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  • 1. Ahlkrona, J.
    et al.
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Lotstedt, P.
    Accuracy of the zeroth- and second-order shallow-ice approximation - numerical and theoretical results2013In: Geoscientific Model Development, ISSN 1991-959X, E-ISSN 1991-9603, Vol. 6, no 6, p. 2135-2152Article in journal (Refereed)
    Abstract [en]

    In ice sheet modelling, the shallow-ice approximation (SIA) and second-order shallow-ice approximation (SOSIA) schemes are approaches to approximate the solution of the full Stokes equations governing ice sheet dynamics. This is done by writing the solution to the full Stokes equations as an asymptotic expansion in the aspect ratio epsilon, i.e. the quotient between a characteristic height and a characteristic length of the ice sheet. SIA retains the zeroth-order terms and SOSIA the zeroth-, first-, and second-order terms in the expansion. Here, we evaluate the order of accuracy of SIA and SOSIA by numerically solving a two-dimensional model problem for different values of epsilon, and comparing the solutions with afinite element solution to the full Stokes equations obtained from Elmer/Ice. The SIA and SOSIA solutions are also derived analytically for the model problem. For decreasing epsilon, the computed errors in SIA and SOSIA decrease, but not always in the expected way. Moreover, they depend critically on a parameter introduced to avoid singularities in Glen's flow law in the ice model. This is because the assumptions behind the SIA and SOSIA neglect a thick, high-viscosity boundary layer near the ice surface. The sensitivity to the parameter is explained by the analytical solutions. As a verification of the comparison technique, the SIA and SOSIA solutions for a fluid with Newtonian rheology are compared to the solutions by Elmer/Ice, with results agreeing very well with theory.

  • 2. Ahlkrona, Josefin
    et al.
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Lötstedt, Per
    A numerical study of scaling relations for non-Newtonian thin film flows with applications in ice sheet modelling2013In: Quarterly Journal of Mechanics and Applied Mathematics, ISSN 0033-5614, E-ISSN 1464-3855, Vol. 66, no 4, p. 417-435Article in journal (Refereed)
    Abstract [en]

    This article treats the viscous, non-Newtonian thin-film flow of ice sheets, governed by the Stokes equations, and the modelling of ice sheets with asymptotic expansion of the analytical solutions in terms of the aspect ratio, which is a small parameter measuring the shallowness of an ice sheet. An asymptotic expansion requires scalings of the field variables with the aspect ratio. There are several, conflicting, scalings in the literature used both for deriving simplified models and for analysis. We use numerical solutions of the Stokes equations for varying aspect ratios in order to compute scaling relations. Our numerically obtained results are compared with three known theoretical scaling relations: the classical scalings behind the Shallow Ice Approximation, the scalings originally used to derive the so-called Blatter-Pattyn equations, and a non-uniform scaling which takes into account a high viscosity boundary layer close to the ice surface. We find that the latter of these theories is the most appropriate one since there is indeed a boundary layer close to the ice surface where scaling relations are different than further down in the ice. This boundary layer is thicker than anticipated and there is no distinct border with the inner layer for aspect ratios appropriate for ice sheets. This makes direct application of solutions obtained by matched asymptotic expansion problematic.

  • 3. Ahlkrona, Josefin
    et al.
    Lötstedt, Per
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Zwinger, Thomas
    Dynamically coupling the non-linear Stokes equations with the shallow ice approximation in glaciology: Description and first applications of the ISCAL method2016In: Journal of Computational Physics, ISSN 0021-9991, E-ISSN 1090-2716, Vol. 308, p. 1-19Article in journal (Refereed)
    Abstract [en]

    We propose and implement a new method, called the Ice Sheet Coupled Approximation Levels (ISCAL) method, for simulation of ice sheet flow in large domains during long time-intervals. The method couples the full Stokes (FS) equations with the Shallow Ice Approximation (SIA). The part of the domain where SIA is applied is determined automatically and dynamically based on estimates of the modeling error. For a three dimensional model problem, ISCAL computes the solution substantially faster with a low reduction in accuracy compared to a monolithic FS. Furthermore, ISCAL is shown to be able to detect rapid dynamic changes in the flow. Three different error estimations are applied and compared. Finally, ISCAL is applied to the Greenland Ice Sheet on a quasi-uniform grid, proving ISCAL to be a potential valuable tool for the ice sheet modeling community.

  • 4.
    Applegate, Patrick J.
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Stone, E. J.
    Keller, K.
    Greve, R.
    An assessment of key model parametric uncertainties in projections of Greenland ice sheet behavior2012In: The Cryosphere, ISSN 1994-0416, E-ISSN 1994-0424, Vol. 6, no 3, p. 589-606Article in journal (Refereed)
    Abstract [en]

    Lack of knowledge about the values of ice sheet model input parameters introduces substantial uncertainty into projections of Greenland Ice Sheet contributions to future sea level rise. Computer models of ice sheet behavior provide one of several means of estimating future sea level rise due to mass loss from ice sheets. Such models have many input parameters whose values are not well known. Recent studies have investigated the effects of these parameters on model output, but the range of potential future sea level increases due to model parametric uncertainty has not been characterized. Here, we demonstrate that this range is large, using a 100-member perturbed-physics ensemble with the SICOPOLIS ice sheet model. Each model run is spun up over 125 000 yr using geological forcings and subsequently driven into the future using an asymptotically increasing air temperature anomaly curve. All modeled ice sheets lose mass after 2005 AD. Parameters controlling surface melt dominate the model response to temperature change. After culling the ensemble to include only members that give reasonable ice volumes in 2005 AD, the range of projected sea level rise values in 2100 AD is similar to 40 % or more of the median. Data on past ice sheet behavior can help reduce this uncertainty, but none of our ensemble members produces a reasonable ice volume change during the mid-Holocene, relative to the present. This problem suggests that the model's exponential relation between temperature and precipitation does not hold during the Holocene, or that the central-Greenland temperature forcing curve used to drive the model is not representative of conditions around the ice margin at this time (among other possibilities). Our simulations also lack certain observed physical processes that may tend to enhance the real ice sheet's response. Regardless, this work has implications for other studies that use ice sheet models to project or hindcast the behavior of the Greenland Ice Sheet.

  • 5.
    Barnett, Jamie
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Holmes, Felicity A.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography. Stockholm University, Faculty of Science, The Bolin Centre for Climate Research (together with KTH & SMHI).
    Modelled dynamic retreat of Kangerlussuaq Glacier, East Greenland, strongly influenced by the consecutive absence of an ice mélange in Kangerlussuaq Fjord2023In: Journal of Glaciology, ISSN 0022-1430, E-ISSN 1727-5652, Vol. 69, no 275, p. 433-444Article in journal (Refereed)
    Abstract [en]

    Mass loss at the Greenland Ice Sheet is influenced by atmospheric processes controlling its surface mass balance, and by submarine melt and calving where glaciers terminate in fjords. There, an ice mélange - a composite matrix of calved ice bergs and sea ice - may provide a buttressing force on a glacier terminus and control terminus dynamics. Kangerlussuaq Glacier is a major outlet of the Greenland Ice Sheet, for which recent major retreat events in 2004/2005 and 2016-2018 coincided with the absence of an ice mélange in Kangerlussuaq Fjord. To better understand the response of Kangerlussuaq Glacier to climatic and oceanic drivers, a 2D flowline model is employed. Results indicate that an ice mélange buttressing force exerts a major control on calving frequency and rapid retreat. When an ice mélange forms in Kangerlussuaq Fjord, it provides stabilising forces and conditions favourable for winter terminus re-advance. When it fails to form during consecutive years, model results indicate that Kangerlussuaq Glacier is primed to retreat into the large overdeepenings in Kangerlussuaq Fjord, and to terminus positions more than 30 km farther inland, implying that excessive mass loss from Kangerlussuaq Glacier by the year 2065 cannot be excluded.

  • 6. Cheng, Gong
    et al.
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Lötstedt, Per
    Sensitivity of ice sheet surface velocity and elevation to variations in basal friction and topography in the full Stokes and shallow-shelf approximation frameworks using adjoint equations2021In: The Cryosphere, ISSN 1994-0416, E-ISSN 1994-0424, Vol. 15, no 2, p. 715-742Article in journal (Refereed)
    Abstract [en]

    Predictions of future mass loss from ice sheets are afflicted with uncertainty, caused, among others, by insufficient understanding of spatiotemporally variable processes at the inaccessible base of ice sheets for which few direct observations exist and of which basal friction is a prime example. Here, we present a general numerical framework for studying the relationship between bed and surface properties of ice sheets and glaciers. Specifically, we use an inverse modeling approach and the associated time-dependent adjoint equations, derived in the framework of a full Stokes model and a shallow-shelf/shelfy-stream approximation model, respectively, to determine the sensitivity of grounded ice sheet surface velocities and elevation to time-dependent perturbations in basal friction and basal topography. Analytical and numerical examples are presented showing the importance of including the time-dependent kinematic free surface equation for the elevation and its adjoint, in particular for observations of the elevation. A closed form of the analytical solutions to the adjoint equations is given for a two-dimensional vertical ice in steady state under the shallow-shelf approximation. There is a delay in time between a seasonal perturbation at the ice base and the observation of the change in elevation. A perturbation at the base in the topography has a direct effect in space at the surface above the perturbation, and a perturbation in the friction is propagated directly to the surface in time.

  • 7. Colleoni, Florence
    et al.
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Niessen, Frank
    Quiquet, Aurelien
    Liakka, Johan
    An East Siberian ice shelf during the Late Pleistocene glaciations: Numerical reconstructions2016In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 147, no SI, p. 148-163Article in journal (Refereed)
    Abstract [en]

    A recent data campaign in the East Siberian Sea has revealed evidence of grounded and floating ice dynamics in regions of up to 1000 m water depth, and which are attributed to glaciations older than the Last Glacial Maximum (21 kyrs BP). The main hypothesis based on this evidence is that a small ice cap developed over Beringia and expanded over the East Siberian continental margin during some of the Late Pleistocene glaciations. Other similar evidence of ice dynamics that have been previously collected on the shallow continental shelves of the Arctic Ocean have been attributed to the penultimate glaciation, i.e. Marine Isotopes Stage 6 (approximate to 140 kyrs BP). We use an ice sheet model, forced by two previously simulated MIS 6 glacial maximum climates, to carry out a series of sensitivity experiments testing the impact of dynamics and mass-balance related parameters on the geometry of the East Siberian ice cap and ice shelf. Results show that the ice cap developing over Beringia connects to the Eurasian ice sheet in all simulations and that its volume ranges between 6 and 14 m SLE, depending on the climate forcing. This ice cap generates an ice shelf of dimensions comparable with or larger than the present-day Ross ice shelf in West Antarctica. Although the ice shelf extent strongly depends on the ice flux through the grounding line, it is particularly sensitive to the choice of the calving and basal melting parameters. Finally, inhibiting a merging of the Beringia ice cap with the Eurasian ice sheet affects the expansion of the ice shelf only in the simulations where the ice cap fluxes are not large enough to compensate for the fluxes coming from the Eurasian ice sheet.

  • 8. Eriksson, Jerker
    et al.
    Löwemark, Ludvig
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Gyllencreutz, Richard
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Holmgren, Hanna
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Andersson, Per
    Schöberg, Hans
    Wallner, Karin
    Persson, Per-Olov
    Weichselian ice dammed lakes: New geochemical evidence for a rapidly deposited event-layer in the Eurasian Arctic Basin2012In: : APEX Sixth International Conference and Workshop: Quaternary Glacial and Climate Extremes / [ed] Ninna Immonen, Martin Jakobsson, Juha Pekka Lunkka, Kari Strand, Oulu: Oulun yliopisto , 2012, p. 48-48Conference paper (Other academic)
  • 9. Ernakovich, J. G.
    et al.
    Eklund, N.
    Varner, Ruth K.
    Stockholm University, Faculty of Science, Department of Physical Geography. University of New Hampshire, USA.
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Jeuring, J.
    Duderstadt, K.
    Granebeck, Annika
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Golubeva, E.
    Is A Common Goal A False Hope in Convergence Research?: Opportunities and Challenges of International Convergence Research to Address Arctic Change2021In: Earth's Future, E-ISSN 2328-4277, Vol. 9, no 5, article id e2020EF001865Article in journal (Refereed)
    Abstract [en]

    The Arctic faces multiple pressures including climate change, shifting demographics, human health risks, social justice imbalances, governance issues, and expanding resource extraction. A convergence of academic disciplines-such as natural and social sciences, engineering and technology, health and medicine-and international perspectives is required to meaningfully contribute to solving the challenges of Arctic peoples and ecosystems. However, successfully carrying out convergent, international research and education remains a challenge. Here, lessons from the planning phase of a convergence research project concerned with the health of Arctic waters developed by the Arctic Science IntegrAtion Quest (ASIAQ) are discussed. We discuss our perspective on the challenges, as well as strategies for success, in convergence research as gained from the ASIAQ project which assembled an international consortium of researchers from disparate disciplines representing six universities from four countries (Sweden, Japan, Russia, and the United States) during 2018-2020.

  • 10. Flink, Anne E.
    et al.
    Hill, Peter
    Noormets, Riko
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Holocene glacial evolution of Mohnbukta in eastern Spitsbergen2018In: Boreas, ISSN 0300-9483, E-ISSN 1502-3885, Vol. 47, no 2, p. 390-409Article in journal (Refereed)
    Abstract [en]

    Submarine geomorphology is one of the main tools for understanding past fluctuations of tidewater glaciers. In this study we investigate the glacial history of Mohnbukta, on the east coast of Spitsbergen, Svalbard, by combining multibeam-bathymetric data, marine sediment cores and remote sensing data. Presently, three tidewater glaciers, Heuglinbreen, Konigsbergbreen and Hayesbreen calve into Mohnbukta. Hayesbreen surged at the end of the Little Ice Age, between 1901 and 1910. The submarine landform assemblage in Mohnbukta contains two large transverse ridges, interpreted as terminal moraines, with debrisflow lobes on their distal slopes and sets of well-preserved geometric networks of ridges, interpreted as crevasse-squeeze ridges inshore of the moraines. The arrangement of crevasse-squeeze ridges suggests that both landform sets were produced during surge-type advances. The terminus position of the 1901-1910 Hayesbreen surge correlates with the inner (R.2) terminal moraine ridge suggesting that the R.1 ridge formed prior to 1901. Marine sediment cores display C-14 ages between 5700-7700cal. a BP derived from benthic foraminifera, from a clast-rich mud unit. This unit represents pre-surge unconsolidated Holocene sediments pushed in front of the glacier terminus and mixed up during the 1901 surge. An absence of retreat moraines in the deeper part of the inner basin and the observation of tabular icebergs calving off the glacier front during retreat suggest that the front of Hayesbreen was close to flotation, at least in the deeper parts of the basin. As the MOH15-01 core does not penetrate into a subglacial till and the foraminifera in the samples were well preserved, the R.1 ridge is suggested to have formed prior to the deposition of the foraminifera. Based on these data we propose that a surge-type advance occurred in Mohnbukta in the early Holocene, prior to 7700cal. a BP, which in turn indicates that glaciers can switch to and from surge mode.

  • 11. Flink, Anne Elina
    et al.
    Noormets, Riko
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Benn, Douglas I.
    Luckman, Adrian
    Lovell, Harold
    The evolution of a submarine landform record following recent and multiple surges of Tunabreen glacier, Svalbard2015In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 108, p. 37-50Article in journal (Refereed)
    Abstract [en]

    This study focuses on the glacial landform record associated with recent surge events of Tunabreen - a calving tidewater glacier in Tempelfjorden, Spitsbergen. Submarine geomorphology and recent terminal fluctuations of Tunabreen's glacier front were studied using high-resolution multibeam-bathymetric data and a range of published and remote-sensing sources, including topographic maps, satellite images and aerial photographs. The retreat moraines in the inner part of Tempelfjorden have been correlated with glacier terminus positions during retreat from the 2004 surge maximum. Glacier surface velocity and ice-front positions derived from high-resolution TerraSAR-X satellite data show ice movements at the glacier front during minor advances of the front in winter when calving is suppressed. This suggests that the moraines have formed annually during quiescent phase winter advances. Tunabreen has experienced three surges since the Little Ice Age (LIA). This is in contrast with most Svalbard surging glaciers which have long quiescent phases and have typically only undergone one or two surges during this time. The landform record in Tempelfjorden is distinguished from previously studied glacier-surge landsystems by four, well-preserved sets of landform assemblages generated by the LIA advance and three subsequent surges, all of which partly modify earlier landform records. Based on the unique landform record in Tempelfjorden, a new conceptual landsystem model for frequently surging glaciers has been put forward improving our understanding of the dynamics of the surging glaciers and, most importantly, how they can be distinguished from the climatically-controlled glaciers in the geological record.

  • 12. Hancke, Manfred
    et al.
    Kirchner, NinaStockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    26th International Forum for Research into Ice Shelf Processes- FRISP, Book of Abstracts: held June 12- 14, 2012, at Utö Värdshus, Stockholm Archipelago, Sweden. Organizers: N. Kirchner, A. Wåhlin, R. Gyllencreutz2012Conference proceedings (editor) (Other academic)
  • 13.
    Heyman, Jakob
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Hubbard, Alun
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Stroeven, Arjen
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Glacier mass balance modelling of the Tibetan Plateau – mesh dependence issues2008Conference paper (Refereed)
    Abstract [en]

    The Tibetan Plateau is an extraordinary topographic feature which exerts a major impact on regional and global climate. Its glacierised mountain ranges attain extreme altitudes and represent an important water resource for more than a billion people in Asia. Understanding the past glacial history of the Tibetan Plateau therefore is important to understanding global and regional climate and glacier hydrological evolution. A regional glacier modelling study has been initiated as part of an umbrella project aiming towards reconstructing the Quaternary palaeoglaciology of the Tibetan Plateau. On the basis of field studies which includes cosmogenic exposure-age dating, it is now generally recognised that former glaciers on the Tibetan Plateau, while more extensive than today, were still restricted to individual mountain areas. In contrast, a handful of previous modelling studies (Kuhle et al. 1989; Calov & Marsiat 1998; Bintanja et al. 2002; Casal et al. 2004) yield a bifurcated result with either 1) the growth of plateau-wide ice sheets (thus overshooting field evidence) or, 2) virtually no ice (which undershoots field evidence).

    We apply and test a positive degree day (PDD) model across the Tibetan Plateau to explore the parameter sensitivity and potential issues of grid-dependence. Utilising the 1km mean monthly (1950 – 2000) distributions of temperature and precipitation from the WorldClim database as a contemporary reference climatology, a suite of PDD experiments are run to predict present day ice cover. At a resolution of 1 km the algorithm nicely identifies zones of positive mass balance (accumulation) across most major contemporary glaciarised areas. Unsurprisingly increased grid resolution yields a significant decrease in the predicted accumulation area with a 40 km grid completely failing to predict accumulation across the domain. Such mesh dependence with larger grid-resolutions yielding less accumulation illustrates a major flaw in large-scale, low resolution ice modelling in areas of high topographical relief where adequate sub-grid parameterisation of accumulation/flow/melt processes have not been accounted for in a meaningful manner (e.g. Marshall & Clarke 1999). The result of the 20 km resolution PDD model can be manipulated to converge by applying extreme perturbations in temperature (c. -10 K) or precipitation (c. + 8000 %) but this yields plateau-wide accumulation areas far exceeding field evidence of glaciation. Our results indicate that the bifurcation in Quaternary ice extent identified in previous ice sheet modelling studies of the Tibetan Plateau are very likely a consequence of grid-resolution related issues implicit to the models applied.

    References

    Bintanja R., van de Wal R.S.W., Oerlemans J. 2002: Global ice volume variations through the last glacial cycle simulated by a 3-D ice-dynamical model. Quaternary International, 95-96, 11-23.

    Calov R, Marsiat I. 1998: Simulations of the Northern Hemisphere through the last glacial-interglacial cycle with a vertically integrated and a three-dimensional thermomechanical ice-sheet model coupled to a climate model. Annals of Glaciology, 27, 169-176.

    Casal T.G.D., Kutzbach J.E., Thompson L.G. 2004: Present And Past Ice-Sheet Mass Balance Simulations For Greenland And The Tibetan Plateau. Climate Dynamics, 23, 407-425.

    Kuhle M., Herterich K., Calov R. 1989: On the Ice Age Glaciation of the Tibetan Highlands and its Transformation into a 3-D Model. GeoJournal, 19, 201-206.

    Marshall S.J., Clarke G.K.C. 1999: Ice sheet inception: subgrid hypsometric parameterization of mass balance in an ice sheet model. Climate Dynamics, 15, 533-550.

  • 14.
    Holmes, Felicity A.
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography. KTH Royal Institute of Technology, Sweden.
    Kuttenkeuler, Jakob
    Krützfeldt, Jari
    Noormets, Riko
    Relating ocean temperatures to frontal ablation rates at Svalbard tidewater glaciers: Insights from glacier proximal datasets2019In: Scientific Reports, E-ISSN 2045-2322, Vol. 9, article id 9442Article in journal (Refereed)
    Abstract [en]

    Fjord-terminating glaciers in Svalbard lose mass through submarine melt and calving (collectively: frontal ablation), and surface melt. With the recently observed Atlantification of water masses in the Barents Sea, warmer waters enter these fjords and may reach glacier fronts, where their role in accelerating frontal ablation remains insufficiently understood. Here, the impact of ocean temperatures on frontal ablation at two glaciers is assessed using time series of water temperature at depth, analysed alongside meteorological and glaciological variables. Ocean temperatures at depth are harvested at distances of 1 km from the calving fronts of the glaciers Kronebreen and Tunabreen, western Svalbard, from 2016 to 2017. We find ocean temperature at depth to control c. 50% of frontal ablation, making it the most important factor. However, its absolute importance is considerably less than found by a 2013-2014 study, where temperatures were sampled much further away from the glaciers. In light of evidence that accelerating levels of global mass loss from marine terminating glaciers are being driven by frontal ablation, our findings illustrate the importance of sampling calving front proximal water masses.

  • 15.
    Holmes, Felicity A.
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Prakash, Abhay
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Stranne, Christian
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Dijkstra, S.
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Calving at Ryder Glacier, Northern Greenland2021In: Journal of Geophysical Research - Earth Surface, ISSN 2169-9003, E-ISSN 2169-9011, Vol. 126, no 4, article id e2020JF005872Article in journal (Refereed)
    Abstract [en]

    Recent evidence has shown increasing mass loss from the Greenland ice sheet, with a general trend of accelerated mass losses extending northwards. However, different glaciers have been shown to respond differently to similar external forcings, constituting a problem for extrapolating and upscaling data. Specifically, whilst some outlet glaciers have accelerated, thinned, and retreated in response to atmospheric and oceanic warming, the behavior of other marine terminating glaciers appears to be less sensitive to climate forcing. Ryder glacier, for which only a few studies have been conducted, is located in North Greenland and terminates with a floating ice tongue in Sherard Osborn Fjord. The persistence or disintegration of floating ice tongues has impacts on glacier dynamics and stability, with ramifications beyond, including sea level rise. This study focuses on understanding the controls on calving and frontal ablation of the Ryder glacier through the use of time-lapse imagery and satellite data. The results suggest that Ryder glacier has behaved independently of climate forcing during recent decades, with fjord geometry exerting a first order control on its calving.

  • 16.
    Holmes, Felicity A.
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    van Dongen, Eef
    Stockholm University, Faculty of Science, Department of Meteorology .
    Noormets, Riko
    Pętlicki, Michał
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Impact of tides on calving patterns at Kronebreen, Svalbard – insights from three-dimensional ice dynamical modelling2023In: The Cryosphere, ISSN 1994-0416, E-ISSN 1994-0424, Vol. 17, no 5, p. 1853-1872Article in journal (Refereed)
    Abstract [en]

    Understanding calving processes and their controls is of importance for reducing uncertainty in sea level rise estimates. The impact of tidal fluctuations and frontal melt on calving patterns has been researched through both modelling and observational studies but remains uncertain and may vary from glacier to glacier. In this study, we isolate various different impacts of tidal fluctuations on a glacier terminus to understand their influence on the timing of calving events in a model of Kronebreen, Svalbard, for the duration of 1 month. In addition, we impose a simplified frontal melt parameterisation onto the calving front in order to allow for an undercut to develop over the course of the simulations. We find that calving events show a tidal signal when there is a small or no undercut, but, after a critical point, undercut-driven calving becomes dominant and drowns out the tidal signal. However, the relationship is complex, and large calving events show a tidal signal even with a large modelled undercut. The modelled undercut sizes are then compared to observational profiles, showing that undercuts of up to ca. 25 m are plausible but with a more complex geometry being evident in observations than that captured in the model. These findings highlight the complex interactions occurring at the calving front of Kronebreen and suggest further observational data and modelling work is needed to fully understand the hierarchy of controls on calving.

  • 17.
    Holmes, Felicity Alice
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography. Stockholm University, Faculty of Science, Department of Geological Sciences.
    van Dongen, Eef
    Stockholm University, Faculty of Science, Department of Meteorology .
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Modelled frontal ablation and velocities at Kronebreen, Svalbard, are sensitive to the choice of submarine melt rate scenario2023In: Journal of Glaciology, ISSN 0022-1430, E-ISSN 1727-5652Article in journal (Refereed)
    Abstract [en]

    Both submarine melt and calving are important for the overall mass balance of marine-terminating glaciers, but uncertainty is rife with regards to the magnitude of the processes. Modelling allows for these processes to be investigated without the need to visit inaccessible ice marginal zones. This study looks at the impact of different submarine melt and sea-ice back pressure scenarios on modelled calving activity and dynamics at Kronebreen, Svalbard, by running separate summer and winter simulations with various submarine melt parameterisations and sea-ice characteristics. It is found that submarine melt is an important driver of seasonal variation in modelled glacier dynamics and calving activity, with the choice of sliding law also exerting a significant influence on results.

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  • 18. How, Penelope
    et al.
    Schild, Kristin M.
    Benn, Douglas I.
    Noormets, Riko
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Luckman, Adrian
    Vallot, Dorothée
    Hulton, Nicholas R. J.
    Borstad, Chris
    Calving controlled by melt-under-cutting: detailed calving styles revealed through time-lapse observations2019In: Annals of Glaciology, ISSN 0260-3055, E-ISSN 1727-5644, Vol. 60, no 78, p. 20-31Article in journal (Refereed)
    Abstract [en]

    We present a highly detailed study of calving dynamics at Tunabreen, a tidewater glacier in Svalbard. A time-lapse camera was trained on the terminus and programmed to capture images every 3 seconds over a 28-hour period in August 2015, producing a highly detailed record of 34 117 images from which 358 individual calving events were distinguished. Calving activity is characterised by frequent events (12.8 events h(-1)) that are small relative to the spectrum of calving events observed, demonstrating the prevalence of small-scale calving mechanisms. Five calving styles were observed, with a high proportion of calving events (82%) originating at, or above, the waterline. The tidal cycle plays a key role in the timing of calving events, with 68% occurring on the falling limb of the tide. Calving activity is concentrated where meltwater plumes surface at the glacier front, and a similar to 5 m undercut at the base of the glacier suggests that meltwater plumes encourage melt-under-cutting. We conclude that frontal ablation at Tunabreen may be paced by submarine melt rates, as suggested from similar observations at glaciers in Svalbard and Alaska. Using submarine melt rate to calculate frontal ablation would greatly simplify estimations of tidewater glacier losses in prognostic models.

  • 19.
    Jakobsson, Martin
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Anderson, John B.
    Nitsche, Frank O.
    Dowdeswell, Julian A.
    Gyllencreutz, Richard
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Mohammad, Rezwan
    O'Regan, Matthew
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Alley, Richard B.
    Anandakrishnan, Sridhar
    Eriksson, Björn
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Kirshner, Alexandra
    Fernandez, Rodrigo
    Stolldorf, Travis
    Minzoni, Rebecca
    Majewski, Wojciech
    Geological record of ice shelf break-up and grounding line retreat, Pine Island Bay, West Antarctica2011In: Geology, ISSN 0091-7613, E-ISSN 1943-2682, Vol. 39, no 7, p. 691-694Article in journal (Refereed)
    Abstract [en]

    The catastrophic break-ups of the floating Larsen A and B ice shelves (Antarctica) in 1995 and 2002 and associated acceleration of glaciers that flowed into these ice shelves were among the most dramatic glaciological events observed in historical time. This raises a question about the larger West Antarctic ice shelves. Do these shelves, with their much greater glacial discharge, have a history of collapse? Here we describe features from the seafloor in Pine Island Bay, West Antarctica, which we interpret as having been formed during a massive ice shelf break-up and associated grounding line retreat. This evidence exists in the form of seafloor landforms that we argue were produced daily as a consequence of tidally influenced motion of mega-icebergs maintained upright in an iceberg armada produced from the disintegrating ice shelf and retreating grounding line. The break-up occurred prior to ca. 12 ka and was likely a response to rapid sea-level rise or ocean warming at that time.

  • 20.
    Jakobsson, Martin
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Andreassen, Karin
    Bjarnadóttir, Lilja Rún
    Dove, Dayton
    Dowdeswell, Julian A.
    England, John H.
    Funder, Svend
    Hogan, Kelly
    Ingólfsson, Ólafur
    Jennings, Anne
    Krog Larsen, Nikolaj
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Landvik, Jon Y.
    Mayer, Larry
    Mikkelsen, Naja
    Möller, Per
    Niessen, Frank
    Nilsson, Johan
    Stockholm University, Faculty of Science, Department of Meteorology .
    O'Regan, Matthew
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Polyak, Leonid
    Nørgaard-Pedersen, Niels
    Stein, Ruediger
    Arctic Ocean glacial history2014In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 92, p. 40-67Article, review/survey (Refereed)
    Abstract [en]

    While there are numerous hypotheses concerning glacial interglacial environmental and climatic regime shifts in the Arctic Ocean, a holistic view on the Northern Hemisphere's late Quaternary ice-sheet extent and their impact on ocean and sea-ice dynamics remains to be established. Here we aim to provide a step in this direction by presenting an overview of Arctic Ocean glacial history, based on the present state-of-the-art knowledge gained from field work and chronological studies, and with a specific focus on ice-sheet extent and environmental conditions during the Last Glacial Maximum (LGM). The maximum Quaternary extension of ice sheets is discussed and compared to LGM. We bring together recent results from the circum-Arctic continental margins and the deep central basin; extent of ice sheets and ice streams bordering the Arctic Ocean as well as evidence for ice shelves extending into the central deep basin. Discrepancies between new results and published LGM ice-sheet reconstructions in the high Arctic are highlighted and outstanding questions are identified. Finally, we address the ability to simulate the Arctic Ocean ice sheet complexes and their dynamics, including ice streams and ice shelves, using presently available ice-sheet models. Our review shows that while we are able to firmly reject some of the earlier hypotheses formulated to describe Arctic Ocean glacial conditions, we still lack information from key areas to compile the holistic Arctic Ocean glacial history.

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  • 21.
    Jakobsson, Martin
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Mayer, Larry A.
    Nilsson, Johan
    Stockholm University, Faculty of Science, Department of Meteorology .
    Stranne, Christian
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Calder, Brian
    O'Regan, Matthew
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Farrell, John W.
    Cronin, Thomas M.
    Brüchert, Volker
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Chawarski, Julek
    Eriksson, Björn
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Fredriksson, Jonas
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Gemery, Laura
    Glueder, Anna
    Holmes, Felicity A.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Jerram, Kevin
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Mix, Alan
    Muchowski, Julia
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Prakash, Abhay
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Reilly, Brendan
    Thornton, Brett
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Ulfsbo, Adam
    Weidner, Elizabeth
    Stockholm University, Faculty of Science, Department of Geological Sciences. University of New Hampshire, USA.
    Åkesson, Henning
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Handl, Tamara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Ståhl, Emelie
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Boze, Lee-Gray
    Reed, Sam
    West, Gabriel
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Padman, June
    Ryder Glacier in northwest Greenland is shielded from warm Atlantic water by a bathymetric sill2020In: Communications Earth & Environment, E-ISSN 2662-4435, Vol. 1, no 1, article id 45Article in journal (Refereed)
    Abstract [en]

    The processes controlling advance and retreat of outlet glaciers in fjords draining the Greenland Ice Sheet remain poorly known, undermining assessments of their dynamics and associated sea-level rise in a warming climate. Mass loss of the Greenland Ice Sheet has increased six-fold over the last four decades, with discharge and melt from outlet glaciers comprising key components of this loss. Here we acquired oceanographic data and multibeam bathymetry in the previously uncharted Sherard Osborn Fjord in northwest Greenland where Ryder Glacier drains into the Arctic Ocean. Our data show that warmer subsurface water of Atlantic origin enters the fjord, but Ryder Glacier's floating tongue at its present location is partly protected from the inflow by a bathymetric sill located in the innermost fjord. This reduces under-ice melting of the glacier, providing insight into Ryder Glacier's dynamics and its vulnerability to inflow of Atlantic warmer water. A bathymetric sill in Sherard Osborn Fjord, northwest Greenland shields Ryder Glacier from melting by warm Atlantic water found at the bottom of the fjord, according to high-resolution bathymetric mapping and oceanographic data.

  • 22.
    Jakobsson, Martin
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Nilsson, Johan
    Stockholm University, Faculty of Science, Department of Meteorology .
    Anderson, Leif
    Backman, Jan
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Björk, Göran
    Cronin, Thomas M.
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Koshurnikov, Andrey
    Mayer, Larry
    Noormets, Riko
    O'Regan, Matthew
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Stranne, Christian
    Stockholm University, Faculty of Science, Department of Geological Sciences. University of New Hampshire, USA.
    Ananiev, Roman
    Barrientos Macho, Natalia
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Cherniykh, Denis
    Coxall, Helen
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Eriksson, Björn
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Flodén, Tom
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Gemery, Laura
    Gustafsson, Örjan
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Jerram, Kevin
    Johansson, Carina
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Khortov, Alexey
    Mohammad, Rezwan
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Semiletov, Igor
    Evidence for an ice shelf covering the central Arctic Ocean during the penultimate glaciation2016In: Nature Communications, E-ISSN 2041-1723, Vol. 7, article id 10365Article in journal (Refereed)
    Abstract [en]

    The hypothesis of a km-thick ice shelf covering the entire Arctic Ocean during peak glacial conditions was proposed nearly half a century ago. Floating ice shelves preserve few direct traces after their disappearance, making reconstructions difficult. Seafloor imprints of ice shelves should, however, exist where ice grounded along their flow paths. Here we present new evidence of ice-shelf groundings on bathymetric highs in the central Arctic Ocean, resurrecting the concept of an ice shelf extending over the entire central Arctic Ocean during at least one previous ice age. New and previously mapped glacial landforms together reveal flow of a spatially coherent, in some regions41-km thick, central Arctic Ocean ice shelf dated to marine isotope stage 6 (similar to 140 ka). Bathymetric highs were likely critical in the ice-shelf development by acting as pinning points where stabilizing ice rises formed, thereby providing sufficient back stress to allow ice shelf thickening.

  • 23.
    Kirchner, N.
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Hutter, Kolumban
    Swiss Federal Institute of Technology.
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Gyllencreutz, Richard
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Capabilities and limitations of numerical ice sheet models: a discussion for Earth-scientists and modelers2011In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 30, no 25-26, p. 3691-3704Article in journal (Refereed)
    Abstract [en]

    The simulation of dynamically coupled ice sheet, ice stream, and ice shelf-systems poses a challenge to most numerical ice sheet models. Here we review present ice sheet model limitations targeting a broader audience within Earth Sciences, also those with no specific background in numerical modeling, in order to facilitate cross-disciplinary communication between especially paleoglaciologists, marine and terrestrial geologists, and numerical modelers. The ‘zero order’(Shallow Ice Approximation, SIA)-,‘higher order’-, and‘full Stokes’ice sheet models are described conceptually and complemented by an outline of their derivations. We demonstrate that higher order models are required to simulate coupled ice sheetice shelf and ice sheet-ice stream systems, in particular if the results are aimed to complement spatial ice flow reconstructions based on higher resolution geological and geophysical data. The zero order SIA model limitations in capturing ice stream behavior are here illustrated by conceptual simulations of a glaciation on Svalbard. The limitations are obvious from the equations comprising a zero order model. However, under certain circumstances, simulation results may falsely give the impression that ice streams indeed are simulated with a zero order SIA model.

  • 24.
    Kirchner, Nina
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Ahlkrona, J.
    Gowan, Evan J.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Lötstedt, P.
    Lea, James M.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Noormets, R.
    von Sydow, L.
    Dowdeswell, J. A.
    Benham, T.
    Shallow ice approximation, second order shallow ice approximation, and full Stokes models: A discussion of their roles in palaeo-ice sheet modelling and development2016In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 135, p. 103-114Article, review/survey (Refereed)
    Abstract [en]

    Full Stokes ice sheet models provide the most accurate description of ice sheet flow, and can therefore be used to reduce existing uncertainties in predicting the contribution of ice sheets to future sea level rise on centennial time-scales. The level of accuracy at which millennial time-scale palaeo-ice sheet simulations resolve ice sheet flow lags the standards set by Full Stokes models, especially, when Shallow Ice Approximation (SIA) models are used. Most models used in paleo-ice sheet modeling were developed at a time when computer power was very limited, and rely on several assumptions. At the time there was no means of verifying the assumptions by other than mathematical arguments. However, with the computer power and refined Full Stokes models available today, it is possible to test these assumptions numerically. In this paper, we review (Ahlkrona et al., 2013a) where such tests were performed and inaccuracies in commonly used arguments were found. We also summarize (Ahlkrona et al., 2013b) where the implications of the inaccurate assumptions are analyzed for two paleo-models - the SIA and the SOSIA. We review these works without resorting to mathematical detail, in order to make them accessible to a wider audience with a general interest in palaeo-ice sheet modelling. Specifically, we discuss two implications of relevance for palaeo-ice sheet modelling. First, classical SIA models are less accurate than assumed in their original derivation. Secondly, and contrary to previous recommendations, the SOSIA model is ruled out as a practicable tool for palaeo-ice sheet simulations. We conclude with an outlook concerning the new Ice Sheet Coupled Approximation Level (ISCAL) method presented in Ahlkrona et al. (2016), that has the potential to match the accuracy standards of full Stokes model on palaeo-timescales of tens of thousands of years, and to become an alternative to hybrid models currently used in palaeo-ice sheet modelling. The method is applied to an ice sheet covering Svalbard.

  • 25.
    Kirchner, Nina
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Ahlkrona, Josefin
    Gowan, Evan J.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Lötstedt, P.
    Lea, James M.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Noormets, R.
    von Sydow, L.
    Dowdeswell, J. A.
    Benham, T.
    Shallow ice approximation, second order shallow ice approximation, and full Stokes models: A discussion of their roles in palaeo-ice sheet modelling and development2016In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 147, p. 136-147Article in journal (Refereed)
    Abstract [en]

    Full Stokes ice sheet models provide the most accurate description of ice sheet flow, and can therefore be used to reduce existing uncertainties in predicting the contribution of ice sheets to future sea level rise on centennial time-scales. The level of accuracy at which millennial time-scale palaeo-ice sheet simulations resolve ice sheet flow lags the standards set by Full Stokes models, especially, when Shallow Ice Approximation (SIA) models are used. Most models used in paleo-ice sheet modeling were developed at a time when computer power was very limited, and rely on several assumptions. At the time there was no means of verifying the assumptions by other than mathematical arguments. However, with the computer power and refined Full Stokes models available today, it is possible to test these assumptions numerically. In this paper, we review (Ahlkrona et al., 2013a) where such tests were performed and inaccuracies in commonly used arguments were found. We also summarize (Ahlkrona et al., 2013b) where the implications of the inaccurate assumptions are analyzed for two paleo-models - the SIA and the SOSIA. We review these works without resorting to mathematical detail, in order to make them accessible to a wider audience with a general interest in palaeo-ice sheet modelling. Specifically, we discuss two implications of relevance for palaeo-ice sheet modelling. First, classical SIA models are less accurate than assumed in their original derivation. Secondly, and contrary to previous recommendations, the SOSIA model is ruled out as a practicable tool for palaeo-ice sheet simulations. We conclude with an outlook concerning the new Ice Sheet Coupled Approximation Level (ISCAL) method presented in Ahlkrona et al. (2016), that has the potential to match the accuracy standards of full Stokes model on palaeo-timescales of tens of thousands of years, and to become an alternative to hybrid models currently used in palaeo-ice sheet modelling. The method is applied to an ice sheet covering Svalbard.

  • 26.
    Kirchner, Nina
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Faria, Sergio
    The multiscale structure of Antarctica. Part II: Ice Shelves.2009In: Low Temperature Science, ISSN 1880-7593, Vol. 68Article in journal (Refereed)
  • 27.
    Kirchner, Nina
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Furrer, R.
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Zwally, H. J.
    Robbins, J. W.
    Statistical modeling of a former Arctic Ocean ice shelf complex using Antarctic analogies2013In: Journal of Geophysical Research: Earth Surface, ISSN 2169-9003, Vol. 118, no 2, p. 1105-1117Article in journal (Refereed)
    Abstract [en]

    Geophysical mapping and coring of the central Arctic Ocean seafloor provide evidence for repeated occurrences of ice sheet/ice shelf complexes during previous glacial periods. Several ridges and bathymetric highs shallower than present water depths of approximate to 1000m show signs of erosion from deep-drafting (armadas of) icebergs, which originated from thick outlet glaciers and ice shelves. Mapped glacigenic landforms and dates of cored sediments suggest that the largest ice shelf complex was confined to the Amerasian sector of the Arctic Ocean during Marine Isotope Stage (MIS) 6. However, the spatial extent of ice shelves can not be well reconstructed from occasional groundings on bathymetric highs. Therefore, we apply a statistical approach to provide independent support for an extensive MIS 6 ice shelf complex, which previously was inferred only from interpretation of geophysical and geological data. Specifically, we assess whether this ice shelf complex comprises a likely source of the deep-draft icebergs responsible for the mapped scour marks. The statistical modeling is based on exploiting relations between contemporary Antarctic ice shelves and their local physical environments and the assumption that Arctic Ocean MIS6 ice shelves scale similarly. Analyzing ice thickness data along the calving front of contemporary ice shelves, a peak over threshold method is applied to determine sources of deep-drafting icebergs in the Arctic Ocean MIS6 ice shelf complex. This approach is novel to modeling Arctic paleoglacial configurations. Predicted extreme calving front drafts match observed deep-draft iceberg scours if the ice shelf complex is sufficiently large.

  • 28.
    Kirchner, Nina
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology (INK).
    Greve, Ralf
    Institute of Low Temperature Sciences, Hokkaido University, Japan.
    Heyman, Jakob
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology (INK).
    Stroeven, Arjen
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology (INK).
    Tibetan Plateau glaciation during the last glacial cycle: widely diverging (LGM-) reconstructions of glacial extents using numerical ice sheet simulations driven by GCM-ensembles of climate forcings2009In: Geophysical Research Abstracts: Vol. 11, EGU2009-1791, 2009, 2009Conference paper (Refereed)
    Abstract [en]

    The Tibetan Plateau is a topographic feature of extraordinary dimension and has an important impact on regional and global climate. Yet, the glacial history of the Tibetan Plateau is less constrained than the history of some other formerly glaciated regions, especially in the Northern Hemisphere (e.g. Laurentide Ice Sheet, Fennoscandian Ice Sheet). Nevertheless, field evidence for extensive valley glaciation indicates that ice sheet glaciation on the Tibetan Plateau did not evolve during the Last Glacial maximum (LGM). This is an important and robust result that has not been widely investigated using numerical ice sheet models, despite potentially important climate ramifications. Perhaps this is because reconstructions of the LGM glacial configurations of the Tibetan Plateau in the framework of numerical simulations covering an entire glacial cycle exhibit a pronounced variability then entire range of which is not supported by field evidence.

    Using the 3d thermomechanical ice sheet model SICOPOLIS, we simulated the evolution of Tibetan Plateau ice configurations during the last 125.000 years. Temperature and precipitation data driving the simulations have been applied in the form of a large ensemble of glacial/interglacial climate scenarios. It is observed that variations in ice sheet configuration resulting from the prescription of different present-day precipitation- and temperature data sets, on the one hand, and different paleoclimates as obtained from reconstructions based on different GCM-model outputs, on the other hand, include as extreme end members an entirely ice free Tibetan Plateau during the last glacial cycle as well as a plateau-scale Tibetan Ice sheet during the LGM. Comparison of such numerical results with available field data indicates that further refinements in the numerical simulations are required, and that these must include atmosphere-ice sheet feedback mechanisms.

    However, because mapped and simulated glacial extents are represented at different spatial scales, this task is not straightforward.

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  • 29.
    Kirchner, Nina
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Greve, Ralf
    Stroeven, Arjen P.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Heyman, Jakob
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Paleoglaciological reconstructions for the Tibetan Plateau during the last glacial cycle: evaluating numerical ice sheet simulations driven by GCM-ensembles2011In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 30, no 1-2, p. 248-267Article in journal (Refereed)
    Abstract [en]

    The Tibetan Plateau is a topographic feature of extraordinary dimension and has an important impact on regional and global climate. However, the glacial history of the Tibetan Plateau is more poorly constrained than that of most other formerly glaciated regions such as in North America and Eurasia. On the basis of some field evidence it has been hypothesized that the Tibetan Plateau was covered by an ice sheet during the Last Glacial Maximum (LGM). Abundant field- and chronological evidence for a predominance of local valley glaciation during the past 300,000 calendar years (that is, 300 kyr), coupled to an absence of glacial landforms and sediments in extensive areas of the plateau, now refute this concept. This, furthermore, calls into question previous ice sheet modeling attempts which generally arrive at ice volumes considerably larger than allowed for by field evidence. Surprisingly, the robustness of such numerical ice sheet model results has not been widely queried, despite potentially important climate ramifications. We simulated the growth and decay of ice on the Tibetan Plateau during the last 125 kyr in response to a large ensemble of climate forcings (90 members) derived from Global Circulation Models (GCMs), using a similar 3D thermomechanical ice sheet model as employed in previous studies. The numerical results include as extreme end members as an ice free Tibetan Plateau and a plateau-scale ice sheet comparable, in volume, to the contemporary Greenland ice sheet. We further demonstrate that numerical simulations that acceptably conform to published reconstructions of Quaternary ice extent on the Tibetan Plateau cannot be achieved with the employed stand-alone ice sheet model when merely forced by paleoclimates derived from currently available GCMs. Progress is, however, expected if future investigations employ ice sheet models with higher resolution, bidirectional ice sheet-atmosphere feedbacks, improved treatment of the surface mass balance, and regional climate data and climate reconstructions.

  • 30.
    Kirchner, Nina
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology (INK).
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    reconstructions and ice sheet modelling2009In: Third Conference on Arctic Palaeoclimate and its Extremes -beyond the frontier, Copenhagen: The Natural History Museum and University of Copenhagen , 2009, p. 42-42Conference paper (Other academic)
  • 31.
    Kirchner, Nina
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography. KTH Royal Institute of Technology, Sweden.
    Kuttenkeuler, Jakob
    Rosqvist, Gunhild
    Stockholm University, Faculty of Science, Department of Physical Geography. University of Bergen, Norway.
    Hancke, Marnie
    Granebeck, Annika
    Weckstrom, Jan
    Weckstrom, Kaarina
    Schenk, Frederik
    Korhola, Atte
    Eriksson, Pia
    Stockholm University, Faculty of Science, Department of Physical Geography.
    A first continuous three-year temperature record from the dimictic arctic-alpine Lake Tarfala, northern Sweden2021In: Arctic, Antarctic and Alpine research, ISSN 1523-0430, E-ISSN 1938-4246, Vol. 53, no 1, p. 69-79Article in journal (Refereed)
    Abstract [en]

    Arctic lakes are exposed to warming during increasingly longer ice-free periods and, if located in glaciated areas, to increased inflow of meltwater and sediments. However, direct monitoring of how such lakes respond to changing environmental conditions is challenging not only because of their remoteness but also because of the scarcity of present and previously observed lake states. At the glacier-proximal Lake Tarfala in the Kebnekaise Mountains, northern Sweden, temperatures throughout the water column at its deepest part (50 m) were acquired between 2016 and 2019. This three-year record shows that Lake Tarfala is dimictic and is overturning during spring and fall, respectively. Timing, duration, and intensity of mixing processes, as well as of summer and winter stratification, vary between years. Glacial meltwater may play an important role regarding not only mixing processes but also cooling of the lake. Attribution of external environmental factors to (changes in) lake mixing processes and thermal states remains challenging owing to for example, timing of ice-on and ice-off but also reflection and absorption of light, both known to play a decisive role for lake mixing processes, are not (yet) monitored in situ at Lake Tarfala.

  • 32.
    Kirchner, Nina
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography. KTH Royal Institute of Technology, Sweden.
    Noormets, Riko
    Kuttenkeuler, Jakob
    Strandell Erstorp, Elias
    Schytt Holmlund, Erik
    Stockholm University, Faculty of Science, Department of Physical Geography. University Centre in Svalbard, Norway.
    Rosqvist, Gunhild
    Stockholm University, Faculty of Science, Department of Physical Geography. University of Bergen, Norway.
    Holmlund, Per
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Wennbom, Marika
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Karlin, Torbjörn
    Stockholm University, Faculty of Science, Department of Physical Geography. n.
    High-resolution bathymetric mapping reveals subaqueous glacial landforms in the Arctic alpine lake Tarfala, Sweden2019In: Journal of Quaternary Science, ISSN 0267-8179, E-ISSN 1099-1417, Vol. 34, no 6, p. 452-462Article in journal (Refereed)
    Abstract [en]

    In Arctic alpine regions, glacio-lacustrine environments respond sensitively to variations in climate conditions, impacting, for example,glacier extent and rendering former ice-contact lakes into ice distal lakes and vice versa. Lakefloors may hold morphological records of past glacier extent, but remoteness and long periods of ice cover on such lakes make acquisition of high-resolution bathymetric datasets challenging. Lake Tarfala and Kebnepakte Glacier, located in the Kebnekaise mountains, northern Sweden, comprise a small, dynamic glacio-lacustrine system holding a climate archive that is not well studied. Using an autonomous surface vessel, a high-resolution bathymetric dataset for Lake Tarfala was acquired in 2016, from which previously undiscovered end moraines and a potential grounding line feature were identified. For Kebnepakte Glacier, structure-from-motion photogrammetry was used to reconstruct its shape from photographs taken in 1910 and 1945. Combining these methods connects the glacial landform record identified at the lakefloor with the centennial-scale dynamic behaviour of Kebnepakte Glacier. During its maximum 20(th) century extent, attained c. 1910, Kebnepakte Glacier reached far into Lake Tarfala, but had retreated onto land by 1945, at an average of 7.9 m year(-1).

  • 33.
    Kirchner, Nina
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography. Stockholm University, Faculty of Science, Department of Geological Sciences. Stockholm University, Faculty of Science, The Bolin Centre for Climate Research (together with KTH & SMHI).
    Weckstrom, Jan
    Jansen, Joachim
    Schenk, Frederik
    Stockholm University, Faculty of Science, Department of Geological Sciences. Stockholm University, Faculty of Science, The Bolin Centre for Climate Research (together with KTH & SMHI). University of Helsinki, Finland.
    Barnett, Jamie
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Granebeck, Annika
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Lepparanta, Matti
    Korhola, Atte
    Water temperature, mixing, and ice phenology in the arctic-alpine Lake Darfáljávri (Lake Tarfala), northern Sweden2024In: Arctic, Antarctic and Alpine research, ISSN 1523-0430, E-ISSN 1938-4246, Vol. 56, no 1, article id 2287704Article in journal (Refereed)
    Abstract [en]

    In the rapidly warming circumpolar Arctic, recent research of lakes has focused on their climatology and ecology but is challenged by sparsity of wintertime data. At the c. 48-m-deep and c. 0.5-km2 large proglacial Darfaljavri (Lake Tarfala), located in an arctic-alpine environment in the Scandinavian Mountains, year-round water temperatures were previously reported for 2016 to 2019. Here, this record is continued for 2019-2020 and 2021-2022, complemented by time-lapse imagery records of the state of the lake surface, as well as degree-day modeling of ice phenology (timing of ice-on and ice-off). Darfaljavri is cryostratified during winter, with interannual variations in the thermocline's thickness and temperature range. The ice season lasts from October to July. Modeled ice-on dates match observed ones reasonably well; however, observed ice-off dates occur much later than modeled ones, likely because of cold impact from Darfaljavri's glacial environment as inferred from a comparison with a close tundra lake. Though new insights into the complex lake mixing and ice phenology are provided, it remains to attribute the characteristics of Darfaljavri's winter stratification to additional potential drivers, such as lake ice thickness, atmospheric heat fluxes, and the water balance of the lake.

  • 34. Kjelldorff, Maria
    et al.
    Kuttenkeuler, Jakob
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Krutzfeldt, Jari
    Sundberg, Mikael
    Water current measurements using oceanographic bottom lander LoTUS2020In: Applied Ocean Research, ISSN 0141-1187, E-ISSN 1879-1549, Vol. 94, article id 101982Article in journal (Refereed)
    Abstract [en]

    LOTUS is a bottom landing, Long Term Underwater Sensing node made for the observation of ocean water temperatures. LoTUS is moored to the seafloor and measures temperature according to a specified time schedule until, at the end of the mission, it surfaces to transmit the collected data to on-shore recipients using an Iridium link. The paper presents an extension of the sensing capability which includes water current velocity (speed and direction) using a robust, reliable and inexpensive Eulerian method. The method is based on the tilting stick principle where a combination of inertial and magnetic measurement data are used. The paper discusses the principal technique, modeling of the system, practical considerations, optimization of the setup for specific flow conditions, and the verification of experimental data.

  • 35. Meier, H. E. Markus
    et al.
    Kniebusch, Madline
    Dieterich, Christian
    Gröger, Matthias
    Zorita, Eduardo
    Elmgren, Ragnar
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Myrberg, Kai
    Ahola, Markus P.
    Bartosova, Alena
    Bonsdorff, Erik
    Börgel, Florian
    Capell, Rene
    Carlén, Ida
    Carlund, Thomas
    Carstensen, Jacob
    Christensen, Ole B.
    Dierschke, Volker
    Frauen, Claudia
    Frederiksen, Morten
    Gaget, Elie
    Galatius, Anders
    Haapala, Jari J.
    Halkka, Antti
    Hugelius, Gustaf
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Hünicke, Birgit
    Jaagus, Jaak
    Jüssi, Mart
    Käyhkö, Jukka
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Kjellström, Erik
    Kulinski, Karol
    Lehmann, Andreas
    Lindström, Göran
    May, Wilhelm
    Miller, Paul A.
    Mohrholz, Volker
    Müller-Karulis, Bärbel
    Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre.
    Pavón-Jordán, Diego
    Quante, Markus
    Reckermann, Marcus
    Rutgersson, Anna
    Savchuk, Oleg P.
    Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre.
    Stendel, Martin
    Tuomi, Laura
    Viitasalo, Markku
    Weisse, Ralf
    Zhang, Wenyan
    Climate change in the Baltic Sea region: a summary2022In: Earth System Dynamics, ISSN 2190-4979, E-ISSN 2190-4987, Vol. 13, no 1, p. 457-593Article, review/survey (Refereed)
    Abstract [en]

    Based on the Baltic Earth Assessment Reports of this thematic issue in Earth System Dynamics and recent peer-reviewed literature, current knowledge of the effects of global warming on past and future changes in climate of the Baltic Sea region is summarised and assessed. The study is an update of the Second Assessment of Climate Change (BACC II) published in 2015 and focuses on the atmosphere, land, cryosphere, ocean, sediments, and the terrestrial and marine biosphere. Based on the summaries of the recent knowledge gained in palaeo-, historical, and future regional climate research, we find that the main conclusions from earlier assessments still remain valid. However, new long-term, homogenous observational records, for example, for Scandinavian glacier inventories, sea-level-driven saltwater inflows, so-called Major Baltic Inflows, and phytoplankton species distribution, and new scenario simulations with improved models, for example, for glaciers, lake ice, and marine food web, have become available. In many cases, uncertainties can now be better estimated than before because more models were included in the ensembles, especially for the Baltic Sea. With the help of coupled models, feedbacks between several components of the Earth system have been studied, and multiple driver studies were performed, e.g. projections of the food web that include fisheries, eutrophication, and climate change. New datasets and projections have led to a revised understanding of changes in some variables such as salinity. Furthermore, it has become evident that natural variability, in particular for the ocean on multidecadal timescales, is greater than previously estimated, challenging our ability to detect observed and projected changes in climate. In this context, the first palaeoclimate simulations regionalised for the Baltic Sea region are instructive. Hence, estimated uncertainties for the projections of many variables increased. In addition to the well-known influence of the North Atlantic Oscillation, it was found that also other low-frequency modes of internal variability, such as the Atlantic Multidecadal Variability, have profound effects on the climate of the Baltic Sea region. Challenges were also identified, such as the systematic discrepancy between future cloudiness trends in global and regional models and the difficulty of confidently attributing large observed changes in marine ecosystems to climate change. Finally, we compare our results with other coastal sea assessments, such as the North Sea Region Climate Change Assessment (NOSCCA), and find that the effects of climate change on the Baltic Sea differ from those on the North Sea, since Baltic Sea oceanography and ecosystems are very different from other coastal seas such as the North Sea. While the North Sea dynamics are dominated by tides, the Baltic Sea is characterised by brackish water, a perennial vertical stratification in the southern subbasins, and a seasonal sea ice cover in the northern subbasins.

  • 36.
    Nilsson, Johan
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Borstad, Chris
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Björk, Göran
    Pierrehumbert, Raymond T.
    Stranne, Christian
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Ice-shelf damming in the glacial Arctic Ocean: dynamical regimes of a basin-covering kilometre-thick ice shelf2017In: The Cryosphere, ISSN 1994-0416, E-ISSN 1994-0424, Vol. 11, no 4, p. 1745-1765Article in journal (Refereed)
    Abstract [en]

    Recent geological and geophysical data suggest that a 1 km thick ice shelf extended over the glacial Arctic Ocean during Marine Isotope Stage 6, about 140 000 years ago. Here, we theoretically analyse the development and equilibrium features of such an ice shelf, using scaling analyses and a one-dimensional ice-sheet-ice-shelf model. We find that the dynamically most consistent scenario is an ice shelf with a nearly uniform thickness that covers the entire Arctic Ocean. Further, the ice shelf has two regions with distinctly different dynamics: a vast interior region covering the central Arctic Ocean and an exit region towards the Fram Strait. In the interior region, which is effectively dammed by the Fram Strait constriction, there are strong back stresses and the mean ice-shelf thickness is controlled primarily by the horizontally integrated mass balance. A narrow transition zone is found near the continental grounding line, in which the ice-shelf thickness decreases offshore and approaches the mean basin thickness. If the surface accumulation and mass flow from the continental ice masses are sufficiently large, the ice-shelf thickness grows to the point where the ice shelf grounds on the Lomonosov Ridge. As this occurs, the back stress increases in the Amerasian Basin and the ice-shelf thickness becomes larger there than in the Eurasian Basin towards the Fram Strait. Using a one-dimensional ice-dynamic model, the stability of equilibrium ice-shelf configurations without and with grounding on the Lomonosov Ridge are examined. We find that the grounded ice-shelf configuration should be stable if the two Lomonosov Ridge grounding lines are located on the opposites sides of the ridge crest, implying that the downstream grounding line is located on a downward sloping bed. This result shares similarities with the classical result on marine ice-sheet stability of Weertman, but due to interactions between the Amerasian and Eurasian ice-shelf segments the mass flux at the downstream grounding line decreases rather than increases with ice thickness.

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  • 37. Noormets, Riko
    et al.
    Flink, Anne
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Glacial dynamics and deglaciation history of Hambergbukta reconstructed from submarine landforms and sediment cores, SE Spitsbergen, Svalbard2021In: Boreas, ISSN 0300-9483, E-ISSN 1502-3885, Vol. 50, no 1, p. 29-50Article in journal (Refereed)
    Abstract [en]

    The submarine landforms and shallow sediment record are presented from Hambergbukta, southeastern Spitsbergen using swath-bathymetric, subbottom acoustic, and sediment core data. The mapped landforms include large terminal and end-moraines with associated debrisflow aprons on their distal flanks, drumlinized till surface, glacial lineations, medial and retreat moraines, crevasse squeeze ridge networks, eskers, as well as iceberg-produced terraces and plough-marks. Analysis of the landforms and landform assemblages in combination with the sediment core data and aerial imagery studies reveal a complex and dynamic glacial history of Hambergbukta. We present a detailed history of Hambergbreen glacier indicating two previously unknown surges as well as new details on the nature of the subsequent ice-margin retreat. The results from two gravity cores combined with the shallow acoustic stratigraphy and high-resolution bathymetry suggest that the c. AD 1900 surge was less extensive than previously thought and the retreat was most likely rapid after the c. AD 1900 and 1957 surges of the Hambergbreen. Mixed benthic foraminifera collected from the outer fjord basin date to 2456 cal. a BP, suggesting older sediments were re-worked by the c. AD 1900 surge. This highlights the importance of exercising caution when using foraminifers for dating surge events in fjord basins enclosed by prominent end-moraines.

  • 38.
    O'Regan, Matt
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Backman, Jan
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Barrientos, Natalia
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Cronin, Thomas M.
    Gemery, Laura
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Mayer, Larry A.
    Nilsson, Johan
    Stockholm University, Faculty of Science, Department of Meteorology .
    Noormets, Riko
    Pearce, Christof
    Stockholm University, Faculty of Science, Department of Geological Sciences. Aarhus University, Denmark.
    Semiletov, Igor
    Stranne, Christian
    Stockholm University, Faculty of Science, Department of Geological Sciences. University of New Hampshire, USA.
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    The De Long Trough: A newly discovered glacial trough on the East Siberian continental margin2017In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 13, no 9, p. 1269-1284Article in journal (Refereed)
    Abstract [en]

    Ice sheets extending over parts of the East Siberian continental shelf have been proposed for the last glacial period and during the larger Pleistocene glaciations. The sparse data available over this sector of the Arctic Ocean have left the timing, extent and even existence of these ice sheets largely unresolved. Here we present new geophysical mapping and sediment coring data from the East Siberian shelf and slope collected during the 2014 SWERUS-C3 expedition (SWERUS-C3: Swedish - Russian - US Arctic Ocean Investigation of Climate-Cryosphere-Carbon Interactions). The multibeam bathymetry and chirp sub-bottom profiles reveal a set of glacial landforms that include grounding zone formations along the outer continental shelf, seaward of which lies a > 65m thick sequence of glacio-genic debris flows. The glacial landforms are interpreted to lie at the seaward end of a glacial trough - the first to be reported on the East Siberian margin, here referred to as the De Long Trough because of its location due north of the De Long Islands. Stratigraphy and dating of sediment cores show that a drape of acoustically laminated sediments covering the glacial deposits is older than similar to 50 cal kyr BP. This provides direct evidence for extensive glacial activity on the Siberian shelf that predates the Last Glacial Maximum and most likely occurred during the Saalian (Marine Isotope Stage (MIS) 6).

  • 39.
    O'Regan, Matt
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Glacial geological implications of overconsolidated sediments on the Lomonosov Ridge and Yermak Plateau2010In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 29, no 25-26, p. 3532-3544Article in journal (Refereed)
    Abstract [en]

    With the coupled use of multibeam swath bathymetry, high-resolution subbottom profiling and sediment coring from icebreakers in the Arctic Ocean, there is a growing awareness of the prevalence of Quaternary ice-grounding events on many of the topographic highs found in present water depths of <1000 m. In some regions, such as the Lomonosov Ridge and Yermak Plateau, overconsolidated sediments sampled through either drilling or coring are found beneath seismically imaged unconformities of glacigenic origin. However, there exists no comprehensive analysis of the geotechnical properties of these sediments, or how their inferred stress state may be related to different glacigenic processes or types of ice-loading. Here we combine geophysical, stratigraphic and geotechnical measurements from the Lomonosov Ridge and Yermak Plateau and discuss the glacial geological implications of overconsolidated sediments. The degree of overconsolidation, determined from measurements of porosity and shear strength, is shown to result from consolidation and/or deformation below grounded ice and, with the exception of a single region on the Lomonosov Ridge, cannot be explained by erosion of overlying sediments. We demonstrate that the amount and depth of porosity loss associated with a middle Quaternary (790–950 thousand years ago – ka) grounding on the Yermak Plateau is compatible with sediment consolidation under an ice sheet or ice rise. Conversely, geotechnical properties of sediments from beneath late Quaternary ice-groundings in both regions, independently dated to Marine Isotope Stage (MIS) 6, indicate a more transient event commensurate with a passing tabular iceberg calved from an ice shelf.

  • 40.
    Otto, Jacqueline
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Holmes, Felicity A.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Supraglacial lake expansion, intensified lake drainage frequency, and first observation of coupled lake drainage, during 1985–2020 at Ryder Glacier, Northern Greenland2022In: Frontiers in Earth Science, E-ISSN 2296-6463, Vol. 10, article id 978137Article in journal (Refereed)
    Abstract [en]

    Along the Greenland Ice Sheet margin, supraglacial lakes store and redistribute ice sheet surface run off, and comprise an important potential hydrological link between the ice surface and the base, with ramifications for subglacial drainage systems and ice flow. As a consequence of increasing global mean surface air temperatures, these lakes have been predicted to expand further inland and to affect larger areas of the ice sheet. However, as contemporary dynamics of such supraglacial lake expansion are not well studied, any assessment of their future implications remains afflicted with uncertainty. Here, recent changes in supraglacial lake distribution and expansion, and in their drainage behavior and frequency, are presented for Ryder Glacier, Northern Greenland, as concluded from a remote sensing based analysis. The 35-year time span covered in the analysis allows for the detection of trends in lake processes and ice velocity, which otherwise were found to exhibit large inter-annual variability. It also reveals the first occurrence of a coupled lake drainage event in 2002. By linking supraglacial lake expansion, drainage modes, and drainage frequency to the efficiency of the subglacial drainage system and ice flow on seasonal and decadal timescales, a contribution is made to better understand the complexity of coupled glacio-hydrological processes, and to help reduce uncertainties in predictions of future mass loss from the Greenland Ice Sheet.

  • 41. Petrini, Michele
    et al.
    Colleoni, Florence
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Hughes, Anna L. C.
    Camerlenghi, Angelo
    Rebesco, Michele
    Lucchi, Renata G.
    Forte, Emanuele
    Colucci, Renato R.
    Noormets, Riko
    Interplay of grounding-line dynamics and sub-shelf melting during retreat of the Bjornoyrenna Ice Stream2018In: Scientific Reports, E-ISSN 2045-2322, Vol. 8, article id 7196Article in journal (Refereed)
    Abstract [en]

    The Barents Sea Ice Sheet was a marine-based ice sheet, i.e., it rested on the Barents Sea floor during the Last Glacial Maximum (21 ky BP). The Bjornoyrenna Ice Stream was the largest ice stream draining the Barents Sea Ice Sheet and is regarded as an analogue for contemporary ice streams in West Antarctica. Here, the retreat of the Bjornoyrenna Ice Stream is simulated by means of two numerical ice sheet models and results assessed against geological data. We investigate the sensitivity of the ice stream to changes in ocean temperature and the impact of grounding-line physics on ice stream retreat. Our results suggest that the role played by sub-shelf melting depends on how the grounding-line physics is represented in the models. When an analytic constraint on the ice flux across the grounding line is applied, the retreat of Bjornoyrenna Ice Stream is primarily driven by internal ice dynamics rather than by oceanic forcing. This suggests that implementations of grounding-line physics need to be carefully assessed when evaluating and predicting the response of contemporary marine-based ice sheets and individual ice streams to ongoing and future ocean warming.

  • 42. Petrini, Michele
    et al.
    Colleoni, Florence
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Hughes, Anna L. C.
    Camerlenghi, Angelo
    Rebesco, Michele
    Lucchi, Renata G.
    Forte, Emanuele
    Colucci, Renato R.
    Noormets, Riko
    Mangerud, Jan
    Simulated last deglaciation of the Barents Sea Ice Sheet primarily driven by oceanic conditions2020In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 238, article id 106314Article in journal (Refereed)
    Abstract [en]

    The Barents Sea Ice Sheet was part of an interconnected complex of ice sheets, collectively referred to as the Eurasian Ice Sheet, which covered north-westernmost Europe, Russia and the Barents Sea during the Last Glacial Maximum (around 21 ky BP). Due to common geological features, the Barents Sea component of this ice complex is seen as a paleo-analogue for the present-day West Antarctic Ice Sheet. Investigating key processes driving the last deglaciation of the Barents Sea Ice Sheet represents an important tool to interpret recent observations in Antarctica over the multi-millennial temporal scale of glaciological changes. We present results from a perturbed physics ensemble of ice sheet model simulations of the last deglaciation of the Barents Sea Ice Sheet, forced with transient atmospheric and oceanic conditions derived from AOGCM simulations. The ensemble of transient simulations is evaluated against the databased DATED-1 reconstruction to construct minimum, maximum and average deglaciation scenarios. Despite a large model/data mismatch at the western and eastern ice sheet margins, the simulated and DATED-1 deglaciation scenarios agree well on the timing of the deglaciation of the central and northern Barents Sea. We find that the simulated deglaciation of the Barents Sea Ice Sheet is primarily driven by the oceanic forcing, with prescribed eustatic sea level rise amplifying the ice sheet sensitivity to sub-shelf melting over relatively short intervals. Our results highlight that the sub-shelf melting has a very strong control on the simulated grounding-line flux, showing that a slow, gradual ocean warming trend is capable of triggering sustained grounded ice discharge over multi-millennial timescales, even without taking into account marine ice sheet or ice cliff instability.

  • 43.
    Prakash, Abhay
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Zhou, Qin
    Hattermann, Tore
    Bao, Weiyang
    Graversen, Rune
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography.
    A nested high-resolution unstructured grid 3-D ocean-sea ice-ice shelf setup for numerical investigations of the Petermann ice shelf and fjord2022In: MethodsX, ISSN 1258-780X, E-ISSN 2215-0161, Vol. 9, article id 101668Article in journal (Refereed)
    Abstract [en]

    Three-dimensional numerical simulation of circulation in fjords hosting marine-terminating ice shelves is challenging because of the complexity of processes involved in such environments. This often requires a comprehensive model setup. The following elements are needed: bathymetry (usually unknown beneath the glacier tongue), ice shelf draft (impacting water column thickness), oceanographic state (including tidal elevation, salinity, temperature and velocity of the water masses), sea ice and atmospheric forcing. Moreover, a high spatial resolution is needed, at least locally, which may be augmented with a coarser and computationally cheaper (nested) model that provides sufficiently realistic conditions at the boundaries. Here, we describe procedures to systematically create such a setup that uses the Finite Volume Community Ocean Model (FVCOM) for the Petermann Fjord, Northwest Greenland. The first simulations are validated against temperature and salinity observations from the Petermann Fjord in September 2019. We provide

    •Complete bathymetry, ice-draft and water column thickness datasets of the Petermann Fjord, with an improved representation of the topography underneath the glacier tongue.

    •Boundary conditions for ocean, atmosphere and sea ice derived from a suite of high-resolution regional models that can be used to initialize and run the regional ocean model with realistic geophysical settings.

  • 44.
    Skelton, Alasdair
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Kockum, I.
    Skewness of Temperature Data Implies an Abrupt Change in the Climate System Between 1985 and 19912020In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 47, no 23, article id e2020GL089794Article in journal (Refereed)
    Abstract [en]

    Instrumental records of mean annual temperature extend back to the seventeenth and eighteenth centuries at multiple sites in Europe. For such long time series, we expect and find that histograms of mean annual temperature data become skewed toward higher temperatures with time because of global warming. However, we also find that skewness changed abruptly and started increasing between 1985 and 1991 (95% confidence) at 17 sites. We argue that this finding may imply an abrupt change in the climate system affecting Europe which probably occurred at this time. We investigate possible causes and find Arctic sea ice loss, potentially linked to reduced sulfate aerosol emissions and coupled to temperature by an albedo feedback mechanism, a likely candidate. This is based on good correlations of sea ice extent and sulfate aerosol emissions with skewness of mean annual temperature data.

  • 45. Stokes, Chris R.
    et al.
    Tarasov, Lev
    Blomdin, Robin
    Stockholm University, Faculty of Science, Department of Physical Geography. Purdue University, USA.
    Cronin, Thomas M.
    Fisher, Timothy G.
    Gyllencreutz, Richard
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Hättestrand, Clas
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Heyman, Jakob
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Hindmarsh, Richard C. A.
    Hughes, Anna L. C.
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Livingstone, Stephen J.
    Margold, Martin
    Stockholm University, Faculty of Science, Department of Physical Geography. Durham University, UK.
    Murton, Julian B.
    Noormets, Riko
    Peltier, W. Richard
    Peteet, Dorothy M.
    Piper, David J. W.
    Preusser, Frank
    Renssen, Hans
    Roberts, David H.
    Roche, Didier M.
    Saint-Ange, Francky
    Stroeven, Arjen P.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Teller, James T.
    On the reconstruction of palaeo-ice sheets: Recent advances and future challenges2015In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 125, p. 15-49Article, review/survey (Refereed)
    Abstract [en]

    Reconstructing the growth and decay of palaeo-ice sheets is critical to understanding mechanisms of global climate change and associated sea-level fluctuations in the past, present and future. The significance of palaeo-ice sheets is further underlined by the broad range of disciplines concerned with reconstructing their behaviour, many of which have undergone a rapid expansion since the 1980s. In particular, there has been a major increase in the size and qualitative diversity of empirical data used to reconstruct and date ice sheets, and major improvements in our ability to simulate their dynamics in numerical ice sheet models. These developments have made it increasingly necessary to forge interdisciplinary links between sub-disciplines and to link numerical modelling with observations and dating of proxy records. The aim of this paper is to evaluate recent developments in the methods used to reconstruct ice sheets and outline some key challenges that remain, with an emphasis on how future work might integrate terrestrial and marine evidence together with numerical modelling. Our focus is on pan-ice sheet reconstructions of the last deglaciation, but regional case studies are used to illustrate methodological achievements, challenges and opportunities. Whilst various disciplines have made important progress in our understanding of ice-sheet dynamics, it is clear that data-model integration remains under-used, and that uncertainties remain poorly quantified in both empirically-based and numerical ice-Sheet reconstructions. The representation of past climate will continue to be the largest source of uncertainty for numerical modelling. As such, palaeo-observations are critical to constrain and validate modelling. State-of-the-art numerical models will continue to improve both in model resolution and in the breadth of inclusion of relevant processes, thereby enabling more accurate and more direct comparison with the increasing range of palaeo-observations. Thus, the capability is developing to use all relevant palaeo-records to more strongly constrain deglacial (and to a lesser extent pre-LGM) ice sheet evolution. In working towards that goal, the accurate representation of uncertainties is required for both constraint data and model outputs. Close cooperation between modelling and data-gathering communities is essential to ensure this capability is realised and continues to progress.

  • 46.
    Tesi, Tommaso
    et al.
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry. Institute of Marine Sciences, National Research Council (ISMAR-CNR), Italy.
    Muschitiello, Francesco
    Stockholm University, Faculty of Science, Department of Geological Sciences. Columbia University, USA; Uni Research Climate, Norway.
    Smittenberg, Rienk H.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences. University Centre in Svalbard (UNIS), Svalbard.
    Vonk, J. E.
    Hill, P.
    Andersson, August
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Noormets, R.
    Dudarev, O.
    Semiletov, I.
    Gustafsson, Örjan
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Massive remobilization of permafrost carbon during post-glacial warming2016In: Nature Communications, E-ISSN 2041-1723, Vol. 7, article id 13653Article in journal (Refereed)
    Abstract [en]

    Recent hypotheses, based on atmospheric records and models, suggest that permafrost carbon (PF-C) accumulated during the last glaciation may have been an important source for the atmospheric CO2 rise during post-glacial warming. However, direct physical indications for such PF-C release have so far been absent. Here we use the Laptev Sea (Arctic Ocean) as an archive to investigate PF-C destabilization during the last glacial–interglacial period. Our results show evidence for massive supply of PF-C from Siberian soils as a result of severe active layer deepening in response to the warming. Thawing of PF-C must also have brought about an enhanced organic matter respiration and, thus, these findings suggest that PF-C may indeed have been an important source of CO2 across the extensive permafrost domain. The results challenge current paradigms on the post-glacial CO2 rise and, at the same time, serve as a harbinger for possible consequences of the present-day warming of PF-C soils.

  • 47.
    van Dongen, Eef C. H.
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography. ETHZ, Switzerland; Delft University of Technology, the Netherlands; Utrecht University, the Netherlands.
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography.
    van Gijzen, Martin B.
    van de Wal, Roderik S. W.
    Zwinger, Thomas
    Cheng, Gong
    Lötstedt, Per
    von Sydow, Lina
    Dynamically coupling full Stokes and shallow shelf approximation for marine ice sheet flow using Elmer/Ice (v8.3)2018In: Geoscientific Model Development, ISSN 1991-959X, E-ISSN 1991-9603, Vol. 11, no 11, p. 4563-4576Article in journal (Refereed)
    Abstract [en]

    Ice flow forced by gravity is governed by the full Stokes (FS) equations, which are computationally expensive to solve due to the nonlinearity introduced by the rheology. Therefore, approximations to the FS equations are commonly used, especially when modeling a marine ice sheet (ice sheet, ice shelf, and/or ice stream) for 103 years or longer. The shallow ice approximation (SIA) and shallow shelf approximation (SSA) are commonly used but are accurate only for certain parts of an ice sheet. Here, we report a novel way of iteratively coupling FS and SSA that has been implemented in Elmer/Ice and applied to conceptual marine ice sheets. The FS-SSA coupling appears to be very accurate; the relative error in velocity compared to FS is below 0.5% for diagnostic runs and below 5% for prognostic runs. Results for grounding line dynamics obtained with the FS-SSA coupling are similar to those obtained from an FS model in an experiment with a periodical temperature forcing over 3000 years that induces grounding line advance and retreat. The rapid convergence of the FS-SSA coupling shows a large potential for reducing computation time, such that modeling a marine ice sheet for thousands of years should become feasible in the near future. Despite inefficient matrix assembly in the current implementation, computation time is reduced by 32 %, when the coupling is applied to a 3-D ice shelf.

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