@article{EgholmJansenBraedstrupetal.2017, author = {Egholm, David L. and Jansen, John D. and Braedstrup, Christian F. and Pedersen, Vivi K. and Andersen, Jane Lund and Ugelvig, Sofie V. and Larsen, Nicolaj K. and Knudsen, Mads F.}, title = {Formation of plateau landscapes on glaciated continental margins}, series = {Nature geoscience}, volume = {10}, journal = {Nature geoscience}, publisher = {Nature Publ. Group}, address = {New York}, issn = {1752-0894}, doi = {10.1038/NGEO2980}, pages = {592 -- +}, year = {2017}, abstract = {Low-relief plateaus separated by deeply incised fjords are hallmarks of glaciated, passive continental margins. Spectacular examples fringe the once ice-covered North Atlantic coasts of Greenland, Norway and Canada, but low-relief plateau landscapes also underlie present-day ice sheets in Antarctica and Greenland. Dissected plateaus have long been viewed as the outcome of selective linear erosion by ice sheets that focus incision in glacial troughs, leaving the intervening landscapes essentially unaffected. According to this hypothesis, the plateaus are remnants of preglacial low-relief topography. However, here we use computational experiments to show that, like fjords, plateaus are emergent properties of long-term ice-sheet erosion. Ice sheets can either increase or decrease subglacial relief depending on the wavelength of the underlying topography, and plateau topography arises dynamically from evolving feedbacks between topography, ice dynamics and erosion over million-year timescales. This new mechanistic explanation for plateau formation opens the possibility of plateaus contributing significantly to accelerated sediment flux at the onset of the late Cenozoic glaciations, before becoming stable later in the Quaternary.}, language = {en} } @article{KnudsenEgholmJacobsenetal.2015, author = {Knudsen, Mads Faurschou and Egholm, David L. and Jacobsen, Bo Holm and Larsen, Nicolaj Krog and Jansen, John D. and Andersen, Jane Lund and Linge, Henriette C.}, title = {A multi-nuclide approach to constrain landscape evolution and past erosion rates in previously glaciated terrains}, series = {Quaternary geochronology : the international research and review journal on advances in quaternary dating techniques}, volume = {30}, journal = {Quaternary geochronology : the international research and review journal on advances in quaternary dating techniques}, publisher = {Elsevier}, address = {Oxford}, issn = {1871-1014}, doi = {10.1016/j.quageo.2015.08.004}, pages = {100 -- 113}, year = {2015}, abstract = {Cosmogenic nuclides are typically used to either constrain an exposure age, a burial age, or an erosion rate. Constraining the landscape history and past erosion rates in previously glaciated terrains is, however, notoriously difficult because it involves a large number of unknowns. The potential use of cosmogenic nuclides in landscapes with a complex history of exposure and erosion is therefore often quite limited. Here, we present a novel multi-nuclide approach to study the landscape evolution and past erosion rates in terrains with a complex exposure history, particularly focusing on regions that were repeatedly covered by glaciers or ice sheets during the Quaternary. The approach, based on the Markov Chain Monte Carlo (MCMC) technique, focuses on mapping the range of landscape histories that are consistent with a given set of measured cosmogenic nuclide concentrations. A fundamental assumption of the model approach is that the exposure history at the site/location can be divided into two distinct regimes: i) interglacial periods characterized by zero shielding due to overlying ice and a uniform interglacial erosion rate, and ii) glacial periods characterized by 100\% shielding and a uniform glacial erosion rate. We incorporate the exposure history in the model framework by applying a threshold value to the global marine benthic delta O-18 record and include the threshold value as a free model parameter, hereby taking into account global changes in climate. However, any available information on the glacial-interglacial history at the sampling location, in particular the timing of the last deglaciation event, is readily incorporated in the model to constrain the inverse problem. Based on the MCMC technique, the model delineates the most likely exposure history, including the glacial and interglacial erosion rates, which, in turn, makes it possible to reconstruct an exhumation history at the site. We apply the model to two landscape scenarios based on synthetic data and two landscape scenarios based on paired Be-10/Al-26 data from West Greenland, which makes it possible to quantify the denudation rate at these locations. The model framework, which currently incorporates any combination of the following nuclides Be-10, Al-26, C-14, and Ne-21, is highly flexible and can be adapted to many different landscape settings. The model framework may also be used in combination with physics-based landscape evolution models to predict nuclide concentrations at different locations in the landscape. This may help validate the landscape models via comparison to measured nuclide concentrations or to devise new effective sampling strategies. (C) 2015 The Authors. Published by Elsevier B.V.}, language = {en} } @misc{EgholmAndersenFaurschouKnudsenetal.2015, author = {Egholm, David L. and Andersen, Jane Lund and Faurschou Knudsen, Mads and Jansen, John D. and Nielsen, S. B.}, title = {The periglacial engine of mountain erosion}, series = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, journal = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, number = {552}, issn = {1866-8372}, doi = {10.25932/publishup-40971}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-409718}, pages = {20}, year = {2015}, abstract = {There is growing recognition of strong periglacial control on bedrock erosion in mountain landscapes, including the shaping of low-relief surfaces at high elevations (summit flats). But, as yet, the hypothesis that frost action was crucial to the assumed Late Cenozoic rise in erosion rates remains compelling and untested. Here we present a landscape evolution model incorporating two key periglacial processes - regolith production via frost cracking and sediment transport via frost creep - which together are harnessed to variations in temperature and the evolving thickness of sediment cover. Our computational experiments time-integrate the contribution of frost action to shaping mountain topography over million-year timescales, with the primary and highly reproducible outcome being the development of flattish or gently convex summit flats. A simple scaling of temperature to marine delta O-18 records spanning the past 14 Myr indicates that the highest summit flats in mid-to high-latitude mountains may have formed via frost action prior to the Quaternary. We suggest that deep cooling in the Quaternary accelerated mechanical weathering globally by significantly expanding the area subject to frost. Further, the inclusion of subglacial erosion alongside periglacial processes in our computational experiments points to alpine glaciers increasing the long-term efficiency of frost-driven erosion by steepening hillslopes.}, language = {en} } @misc{AndersenEgholmFaurschouKnudsenetal.2015, author = {Andersen, Jane Lund and Egholm, David L. and Faurschou Knudsen, Mads and Jansen, John D. and Nielsen, S. B.}, title = {The periglacial engine of mountain erosion}, series = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, journal = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, number = {530}, issn = {1866-8372}, doi = {10.25932/publishup-40965}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-409656}, pages = {16}, year = {2015}, abstract = {With accelerating climate cooling in the late Cenozoic, glacial and periglacial erosion became more widespread on the surface of the Earth. The resultant shift in erosion patterns significantly changed the large-scale morphology of many mountain ranges worldwide. Whereas the glacial fingerprint is easily distinguished by its characteristic fjords and U-shaped valleys, the periglacial fingerprint is more subtle but potentially prevails in some mid- to high-latitude landscapes. Previous models have advocated a frost-driven control on debris production at steep headwalls and glacial valley sides. Here we investigate the important role that periglacial processes also play in less steep parts of mountain landscapes. Understanding the influences of frost-driven processes in low-relief areas requires a focus on the consequences of an accreting soil mantle, which characterises such surfaces. We present a new model that quantifies two key physical processes: frost cracking and frost creep, as a function of both temperature and sediment thickness. Our results yield new insights into how climate and sediment transport properties combine to scale the intensity of periglacial processes. The thickness of the soil mantle strongly modulates the relation between climate and the intensity of mechanical weathering and sediment flux. Our results also point to an offset between the conditions that promote frost cracking and those that promote frost creep, indicating that a stable climate can provide optimal conditions for only one of those processes at a time. Finally, quantifying these relations also opens up the possibility of including periglacial processes in large-scale, long-term landscape evolution models, as demonstrated in a companion paper.}, language = {en} } @article{AndersenEgholmKnudsenetal.2015, author = {Andersen, Jane Lund and Egholm, D. L. and Knudsen, M. F. and Jansen, John D. and Nielsen, S. B.}, title = {The periglacial engine of mountain erosion - Part 1: Rates of frost cracking and frost creep}, series = {Earth surface dynamics}, volume = {3}, journal = {Earth surface dynamics}, number = {4}, publisher = {Copernicus}, address = {G{\"o}ttingen}, issn = {2196-6311}, doi = {10.5194/esurf-3-447-2015}, pages = {447 -- 462}, year = {2015}, abstract = {With accelerating climate cooling in the late Cenozoic, glacial and periglacial erosion became more widespread on the surface of the Earth. The resultant shift in erosion patterns significantly changed the large-scale morphology of many mountain ranges worldwide. Whereas the glacial fingerprint is easily distinguished by its characteristic fjords and U-shaped valleys, the periglacial fingerprint is more subtle but potentially prevails in some mid- to high-latitude landscapes. Previous models have advocated a frost-driven control on debris production at steep headwalls and glacial valley sides. Here we investigate the important role that periglacial processes also play in less steep parts of mountain landscapes. Understanding the influences of frost-driven processes in low-relief areas requires a focus on the consequences of an accreting soil mantle, which characterises such surfaces. We present a new model that quantifies two key physical processes: frost cracking and frost creep, as a function of both temperature and sediment thickness. Our results yield new insights into how climate and sediment transport properties combine to scale the intensity of periglacial processes. The thickness of the soil mantle strongly modulates the relation between climate and the intensity of mechanical weathering and sediment flux. Our results also point to an offset between the conditions that promote frost cracking and those that promote frost creep, indicating that a stable climate can provide optimal conditions for only one of those processes at a time. Finally, quantifying these relations also opens up the possibility of including periglacial processes in large-scale, long-term landscape evolution models, as demonstrated in a companion paper.}, language = {en} } @article{EgholmAndersenKnudsenetal.2015, author = {Egholm, D. L. and Andersen, Jane Lund and Knudsen, M. F. and Jansen, John D. and Nielsen, S. B.}, title = {The periglacial engine of mountain erosion - Part 2: Modelling large-scale landscape evolution}, series = {Earth surface dynamics}, volume = {3}, journal = {Earth surface dynamics}, number = {4}, publisher = {Copernicus}, address = {G{\"o}ttingen}, issn = {2196-6311}, doi = {10.5194/esurf-3-463-2015}, pages = {463 -- 482}, year = {2015}, abstract = {There is growing recognition of strong periglacial control on bedrock erosion in mountain landscapes, including the shaping of low-relief surfaces at high elevations (summit flats). But, as yet, the hypothesis that frost action was crucial to the assumed Late Cenozoic rise in erosion rates remains compelling and untested. Here we present a landscape evolution model incorporating two key periglacial processes - regolith production via frost cracking and sediment transport via frost creep - which together are harnessed to variations in temperature and the evolving thickness of sediment cover. Our computational experiments time-integrate the contribution of frost action to shaping mountain topography over million-year timescales, with the primary and highly reproducible outcome being the development of flattish or gently convex summit flats. A simple scaling of temperature to marine delta O-18 records spanning the past 14 Myr indicates that the highest summit flats in mid-to high-latitude mountains may have formed via frost action prior to the Quaternary. We suggest that deep cooling in the Quaternary accelerated mechanical weathering globally by significantly expanding the area subject to frost. Further, the inclusion of subglacial erosion alongside periglacial processes in our computational experiments points to alpine glaciers increasing the long-term efficiency of frost-driven erosion by steepening hillslopes.}, language = {en} }