Drag forces at the ice-sheet bed and resistance of hard-rock obstacles:The physics of glacial ripping

Glacial ripping involves glaciotectonic disintegration of rock hills and extensive removal of rock at the ice-sheet bed, triggered by hydraulic jacking caused by fluctuating water pressures. Evidence from eastern Sweden shows that glacial ripping caused significant subglacial erosion during the final deglaciation of the Fennoscandian ice sheet, distinct from abrasion and plucking (quarrying). Here we analyse the ice drag forces exerted onto rock obstacles at the base of an ice sheet, and the resisting forces of such rock obstacles: glaciotectonic disintegration requires that ice drag forces exceed the resisting forces of the rock obstacle. We consider rock obstacles of different sizes, shapes and fracture patterns, informed by natural examples from eastern Sweden. Our analysis shows that limited overpressure events, unfavourable fracture patterns, low-Transmissivity fractures, slow ice and streamlined rock hamper rock hill disintegration. Conversely, under fast ice flow and fluctuating water pressures, disintegration is possible if the rock hill contains subhorizontal, transmissive fractures. Rock steps on previously smooth, abraded surfaces, caused by hydraulic jacking, also enhance drag forces and can cause disintegration of a rock hill. Glacial ripping is a physically plausible erosion mechanism, under realistic glaciological conditions prevalent near ice margins.</p

The last glaciation in Caithness, Scotland: revised till stratigraphy and ice-flow paths indicate multiple ice flow phases

This paper reviews existing information on the last glaciation of Caithness and presents new evidence for additional till units and for long distance ice-flow paths based on till palynomorphs, indicator erratics and striae. Early, radial expansion of Northern Highland ice probably occurred at 31 – 29 ka. After ice withdrawal from the north coast, Moray Firth ice returned before a second withdrawal. Thereafter Moray Firth ice advanced to limits close to the Atlantic shelf edge between 21 and 18 ka. Deglaciation of hill summits was completed by 18.4 – 17.8 ka but the low ground south of Wick only became ice free after 16.5 ka. Recognition of these multiple ice-flow events is consistent with the dynamic behaviour of the last ice sheet exhibited in mathematical simulations. The event sequence differs, however, from nineteenth century and recent two-stage flow set reconstructions as each main flow set is shown to represent multiple events following similar paths but under different ice sheet configurations. Various configurations allow ice to flow northwards across Caithness and so remove any requirement for the Fennoscandian Ice Sheet to directly block and divert the outflow of ice from the Moray Firth during the last glaciation. During the Late Pleistocene, the British and Irish Ice Sheet (BIIS) discharged ice via the Moray Firth from a source area that covered c. 1700 km2 in the Northern and Grampian Highlands of Scotland (Geikie 1877) (Fig. 1). Part of the flow of Moray Firth ice moved across Caithness (Peach & Horne 1880) to deposit ‘shelly boulder clay’, a mud-rich till unit including erratics of Pleistocene marine shells and Mesozoic rocks derived from glacial erosion of the bed of the inner Moray Firth (Croll 1870). Ice flow towards the north over Caithness (Fig. 2a) has long been viewed as a result of deflection of the outflow of the Moray Firth glacier by the presence of the Fennoscandian Ice Sheet (FIS) in the North Sea (Croll 1870)

Closed-form Jones matrix of dual-polarized inverted-vee dipole antennas over lossy ground

This paper presents a closed-form expression for the Jones matrix of a dual-polarized inverted-vee dipole antenna based on the Lorentz reciprocity theorem and the basic rules of electromagnetic refraction. The expression is used to determine the intrinsic cross-polarization ratio (IXR) as a function of droop angle, position of the source in the sky, antenna height, frequency, and reflection coefficient of the underlying ground. The expression is verified using full-wave simulations with a method-of-moments solver, showing very good agreement. It explains the increase in the IXR when the antenna is placed over a perfect electric ground plane. This result is used to explain the polarization properties of the Square Kilometre Array Log-periodic Antenna. Through the LOw-Frequency ARray Low-Band Antenna (LOFAR-LBA), the importance of the size of the ground plane is explained. Finally, design consideration for high polarization purity antennas is discussed

Glacial Ripping in Sedimentary Rocks: Loch Eriboll, NW Scotland

Glacial ripping is a newly recognized process sequence in which subglacial erosion is triggered by groundwater overpressure. Investigations in gneiss terrain in lowland Sweden indicate that ripping involves three stages of (i) hydraulic jacking, (ii) rock disruption under subglacial traction, and (iii) glacial transport of rock blocks. Evidence for each stage includes, respectively, dilated fractures with sediment fills, disintegrated roches moutonnées, and boulder spreads. Here, we ask: can glacial ripping also occur in sedimentary rocks, and, if so, what are its effects? The case study area is in hard, thinly bedded, gently dipping Cambrian quartz-arenites at Loch Eriboll, NW Scotland. Field surveys reveal dilated, sediment filled, bedding-parallel fractures, open joints, and brecciated zones, interpreted as markers for pervasive, shallow penetration of the quartz-arenite by water at overpressure. Other features, including disintegrated rock surfaces, boulder spreads, and monomict rubble tills, indicate glacial disruption and short distance subglacial transport. The field results together with cosmogenic isotope ages indicate that glacial ripping operated with high impact close to the former ice margin at Loch Eriboll at 17.6–16.5 ka. Glacial ripping thus can operate effectively in bedded, hard sedimentary rocks, and the accompanying brecciation is significant—if not dominant—in till formation. Candidate markers for glacial ripping are identified in other sedimentary terrains in former glaciated areas of the Northern Hemisphere

Bradykinesia Is Driven by Cumulative Beta Power During Continuous Movement and Alleviated by Gabaergic Modulation in Parkinson's Disease

Spontaneous and "event-related" motor cortex oscillations in the beta (15-30 Hz) frequency range are well-established phenomena. However, the precise functional significance of these features is uncertain. An understanding of the specific function is of importance for the treatment of Parkinson's disease (PD), where attenuation of augmented beta throughout the motor network coincides with functional improvement. Previous research using a discrete movement task identified normalization of elevated spontaneous beta and postmovement beta rebound following GABAergic modulation. Here, we explore the effects of the gamma-aminobutyric acid type A modulator, zolpidem, on beta power during the performance of serial movement in 17 (15M, 2F; mean age, 66 ± 6.3 years) PD patients, using a repeated-measures, double-blinded, randomized, placebo-control design. Motor symptoms were monitored before and after treatment, using time-based Unified Parkinson's Disease Rating Scale measurements and beta oscillations in primary motor cortex (M1) were measured during a serial-movement task, using magnetoencephalography. We demonstrate that a cumulative increase in M1 beta power during a 10-s tapping trial is reduced following zolpidem, but not placebo, which is accompanied by an improvement in movement speed and efficacy. This work provides a clear mechanism for the generation of abnormally elevated beta power in PD and demonstrates that perimovement beta accumulation drives the slowing, and impaired initiation, of movement. These findings further indicate a role for GABAergic modulation in bradykinesia in PD, which merits further exploration as a therapeutic target.Peer reviewe

Plucking enhanced beneath ice sheet margins: evidence from the Grampian Mountains, Scotland

Concentrations of boulders are a common feature of landscapes modified by former mid-latitude ice sheets. In many cases, the origin of the boulders can be traced in the up-ice direction to a cliff only tens to hundreds of metres distant. The implication is that a pulse of plucking and short boulder transport occurred beneath thin ice at the end of the last glacial cycle. Here we use a case study in granite bedrock in the Dee Valley, Scotland, to constrain theory and explore the factors involved in such a late phase of plucking. Plucking is influenced by ice velocity, hydrology, effective ice pressure, the extent of subglacial cavities and bedrock characteristics. The balance between these factors favours block removal beneath thin ice near a glacier margin. At Ripe Hill in the Dee Valley, a mean exposure age of 14.2 ka on blocks supports the view that the boulder train formed at the end of ice sheet glaciation. The late pulse of plucking was further enhanced by ice flowing obliquely across vertical joints and by fluctuations in sub-marginal meltwater conditions. An implication of the study is that there is the potential for a wave of ice-marginal plucking to sweep across a landscape as an ice sheet retreats

Drag forces at the ice-sheet bed and resistance of hard-rock obstacles: the physics of glacial ripping

Glacial ripping involves glaciotectonic disintegration of rock hills and extensive removal of rock at the ice-sheet bed, triggered by hydraulic jacking caused by fluctuating water pressures. Evidence from eastern Sweden shows that glacial ripping caused significant subglacial erosion during the final deglaciation of the Fennoscandian ice sheet, distinct from abrasion and plucking (quarrying). Here we analyse the ice drag forces exerted onto rock obstacles at the base of an ice sheet, and the resisting forces of such rock obstacles: glaciotectonic disintegration requires that ice drag forces exceed the resisting forces of the rock obstacle. We consider rock obstacles of different sizes, shapes and fracture patterns, informed by natural examples from eastern Sweden. Our analysis shows that limited overpressure events, unfavourable fracture patterns, low-transmissivity fractures, slow ice and streamlined rock hamper rock hill disintegration. Conversely, under fast ice flow and fluctuating water pressures, disintegration is possible if the rock hill contains subhorizontal, transmissive fractures. Rock steps on previously smooth, abraded surfaces, caused by hydraulic jacking, also enhance drag forces and can cause disintegration of a rock hill. Glacial ripping is a physically plausible erosion mechanism, under realistic glaciological conditions prevalent near ice margins

Gender, age and the MBA: An analysis of extrinsic and intrinsic career benefits

Against the background of an earlier UK study, this paper presents the findings of a Canadian based survey of career benefits from the MBA. Results indicate firstly that gender and age interact to influence perceptions of career outcomes (young men gain most in terms of extrinsic benefits of career change and pay), and secondly that both men and women gain intrinsic benefits from the MBA. However, intrinsic benefits vary by gender: men in the study were more likely to say they gained confidence from having a fuller skill set while women were more likely to say they gained confidence from feelings of self worth; men emphasised how they had learned to give up control while women argued that they had gained a ‘voice’ in the organization. The role of the MBA in career self- management and the acquisition of key skills are examined as well as the implications for the design of programmes in meeting the varied need of men and women in different age groups

Block and thread intercultural narratives and positioning: conversations with newly arrived postgraduate students

This paper considers how, in the process of positioning that is implicit in every interaction, all of us employ multiple and often competing narratives when we talk about cultural identity and our relationships with new cultural environments. In interviews with newly arrived postgraduate students about their experience of travelling to study abroad, the students employ competing block and thread narratives. Block narratives represent an essentialist discourse of culture. As such, they are easily converted into cultural prejudice by blocking the possibility for understanding and sharing at the point of tolerating an Other who can never be like ‘us’. These are default narratives because of the way in which we are brought up in our societies within a global positioning and politics. Thread narratives instead support a critical cosmopolitan discourse of cultural travel and shared meanings across structural boundaries that act against cultural prejudice. Threads need to be nurtured as alternative forms of engagement. Therefore, there is a place for the researchers to intervene with their own thread narratives. This intervention is both allowed within and supported by an understanding that researchers join with their participants in the creative intercultural events of the interview

Tambora and the Mackerel Year: Phenology and Fisheries During an Extreme Climate Event

Global warming has increased the frequency of extreme climate events, yet responses of biological and human communities are poorly understood, particularly for aquatic ecosystems and fisheries. Retrospective analysis of known outcomes may provide insights into the nature of adaptations and trajectory of subsequent conditions. We consider the 1815 eruption of the Indonesian volcano Tambora and its impact on Gulf of Maine (GoM) coastal and riparian fisheries in 1816. Applying complex adaptive systems theory with historical methods, we analyzed fish export data and contemporary climate records to disclose human and piscine responses to Tambora’s extreme weather at different spatial and temporal scales while also considering sociopolitical influences. Results identified a tipping point in GoM fisheries induced by concatenating social and biological responses to extreme weather. Abnormal daily temperatures selectively affected targeted fish species—alewives, shad, herring, and mackerel—according to their migration and spawning phenologies and temperature tolerances. First to arrive, alewives suffered the worst. Crop failure and incipient famine intensified fishing pressure, especially in heavily settled regions where dams already compromised watersheds. Insufficient alewife runs led fishers to target mackerel, the next species appearing in abundance along the coast; thus, 1816 became the “mackerel year.” Critically, the shift from riparian to marine fisheries persisted and expanded after temperatures moderated and alewives recovered. We conclude that contingent human adaptations to extraordinary weather permanently altered this complex system. Understanding how adaptive responses to extreme events can trigger unintended consequences may advance long-term planning for resilience in an uncertain future