland migration of these systems copyright 2004 john wiley sons ltd

velocity and attenuation in young oceanic crust new downhole log results from dsdp odp iodp holes 504b and 1256d 1 to characterize the acoustic properties of oceanic crust and investigate the nature of the oceanic layer 2 3 seismic boundary we analyze sonic logging waveforms recorded in the two deepest holes ever drilled in intact oceanic crust ocean drilling program odp integrated ocean drilling program iodp hole 1256d and deep sea drilling program dsdp odp hole 504b hole 1256d penetrated 15 ma old crust on a superfast spreading segment of the cocos plate while hole 504b was drilled in a 6 ma old section of an intermediate spreading segment of the nazca plate careful reprocessing of different sets of waveforms allows us to draw depth profiles of the primary parameters controlling wave propagation in the crust compressional and shear velocities vp and vs and attenuation the results underline the importance of proper filtering and processing parameters for the acquisition of acoustic logs in hard rocks while the reprocessed vp logs improve previous data particularly at the bottom of hole 1256d the vs logs represent significant enhancements in quality and in extent of previous profiles in the two holes attenuation results provide the first measure of intrinsic attenuation in these holes the velocity and attenuation logs confirm that hole 504b had reached oceanic layer 3 without recovering gabbros conversely while gabbros were recovered in hole 1256d no layer 3 velocity values were recorded in the interval logged however the reprocessed data suggest that the transition to layer 3 might occur within the deepest 87 m of the hole that were not logged or immediately below accordingly the layer 2 3 seismic boundary does not appear to be related to the top of the plutonic crust despite different ages spreading rates and mineralogy however the two sites display remarkably similar velocity and attenuation profiles through the entire crustal sections logged suggesting that they can be reliably considered representative of the acoustic properties of the nazca cocos spreading system and more generally of undisturbed oceanic crust

seismic attenuation structure of the east pacific rise near 9 30 n we describe a spectral technique to measure the apparent attenuation of compressional waveforms recorded during an active seismic tomography experiment centered at 9 32 n on the east pacific rise over 3500 estimates of t are obtained from 0 4 to 0 7 s long windows aligned with crustal p phases including diffractions above and below a midcrustal magma chamber and moho reflections which cross the rise axis at a range of lower crustal depths we apply a smoothest model inversion algorithm to the t measurements to derive images of apparent crustal q 1 both 20 km off axis and within a 16 16 km area centered on the rise crest the models resolve regions of high attenuation in the uppermost crust and in a low q zone which extends from midcrustal to lower crustal depths beneath the rise axis off axis q values in the upper 1 km average 35 50 while at depths greater than 2 3 km q is at least 500 1000 the high levels of attenuation in the uppermost crust probably result from the combined effect of factional fluid flow and scattering mechanisms within 1 3 km of the rise axis q increases markedly in the uppermost 1 km to about 65 if the increase in attenuation off axis is entirely due to the 300 m increase in the thickness of layer 2a extrusives required by seismic velocity measurements then q in layer 2a must be about 10 20 no measurements of q are obtained in the immediate vicinity of the 1 6 km deep axial magma lens because no wave paths cross the rise axis through this region the diffractions beneath the magma chamber and the moho reflections require a low q region with minimum q values of 20 50 which extends from no more than 2 5 km depth to the base of the crust these values are similar to laboratory measurements of q obtained at solidus temperatures and constrain the low q region to contain no more than a few percent melt the axial magma chamber which comprises a melt lens and an underlying crystal mush zone must be confined to a narrow 1 km thick region through which no rise crossing paths pass inversions for along axis structure in the low q anomaly show a 20 25 increase in attenuation at 2 3 km depth north of 9 34 n but resolve no such trend at 4 6 km depth the along axis variations may reflect the recent history of volcanic eruptions and hydrothermal cooling and do not require systematic along axis variations in magma supply

biofilms and case hardening on sandstones from al quwayra jordan on cambrian and ordovician sandstone inselbergs at al quwayra in southern jordan many near vertical rock walls exhibit marked case hardening detailed examination of the case hardened layer from four sites on both red and white sandstones has been carried out using schmidt hammer simple chemical analyses scanning electron microscopy energy dispersive spectroscopy and inductively coupled plasma mass spectrometry these analyses reveal a range of types of case hardening often in association with cryptoendolithic bio lms containing cyanobacteria and fungi iron manganese and calcium are all found to be common constituents of cements within the case hardened layer micro organic bio lms may assist in the formation of case hardening through aiding cementation but are by no means a necessary component of all the hardened layers in the wadi rum area copyright 2004 john wiley sons ltd

subglacial controls on the flow of institute ice stream west antarctica the institute ice stream iis rests on a reverse sloping bed extending 150 km upstream into the 1 8 km deep robin subglacial basin placing it at the threshold of marine ice sheet instability understanding iis vulnerability has focused on the effect of grounding line melting which is forecast to increase significantly this century changes to ice flow dynamics are also important to iis stability yet little is known about them here we reveal that the trunk of the iis occurs downstream of the intersection of three discrete subglacial features a large active subglacial lake a newly discovered sharp transition to a zone of weak basal sediments and a major tectonic rift the border of iis trunk flow is confined by the sediment on one side and by a transition between basal melting and freezing at the border with the bungenstock ice rise by showing how basal sediment and water dictate present day flow of iis we reveal that ice sheet stability here is dependent on this unusual arrangement

how much gabbro is in ocean seismic layer 3 ocean seismic layer 3 is distinguished from layer 2 by higher velocities lower variability and lower gradients in velocity with depth based on studies of rocks recovered from ophiolites and walls of fracture zones most models of lower ocean crust correlate seismic layer 3 with sequences of gabbroic lithologies the average velocity 6 5 km s for a vertical seismic profile in a gabbro sequence at ocean drilling program odp hole 735b is consistent with refraction values for layer 3 however the q inverse attenuation obtained from these data after correction to temperature and pressure conditions in lower crust are too low to be consistent with amplitudes of signals observed in refraction experiments laboratory measurements of q on gabbros from hole 735b and ophiolites are also one to two orders of magnitude lower than seismic refraction q these results indicate that the gabbro sequence at hole 735b cannot be typical of seismic layer 3 based on q upper layer 3 may be metadolerite and lower layer 3 may include interbedded gabbros and ultramafics serpentine is highly attenuating and is unlikely to be a major component

basal roughness of the institute and moller ice streams west antarctica process determination and landscape interpretation we present a detailed analysis of bed roughness beneath institute and moller ice streams west antarctica using radio echo sounding data res acquired in the austral summer of 2010 11 we assess roughness using a two parameter approach and also assess the directionality of roughness relative to present day ice flow our work highlights the wealth of additional information that resides in analyses of bed roughness employing these multiple approaches we show that spatially variable roughness patterns are partly a consequence of the ability of flowing ice not only to smooth the bed but also to redistribute and remove sediments and to do this along flow accordingly we identify some fast flow tributaries underlain by topography that has been streamlined and other tributaries that are underlain by sediments we also identify locations that are currently protected from erosion but where more ancient erosion may once have occurred we conclude that detailed roughness analysis is a useful tool for landscape interpretation and we suggest that the roughness of an ice sheet s bed should be viewed not only as the consequence of ancient marine sedimentation but also as a product of more contemporary erosion and redistribution of sediments as well as bedrock smoothing that is ongoing because of continuing dynamic activity in this way basal roughness has the potential to evolve continually with ice sheet form and flow and should not be viewed simply as a snapshot of either present day or palaeo basal conditions

controls on last glacial maximum ice extent in the weddell sea embayment antarctica the weddell sea sector of the antarctic ice sheet is hypothesized to have made a significant contribution to sea level rise since the last glacial maximum using a numerical flowline model we investigate the controls on grounding line motion across the eastern weddell sea and compare our results with field data relating to past ice extent specifically we investigate the influence of changes in ice temperature accumulation sea level ice shelf basal melt and ice shelf buttressing on the dynamics of the foundation ice stream we find that ice shelf basal melt plays an important role in controlling grounding line advance while a reduction in ice shelf buttressing is found to be necessary for grounding line retreat there are two stable positions for the grounding line under glacial conditions at the northern margin of berkner island and at the continental shelf break global mean sea level contributions associated with these two scenarios are 50 mm and 130 mm respectively comparing model results with field evidence from the pensacola mountains and the shackleton range we find it unlikely that ice was grounded at the continental shelf break for a prolonged period during the last glacial cycle however we cannot rule out a brief advance to this position or a scenario in which the grounding line retreated behind present during deglaciation and has since re advanced better constraints on past ice sheet and ice shelf geometry ocean temperature and ocean circulation are needed to reconstruct more robustly past behavior of the foundation ice stream

sandstone dissolution landforms in the furnas formation southern brazil the furnas formation silurian to devonian of the parana basin consists of medium to coarse sandstones quartzarenites cemented by kaolinite which reduces its porosity and makes the rock more resistant a mesozoic crustal upwarping called the ponta grossa arch and subsequent erosion gave origin to the devonian scarp which is a cuesta supported by the sandstones the scarp serves as a boundary line for the second parana plateau a step of the en echelon relief in the state of parana in southern brazil in the outcropping areas the sandstones display a ubiquitous phenomenon of superficial exfoliation by dissolution of the kaolinitic mesodiagenetic cement and reprecipitation of microcrystalline telodiagenetic kaolinite the dissolution of kaolinite contributes to the development of small scale superficial landforms such as alveoles runnels dissolution basins concavities at the base of rocky walls polygonal joints anastomosing tunnels and erosion columns in the quebra perna river area near the devonian scarp the sandstones of the furnas formation show many features of significant subterranean erosion such as furnas collapsed hollows ponds wet and dry depressions sinkholes tunnels and grikes such features are associated with the presence of brittle structures faults fractures dikes in two directions the main one ne sw corresponds to reactivations of the proterozoic basement of the parana basin and the secondary one nw se runs parallel to the ponta grossa arch axis furnas and other associated features frequently occur at the crossing of such structures these findings indicate that the great features of subterranean erosion are the result of kaolinite dissolution which intensifies along structural directions in a region of scarped relief and steep hydraulic gradient as a consequence the furnas formation displays landforms that are characteristic of a both structural and karstic aquifer copyright 2007 john wiley sons ltd

deep structures of slopes connected with sandstone crags in the upland area of the świetokrzyskie holy cross mountains central poland abstract deep seated gravitational deformations of slopes are commonly observed and described in mountains but faintly manifested and thus neglected in upland areas thanks to the geophysical methods e g ert as well as detailed surface surveys and studies in caves such deformations have been discovered and described in the region of the wietokrzyskie holy cross mountains central poland which is an upland area of usually gentle slopes of a height of several tens of metres the deformations which have been studied on 20 30 m high slopes formed of sandstone series alternating with claystone siltstone and heterolithe series represent various movements from slight rotational and translational ca 1 m movements through horizontal and vertical shifts of several metres to significant dislocations of sandstone corestones embedded within the regolith forming the slopes the last mentioned structures are currently relics completely masked by subsequent slope processes and their origin should be related to specific periglacial and even sub glacial conditions of the pleistocene on the other hand as confirmed by radiocarbon dating and other mapping slight movements of relatively low slopes can even be active nowadays these deformations that occur in regions of a relatively low relief and have so far been neglected or considered to be unique are possibly even more frequent and should be studied using modern geophysical and other methods

periglacial geomorphology of the appalachian highlands and interior highlands south of the glacial border a review abstract many features reported in the appalachian region have been assigned a paleoperiglacial origin based on field relationships and their similarities with analogs active in present day actuoperiglacial environments these forms include but are not limited to sorted and nonsorted varieties of patterned ground grezes litees block fields block slopes and block streams cryoplanation terraces and hillslope and river terrace landscapes although very small scale features generally less than 1 m in plan or section dimension and some larger forms on steep hillslopes are known to be at least sporadically active today the large scale features discussed here are interpreted as either inactive or truly fossil periglacial phenomena thus they hav implications for paleoclimatic reconstruction and significance as indicators of relative landscape stability since the time s of their development this paper briefly reviews the historical dimension of early researchs work present selected examples of recent results and gives authors conclusions

in situ evidence for the nature of the seismic layer 2 3 boundary in oceanic crust the igneous oceanic crust is typically thought of as comprising two layers an upper crust seismic layer 2 characterized by a rapid increase in seismic velocity with depth and a thicker lower crust seismic layer 3 which is distinguished from layer 2 by both a higher p wave velocity 6 69 0 26 km s l and a much smaller vertical velocity gradient 1 8 km continuous hole drilled into the oceanic crust11 13 and find that at this location the seismic layer 2 3 boundary lies within the sheeted dyke complex where it is associated with gradual downhole changes in crustal porosity and alteration not a litho logical transition from sheeted dykes to gabbro

seismic wave dispersion and attenuation in aheim dunite an experimental study summary a machine has been developed which provides for the study of both shear modulus dispersion and internal friction in geological materials through the observation of forced torsional oscillations of low frequency 10 1000 mhz and strain amplitude 1000 c the eventual goal is an understanding of the dispersion and attenuation of seismic shear waves in the earth s upper mantle measurements have been made on cylindrical specimens of an olivine rich rock from aheim norway which contains in addition to olivine about 10 per cent pyroxene and 5 10 per cent of hydrous silicate phases dominantly clinochlore serpentine and talc in order to separate aspects of the mechanical behaviour associated with the olivine aggregate from those attributable to the hydrous phases and or their dehydration products the specimens were either previously fired under controlled oxygen fugacity at 1200 c for 24 hours in order to effect complete dehydration within the olivine stability field or simply oven dried at 110 c linearity of the mechanical behaviour has been demonstrated by the amplitude independence of the results for the strain amplitude range 10 8 10 6 and by the quantitative consistency between the observed modulus dispersion and that calculated from the measured internal friction through the kramers kronig relations of linear theory both the pressure dependence at room temperature and the temperature dependence at 300 mpa of the shear modulus and internal friction have been investigated much of the variation of the shear modulus is attributed to changes of crack porosity associated with changes in pressure and temperature and with in situ dehydration temporal evolution of q 1 towards a lower asymptotic value over periods of hours of exposure to given conditions of high pressure and temperature is tentatively attributed to the gradual diminution of enhanced anelastic relaxation associated with regions of decaying stress concentration at asperities on cracks and or grain boundaries at the highest pressures and temperatures achieved in this study 300 mpa and 1000 c marked dispersion of the shear modulus amounting to 5 per cent between periods of 3 and 100 s and concomitant strong internal friction varying with oscillation period to approximately as 0 011to1 6 are observed the energy dissipation appears to be concentrated within rather than at the boundaries between the olivine grains these results provide the clearest indication yet that solid state probably intragranular anelastic relaxation in ultramafic rocks gives rise to losses comparable with those observed seismologically in the earth s upper mantle although the mechanistic basis for the observed anelasticity remains to be established

widespread rapid grounding line retreat of pine island thwaites smith and kohler glaciers west antarctica from 1992 to 2011 we measure the grounding line retreat of glaciers draining the amundsen sea sector of west antarctica using earth remote sensing ers 1 2 satellite radar interferometry from 1992 to 2011 pine island glacier retreated 31 km at its center with most retreat in 2005 2009 when the glacier ungrounded from its ice plain thwaites glacier retreated 14 km along its fast flow core and 1 to 9 km along the sides haynes glacier retreated 10 km along its flanks smith kohler glaciers retreated the most 35 km along its ice plain and its ice shelf pinning points are vanishing these rapid retreats proceed along regions of retrograde bed elevation mapped at a high spatial resolution using a mass conservation technique that removes residual ambiguities from prior mappings upstream of the 2011 grounding line positions we find no major bed obstacle that would prevent the glaciers from further retreat and draw down the entire basin 2014 american geophysical union all rights reserved

the relationship between sticky spots and radar reflectivity beneath an active west antarctic ice stream isolated areas of high basal drag or sticky spots are important and poorly understood features in the force balance and dynamics of west antarctic ice streams characterizing sticky spots formed by thin or drying subglacial till using ice penetrating radar is theoretically possible as high radar bed returned power brp is commonly related to an abundance of free water at the ice bed interface provided losses from englacial attenuation can be estimated in this study we use airborne radar data collected over evans ice stream to extract brp profiles and test the sensitivity of brp to the adopted englacial attenuation correction we analyse 11 20 km profiles in four fast flow areas where sticky spots have been inferred to exist on the basis of model and surface data inversions in the majority of profiles we note that the increase in basal drag is accompanied by a decrease in brp and suggest that this is evidence both for the presence of a sticky spot in those locations and that local variations in subglacial hydrology are responsible for their existence a comparison is made between empirical and numerical modelling approaches for deriving englacial attenuation and our findings generally support previous studies that advocate a modelling approach

mechanics of graben evolution in canyonlands national park utah results of numerical models and field observations of regularly spaced grabens in canyonlands national park utah demonstrate that salt flow beneath a brittle overburden accommodated recent and ongoing westward gravity spreading erosion of the colorado river canyon differentially loaded the underlying viscous salt in our models the overlying brittle strata flexed downward toward the canyon initiating faults near the surface that propagated downward toward the salt contact modeled grabens developed sequentially away from the canyon eastward as salt was expelled from beneath undeformed strata after their eastern boundary faults broke through horst blocks tilted in the opposite direction of initial flexure resulting in increased symmetry of older grabens closer to the canyon continued extension formed a reactive diapir beneath each graben field observations show that multiple faults bound grabens indicating reactive diapirs beneath them topographic profiles and surveyed points along a stratigraphic layer show that horst blocks subsided as salt migrated toward the river canyon and into the diapirs field data from less evolved horsts imply that individual horst blocks responded to differential loading by progressive flexure and tilt similar to the models horst block flexures also vary along strike and localized folds and faults formed where fault displacement changes abruptly

glacier naled evolution and relation to the subglacial drainage system based on water chemistry and gpr surveys werenskioldbreen sw svalbard glacier naledi are extrusive ice masses that appear in front of glaciers as a consequence of refreezing of meltwater seepage during the accumulation season these structures provide a unique opportunity to understand subglacial drainage activity during the accumulation season however only few detailed studies have previously focused on their characteristics here we investigated glacier derived naled assemblages in the proglacial zone of the polythermal glacier werenskioldbreen 27 4 km 2 in sw svalbard we determined the spatial distribution of naledi using ground penetrating radar surveys the main subglacial drainage pattern was related to a channel under the medial moraine and three sources are linked to a distributed subglacial drainage network the relation between atmospherically corrected ca 2 mg 2 and so 4 2 in sub naled waters was closely related to sulphide oxidation coupled with carbonate dissolution r 0 99 slope 1 6 this is consistent with the local lithology which is dominated by schist containing carbonates we also found high carbonate saturation indices in pale white ice layers within the naled we conclude that sulphide oxidation coupled with carbonate dissolution is the dominant chemical weathering process in the subglacial drainage system of werenskioldbreen during the accumulation season

characterization of subglacial landscapes by a two parameter roughness index previous studies using fourier transformation ft methods to analyze subglacial roughness have shown promise for distinguishing between different types of subglacial landscape from raw subglacial elevation data we derive a two parameter ft roughness index whereis based on the ft of elevation as previously considered in isolation andis based on both the ft of elevation and the ft of bed slope profile in this way we take account of both vertical and horizontal irregularities in subglacial surfaces we demonstrate the statistical veracity of using to consider roughness in terms of obstacle amplitudes and spacing and consider the use of in studies of ice dynamics and subglacial geomorphological interpretation we show that can be linked to basal sliding rates on the metre scale and can be used to differentiate further than single parameter roughness indices between different classes of subglacial landscape in particular between erosional and depositional settings

a subglacial water flow model for west antarctica the current generation of continental scale ice sheet models cannot successfully reproduce the complex ice water sediment interactions of the west antarctic ice sheet wais in a physically meaningful manner the potential of a thin film based subglacial water flow model for incorporation into a continental scale coupled ice water flow model of the wais is evaluated in this paper the subglacial water flow model is applied to the ross sea sector of the wais in both a steady state and time dependent form to derive the equilibrium water depth for the present day configuration the potential for coupling the model to an ice flow model is then demonstrated using a variable sliding parameter that is a function of the subglacial water depth a coupled ice water flow model using the parameterization tested in this paper could have the potential for reproducing the surface elevation velocity and thermal regime of the wais successfully these requirements are crucial in modelling the evolution of the wais and must be addressed before reliable continental scale predictive models can be utilized

old pacific crust near hawaii a seismic view an expanding spread profile esp over 80 ma pacific ocean crust near hawaii was modeled with a one dimensional solution for travel times relative amplitudes and sometimes frequency content the resulting profiles of compressional and shear wave velocities and poisson s ratio are similar to those determined by other ocean crustal studies except that the shear wave q in the sediments is very low 10 and some unusual high frequency high phase velocity arrivals require a laminated structure within the mid crustal section at about 3 km below the base of the sediment the laminations are probably alternating layers of gabbro and ultramafic cumulates the cumulates may be fractionation products from an upper level magma chamber that was present at the time of crustal formation reflections which may correspond to these laminations have been observed in reflection records both near this esp and in the north atlantic comparison of different ophiolite models for ocean crust reveal that the most commonly used velocity lithology model is probably in error for the pillow and dike basalts resulting in an interpretation of overly thick layers in previous studies of refraction lines the preferred interpretation is that the average total thickness of the pillow and dike sequences is between 1 0 and 1 5 km

case hardening of sandstone the case hardened crust developed on the aztec sandstone in the valley of fire nevada has been characterized by a variety of techniques including electron microscopy and infrared spectroscopy the case hardened crust consists mainly of host rock with a fine grained cement and wind deposited kaolinite the cement is usually calcite but in some cases the hydrated calcium borate colemanite was found to be the case hardening cement

radar based subglacial lake classification in antarctica 1 subglacial lakes in east antarctica can be separated into four categories specified by radar reflection properties definite lakes are brighter than their surroundings by at least 2 db relatively bright and both are consistently reflective specular and have a reflection coefficient greater than 10 db absolutely bright dim lakes are relatively bright and specular but not absolutely bright indicating nonsteady ice dynamics fuzzy lakes are both relatively and absolutely bright but not specular and may indicate saturated sediments or swamps indistinct lakes are absolutely bright and specular but no brighter than their surroundings lakes themselves and the different classes of lakes are not arranged randomly throughout antarctica but are clustered around ice divides ice stream onsets and prominent bedrock troughs with each cluster demonstrating a different characteristic lake classification distribution the lake classification algorithm expands on previous studies and demonstrates a novel way to characterize ice water interactions in east antarctica

can seismic observations of bed conditions on ice streams help constrain parameters in ice flow models we investigate correlations between seismically derived estimates of basal acoustic impedance and basal slipperiness values obtained from a surface to bed inversion using a stokes ice flow model using high resolution measurements along several seismic profiles on pine island glacier pig we find no significant correlation at kilometer scale between acoustic impedance and either retrieved basal slipperiness or basal drag however there is a stronger correlation when comparing average values along the individual profiles we hypothesize that the correlation appears at the length scales over which basal variations are important to large scale ice sheet flow although the seismic technique is sensitive to the material properties of the bed at present there is no clear way of incorporating high resolution seismic measurements of bed properties on ice streams into ice flow models we conclude that more theoretical work needs to be done before constraints on mechanical conditions at the ice bed interface from acoustic impedance measurements can be of direct use to ice sheet models

marine ice sheet collapse potentially under way for the thwaites glacier basin west antarctica resting atop a deep marine basin the west antarctic ice sheet has long been considered prone to instability using a numerical model we investigated the sensitivity of thwaites glacier to ocean melt and whether its unstable retreat is already under way our model reproduces observed losses when forced with ocean melt comparable to estimates simulated losses are moderate 1 mm per year of sea level rise collapse in the different simulations within the range of 200 to 900 years

inland extent of the weddell sea rift imaged by new aerogeophysical data the weddell sea rift was a major focus for jurassic extension and magmatism during the early stages of gondwana break up and underlies the weddell sea embayment which separates east antarctica from a collage of crustal blocks in west antarctica newly collected aerogeophysical data over the catchments of institute and moller ice streams reveal the inland extent of the weddell sea rift against the ellsworth whitmore block and a hitherto unknown major left lateral strike slip boundary between east and west antarctica aeromagnetic and gravity anomalies define the regional subglacial extent of proterozoic basement middle cambrian rift related volcanic rocks jurassic intrusions and sedimentary rocks of inferred post jurassic age 2d and 3d magnetic depth to source estimates were used to help constrain joint magnetic and gravity models for the region the models reveal that proterozoic crust similar to that exposed at haag nunataks extends southeast of the ellsworth mountains to the margin of the coastal basins thick granitic jurassic intrusions are modelled at the transition between the ellsworth whitmore block and the thinner crust of the weddell sea rift and within the pagano shear zone the crust beneath the inland extension of the weddell sea rift is modelled as being either 4 km thinner compared to the adjacent ellsworth whitmore block or as underlain by an up to 8 km thick mafic underplate

geomorphology of the gran sabana guayana shield southeastern venezuela abstract the forms of the gran sabana have been developed mainly on rocks of the gently folded precambrian roraima group a sequence of quartzites and siliceous conglomerates with intercalated jasper chert arkose siltstone and shale locally intruded by diabase dikes and sills two planation surfaces are preserved on the gran sabana the auyan tepui 2000 2900 m elevation and the wonken 900 1200 m surfaces with a speculative mesozoic initial age they are still extending table mountains dominate the landscape they comprise pediments foothills precipitous scarps hundreds of meters high a rim and a flattish summit the table mountain topography is controlled by joint systems water percolates along joints and bedding planes dissolves the siliceous cement the quartzite disaggregates and large blocks are isolated and collapse accumulating at the foot of the scarps and on top of the table mountain summits the percolating water forms large and intricate cave systems which frequently emerge in the scarp zone as high waterfalls these mark the lowest level of erosion of the table mountains karstic structures are prominent in the summit zone the evolution of the table mountain topography has been controlled by a combination of tectonic structural stratigraphic climatic and temporal factors

role of biological soil crust cover in bioweathering and protection of sandstones in a semi arid landscape torrollones de gabarda huesca spain sandstone structural landscapes in the semi arid torrollones de gabarda area province of huesca ne spain are often covered by a well developed biological soil crust of lichens mosses and cyanobacteria and black coatings on vertical surfaces by using scanning electron microscopy with backscattered detector imaging the biological soil crust studied evidenced high activity in the sandstone crust interface processes such as physical disintegration etching and dwelling as well as biomineralization by calcium oxalate and xation of mineral particles by extracellular polymeric substances were observed on the horizontal sandstone surfaces these processes may cause the occurrence of gnammas and the development of a protective coating that favours intense aking when the crust is disturbed on the sandstone cliffs columnar and tafoni weathering development is clearly guided by the protective action of the biological soil crust these qualitative observations are important to develop methodologies to address their quantitative importance in geomorphological processes in semi arid landscapes copyright 2004 john wiley sons ltd

geochemistry of surface and subsurface waters in quartz sandstones significance for the geomorphic evolution of tepui table mountains gran sabana venezuela summary in situ measurements of discharge ph electric conductivity ec temperature and sio2 content have been carried out during five expeditions in the last 20 years on the summit plateaus inside caves and along the rivers of the surrounding lowlands of three tepui massifs in venezuela auyan roraima and chimanta additionally detailed chemical analyses were performed on waters sampled in a newly discovered extensive quartz sandstone cave system on the auyan tepui rock samples of the quartz sandstone bedrock from different locations have been analysed to obtain their chemical composition with a wavelength dispersive x ray fluorescence spectrometer these data show that the majority of silica present in surface and subsurface water comes from dissolution of quartz and only in minor amount from hydrolysis of other silicate minerals probably the presence of a hardened crust of iron hydroxides limits the dissolution of silica on the top surface of tepuis dissolution in the subsurface instead is more significant and causes in the long term the arenisation of the quartz sandstone and its subsequent removal by mechanical erosion on the other hand waters flowing on the arkosic rock outcropping on the lowland below the tepuis obtain their high dissolved silica content mainly from hydrolysis of silicates the morphological evolution of these table mountains appears thus to be controlled mainly by the underground weathering of the quartz sandstone with the opening of deep fractures grietas and the collapse of large underground horizontal cave systems scarp retreat instead seems to be related to the higher weathering rate of the more arkosic formations underlying the quartz sandstones

structural and lithological guidance on speleogenesis in quartz sandstone evidence of the arenisation process a detailed petrographic structural and morphometric investigation of different types of caves carved in the quartz sandstones of the tepui table mountains in venezuela has allowed identification of the main speleogenetic factors guiding cave pattern development and the formation of particular features commonly found in these caves such as funnel shaped pillars pendants and floor bumps samples of fresh and weathered quartz sandstone of the mataui formation roraima supergroup were characterised through wds dispersive x ray chemical analyses picnometer measurements edax analyses sem and thin section microscopy in all the caves two compositionally different strata were identified almost pure quartz sandstones with content of silica over 95 and high primary porosity around 4 and phyllosilicate rich quartz sandstone with contents of aluminium over 10 and low primary porosity lower than 0 5 phyllosilicates are mainly pyrophyllite and kaolinite sem images on weathered samples showed clear evidence of dissolution on quartz grains to different degrees of development depending on the alteration state of the samples grain boundary dissolution increases the rock porosity and gradually releases the quartz grains suggesting that arenisation is a widespread and effective weathering process in these caves the primary porosity and the degree of fracturing of the quartz sandstone beds are the main factors controlling the intensity and distribution of the arenisation process weathering along iron hydroxide or silt layers which represent inception horizons or a strata bounded fracture network predisposes the formation of horizontal caves in specific stratigraphic positions the loose sands produced by arenisation are removed by piping processes gradually creating anastomosing open fracture systems and forming braided mazes geometric networks or main conduit patterns depending on the local lithological and structural guidance on the weathering process this study demonstrates that all the typical morphologies documented in these quartz sandstone caves can be explained as a result of arenisation which is guided by layers with particular petrographic characteristics primary porosity content of phyllosilicates and iron hydroxides and different degrees of fracturing strata bounded fractures or continuous dilational joints

seismic structure of the upper oceanic crust revealed by in situ q logs in situ seismic attenuation is computed through 2 1 km of the upper oceanic crust in the vicinity of odp hole 504b the results strongly tie crustal properties to seismic measurables and observed geological structures we find that the attenuation can be used to define seismic layer boundaries and is closely related to the intensity of vertical heterogeneity the in situ attenuation q 1 consists of both intrinsic and scattering contributions but is dominated by the scattering attenuation unless porosities are near zero when it approaches typical estimates from seismic refraction studies the attenuation is analytically modeled by multiple backscattering from heterogeneities observed in a sonic vp log and is found to decrease step wise in relatively homogeneous layers from q 25 to q 300 between the top of seismic layer 2a and a sharp discontinuity at 1 3 km depth these changes correspond with heterogeneities at 1 0 1 3 m and at 5 6 10 0 m wavelengths that we interpret to be associated with fracturing and structure of pillow basalts and lava flows in seismic layers 2a and 2b although seismic velocity studies suggest that the layer 2 3 boundary also occurs at about 1 3 km the large variation in q 140 to 460 below this depth indicates that a seismically homogeneous and uniform layer 3 has not been reached in hole 504b we derive an empirical relationship between attenuation and porosity q 1 q0 1 e where q0 1 0 004 and 25 that may be applicable at other oceanic crust locations and useful for constraining seismic inversion models

estuaries beneath ice sheets interactions between subglacial hydrology and the ocean make the existence of estuaries at the grounding zones of ice sheets likely here we present geophysical observations of an estuary at the downstream end of the hydrologic system that links the active subglacial lakes beneath whillans ice stream to the ocean beneath the ross ice shelf antarctica this subglacial estuary consists of a hydropotential low upstream of the grounding zone which is linked to the ocean by a hydropotential trough and a large subglacial channel this subglacial channel which is imaged using active source seismic methods has an apparent width of 1 km and a maximum depth of 7 m the hydropotential trough continues upstream of the grounding zone and results from an along flow depression in surface elevations pressure differences along the trough axis are within a range that can be overcome by tidally induced processes making the interaction of subglacial and ocean water likely

phreatic drainage conduits within quartz sandstone evidence from the jurassic precipice sandstone carnarvon range queensland australia abstract discrete underground drainage conduits in quartz sandstones are far less common than in limestones this paper provides field evidence from the quartzose precipice sandstone in the carnarvon range of south central queensland australia for tubular underground drainage networks similar in many ways to limestone conduits diameters range from less than 1 or 2 cm to over 1 5 m most display a near circular to oval cross section that seems to suggest phreatic or epiphreatic development and the internal surfaces of many are case hardened by secondary silica deposits a number of the region s perennial springs appear to be fed by such tubes the dominant vertical jointing of the quartz sandstone and relatively high permeability of the sandstone are important controls on tube formation solutional weathering of the sandstone is widespread and is followed by the removal of loosened sand grains by flowing underground water the process of arenisation tube development would appear to have been happening for a very long time and may still be occurring a model for tube network formation is proposed these findings highlight our potentially poor understanding of groundwater flow within some quartz sandstones and may have important groundwater management implications

attenuation differences in layer 2a in intermediate and slow spreading oceanic crust in situ seismic attenuationq 1logs are derived from borehole velocity profiles and reveal sharp boundaries between morphologies of the extrusive volcanic layers in intermediate and slow spreading oceanic crust q 1logs are calculated from the scattering attenuation associated with vertical velocity heterogeneity in ocean drilling program holes 504b and 896a and in hole 395a located in 5 9 7 3 ma crust on the pacific and atlantic plates respectively our results strongly tie crustal properties to seismic measurables and observed geological structures we find that the scattering attenuation can be used to identify the extrusive volcanic sequence because it is closely related to changes in the degree of vertical heterogeneity we interpret a distinct decrease in the q 1log at the transition below the extrusive volcanic layer to correspond with the seismic layer 2a 2b boundary the boundary is located at 465 m depth below the sea floor in both hole 395a and 504b although this is likely to be a coincidence of the sediment thickness at these sites layer 2a is estimated to be approximately 150 m thick in hole 504b and 300 m thick in hole 395a cyclic sequences of high porosity pillows and low porosity massive units in the uppermost 100 m of volcanics in hole 395a result in large velocity heterogeneities which cause 5 times more attenuation in this layer than in hole 504b in hole 896a by contrast fewer pillows more massive flows and a greater volume of carbonate veins decrease the velocity heterogeneity and attenuation significantly over only 1 km distance from hole 504b we conclude that the attenuation in the extrusive volcanics of the ocean crust is largely controlled by variation in local heterogeneity and morphology as well as by subsequent hydrothermal alteration the observed differences inq 1profiles and layer 2a thickness at these sites may be attributed to variations in the volume and duration of volcanic activity at mid ocean spreading centers for these pacific and atlantic ridge segments

high temperature experiments on the elastic and anelastic behaviour of magmatic rocks measurements of the complex shear modulus have been performed on various ultrabasic and basic rocks dunite peridotite gabbro and basalts and a synthetic forsterite at temperatures between 600 and 1500 c depending on the sample material and in the frequency range from 0 003 to 30 hz under forced torsional oscillations the maximum shear stress was 0 3 mpa and the strains were of the order 5 10 6 to 5 10 5 measurements of the electrical resistivity as a function of temperature have been carried out together with the mechanical experiments to indicate the beginning of partial melting the absolute values of the shear modulus below tm are comparable with the results of other authors the onset of melting shows up in a rapid decrease of both shear modulus and electrical resistivity at high temperatures t 0 8 tm the absorption factor q 1 follows a power law q 1 exp a rt with 0 15 0 30 and a between 100 and 200 kj mol 1 towards lower temperatures this high temperature background is superimposed by a flat absorption peak which shifts from high to low frequencies with falling temperature this peak can often be described by a lognormal distribution of simple relaxation processes with a standard deviation of about three and a mean relaxation time exp a rt with a 400 600 kj mol 1 above tm a third contribution to q 1 is observed that can be regarded as independent of frequency between 1 and 30 hz this last component which contributes 50 to the total absorption is attributed to the melt phase in the samples an explanation for the high temperature background and the peak is proposed which is based on the relaxation of pinned dislocation lines by some coupling mechanism via the reorganisation of the stress field with increasing deformation this process may be responsible for both features at very high temperatures and low frequencies a small influence of steady state flow on q 1 is observed and below t 0 6 tm thermal cracking or thermoelastic losses may contribute to q 1 the influence of lithostatic pressure on q 1 is estimated and the extrapolation of the experimental data to the p t conditions of the uppermost mantle are in good agreement with published earth models an attempt is made to establish a general q t relationship from the experimental data

seismic attenuation in the crust at the mid atlantic plate boundary in south west iceland summary very little seismic attenuation occurs in the mid to lower crust of south west iceland the lowest path averaged quality factor for a wave turning in the mid to lower crust 12 20 km is qp 110 for p waves and q5 250 for s waves with most of the data having higher values typically qp 200 300 and qr 400 600 attenuation estimates based on a 1 d layered inversion give correspondingly high values qp 800 and q 800 2000 these q values are inconsistent with thermal models that predict a broad 100 km wide region of above solidus temperatures centred on the volcanic zones the observed attenuation implies an upper limit for mid to lower crustal temperature in the 700 775 c range assuming a gabbroic lithology much higher attenuation qp 60 qr 100 occurs in the uppermost 4 km of crust this is most likely apparent attenuation caused by strong near surface seismic heterogeneity resulting from fissures faults and extreme changes in porosity up to 20 30 per cent the quality factor of the near surface layer varies regionally and is lower in volcanic zones than in either the reykjanes peninsula or the south iceland lowland

velocity structure near iodp hole u1309d atlantis massif from waveform inversion of streamer data and borehole measurements seismic full waveform inversion fwi is a promising method for determining the detailed velocity structure of the igneous oceanic crust especially for locations such as the mid atlantic ridge with significant lateral heterogeneity and seafloor topography we examine the accuracy of fwi by inverting after downward continuation to datum just above the seafloor a multichannel seismic mcs profile from atlantis massif oceanic core complex at 30 n that passes close to integrated ocean drilling program iodp hole u1309d and comparing the results against borehole measurements and existing on bottom refraction data the comparisons include the results of iodp expedition 340t which extended the sonic logging and vertical seismic profiling to the bottom of the borehole at 1400 m below seafloor compared to travel time tomography the refinement in velocity and velocity gradient produced by fwi significantly improves the overall match to the borehole measurements and allows the multilevel pattern of deformation and alteration of the detachment footwall seen in hole u1309d to be extrapolated across the central dome prestack depth migration of the profile using the fwi velocities reveals the top and edges of the high velocity gabbroic core of the massif it also indicates that the comparatively uniform gabbroic rocks drilled at hole u1309d extend to 2 5 km below seafloor but overlie an extended 2 km thick mantle transition zone

measurements of attenuation from vertical seismic profiles values of attenuation have been extracted from five detailed velocity surveys significant amounts of attenuation are observed in all five wells measured values of attenuation vary by a factor of 10 ranging from less than 0 1 to 0 9 db wavelength correlation of attenuation with lithology is good considering the resolution limits of the surveys one of the surveys was performed in west texas and the other four were carried out in the gulf coast area the total number of recording depths in each survey varied from 31 to 208 with downhole recordings taken every 10 or 20 ft over key stratigraphic zones in all five surveys a monitor geophone was placed near the well to check for source consistency our procedure for measuring attenuation involved constructing the spectral ratio between each downhole pulse and one particular reference pulse a value of cumulative attenuation ca was then extracted from each spectral ratio and plotted as a function of geophone depth this method allowed us to use many m

rock anelasticity measurements at high pressure low strain amplitude and seismic frequency a new apparatus has been constructed which will ultimately facilitate the study of rock anelasticity under conditions which closely approach those of teleseismic wave propagation simultaneous high pressure and temperature low frequency and strain amplitude and controlled pore pressure of volatiles its performance has been tested in a series of preliminary experiments conducted at room temperature measurements on a steel standard demonstrate the sensitivity of the apparatus to very small departures from ideal elasticity q 1000 experimental data for a fine grained granitic rock show that both the shear modulus g and quality factor q increase sharply with increasing pressure below 100 mpa beyond which pressure both parameters become markedly less pressure sensitive these observations are in accord with those of previous studies at higher frequencies and larger strains and are consistent with the view that the anelasticity of rocks at ambient pressure is dominated by mechanisms operative at open cracks and grain boundaries

the role of downhole measurements in marine geology and geophysics during the last 25 years downhole measurements have been increasingly used for scientific applications in marine geology and geophysics particularly in deep sea drilling operations used mostly by the oil industry to map promising formations for exploration and production of hydrocarbons a variety of instruments have been developed that can be lowered down drill holes to extract information about the subsurface geology in the last decade advances in computers software and data transmission have greatly increased the amount and quality of data that such instruments can provide relatively new instruments that image the borehole wall with high resolution can reveal layers and faults that previously could be seen only in core sections downhole measurements play a crucial role in linking core data with regional geophysical surveys and in providing data where core sections could not be obtained examples of recent scientific applications and approaches are presented that address previous problems with data quality and changes in properties over time after a hole is drilled the role of downhole measurements is discussed for two broad areas of research the structure and composition of the earth s crust most of which is formed at mid ocean ridges and past changes in earth s environment recorded in the deep sea sediments overlying the crust finally new emerging technologies and experiments that promise significant advantages over current methods for downhole measurements in marine geology and geophysics are discussed