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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, 2135-2152 p.Article 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, 417-435 p.Article 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, 1-19 p.Article 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. 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, 148-163 p.Article 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.

  • 5. 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, 48-48 p.Conference paper (Other academic)
  • 6. 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, 37-50 p.Article 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.

  • 7. 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)
  • 8.
    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.

  • 9.
    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, 691-694 p.Article 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.

  • 10.
    Jakobsson, Martin
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Andreassen, Karin
    Bjarnadóttir, Lilja Rün
    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, Vol. 92, 40-67 p.Article in journal (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.

  • 11.
    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, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 7, 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.

  • 12.
    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, Vol. 30, no 25-26, 3691-3704 p.Article 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.

  • 13.
    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, 103-114 p.Article, 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.

  • 14.
    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, 136-147 p.Article 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.

  • 15.
    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)
  • 16.
    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, 1105-1117 p.Article 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.

  • 17.
    Kirchner, Nina
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Furrer, Reinhard
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Zwally, Jay
    NASA.
    Robbins, John W.
    Statistical modeling of a former Arctic Ocean ice shelf complex using Antarctic analogies2012In: APEX Sixth International Conference and Workshop: Quaternary Glacial and Climate Extremes, 2012, 58-58 p.Conference paper (Other academic)
  • 18.
    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.

  • 19.
    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, Vol. 30, no 1-2, 248-267 p.Article 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.

  • 20.
    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, 42-42 p.Conference paper (Other academic)
  • 21.
    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, 1745-1765 p.Article 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.

  • 22.
    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, 1269-1284 p.Article 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).

  • 23.
    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, Vol. 29, no 25-26, 3532-3544 p.Article 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.

  • 24. 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, 15-49 p.Article, 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.

  • 25.
    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, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 7, 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.

1 - 25 of 25
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