the last british ice sheet a review of the evidence utilised in the compilation of the glacial map of britain this paper reviews the evidence presently available as at december 2003 for the compilation of the glacial map of britain see clark c d evans d j a khatwa a bradwell t jordan c j marsh s h mitchell w a bateman m d 2004 map and gis database of glacial landforms and features related to the last british ice sheet boreas 33 359 375 and http www shef ac uk geography staff clark chris britice html in an effort to stimulate further research on the last british ice sheet and promote a reconstruction of ice sheet behaviour based on glacial geology and geomorphology the wide range of evidence that has been scrutinized for inclusion on the glacial map is assessed with respect to the variability of its quality and quantity and the existing controversies in ice sheet reconstructions landforms interpreted as being of unequivocal ice marginal origin moraines ice contact glacifluvial landforms and lateral meltwater channels and till sheet margins are used in conjunction with available chronological control to locate former glacier and ice sheet margins throughout the last glacial cycle subglacial landforms drumlins flutings and eskers have been used to demarcate former flow patterns within the ice sheet the compilation of evidence in a regional map is crucial to any future reconstructions of palaeo ice sheet dynamics and will provide a clearer understanding of ice sheet configuration ice divide migration and ice thickness and coverage for the british ice sheet as it evolved through the last glacial cycle

testing the ability of surface arrays to monitor microseismic activity recently there has been much interest in the use of data from surface arrays in conjunction with migration based processing methods for passive seismic monitoring in this study we use an example of this kind of data recorded whilst 18 perforation shots with a variety of positions and propellant amounts were detonated in the subsurface as the perforation shots provide signals with known source positions and origin times the analysis of these data is an invaluable opportunity to test the accuracy and ability of surface arrays to detect and locate seismic sources in the subsurface in all but one case the signals from the perforation shots are not visible in the raw or preprocessed data however clear source images are produced for 12 of the perforation shots showing that arrays of surface sensors are capable of imaging microseismic events even when the signals are not visible in individual traces we find that point source locations are within typically 45 m laterally of the true shot location however the depths are less well constrained 150 m we test the sensitivity of our imaging method to the signal to noise ratio in the data using signals embedded in realistic noise we find that the position of the imaged shot location is quite insensitive to the level of added noise the primary effect of increased noise being to defocus the source image given the migration approach the array geometry and the nature of coherent noise during the experiment signals embedded in noise with ratios 0 1 can be used to successfully image events furthermore comparison of results from data and synthetic signals embedded in noise shows that in this case prestack corrections of traveltimes to account for near surface structure will not enhance event detectability although the perforation shots have a largely isotropic radiation pattern the results presented here show the potential for the use of surface sensors in microseismic monitoring as a viable alternative to classical downhole methods

were deforming subglacial beds beneath past ice sheets really widespread abstract the concept of widespread large strain deformation of subglacial unconsolidated unfrozen sediments during pleistocene glaciations has inconsistencies in the geological record numerous properties of tills and related sediments are difficult to reconcile with pervasive strains as predicted by the deforming bed theory while accepting glacier bed deformation as a geological process occurring under certain circumstances we propose that it was much less widespread than believed by some instead we believe that basal sliding and englacial transport are major ice movement and debris transport mechanisms

the minch palaeo ice stream nw sector of the british irish ice sheet geophysical data from the uk continental shelf off nw scotland reveal strongly parallel subglacial bedforms on the sea bed field mapping on the adjacent landmass has identified closely spaced bedrock megagrooves and highly elongate drumlinoid forms all these mega scale glacial lineations probably formed beneath the same fast flowing zone of the last british irish ice sheet the minch palaeo ice stream this ice stream drained the nw sector of the british irish ice sheet during the late devensian glaciation marine isotope stage 2 and terminated near the edge of the continental shelf the size of the adjacent trough mouth sula sgeir fan and the presence of buried mega scale glacial lineations within the quaternary stratigraphic record imply that fast ice sheet flow in the minch has been a feature of several mid to late pleistocene glaciations

glacier response to climatic change during the loch lomond stadial and early flandrian geomorphological and palynological evidence from the isle of skye scotland the deglaciation of skye at the close of the loch lomond stadial is assessed on the basis of detailed geomorphological mapping and pollen stratigraphic correlations it is concluded that deglaciation proceeded in two distinct stages the first was marked by numerous glacier stillstands and readvances while uninterrupted retreat and local glacier stagnation occurred during the second and final stage the pollen evidence indicates that the first stage was well advanced before the marked thermal improvement at the start of the flandrian and it is inferred that initial glacier retreat occurred in response to a decline in precipitation in the later part of the loch lomond stadial the first stage of glacier retreat continued into the early flandrian during which climatic amelioration was interrupted briefly final deglaciation appears to have occurred rapidly in response to sustained temperature increases the collective evidence also indicates spatial variations in the timing of deglaciation which appear to reflect differences in glacier morphology

dating of late quaternary faulting in northern sweden fault scarps interpreted as post or late glacial in age occur in northern sweden and adjacent parts of finland and norway in the lansjarv area in northern sweden attempts have been made to date fault displacement relative to the glacial and postglacial stratigraphy by trenching across fault scarps it is shown that the faulting occurred soon after the local deglaciation some 9000 years ago with no signs of movements since the faulting appears to have been associated with violent earthquakes because landslides and soil deformation interpreted to be seismically induced and dating from the same period as the faulting are frequently found in the area

benthic megafauna assemblage change over three decades in the abyss variations from species to functional groups abstract megafaunal seafloor assemblages on the monterey fan in the ne pacific station m 4000 m depth were studied between 2006 2018 using remotely operated vehicles rovs as part of a continuing time series study that began in 1989 since 2006 we identified nearly 120 000 individual animals representing over 142 morphospecies and observed continuous changes in the megafaunal assemblage this study which tracked variation in observed morphospecies over a 13 year period is one of the most detailed long term records of megafaunal change for abyssal depths our investigation shows that new variations continued to emerge reinforcing the concept that the deep sea is dynamic over short time scales rather than static over long periods some species were uncommon but later observed in high numbers then decreased to very low or undetectable levels e g elpidia sp a while others e g psychropotes longicauda exhibited a relatively persistent presence with less fluctuation in abundance decreasing total echinoderm density from 2013 2018 did not correspond with the continued occurrence of large episodic poc flux events between 2016 2018 this may be attributed to the quality of food supply arriving at the seafloor and the varied ability of organisms to utilize it long term tracking 30 years of 10 specific epibenthic echinoderm species originally quantified from camera sled images shows a pattern of assemblage structure perhaps returning toward the composition observed in the 1990s and early 2000s many questions remain as to how this abyssal site and others will change with continued and potentially increasing anthropogenic change in the upper ocean for example the marine heat anomaly known as the warm blob may have influenced major ecological processes at the abyssal seafloor in terms of morphospecies and functional group composition due to changes in poc flux the degree of dynamism continues to indicate that ad hoc or short term investigations provide a limited perspective for assessing community structure in conservation or resource exploitation impact assessment studies in the deep sea

reverse time migration based reflection tomography using teleseismic free surface multiples converted and multiply reflected phases from teleseismic events are routinely used to create structural images of the crust mantle boundary moho and the elasticity contrasts within the crust and upper mantle the accuracy of these images is to a large extent determined by the background velocity model used to propagate these phases to depth in order to improve estimates of 3 d velocity variations and hence improve imaging we develop a method of reverse time migration based reflection tomography for use with wavefields from teleseismic earthquakes recorded at broad band seismograph arrays reflection tomography makes use of data redundancy that is the ability to generate numerous structural images of the subsurface with different parts of the wavefield in exploration seismology where it is known as migration velocity analysis reflection tomography typically involves the generation of an extended image e g offsetor angle gathers and the fitness of the background model is evaluated through the application of image domain annihilators in regional scale passive source seismology however annihilation based methods are inadequate because the sparse and irregular distribution of teleseismic sources is not likely to produce illumination over a sufficient range of angles to overcome this problem we turn towards a source indexed moveout scheme instead of extended image annihilation we determine the success of the tomographic velocity model by cross correlating images produced with multiply scattered waves from different teleseismic sources the optimal velocity model is the one that minimizes correlation power between windowed images away from zero depth shift we base our inversion scheme on the seismic adjoint method and a conjugate gradient solver for each image pair the update direction is determined by correlations between downgoing wavefields with upgoing adjoint wavefields for both images the sensitivity kernels used in this method is similar to those found in other forms of adjoint tomography but their shapes are controlled by the spatial distribution of the error function we present the method and a proof of concept with 2 d synthetic data

glacial deposits in great britain and ireland with the increasing awareness of environmental problems there is a growing desire for a de tailed understanding of superficial deposits in general and glacial deposits in particular this is because these sediments have a profound influence on groundwater protection waste management and nature conservation they are also of vital importance to civil engineering because they provide the foundation for as well as the materials extracted to build our roads and buildings the greater part of the british isles was glaciated at least once during the quater nary era in the regions subjected to glaciation glacial deposits underlie much of the present land surface although there have been many recent publica tions on various quaternary geological topics the present volume is the first dedicated to a detailed assessment of the glacial deposits of britain and ireland after introductory chapters presenting the glacial history the sedimentary sequences in 24 critical regions are discussed these regions include all of ireland the gla ciated area of great britain and the adjoining offshore region of the north sea the controver sial evolution of the irish sea basin during the last glaciation is discussed from various view points a collection of critical topics presented in the later part of the book range from the classification of glacigenic landforms and de posits to the results of geophysical geotechnical and geochemical analyses the book includes 368 figures 40 tables and 51 colour photographs a detailed index and a list of over 1000 references with 45 contributions by 48 scientists this volume represents a truly contemporary view of this field of research to gether with its companion volume the glacial deposits in north west europe this book pro vides an excellent textbook for the advanced student or the amateur as well as an indispen sible source and guidebook for the professional scientist

the younger dryas cold interval as viewed from central greenland abstract greenland ice core records provide an exceptionally clear picture of many aspects of abrupt climate changes and particularly of those associated with the younger dryas event as reviewed here well preserved annual layers can be counted confidently with only 1 errors for the age of the end of the younger dryas 11 500 years before present ice flow corrections allow reconstruction of snow accumulation rates over tens of thousands of years with little additional uncertainty glaciochemical and particulate data record atmospheric loading changes with little uncertainty introduced by changes in snow accumulation confident paleothermometry is provided by site specific calibrations using ice isotopic ratios borehole temperatures and gas isotopic ratios near simultaneous changes in ice core paleoclimatic indicators of local regional and more widespread climate conditions demonstrate that much of the earth experienced abrupt climate changes synchronous with greenland within thirty years or less post younger dryas changes have not duplicated the size extent and rapidity of these paleoclimatic changes

ice rafted debris associated with binge purge oscillations of the laurentide ice sheet the north atlantic sediment record suggests quasi periodic 7000 to 12 000 year period ice rafted debris ird depositions during at least the last glacial period the cause of these heinrich events as they are commonly known is not fully understood however they may point to surges of the ice stream that drained the hudson bay hudson strait region of the laurentide ice sheet we investigate a simple conceptual model of ice stream instability the binge purge model to suggest ways in which the ice stream could have entrained sufficient debris to account for the estimated mass of ird associated with a typical heinrich ird layer in the north atlantic 1 0 0 3 1015 kg we find that freezing of debris laden ice at the bed of the ice stream during the brief 750 years surge phase of the ice stream s hypothesized binge purge cycle can incorporate up to 5 1 1015 kg this amount is sufficient to meet the constraints of the north atlantic sediment record but by no means verifies the binge purge model as the cause of heinrich events

modelled ice sheet margins of three greenland ice sheet models compared with a geological record from ice marginal deposits in central west greenland ice sheet modelling is an essential tool for estimating the effect of climate change on the greenland ice sheet the large spatial and long term temporal scale ofice sheet models limits the amount of data which can be used to test model results the geological record is useful because it provides test material on the timescales typical for the memory of ice sheets millennia this paper compares modelled ice marginal positions with a geological scenario of ice marginpositions since the last glacial maximum to the present in west greenland morphological evidence of ice margin positions is provided by moraines moraine systems are dated by 14c dated marine shells and terrestrial peat we compared three greenland ice sheet models there are distinct differences inmodelled ice margin positions between the models and between model results and the geological record disagreement between models and the geologicalrecord in the near coastal area are explained by the insufficient treatment of marginal processes in a tidewater environment a smaller than present ice sheetaround the warm period in the holocene holocene climatic optimum only pops up when an unsmooth forcing is used this underlines the importance ofshort term variations in climatic variables in determining ice margin positions in the past but also in the future

glacier hydrology in svalbard norwegian high arctic abstract progress in glacier hydrology has focused on temperate glaciers although partly or wholly non temperate glaciers are found in many parts of the world and significant areas of former ice sheets were probably non temperate it has usually been assumed that drainage from non temperate glaciers consists of relatively dilute and invariable supraglacial runoff in this paper the hydrology of glaciers in the high arctic archipelago of svalbard is examined variations in the thermal regimes of glaciers and their implications for drainage are first addressed results from svalbard concerning meltwater discharge and storage and solute and suspended sediment dynamics are then reviewed in general meltwater penetration of non temperate ice is limited and drainage from non temperate glaciers in svalbard has many of the characteristics of sub aerial drainage however it would be erroneous to view this drainage simply as dilute supraglacial runoff as significant material transport occurs in suspension and solution furthermore twentieth century climate change has probably led to changes in the thermal regimes of some svalbard glaciers such that relict drainage structures are sometimes observed while meltwater yields are augmented by runoff which can be attributed to the loss of mass from glaciers at large spatial scales turbid plumes observed at the marine margins of polythermal ice caps indicate that meltwater can be routed subglacially through non temperate ice

how glaciers entrain and transport basal sediment physical constraints abstract simple insights from the physics of ice water and sediment place constraints on the possible sediment transport behavior of glaciers and ice sheets because glaciers concentrate runoff streams generated by glaciers transport much sediment and may erode bedrock rapidly deforming glacier beds also can transport much sediment particularly in marginal regions rapid sediment entrainment producing thick debris rich basal zones may occur by regelation into subglacial materials and by freeze on from rising supercooled waters numerous other mechanisms may be important but primarily near ice margins especially those of advancing or fluctuating glaciers several sediment entrainment mechanisms may be active beneath a single glacier but one process is likely to be dominant at any place and time

till as a glacial tectomict its internal architecture and the development of a typing method for till differentiation abstract till has been studied for over a century yet still a clear understanding of the mechanics of deposition and or emplacement of till eludes us tills in the past have been generally subdivided into sediments formed in various sub environments of the glacial system as such it has been generally agreed that tills can be separated in terms of these types based on various attributes such as clast fabric increasingly such separation into various sub types of till has become however highly debatable and tenuous based upon the use of micromorphology and a greater in depth appreciation of the internal architecture of till it has become apparent that previously established classifications of till are erroneous micromorphology reveals that deposition and or emplacement of tills is a complex tectonic process such that lodgement melt out and flow tills as previously defined do not appear to exist rather a new term better describes tills as glacial tectomicts by using micromorphology in establishing the presence of sets of microstructures a statistical method is introduced that permits differentiation of tills from other diamictons the implications for this new understanding of tills allied to statistical typing is considerable in terms of till formation glacial stratigraphy and understanding glacial environments in terms of stress histories porewater transmission subglacial deformation mechanics till rheological phase changes and the timeline of till formation

an ice cap landsystem for palaeoglaciological reconstructions characterizing the younger dryas in western scotland abstract this paper reviews the contrasting behaviours of ice caps and icefields defines a generic landsystem model that can effectively discriminate between them and applies the model to landform sediment assemblages in an area of western scotland such a model is necessary since many palaeoenvironmental inferences from formerly glaciated terrains are based on the geometry extent and dynamics of reconstructed ice masses the validity of these glacier reconstructions is dependent on the accurate initial interpretation of relict landforms and sediments and their inter relationships a new landsystems model is presented here in which individual geological and geomorphological elements are checked against a set of eight theoretical diagnostic criteria that characterize the style of former glaciation when applied to a 1200 km 2 area of the western scottish highlands the landsystem tool predicts 1 an extensive younger dryas ice cap with a maximum surface elevation of 900 m above sea level implying colder or wetter conditions than previously thought and 2 the survival of an independent mountain icefield in part of the area during deglaciation glaciological theory proxy palaeoenvironmental data and established glacier climate topography relationships support these predictions thereby giving credibility to the landsystem methodology as a generic tool for palaeoglaciological reconstructions

younger dryas glacial landsystems in north west scotland an assessment of modern analogues and palaeoclimatic implications abstract by explicitly linking process to form landsystem models developed in modern environments provide powerful tools for the reconstruction of past environments from the sedimentological and geomorphological record some landform characteristics however may originate by more than one process equifinality so although a particular depositional model may fit the data it is not necessarily the correct one here we critically examine the use of modern analogues to interpret glacial landforms focusing on hummocky moraine a distinctive landform association consisting of numerous sharp crested moraine mounds and ridge fragments which occurs on many valley floors in upland britain detailed studies of examples of younger dryas age in the north west highlands of scotland show that they represent ice contact fans which have been affected to varying degrees by ice push and subglacial deformation and that they formed at active oscillating ice margins the sedimentological data are inconsistent with an origin by englacial thrusting as has been proposed for similar moraines elsewhere in upland britain on the basis of morphological criteria comparison with modern glacial landsystems shows that although aspects of hummocky moraines in the study area have close modern counterparts none provides an exact analogue in all respects this may reflect the distinctive climate of western scotland during the younger dryas estimates of mean july temperatures in the younger dryas in scotland based on chironomid assemblages are c 8 c at sea level when combined with reconstructed equilibrium line altitudes for contemporaneous glaciers these estimates indicate mean annual precipitation totals in the range of 2358 337 mm yr 1 at sea level in nw scotland c 26 higher than present day values the implied heavy snowfall suggests that glaciers in western scotland during the younger dryas were of high activity type unlike modern glaciers in svalbard for example high glacier velocities and high winter precipitation would suppress permafrost development despite low mean annual temperatures the climatic environment also favoured rapid oscillations of glacier margins during both glacier build up and retreat phases climatic factors combined with the availability of large quantities of subglacial debris led to the development of distinctive glacial landsystems which may have no direct modern analogue

megafaunal variation in the abyssal landscape of the clarion clipperton zone abstract the potential for imminent polymetallic nodule mining in the clarion clipperton fracture zone ccz has attracted considerable scientific and public attention this concern stems from both the extremely large seafloor areas that may be impacted by mining and the very limited knowledge of the fauna and ecology of this region the environmental factors regulating seafloor ecology are still very poorly understood in this study we focus on megafaunal ecology in the proposed conservation zone area of particular environmental interest 6 study area centred 17 16 n 122 55 w we employ bathymetric data to objectively define three landscape types in the area a level bottom flat an elevated ridge a depressed trough water depth 3950 4250 m that are characteristic of the wider ccz we use direct seabed sampling to characterise the sedimentary environment in each landscape detecting no statistically significant differences in particle size distributions or organic matter content additional seafloor characteristics and data on both the metazoan and xenophyophore components of the megafauna were derived by extensive photographic survey from an autonomous underwater vehicle image data revealed that there were statistically significant differences in seafloor cover by nodules and in the occurrence of other hard substrata habitat between landscapes statistically significant differences in megafauna standing stock functional structuring diversity and faunal composition were detected between landscapes the flat and ridge areas exhibited a significantly higher standing stock and a distinct assemblage composition compared to the trough geomorphological variations presumably regulating local bottom water flows and the occurrence of nodule and xenophyophore test substrata between study areas may be the mechanism driving these assemblage differences we also used these data to assess the influence of sampling unit size on the estimation of ecological parameters we discuss these results in the contexts of regional benthic ecology and the appropriate management of potential mining activities in the ccz and elsewhere in the deep ocean

decomposition of multicomponent sea floor data into upgoing and downgoing p and s waves a method for decomposing multicomponent sea floor measurements into upgoing and downgoing p and s waves is presented we assume that a marine survey employing a marine source in the water layer is conducted over a plane layered medium from recordings of the pressure just above the sea floor and the particle velocity vector just below the sea floor decomposition filters can be determined by plane wave analysis the decomposition filter coefficients depend on the p and s wave velocities and the density at the sea bottom we show how to decompose the multicomponent measurements into upgoing and downgoing p and s vertical traction components vertical particle velocity components and horizontal particle velocity components the decomposition filters are applied with good results to synthetic data modeled in a plane layered medium

azimuthal anisotropy in microseismic monitoring a bakken case study abstracthydraulic fracturing routinely applied for enhancing the permeability of unconventional oil and gas reservoirs is one of the possible causes for azimuthal anisotropy of the treated formations accounting for both naturally occurring and completion induced azimuthal anisotropy leads to marked improvements in the results of microseismic data processing as illustrated on a data set acquired in the bakken field north dakota usa those improvements include the possibility of modeling the observed shear wave splitting reduction of misfit between the picked and modeled traveltimes of microseismic events and relocation and tightening of the spatial distribution of the event hypocenters in addition and perhaps most importantly for the development of microseismic technology the feasibility of joint inversion of field microseismic data for the event locations and azimuthally anisotropic velocity model containing triclinic layers is demonstrated

joint location and source mechanism inversion of microseismic events benchmarking on seismicity induced by hydraulic fracturing summary seismic monitoring can greatly benefit from imaging events with a low signal to noise ratio snr as the number of the events with a low signal grows exponentially one way to detect weaker events is improvement of a snr by migration type stacking of waveforms from multiple stations we have developed a new method of location of seismic events that involves stackingofseismicphasesandamplitudesalongdiffractiontraveltimecurvestosuppressnoise and detect seismic events with a snr lower than that on individual receivers the stacking includes polarity correction based on a simultaneous seismic moment tensor inversion and detection algorithm on the stack function we applied this method to locate microseismicity induced by hydraulic fracturing first we calibrated the velocity model by locating perforation shots at known locations then we processed 3 d of data from microseismic monitoring of shale stimulation and benchmarked migration type locations of the largest events that were manually located the detected and located events induced by hydraulic fracturing in this case study are mostly shear events forming narrow bands along the maximum horizontal stress direction approximately 100 m above the injection intervals the proposed technique is fully automated and feasible for real time seismic monitoring

subglacial hydraulic conditions and their possible impact upon subglacial bed formation abstract in reviewing hypotheses of subglacial bed formation it becomes apparent that a close relationship must exist between the development of these bedforms and subglacial hydraulic conditions a speculative classification of subglacial temperate beds is presented subglacial beds are subdivided into h q and m beds h beds are typically hard beds in which upper interface meltwater activity dominates m beds are those in whichd soft mobile subglacial debris conditions prevail and in which advective pervasive f flow of saturated debris occurs in q beds both of the other bed types occur but are of a spatially and temporally transitory nature thus this latter bed type may be the most common beneath ice masses it is under an m bed state that many drumlin subglacial bedforms and associated forms are thought to develop discussion on the processes involved in mobile debris entrainment and the effect of changing rheological parameters are developed it is argued that such bedform initiation occurs due to anisotropic rheological variations within the mobile debris setting up a feedback effect such conditions are thought to be found most likely at the lower interface of a fast moving ice mass possibly typical of an ice stream in using empirical analyses of mobile debris conditions a limited set of relationships between subglacial meltwater flux rates basal ice velocity debris porosity and bed width are established with reference to mobile debris thicknesses using this relationship a set of possible parameter values that would allow bed formation to be achieved are presented the impact of bed formation under these conditions upon possible sedimentological facies and deformation structures is postulated the wider effect of m bedd conditionsd on subglacial sedimentology is introduced and the need to investigate subglacial hydraulic and bed conditions under past glaciations from the evidence of their glacial sediments and landforms emphasised

evidence against pervasively deformed bed material beneath rapidly moving lobes of the southern laurentide ice sheet abstract during the wisconsin glaciation the ice lobes of the southern part of the laurentide ice sheet moved rapidly as the result of elevated subglacial water pressure the rapid movement was probably not accompanied by wide spread pervasive deformation of unfrozen material under the ice the till stratigraphy of much of this area is well known and it is clear that the stratigraphic sequence is generally intact therefore such deformation if it occurred would have been restricted to the till sheet that was currently being deposited however this seems unlikely as well because the till of the region commonly contains lenses and clasts of unlithified bedded sand that should have been destroyed by pervasive shearing if unfrozen deformed till layers were widely present they must have been confined to the thin upper parts of till sheets

drumlins in ice sheet reconstructions with reference to the western pennines northern england abstract drumlin morphology and distribution can be used to determine the palaeoglaciological conditions under which subglacial bedforms were created by late pleistocene ice sheets detailed field mapping of part of the large drumlin fields in northern england in particular the identification of areas of superimposed drumlins has been shown to be of significance in the delimitation of the ice divide within the area the overall distribution of the subglacial bedforms has also been used to define the extent of ice streams within this part of the late devensian dimlington stadial ice sheet based on the physical principles which underlie the deformational model of drumlin formation and present day glaciological information on subglacial ice sheet conditions variations in drumlin morphology have allowed a first attempt at reconstructing areas of high 100 kpa and low

the interpretation and classification of subglacially deformed materials abstract a general classification of subglacially deformed materials is proposed based on sediment properties and their relationship to styles of subglacial strain deformation till is defined as homogenized usually diamictic material formed by glacially induced shear of subsole materials three types are recognized type a formed by pervasive ductile deformation type b formed by brittle shear and comminution till produced by the reduction of void space by in situ crushing and abrasion the term glacitectonite is adopted for materials that have undergone subglacial shear but retain some of the structural characteristics of the parent material original structures may be truncated by glacitectonic fabric elements type a or distorted but not truncated type b the principles of strain and material response particle orientation mechanisms fabric development and drainage conditions are reviewed examples of deformation tills and glacitectonites from breidamerkurjokull iceland slettmarkbreen norway loch lomond scotland and east yorkshire england are used to illustrate the nature of subglacially deformed materials and their relationship to former subglacial strain and drainage conditions the geologic evidence suggests a continuum of bed strengths can be recognized in deforming substrata ranging from high strength low strain type b deformation till and comminution till to low strength high strain type a deformation till

frozen bed fennoscandian and laurentide ice sheets during the last glacial maximum the areal extents of the laurentide and fennoscandian ice sheets during the last glacial maximum about 20 000 years ago are well known1 but thickness estimates range widely from high domed2 to thin3 with large implications for our reconstruction of the climate system regarding for example northern hemisphere atmospheric circulation and global sea levels this uncertainty stems from difficulties in determining the basal temperatures of the ice sheets and the shear strength of subglacial materials4 a knowledge of which would better constrain reconstructions of ice sheet thickness here we show that in the absence of direct data the occurrence of ribbed moraines in modern landscapes can be used to determine the former spatial distribution of frozen and thawed bed conditions we argue that ribbed moraines were formed by brittle fracture of subglacial sediments induced by the excessive stress at the boundary between frozen and thawed bed conditions resulting from the across boundary difference in basal ice velocity maps of glacial landforms from aerial photographs of canada and scandinavia reveal a concentration of ribbed moraines around the ice sheet retreat centres of quebec keewatin newfoundland and west central fennoscandia together with the evidence from relict landscapes that mark glacial areas with frozen bed conditions the distribution of ribbed moraines on both continents suggest that a large area of the laurentide and fennoscandian ice sheets was frozen based and therefore high domed and stable during the last glacial maximum

ice proximal glaciomarine sedimentation and sea level change in the inverness area scotland a review of the deglaciation of a major ice stream of the british late devensian ice sheet abstract evidence of both rising and falling relative sea levels and glacitectonic movements is preserved in two formations of raised glaciomarine deposits that were laid down in front of an oscillating grounded tidewater glacier in the inverness firth these changes occurred during the latter stages of the disintegration of the moray firth ice stream one of the major ice streams that drained the british main late devensian ice sheet most of the glaciomarine deposits antedate a sequence of glacio isostatically tilted late devensian marine shorelines and associated littoral and estuarine deposits the shorelines began forming at about 13 000 bp and record a progressive fall in relative sea level a new model for the deglaciation of the moray firth region is proposed after a critical appraisal of published accounts of both onshore and offshore quaternary sequences the disintegration of the moray firth ice stream involved several rapid phases of retreat to pinning points caused by iceberg calving and triggered by rising global sea level each retreat was followed by minor readvances or stillstands possibly caused by short lived accelerated periods of glacio isostatic rebound and concomitant temporary falls in relative sea level two such events occurred in the inverness firth the ardersier oscillation and the alturlie stillstand substantial differences lower relative sea levels later deglaciation are apparent between the pattern of ice retreat in the moray firth region and published accounts of the deglaciation of the irish sea basin these differences require a reassessment of some current hypotheses concerning the disintegration of major ice streams associated with high relative sea levels furthermore geological and geomorphological evidence suggesting both rising and falling sea levels in the inverness area prior to ca 13 500 bp is not fully compatible with recently published computer simulations of the dissolution of the british main late devensian ice sheet

offset and angle domain common image point gathers for shot profile migration prestack depth migration of shot profiles by downward continuation is a practical imaging algorithm that is especially cost effective for sparse shot wide azimuth geometries the interpretation of offset as the displacement between the downward propagating shot wavefield and upward propagating receiver wavefield enables us to extract offset domain common image point cip gathers during shot profile migration the offset domain gathers can then be transformed to the angle domain with a radial trace mapping originally introduced for shot geophone migration the computational implications of this procedure include both the additional cost of multioffset imaging and an implicit transformation from shot geophone to midpoint offset coordinates although this algorithm provides a mechanism for imaging angle dependent reflectivity via shot profile migration for sparse shot geometries the fundamental problem of shot aliasing may severely impact the quality of cip gathers

reflections on soft subglacial beds as a mosaic of deforming and stable spots abstract much discussion regarding subglacial processes is centred around the question of how soft beds respond to ice overriding in particular how widespread and penetrative is their deformation how does it affect the ice stability and what are the consequences for large scale sediment redistribution patterns contrasting views have been proposed which are difficult to verify and reconcile in the light of scarcity of agreed criteria to identify deforming beds here we suggest that soft beds under warm based glaciers can be envisaged as a mosaic of deforming and stable spots transient in time and space according to changes in the control parameters at the ice bed interface the subglacial mosaic model is consistent with field data showing heavily deformed beds interspersed with largely undisturbed beds it is suggested that deformation found in tills is a cumulative effect of strain within a relatively thin ephemeral deforming spots under the ice sole affecting the till as it accretes during ice overriding in accord with the coulomb plastic rheology the role of clast ploughing in bed deformation is emphasised

constrained tomography of realistic velocity models in microseismic monitoring using calibration shots the knowledge of the velocity model in microseismic jobs is critical to achieving statistically reliable microseismic event locations the design of microseismic networks and the limited sources for calibration do not allow for a full tomographic inversion we propose optimizing a priori velocity models using a few active shots and a nonlinear inversion suitable to poorly constrained systems the considered models can be described by several layers with different p and s wave velocities the velocities may be constant or have 3d gradients the layer interfaces may be simple dipping planes or more complex 3d surfaces in this process the p and s wave arrival times and polarizations measured on the seismograms constitute the observed data set they are used to estimate two misfit functions i one based on the measurement residuals and ii one based on the inaccuracy of the source relocation these two functions are minimized thanks to a simulated annealing scheme which decreases the risk of converging to a local solution within the velocity model the case study used to illustrate this methodology highlights the ability of this technique to constrain a velocity model with dipping layers this was performed by jointly using sixteen perforation shots recorded during a multi stage fracturing operation from a single string of 3c receivers this decreased the location inaccuracies and the residuals by a factor of six in addition the retrieved layer dip was consistent with the pseudo horizontal trajectories of the wells and the background information provided by the customer finally the theoretical position of each calibration shot was contained in the uncertainty domain of the relocation of each shot in contrast single stage inversions provided different velocity models that were neither consistent between each other nor with the well trajectories this example showed that it is essential to perform a multi stage inversion to derive a better updated velocity model

subglacial conditions under the last ice sheet in northwest germany ice bed separation and enhanced basal sliding abstract theoretical reconstruction of ice thickness during the weichselian glaciation in a portion of northwest germany yields maximum thicknesses of about 280 m some 50 km into the ice sheet from the margin this corresponds to averaged driving stress at the glacier bed in the order of 17 kpa the increase in ice thickness away from the margin was non uniform caused by undulations in pre existing topography steady state subglacial water pressure heads were high up in the ice sheet reaching the flotation point in places hydraulic lifting of the ice sheet probably occurred causing non deforming bed conditions where transformation of basal shear stresses to soft subglacial sediments was prevented by a water film thin stringers of undeformed sorted sediments occurring in tills are attributed to basal meltwater washing during phases of ice bed separation the basal water film was over 5 mm thick using a quantitative approach coupled with field data we demonstrate that neither internal ice deformation nor bed deformation contributed much to the ice motion instead the rapid movement of the weichselian ice sheet was caused by basal sliding with ploughing the ice sheet was probably characterized by an unstable behaviour

high resolution modeling of the advance of the younger dryas ice sheet and its climate in scotland abstract ice sheet modeling tightly constrained by empirical studies provides an effective framework to reconstruct past climatic and environmental conditions scotland was severely affected by the abrupt climate change associated with the younger dryas stade during which an extensive ice sheet formed across the west highlands after a period of ice free conditions here a quasi three dimensional time dependent ice flow mass balance model is developed and applied to scotland at 1 km resolution the flow model is based on the driving stress approximation with an additional longitudinal correction term essential at this scale of operation surface mass balance is driven by temperature and precipitation changes and further mass wastage is achieved through an empirically defined calving term the ice dynamics and mass balance components are coupled through the equation for mass continuity which is integrated through time over a finite difference grid which yields the geometric evolution of the ice sheet initial experiments reveal the model to be relatively insensitive to internal parameters but highly sensitive to mass balance furthermore these experiments indicate that scotland is readily susceptible to glaciation with large glaciers building up on the flanks of ben nevis after a temperature depression of 2 5 c under present day precipitation the younger dryas is modeled using a grip temperature series locally adjusted for amplitude and a systematic series of runs enables the isolation of the climate which best matches mapped ice limits this optimum fit configuration requires an annual temperature cooling of 8 c and the introduction of substantial west east and south north precipitation gradients of 40 and 50 respectively to the present day regime under these conditions a series of substantial independent regional ice centers develop in agreement with trimline studies and after 550 years the modeled ice sheet closely resembles the maximum limits as indicated by field mapping however modeled ice continues to expand beyond 550 yr in conflict with the mapped ice limits which suggest a prolonged period of stability this discrepancy may be explained by the onset of extreme aridity ca 400 yr into the stade associated with a southern migration of the polar front leading to a reduction in atmospheric circulation which effectively starved the ice sheet of its moisture source preventing further expansion introduction of an additional 20 reduction in precipitation to the optimum fit regime after 350 yr brings the modeled ice sheet to equilibrium substantiating this conclusion

fluted moraine formation and till genesis below a temperate valley glacier slettmarkbreen jotunheimen southern norway clast fabric and morphological data have been used to determine the origin of fluted subglacial tills exposed by recent retreat of the slettmarkbreen glacier norway a new method for the interpretation of clast fabric data allows aspects of the strain and depositional history of the till to be reconstructed the till formed by a combination of lodgement and subsole deformation by slip along discrete shear planes lodgement was dominant for the larger size fractions 125 mm while the smaller material was more susceptible to deformation the fluted till surface reflects the tendency for the till matrix to deform into regions of low confining pressure in the lee of lodged boulders downglacier components of till flow are thought to have resulted in significant sediment transfer towards the margin

inversion of microseismic data for triclinic velocity models modern downhole microseismic surveys often employ geometries in which ray trajectories generated by a collection of locatable events provide full polar and azimuthal coverage making it possible to estimate the in situ seismic anisotropy we show that traveltimes and particle motions of the direct p and shear waves acquired in such geometries can constrain stiffness tensors of triclinic media while obtaining all 21 stiffness coefficients of a homogeneous triclinic space simultaneously with locating pertinent microseismic events from data recorded in a single vertical well is relatively straightforward the same methodology does not necessarily succeed in layered formations because the combination of their vertical heterogeneity and azimuthal anisotropy might invalidate the commonly adopted approximation of the event azimuths by those of the p wave polarization vectors when the event azimuths cannot be derived from the particle motions traveltimes observed in two or more wells are required to locate the events and build layered triclinic or higher symmetry azimuthally anisotropic velocity models as our numerical tests indicate the multi well event location methods are expected to perform better than their single well counterparts because they rely solely on triangulation and eliminate the usually pronounced azimuthal uncertainties in the event locations that stem from noises adversely affecting hodogram analysis

lithofacies types and vertical profile models an alternative approach to the description and environmental interpretation of glacial diamict and diamictite sequences abstract increased knowledge of modern glacial depositional environments has resulted in rapidly evolving classifications of glacial tills these are based to a large degree on theoretical considerations of likely depositional processes the classifications are sophisticated and more advanced than the establishment of simple field criteria whereby individual till facies can be identified in quaternary and pre quaternary successions this situation is compounded in many quaternary terrains by the continued description of tills in terms of laboratory derived analytical data only reflecting a traditional interest in stratigraphic correlation rather than reconstruction of depositional environment detailed sedimentological logging of lithofacies is rarely undertaken there is thus considerable confusion as to what is being described or sampled when analytical data are presented for many pleistocene tills the same remarks apply to pre pleistocene tillites a lithofacies code is presented here for the rapid description and visual appraisal of field sequences or drill cores containing unconsolidated diamicts or lithified diamictites the term till is not used as it has a strict genetic definition referring to direct aggregation and deposition by glacier ice use of a four part code in conjunction with codes already published for fluvial sediments allows fundamental field properties to be depicted independent of genetic terminology and provides a firm basis for subsequent environmental interpretation and analytical work the value of this approach is illustrated by comparing a representative suite of vertical profiles of diamict assemblages deposited by modern grounded glaciers with a classic late pleistocene glacigenic sequence at scarborough bluffs ontario

estimating geomechanical parameters from microseismic plane focal mechanisms recorded during multistage hydraulic fracturing abstractwe extend a full waveform modeling method to invert source focal plane mechanisms for microseismic data recorded with dual borehole seismic arrays combining inverted focal plane mechanisms with geomechanics knowledge we map the pore pressure distribution in the reservoir determining focal mechanisms for microseismic events is challenging due to poor geometry coverage we use the p wave polarities the p and s wave similarities the sv p amplitude ratio and the sh p amplitude ratio to invert the focal plane mechanisms a synthetic study proves that this method can effectively resolve focal mechanisms with dual array geometry we apply this method to 47 relatively large events recorded during a hydraulic fracturing operation in the barnett shale the focal mechanisms are used to invert for the orientation and relative magnitudes of the principal stress axes the orientation of the planes slipping in shear and the approximate pore pressure perturbation that caused the slip the analysis of the

image domain velocity inversion and event location for microseismic monitoring abstractmicroseismic event locations obtained from seismic monitoring data sets are often a primary means of determining the success of fluid injection programs such as hydraulic fracturing for oil and gas extraction geothermal projects and wastewater injection event locations help the decision makers to evaluate whether operations conform to expectations or parameters need to be changed and may be used to help assess and reduce the risk of induced seismicity however obtaining accurate event location estimates requires an accurate velocity model which is not available at most injection sites common velocity updating techniques require picking arrivals on individual seismograms this can be problematic in microseismic monitoring particularly for surface acquisition due to the low signal to noise ratio of the arrivals we have developed a full wavefield adjoint state method for locating seismic events while inverting for p and s wave velocity models that optimally focus multiple complementary ima

microseismic image domain velocity inversion marcellus shale case study abstractseismic monitoring at injection wells relies on generating accurate location estimates of detected micro seismicity event location estimates assist in optimizing well and stage spacings assessing potential hazards and establishing causation of larger events the largest impediment to generating accurate location estimates is an accurate velocity model for surface based monitoring the model should capture 3d velocity variation yet rarely is the laterally heterogeneous nature of the velocity field captured another complication for surface monitoring is that the data often suffer from low signal to noise levels making velocity updating with established techniques difficult due to uncertainties in the arrival picks we use surface monitored field data to demonstrate that a new method requiring no arrival picking can improve microseismic locations by jointly locating events and updating 3d p and s wave velocity models through image domain adjoint state tomography this approach creates a co

distribution source and evolution of nitrate in a glacial till of southern alberta canada this paper addresses the distribution origin and controls upon nitrate in a 30 km2 area of the interior great plains region of southern alberta canada high concentrations of nitrate 100 mg l 1 no 3 n occurred in several isolated enclaves below the water table in brown weathered till nitrate concentrations of over 300 mg l 1 n were encountered in groundwater samples collected from these enclaves low nitrate concentrations 1 1 mg l 1 no 3 n were also encountered in the weathered till upgradient and downgradient of the nitrate enclaves groundwater samples collected from the underlying grey nonweathered till and bedrock had no 3 n concentrations of 1 1 mg l 1 through the application of geochemical no 3 n and nh 4 n studies environmental isotope studies tritium microbial analyses nitrifiers and laboratory experiments it was shown that the high nitrates found in the weathered till are the result of the oxidation of ammonium present within the tills it is postulated that this oxidation occurred during the holocene epoch when water tables were much lower than present day levels 5 18 m and 2 m below ground respectively through the use of eh measurements the enumeration of denitrifying bacteria and laboratory experiments the potential for denitrification was shown to exist below the present day water table in the weathered till as well as in the nonweathered till and bedrock isotopic data showed that less denitrification may be occurring within the nitrate enclaves than in adjacent downgradient areas

extended wave equation imaging conditions for passive seismic data abstractthe information obtained from seismic monitoring at injection sites is often limited to seismic event properties e g location origin time moment tensor the accuracy of which greatly depends on the assumed or estimated elastic velocity models current traveltime based methods to calibrate velocity models rely on having distinct arrivals in surface seismic monitoring however the signal to noise ratio of arrivals is often too poor for pick based methods and migration techniques are needed wave equation migration can image microseismic sources without the need for picking or knowledge of the onset time by reconstructing the wavefield through the model space and applying an imaging condition we have devised extended imaging conditions for passive seismic wave equation imaging algorithms and we have found that they represent a key step toward verifying and updating elastic velocity models by extending imaging conditions away from zero lag in time and space we can better evaluate the focu

late devensian ice sheet in the western grampians scotland a radial pattern of ice flow of the last ice sheet in the largest source area of ice c 5000 km2 in the british isles in western scotland is demonstrated by the dispersal of indicator erratics and by patterns of striae friction cracks and ice moulded landforms three major ice domes and the principal ice divide are identified in the western grampians the ice domes coincided with the highest mountain blocks while it is inferred that the alignment of many of the pre existing valleys controlled much of the outflow of ice forming ice streams within the ice sheet the importance of the rannoch moor basin as a radial provider of ice to surrounding areas was apparently less significant than has hitherto been considered ice flowed into and across the southern part of the basin from the principal ice divide located to the west and south no evidence has yet been found that would support a model of an eastward migrating ice divide either during the build up or during the deglaciation of the late devensian ice sheet

errors in hypocenter location picking model and magnitude dependence abstract the location procedures of seismic events are influenced by two major classes of errors the error in picking individual seismic phases and modeling error due to the departure of the real earth from the reference model used in the location both classes of error influence the estimate of location and it is difficult to separate them the role of picking errors can be assessed by a nonlinear analysis using a monte carlo procedure arrivals times are perturbed with random numbers drawn from a normal distribution and the event is relocated using these perturbed arrival times by repeating the procedure many times a cluster of locations is obtained which can be used to investigate the effects of picking errors on the hypocenter this analysis is insensitive to velocity model errors as these are fixed for a given combination of stations and phases some care must be exercised when analysing multidimensional distributions in two dimensional slices because of a projection effect the modeling error due to the influence of lateral heterogeneity in the earth is examined by comparing the locations of the same event using different combinations of phases and network geometries which reinforces the need to use arrivals other than p for accurate depth resolution the sensitivity of p arrivals to changes in depth are swamped by model errors and inclusion of depth sensitive phases such as pp is highly recommended the effect of picking errors on location is found to be much smaller than the mislocation caused by neglecting lateral heterogeneity when only p arrivals are used consequently the monte carlo analysis which is primarily aimed at picking errors only is most appropriate when multiple phases have been used to more accurately constrain the hypocenter especially for the depth component altering the type of phase data used in the location plays a similar role in changing the network geometry in that both are mechanisms that influence the nature of the constraint on the hypocenter by relocating events with network geometries corresponding to the different magnitudes it is found that the location of the event can be affected significantly by the magnitude and when using robust statistics to describe earthquake residuals the mislocation can occur in a systematic manner the effect is marked in regions with significant lateral variations in seismic velocities for example low magnitude events in the flores sea are found to be dragged toward australia as a result of the fast paths to australian stations relative to the iasp 91 reference velocity model

evidence for abundant subglacial meltwater beneath the paleo ice sheet in pine island bay antarctica marine geological and geophysical data collected from the continental shelf in pine island bay antarctica reveal a complex paleo subglacial drainage system controlled by bedrock topography and subglacial meltwater discharge significant amounts of freely flowing meltwater existed beneath former ice sheets in pine island bay subglacial drainage is characterized by descriptions of glacial landforms imaged on the sea floor and sedimentary deposits collected in piston cores bedrock geology is characterized using seismic data large scale landforms on the shelf include channels and cavities incised into impermeable crystalline bedrock there is a transition from randomly oriented channels on the inner shelf to a dendritic pattern of elongate channels on the middle shelf on the outer shelf a change in basal conditions occurs where sedimentary deposits bury crystalline bedrock no evidence for flowing meltwater exists on sedimentary substrates instead meltwater formed a the ice sediment contact was incorporated into the sediments contributing to development of a deforming lied which was sampled in piston cores characterization of subglacial meltwater processes that occurred in the past may aid in understanding the role meltwater plays in stability of the west antarctic ice sheet today

climate variations in britain during the last glacial holocene transition 15 0 11 5 cal ka bp comparison with the grip ice core record stratigraphic records from four sequences in britain that span the last glacial holocene transition c 15 0 11 5 cal ka bp are summarized and compared using a calibrated timescale palaeoclimatic changes are defined using temperature estimates based on coleopteran data at two of the sites and chironomid assemblages at the other sites an attempt is made to distingish between the timing of a climate shifts and b ecosystem responses at all four sites the british data are compared with the grip ice core record recently advocated by bjorck et al as the type sequence for the last termination in the north atlantic region there is a degree of compatibility between the british and grip palaeoclimatic records the clearest climatic signals in the british lake records are a a period of warm conditions a thermal maximum between 15 0 and 14 5 cal ka bp which equates approximately with grip zone gi le b the onset of climatic cooling which appears to have commenced during grip zone gi le at c 14 5 cal ka bp c a period of marked climatic instability from 14 4 to 12 7 cal ka bp the regional effects of which are difficult to resolve using the evidence presently available d further climatic cooling which commenced at around 13 0 cal ka bp after which the lake biota catchment soils and vegetation cover at least in some parts of britain did not recover until the onset of the holocene e continued climatic cooling with the most severe impact on lake ecosystems occurring during the period c 12 6 to 11 5 cal ka bp

preservation of landforms under ice sheets and ice caps abstract this article addresses the question of whether or not distinct glacial and non glacial landforms can survive beneath ice sheets and ice caps with little or no morphological alteration a review of recent work documents the existence of pre last stadial landforms and landscapes in areas covered by the fennoscandian and laurentide ice sheets a substantial number of independent works indicate that landforms such as eskers drainage channels and boulder fields have escaped destruction despite complete ice overriding during several tens of millenia full preservation of former ground surfaces or delicate landforms probably is linked to areas where the ice sheet base was continuously frozen to its bed larger robust landforms such as large drumlins appear to have been preserved even under wet based conditions in glaciated areas patches preserved under dry cold based conditions provide important windows towards the past showing landscapes that were destroyed in surrounding areas affected by wet based and eroding ice some consequences for the research fields of non glacial geomorphology archaeology and botany include the possibility of subglacial museums and refugia a time space model describes geomorphological access to information from older events in glaciated areas

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

moment tensor migration imaging we develop and apply an imaging procedure for simultaneous location and characterization of seismic source properties called moment tensor migration imaging the procedure constructs images for moment tensor components using a weighted diffraction stack migration and combines ray theoretical green s functions with a reverse time moment tensor imaging methodology by applying an approximation we term the ray angles only approximation we form an expression for moment tensor migration imaging where the migration weights depend only on the take off and arrival angles for rays leaving receiver positions and incident upon the image points moment tensor migration imaging retains the benefits of diffraction stack procedures for source location and characterization namely speed flexibility and the potential for incorporating non linear stacking procedures whilst also providing the benefits of moment tensor imaging such as the inclusion of multiple phase and multiple component data the collapsing of the source radiation pattern estimation of the moment tensor we examine variations of the imaging procedure through a synthetic test we show that although the assumptions required for the imaging and ray angles only approximation may not be strictly valid for realistic survey geometries a simple weight adjustment can be used to obtain more accurate and stable results in these situations in our synthetic example we find that the use of a p wave only migration without this reweighting structure produces poor results whereby the resulting images show activity upon incorrect moment tensor components however many of these effects are mitigated by use of the reweighting scheme and the results are further improved through the introduction of non linear stacking operators such as semblance weighted stacks the highest quality moment tensor images for the synthetic test examined here are obtained through the use of both p wave and s wave wave fields this highlights the importance of multicomponent data and multiphase modelling when characterizing seismic sources we also find that the imaged moment tensor components vary proportionately when the input velocities are perturbed by a scale factor this suggests for the geometry investigated here derived source properties such as fault plane solutions and shear tensile components will not be influenced by bulk changes in seismic velocities finally we show the application to a real microseismic event observed using a surface array during hydraulic fracturing we find that the procedure collapses the seismic radiation pattern into an anomaly with a maximum at the hypocentre and our derived mechanism is consistent with the observed radiation pattern from the source

palaeoclimatic reconstruction from loch lomond readvance glaciers in the west drumochter hills scotland geomorphological mapping in the west drumochter hills provides evidence of a readvance of locally nourished glaciers during the loch lomond younger dryas stade in the form of an icefield 67 7 km2 in area drained by outlet glaciers the icefield limits accord broadly with those proposed by sissons 1980 but all geomorphic stratigraphic and sedimentological evidence conflicts with a recent proposal that the landforms in the area reflect southwestwards retreat of the last ice sheet up valley continuity of recessional moraines indicates that the ice remained active and close to climatic equilibrium during the earlier stages of glacier retreat consistent with slow warming following the coldest part of the stade the pattern of equilibrium line altitudes elas across the icefield is consistent with transfer of snow by westerly and southerly winds the ela of the reconstructed icefield as a whole is 622 629 m although this figure is likely to be lower than the regional climatic ela because the icefield probably received additional snow blown from adjacent plateau surfaces and slopes inclusion of potential snow blow areas in the ela calculation yields a value of 648 656 m the climatic ela is therefore likely to have lain between 622 and 656 m mean june to august temperature at the ela based on chironomid assemblages at two sites falls within the range 4 0 0 7 c empirical relationships between temperature and precipitation at modern glacier elas indicate that mean annual precipitation map at the ela was 1977 464 mm statistically indistinguishable from modern values comparison with precipitation values calculated for the isle of mull on the west coast suggest that the precipitation gradient across the central highlands of scotland was steeper during the loch lomond stade than at present probably as the result of efficient scavenging of precipitation from westerly airflows by the west highland icefield copyright 2005 john wiley sons ltd

the sustained observatory over the porcupine abyssal plain pap insights from time series observations and process studies oceanographic research in the past 20 years has highlighted the importance of understanding decadal scale variation in the oceans in order to predict how the ocean will respond to climate change climate factors control many of the processes that regulate the input of nutrients to the euphotic zone primary productivity zooplankton responses the downward flux of organic matter the biomass and composition of seabed communities and ultimately the burial of carbon in deep sea sediments a critical part of our understanding of the interdependence of these many elements comes from long term eulerian observations such as the hawaii ocean time series hot karl et al 2003 the bermuda atlantic time series bats steinberg et al 2001 the ne pacific station m smith and druffel 1998 and the ne atlantic porcupine abyssal plain site pap billett and rice 2001 and lampitt et al 2001 each time series site is considered as representing a different oceanic setting and each has a different suite of observations and history however the unifying theme of them all is to understand the time varying properties and processes of the oceanic environment only at station m and pap have water column and seabed observations been integrated a feature of some considerable advantage this special issue presents the latest work at pap

subglacial till formation sedimentary characteristics and classification we review the major subglacial till forming processes as presently understood by glacial researchers and define the parameters within which tills are produced and reconcile them with sedimentary end members processes of deformation flow sliding lodgement and ploughing coexist at the base of temperate glacier ice and act to mobilize and transport sediment and deposit it as various end members ranging from glacitectonically folded and faulted stratified material to texturally homogeneous diamicton the dominance of any one subglacial process varies both spatially and temporally giving rise to the possibility that a till or complex till sequence contains a superimposed signature of former transportation deposition at the ice bed interface we recommend that while glacial geologists and geomorphologists should be able to recognize the sedimentary imprints of various subglacial processes genetic fingerprinting of subglacial tills should be less process specific and till classification must reflect the range of products encompassed by the subglacial till production continuum glacial geologists can presently unequivocally identify a glacitectonite rock or sediment that has been deformed by subglacial shearing deformation but retains some of the structural characteristics of the parent material b subglacial traction till sediment deposited by a glacier sole either sliding over and or deforming its bed the sediment having been released directly from the ice by pressure melting and or liberated from the substrate and then disaggregated and completely or largely homogenised by shearing and support the theoretical case for c melt out till sediment released by the melting of stagnant or slowly moving debris rich glacier ice and directly deposited without subsequent transport or deformation because observations on contemporary glaciers reveal that their beds are most likely to be mosaics of deformation and sliding and warm based and cold based conditions the patterns of which change temporally and spatially it is extremely unlikely that subglacial till end members in the geological record will be anything but hybrids produced by the range of processes operative in the subglacial traction zone

wave equation migration velocity analysis ii subsalt imaging examples subsalt imaging is strongly dependent on the quality of the velocity model however rugose salt bodies complicate wavefield propagation and lead to subsalt multipathing illumination gaps and shadow zones which cannot be handled correctly by conventional traveltime based migration velocity analysis mva we overcome these limitations by the wave equation mva technique introduced in a companion paper and demonstrate the methodology on a realistic synthetic data set simulating a salt dome environment and a gulf of mexico data set we model subsalt propagation using wave paths created by one way wavefield extrapolation those wave paths are much more accurate and robust than broadband rays since they inherit the frequency dependence and multipathing of the underlying wavefield we formulate an objective function for optimization in the image space by relating an image perturbation to a perturbation of the velocity model the image perturbations are defined using linearized prestack residual migration thus ensuring stability relative to the first order born approximation assumptions synthetic and real data examples demonstrate that wave equation mva is an effective tool for subsalt velocity analysis even when shadows and illumination gaps are present

separation of p and s waves in combined p and sv observations with vertical and in line horizontal geophones recognition and separation of the individual p and s arrivals may form a considerable problem due to the geophone receiving characteristics p and s waves are observed on both horizontal and vertical geophones a new method of separating p and s waves is presented that inverts the geophone records for the receiving characteristics thereby separating the two wavetypes the filter coefficients are determined from the near surface p and s wave velocity and from the geometry of the geophone groups the p s separation filter was tested on synthetic data with good results even ps and sp converted reflections which are completely coinciding for horizontal reflectors could be well separated the filter appears to be stable and to have low sensitivity to noise in the data or to errors in the near surface p and s wave velocity the filter is superior to other separation methods which were applied to the same dataset for comparison this method can also be applied to vsp data

temporal variability in phytodetritus and megabenthic activity at the seabed in the deep northeast atlantic we report a ten year study of the abundance and activity of megabenthos on the porcupine abyssal plain northeast atlantic together with observations on the occurrence of phytodetritus at the deep sea floor 4850 m using the southampton oceanography centre time lapse camera system bathysnap we have recorded a radical change in the abundance and activity of megabenthos between the two periods of study 1991 1994 and 1997 2000 in 1991 1994 the larger megabenthos occurred at an abundance of c 71 6 ha and were dominated by large holothurians in addition there were very substantial populations of smaller megabenthic ophiuroids c 4979 ha together the total megabenthos are estimated to track over some 17 cm2 m2 d exploiting 100 of the surface of the seabed in c 2 5 years in 1997 2000 the larger megabenthos increased to an abundance of c 204 ha and were joined by exceptional numbers of a small holothurian species amperima rosea 6457 ha and ophiuroids principally ophiocten hastatum 53 539 ha the total megabenthos population was tracking at an estimnated rate of c 247 cm2 m2 d exploiting 100 of seabed in just 6 weeks coincident with these increases in the abundance and activity of the megabenthos there were apparently no mass depositions of aggregated phytodetritus to the seabed in the summers of 1997 1999 mass occurrences of phytodetritus had been noted during the summer months of the three years previously studied 1991 1993 and 1994 with covering between 50 and 96 of the sediment surface there is a statistically significant p 0 02 negative correlation between maximum extent of this seabed cover of phytodetritus and seabed tracking by megabenthos additional studies lampitt et al progr ocean 50 2001 indicate that there were no substantial changes in surface ocean primary productivity in export flux or in the composition of the flux that might otherwise account for the apparent absence of observable concentrations of phytodetritus during the summers of 1997 1999 we postulate that the marked increase in megabenthic tracking activity resulted in the removal via consumption disaggregation burial etc of the bulk of the incoming phytodetrital flux during these years a simple conceptual model based on the apparent phytodetrital fluxes observed in 1991 and 1993 suggests that the megabenthos tracking rates estimated for 1997 1999 are sufficient to account for near total removal of this flux however we are not able to estimate other processes removing phytodetritus i e other elements of the benthos that may also have increased between 1991 1994 and 1997 1999 other independent studies e g ginger et al progr ocean 50 2001 of flux constituents support the possibility that just a few species of megabenthos e g a rosea and o hastatum could well have consumed a major proportion of the incoming flux and so substantially modified the composition of the organic matter available to other components of the benthos

sedimentology stratigraphy and glacier dynamics western scottish highlands glacial sediments of the western scottish highlands are comprehensively described and characterized here for the first time enabling the first glacial stratigraphy for the area to be proposed this classification is based on the results of extensive geological mapping and field investigation of sedimentary sequences and their structures x ray diffraction and particle size distribution analyses and comparison with deposits formed in contemporary glaciated environments these new data are subsequently appraised in terms of their implications for late pleistocene glacier evolution and dynamics together the data suggest that much of the landscape is palimpsest and can be attributed to the weichselian late devensian glaciation subsequent glacier advance during the younger dryas did little to modify the area suggesting that ice flow was dominated by sliding on a meltwater lubricated rigid bed with deformation of basal sediments playing a more limited role final deglaciation was marked by a significant increase in basal meltwater flux reflecting the warming climate and increasing precipitation these new palaeoglaciological and palaeoenvironmental insights advance our understanding of former glacier dynamics in the western scottish highlands improve our knowledge of pleistocene landscape evolution of this area and enable comparisons to be made with sedimentary sequences elsewhere