Till deformation beneath Black Rapids Glacier, Alaska, and its implication on glacier motion

Thesis (Ph.D.) University of Alaska Fairbanks, 1999The motion of a glacier is largely determined by the nature of its bed. The basal morphology and its reaction to the overlying ice mass have been subject to much speculation, because the glacier bed is usually difficult to access, and good field data are sparse. In spring 1997 a commercial wireline drill rig was set up on Black Rapids Glacier, Alaska, to extract cores of basal ice, subglacial till, and underlying bedrock. One of the boreholes was equipped with three tiltmeters to monitor till deformation, and a piezometer to record pore water pressure. The surface velocity and ice deformation in a borehole were also measured. The drill successfully reached bedrock twice after penetrating a till layer, some 5 to 7 m in thickness, confirming an earlier seismic interpretation. The till consisted of a sandy matrix containing clasts up to boulder size. Bedrock and till lithology indicated that all the drill holes were located to the north of the Denali Fault, a major tectonic boundary along which the glacier flows. The mean annual surface velocity of the glacier was 60 ma-1 , of which 20 to 30 ma-1 were ice deformation, leaving 30 to 40 ma-1 of basal motion. The majority of this basal motion occurred at a depth of more than 2 m in the till, contradicting previously held ideas about till deformation. Basal motion could occur as sliding of till over the underlying bedrock, or on a series of shear layers within the till. This finding has implications for the interpretation of the geologic record of former ice sheets, for geomorphology, and for glacier dynamics. The effect of a thick till layer on ice flow and on quantities observable at the glacier surface was calculated. These include velocity changes on secular, seasonal, and shorter time scales. A mechanism for uplift events and dye tracing responses was suggested. An easy surface observation that could serve to clearly distinguish a glacier underlain by till from the more traditional view of a glacier underlain by bedrock could not be identified

A unifying framework for iceberg-calving models

We propose a general framework for iceberg-calving models that can be applied to any calving margin.We propose a general framework for iceberg-calving models that can be applied to any calving margin. The framework is based on mass continuity, the assumption that calving rate and terminus velocity are not independent and the simple idea that terminus thickness following a calving event is larger than terminus thickness at the event onset. The theoretical, near steady-state analysis used to support and analyze the framework indicates that calving rate is governed, to first order, by ice thickness, thickness gradient, strain rate, mass-balance rate and backwards melting of the terminus; the analysis furthermore provides a physical explanation for a previously derived empirical relationship for ice-shelf calving (Alley and others, 2008). In the calving framework the pre- and post-calving terminus thicknesses are given by two unknown but related functions. The functions can vary independently of changes in glacier flow and geometry, and can therefore account for variations in calving behavior due to external forcings and/or self-sustaining calving processes (positive feedbacks). Although the calving framework does not constitute a complete calving model, any thickness-based calving criterion can easily be incorporated into the framework. The framework should be viewed as a guide for future attempts to parameterize calving.Support for this project was provided by NASA’s Cryospheric Sciences Program (NNG06GB49G), the US National Science Foundation (ARC0531075 and ARC0909552) and an International Polar Year student traineeship funded by the Cooperative Institute for Arctic Research (CIFAR) through cooperative agreement NA17RJ1224 with the US National Oceanic and Atmospheric Administration. The paper was inspired by discussions with E. Bueler, M. Fahnestock, M.P. Lu ̈thi, R.J. Motyka, J. Brown and D. Podrasky. We thank A. Vieli, an anonymous reviewer and the scientific editor, R. Greve, for thorough reviews that helped to focus the manuscript.Ye

Seasonal and interannual variations in ice melange and its impact on terminus stability, Jakobshavn Isbræ, Greenland

We used satellite-derived surface temperatures and time-lapse photography to infer temporal variations in the proglacial ice melange at Jakobshavn Isbræ, a large and rapidly retreating outlet glacier in Greenland.We used satellite-derived surface temperatures and time-lapse photography to infer temporal variations in the proglacial ice melange at Jakobshavn Isbræ, a large and rapidly retreating outlet glacier in Greenland. Freezing of the melange-covered fjord surface during winter is indicated by a decrease in fjord surface temperatures and is associated with (1) a decrease in ice melange mobility and (2) a drastic reduction in iceberg production. Vigorous calving resumes in spring, typically abruptly, following the steady up-fjord retreat of the sea-ice/ice-melange margin. An analysis of pixel displacement from time-lapse imagery demonstrates that melange motion increases prior to calving and subsequently decreases following several events. We find that secular changes in ice melange extent, character and persistence can influence iceberg calving, and therefore glacier dynamics over daily-to-monthly timescales, which, if sustained, will influence the mass balance of an ice sheet.This research was supported by funds from the Gordon and Betty Moore Foundation (GBMF2627), NASA (NNX08AN74G), the US National Science Foundation (ANT0944193 and ANS0909552) and the New Hampshire Space Grant Consortium (NNX10AL97H). We thank CH2M HILL Polar Services and Air Greenland for logistics support, and PASSCAL (Program for the Array Seismic Studies of theContinental Lithosphere) for the use of seismic instrumentation. Ian Joughin derived TerraSAR-X velocities and terminus positions from images provided by the German (DLR) space agency under NASA grant NNX08AL98A. We acknowledgethe use of Rapid Response imagery from the Land Atmosphere Near-real time Capability for EOS (LANCE) system operated by the NASA/GSFC/Earth Science Data and Information System (ESDIS) with funding provided by NASA HQ. Glacier surface elevations were provided by CReSIS, and bed elevations by CReSIS and Mathieu Morlighem. The manuscript was significantly improved by comments from Tim Bartholomaus and an anonymous reviewer.Ye

Time-dependent basal stress conditions beneath Black Rapids Glacier, Alaska, USA, inferred from measurements of ice deformation and surface motion

Observations of surface motion and ice deformation from 2002–03 were used to infer mean stress fields in a cross-section of Black Rapids Glacier, Alaska, USA, over seasonal timescalesObservations of surface motion and ice deformation from 2002–03 were used to infer mean stress fields in a cross-section of Black Rapids Glacier, Alaska, USA, over seasonal timescales. Basal shear stresses in a well-defined zone north of the center line (orographic left) were approximately 7% and 16% lower in spring and summer, respectively, than in winter. Correspondingly higher stresses were found near the margins. These changes in the basal shear stress distribution were sufficiently large to cause mean surface velocities to be 1.2 and 1.5 times larger in spring and summer than in winter. These results were inferred with a simple inverse finite-element flow model that can successfully reproduce bulk surface velocities and tiltmeter data. Stress redistribution between the well-defined zone and the margins may also occur over much shorter time periods as a result of rapidly changing basal conditions (ice–bed decoupling or enhanced till deformation), thereby causing large variations in surface velocity and strongly influencing the glacier’s net motion during summer.This project was supported by grants OPP-0115819 and OPP-0414128 of the US National Science Foundation. The fieldwork could not have been completed without the help of A. Arendt, A. Behar, J. Brown, A. Bucki, S. Campbell, T. Clarke, L. Cox, K. Echelmeyer, D. Elsberg, W. Harrison, U. Korotkova, A. Mahoney, D. Moudry, M. Parrish, D. Pomraning, B. Valentine, R. Woodard and S. Zirnheld. C. Larsen provided important, last-minute assistance with instrument assembly. Logistics support was by Veco Polar Resources, Tundra Helicopters and Ultima Thule Air Service. Discussions with W. Harrison, K. Echelmeyer, R. Motyka and A. Arendt improved the manuscript. We would also like to thank the scientific editor, J. Walder, and J. Kavanaugh and D. Cohen for insightful reviews.Ye

Tidewater glacier response to individual calving events

Tidewater glaciers have been observed to experience instantaneous, stepwise increases in velocity during iceberg-calving events due to a loss of resistive stresses. These changes in stress can potentially impact tidewater glacier stability by promoting additional calving and affecting the viscous delivery of ice to the terminus. Using flow models and perturbation theory, we demonstrate that calving events and subsequent terminus readvance produce quasi-periodic, sawtooth oscillations in stress that originate at the terminus and propagate upstream. The stress perturbations travel at speeds much greater than the glacier velocities and, for laterally resisted glaciers, rapidly decay within a few ice thickness of the terminus. Consequently, because terminus fluctuations due to individual calving events tend to be much higher frequency than climate variations, individual calving events have little direct impact on the viscous delivery of ice to the terminus. This suggests that the primary mechanism by which calving events can trigger instability is by causing fluctuations in stress that weaken the ice and lead to additional calving and sustained terminus retreat. Our results further demonstrate a stronger response to calving events in simulations that include the full stress tensor, highlighting the importance of accounting for higher order stresses when developing calving parameterizations.© The Author(s), 2022. Published by Cambridge University Press. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited.Ye

Runaway thinning of the low-elevation Yakutat Glacier, Alaska, and its sensitivity to climate change

Lake-calving Yakutat Glacier in southeast Alaska, USA, is undergoing rapid thinning and terminus retreat. We use a simplified glacier model to evaluate its future mass loss. In a first step we compute glacier-wide mass change with a surface mass-balance model, and add a mass loss component due to ice flux through the calving front. We then use an empirical elevation change curve to adjust for surface elevation change of the glacier and finally use a flotation criterion to account for terminus retreat due to frontal ablation. Surface mass balance is computed on a daily timescale; elevation change and retreat is adjusted on a decadal scale. We use two scenarios to simulate future mass change: (1) keeping the current (2000–10) climate and (2) forcing the model with a projected warming climate. We find that the glacier will disappear in the decade before 2110 or 2070 under constant or warming climates, respectively. For the first few decades, the glacier can maintain its current thinning rates by retreating and associated loss of high-ablating, low-elevation areas. However, once higher elevations have thinned substantially, the glacier can no longer counteract accelerated thinning by retreat and mass loss accelerates, even under constant climate conditions. We find that it would take a substantial cooling of 1.5°C to reverse the ongoing retreat. It is therefore likely that Yakutat Glacier will continue its retreat at an accelerating rate and disappear entirely

Tracking icebergs with time-lapse photography and sparse optical flow, LeConte Bay, Alaska, 2016–2017

We present a workflow to track icebergs in proglacial fjords using oblique time-lapse photos and the Lucas-Kanade optical flow algorithm. We employ the workflow at LeConte Bay, Alaska, where we ran five time-lapse cameras between April 2016 and September 2017, capturing more than 400 000 photos at frame rates of 0.5–4.0 min−1. Hourly to daily average velocity fields in map coordinates illustrate dynamic currents in the bay, with dominant downfjord velocities (exceeding 0.5 m s−1 intermittently) and several eddies. Comparisons with simultaneous Acoustic Doppler Current Profiler (ADCP) measurements yield best agreement for the uppermost ADCP levels (∼ 12 m and above), in line with prevalent small icebergs that trace near-surface currents. Tracking results from multiple cameras compare favorably, although cameras with lower frame rates (0.5 min−1) tend to underestimate high flow speeds. Tests to determine requisite temporal and spatial image resolution confirm the importance of high image frame rates, while spatial resolution is of secondary importance. Application of our procedure to other fjords will be successful if iceberg concentrations are high enough and if the camera frame rates are sufficiently rapid (at least 1 min−1 for conditions similar to LeConte Bay).This work was funded by the U.S. National Science Foundation (OPP-1503910, OPP-1504288, OPP-1504521 and OPP-1504191).Ye

Production of knowledge revisited - the impact of academic spin-offs on public research performance in Europe (PROKNOW): abbreviated description of the research project funded by the European Commission

"The EU-funded project 'Production of Knowledge Revisited: The Impact of Academic Spin-Offs on Public Research Performance in Europe (PROKNOW)' aims at analysing the interactions between public research institutions and academic spin-offs focussing on the impact of entrepreneurial activities on the academic research system. Based upon approaches in organisational sociology, science policy studies and science studies and analysing the gains and losses of spin-off activities for public research institutions, PROKNOW examines the relevance of public and private forms of knowledge in innovative processes of knowledge production. Academic spin-offs often epitomise innovative forms of knowledge production and are thus an exemplary topic to study innovation processes in the interaction of science, economy and society. PROKNOW proposes a European-wide comparison of research institutions in seven countries, including the three biggest research systems, Germany, France and the UK, and the -often considered to be innovative -systems of the Netherlands, Switzerland and Finland, and the associated candidate country Bulgaria. Institutionally, PROKNOW analyses different forms of public sector research institutions, university and extrauniversity institutions. In terms of economic sectors, the project focuses on life sciences, information sciences and nanotechnology. Thus, PROKNOW can help provide the institutional and organisational conditions for a profitable interaction between public research institutions and academic spin-offs." (author's abstract)"Das Eu-geförderte Projekt 'Production of Knowledge Revisited: The Impact of Academic Spin-Offs on Public Research Performance in Europe (PROKNOW)' analysiert Interaktionen zwischen öffentlichen Forschungseinrichtungen und deren akademischen Ausgründungen ('Spin-offs') und hat dabei die Folgen der unternehmerischen Aktivitäten auf das akademische Forschungssystem im Fokus. Auf der Grundlage von Ansätzen aus der Organisationssoziologie und der neueren Wissenschaftsforschung fragt das Projekt nach Gewinnen und Verlusten von Spin-off-Aktivitäten für öffentliche Forschungseinrichtungen und leistet damit einen Beitrag zur Erforschung zum Verhältnis öffentlicher und privater Wissensformen in innovativen Prozessen der Wissensproduktion. Anhand von akademischen Ausgründungen lassen sich Innovationsprozesse als Interaktion von Wissenschaft, Wirtschaft und Gesellschaft in exemplarischer Weise untersuchen. PROKNOW wird einen europaweiten Vergleich der Forschungseinrichtungen in sieben Ländern unternehmen. Ausgewählt wurden die drei größten Forschungssysteme, Deutschland, Frankreich und Großbritannien sowie die vielfach als innovativ eingeschätzten Systeme der Niederlande, der Schweiz und Finnlands und des EU-Beitrittskandidaten Bulgarien, die jeweils für avancierte Ansätze stehen. Dabei wird PROKNOW verschiedene Formen von öffentlichen Forschungseinrichtungen, universitäre und außeruniversitäre Einrichtungen analysieren. Das Projekt wird sich auf die Bereiche Biowissenschaften, Informations- und Nanotechnologien konzentrieren. Damit kann PROKNOW dazu beitragen, die institutionellen und organisatorischen Rahmenbedingungen für eine fruchtbare Interaktion von öffentlichen Forschungseinrichtungen und akademischen Spin-offs zu optimieren." (Autorenreferat

Bucki (2006), Rapid erosion of soft sediments by tidewater glacier advance: Taku Glacier

[1] Taku Glacier in southeast Alaska has advanced 7.5 km over the last 115 years, overriding its own glaciomarine and outwash sediments. We have documented rapid erosion of these sediments by comparing radio echo soundings (RES) along five transects (2003)(2004)(2005) to earlier RES surveys (1989 and 1994) and to early bathymetric surveys of the proglacial fjord. Erosion rates, _ E, reached 3.9 ± 0.8 m

Ice-front variation and tidewater behavior on Helheim and Kangerdlugssuaq Glaciers, Greenland

We used satellite images to examine the calving behavior of Helheim and Kangerdlugssuaq Glaciers, Greenland, from 2001 to 2006, a period in which they retreated and sped up. These data show that many large iceberg-calving episodes coincided with teleseismically detected glacial earthquakes, suggesting that calving-related processes are the source of the seismicity. For each of several events for which we have observations, the ice front calved back to a large, pre-existing rift. These rifts form where the ice has thinned to near flotation as the ice front retreats down the back side of a bathymetric high, which agrees well with earlier theoretical predictions. In addition to the recent retreat in a period of higher temperatures, analysis of several images shows that Helheim retreated in the 20th Century during a warmer period and then re-advanced during a subsequent cooler period. This apparent sensitivity to warming suggests that higher temperatures may promote an initial retreat off a bathymetric high that is then sustained by tidewater dynamics as the ice front retreats into deeper water. The cycle of frontal advance and retreat in less than a century indicates that tidewater glaciers in Greenland can advance rapidly. Greenland's larger reservoir of inland ice and conditions that favor the formation of ice shelves likely contribute to the rapid rates of advance