181 research outputs found
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
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
Recent advances in TIS research: towards a new phase in transition studies
The technological innovation systems (TIS) approach has become one of the key frameworks for the study of emerging technologies in and beyond the context of sustainability transitions. It focuses on understanding the dynamics of an innovation system associated with a specific technology. The approach is often used to assess the performance of a TIS, to identify shortcomings and to derive policy recommendations for the support of a selected technology (Bergek et al., 2008; Hekkert and Negro, 2009). Since its inception, the framework has seen
several conceptual developments, including a clarification of scoping issues, TIS functions as a central tool for performance assessment, a strategic perspective on system building, international and global ties within TIS, and suggestions for the analysis of TIS contexts (Bergek et al., 2015; Binz et al., 2014; Markard et al., 2015). At the same time, however, there are also new conceptual challenges, especially when the TIS is used for sustainability transition studies. One of these challenges is how to study whole system reconfigurations, i.e. to move beyond the focus on specific technologies. Ongoing transitions such as the energy transition currently enter into a new phase of accelerated development, in which multiple emerging
and mature technologies interact. Other conceptual challenges include the decline
of incumbent technologies, intensified struggles among actors or transition
processes transcending sectoral and national boundaries
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
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
An automated microreactor for semi-continuous biosensor measurements.
Living bacteria or yeast cells are frequently used as bioreporters for the detection of specific chemical analytes or conditions of sample toxicity. In particular, bacteria or yeast equipped with synthetic gene circuitry that allows the production of a reliable non-cognate signal (e.g., fluorescent protein or bioluminescence) in response to a defined target make robust and flexible analytical platforms. We report here how bacterial cells expressing a fluorescence reporter ("bactosensors"), which are mostly used for batch sample analysis, can be deployed for automated semi-continuous target analysis in a single concise biochip. Escherichia coli-based bactosensor cells were continuously grown in a 13 or 50 nanoliter-volume reactor on a two-layered polydimethylsiloxane-on-glass microfluidic chip. Physiologically active cells were directed from the nl-reactor to a dedicated sample exposure area, where they were concentrated and reacted in 40 minutes with the target chemical by localized emission of the fluorescent reporter signal. We demonstrate the functioning of the bactosensor-chip by the automated detection of 50 μgarsenite-As l(-1) in water on consecutive days and after a one-week constant operation. Best induction of the bactosensors of 6-9-fold to 50 μg l(-1) was found at an apparent dilution rate of 0.12 h(-1) in the 50 nl microreactor. The bactosensor chip principle could be widely applicable to construct automated monitoring devices for a variety of targets in different environments
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
Glacier, fjord, and seismic response to recent large calving events, Jakobshavn Isbræ, Greenland
The recent loss of Jakobshavn Isbræ’s extensive
floating ice tongue has been accompanied by a change in
near terminus behavior.The recent loss of Jakobshavn Isbræ’s extensive
floating ice tongue has been accompanied by a change in
near terminus behavior. Calving currently occurs primarily
in summer from a grounded terminus, involves the
detachment and overturning of several icebergs within
30 – 60 min, and produces long-lasting and far-reaching
ocean waves and seismic signals, including ‘‘glacial
earthquakes’’. Calving also increases near-terminus glacier
velocities by 3% but does not cause episodic rapid glacier
slip, thereby contradicting the originally proposed glacial
earthquake mechanism. We propose that the earthquakes are
instead caused by icebergs scraping the fjord bottom during
calving.We thank J. Brown and D. Maxwell for field
assistance, and S. Anandakrishnan, A. Behar, and R. Fatland for loaning
GPS receivers. Comments from editor E. Rignot and reviewers S. O’Neel
and T. Pfeffer improved the manuscript. Logistics and instrumental support
were provided by VECO Polar Resources, UNAVCO, and PASSCAL.
Seismic analysis was done with the Matlab waveform object package written
by C. Reyes (http://www.giseis.alaska.edu/Seis/EQ/tools/matlab/). Funding
was provided by NASA’s Cryospheric Sciences Program (NNG06GB49G),
the U.S. National Science Foundation (ARC0531075), the Swiss National
Science Foundation (200021-113503/1), the Comer Science and Education
Foundation, and a CIFAR IPY student fellowship under NOAA cooperative
agreement NA17RJ1224 with the University of Alaska.Ye
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
Active seismic studies in valley glacier settings: strategies and limitations
Subglacial tills play an important role in glacier dynamics but are difficult to characterize in situ. Amplitude Variation with Angle (AVA) analysis of seismic reflection data can distinguish between stiff tills and deformable tills. However, AVA analysis in mountain glacier environments can be problematic: reflections can be obscured by Rayleigh wave energy scattered from crevasses, and complex basal topography can impede the location of reflection points in 2-D acquisitions. We use a forward model to produce challenging synthetic seismic records in order to test the efficacy of AVA in crevassed and geometrically complex environments. We find that we can distinguish subglacial till types in moderately crevassed environments, where ‘moderate’ depends on crevasse spacing and orientation. The forward model serves as a planning tool, as it can predict AVA success or failure based on characteristics of the study glacier. Applying lessons from the forward model, we perform AVA on a seismic dataset collected from Taku Glacier in Southeast Alaska in March 2016. Taku Glacier is a valley glacier thought to overlay thick sediment deposits. A near-offset polarity reversal confirms that the tills are deformable
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