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Automated Mapping of Glacial Overdeepenings Beneath Contemporary Ice Sheets: Approaches and Potential Applications

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Automated Mapping of Glacial Overdeepenings Beneath Contemporary Ice Sheets: Approaches and Potential Applications This is a repository copy of Automated mapping of glacial overdeepenings beneath contemporary ice sheets: Approaches and potential applications. White Rose Research Online URL for this paper: http://eprints.whiterose.ac.uk/84492/ Version: Accepted Version Article: Patton, H., Swift, D.A., Clark, C.D. et al. (3 more authors) (2015) Automated mapping of glacial overdeepenings beneath contemporary ice sheets: Approaches and potential applications. Geomorphology, 232. 209 - 223. ISSN 0169-555X https://doi.org/10.1016/j.geomorph.2015.01.003 Article available under the terms of the CC-BY-NC-ND licence (https://creativecommons.org/licenses/by-nc-nd/4.0/) Reuse Unless indicated otherwise, fulltext items are protected by copyright with all rights reserved. The copyright exception in section 29 of the Copyright, Designs and Patents Act 1988 allows the making of a single copy solely for the purpose of non-commercial research or private study within the limits of fair dealing. The publisher or other rights-holder may allow further reproduction and re-use of this version - refer to the White Rose Research Online record for this item. Where records identify the publisher as the copyright holder, users can verify any specific terms of use on the publisher’s website. Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing [email protected] including the URL of the record and the reason for the withdrawal request. [email protected] https://eprints.whiterose.ac.uk/ Automated mapping of glacial overdeepenings beneath contemporary ice sheets: approaches and potential applications Henry Pattona,b,1,†, Darrel A. Swifta,*,†, Chris D. Clarka, Stephen J. Livingstonea, Simon J. Cookc, Alun Hubbardb aDepartment of Geography, University of Sheffield, Winter Street, Sheffield, S10 2TN, U bDepartment of Geology, University of Troms! " The Arctic University of Norway, N-9037 Troms!, Norway cSchool of Science and the Environment, Manchester Metropolitan University, ,hester Street, Manchester, M1 5GD, U 1Present address. Department of Geology, University of Troms! " The Arctic University of Norway, N-9037 Troms!, Norway / ,orresponding author. Email d.a.swift2sheffield.ac.u3. Telephone 444 114 222 7959. †These authors contributed e6ually to this wor3. Abstract Awareness is growing of the significance of overdeepenings in ice sheet systems. However, a complete understanding of overdeepening formation is lacking, meaning observations of overdeepening location and morphometry are urgently required to motivate process understanding. Subject to the development of appropriate mapping approaches, high resolution subglacial topography data sets covering the whole of Antarctica and Greenland offer significant potential to acquire such observations and to relate overdeepening characteristics to ice sheet parameters. We explore a possible method for mapping overdeepenings beneath the Antarctic and Greenland ice sheets and illustrate a potential application of this approach by testing a possible relationship between overdeepening elongation ratio and ice sheet flow velocity. We find that hydrological and terrain filtering approaches are unsuited to mapping overdeepenings and develop a novel rule-based GIS methodolo y that delineates overdeepenin perimeters by analysis of closed-contour properties. (e then develop GIS procedures that provide information on overdeepenin morpholo y and topo raphic conte)t. Limitations in the accuracy and resolution of bed-topo raphy data sets mean application to laciolo ical problems requires consideration of quality-control criteria to ,a- remove potentially spurious depressions and ,b- reduce uncertainties that arise from the inclusion of depressions of non lacial ori in, or those in re ions where empirical data are sparse. .o address the problem of overdeepenin elon ation, potential quality control criteria are introduced/ and discussion of this e)ample serves to hi hli ht the limitations that mappin approaches 0 and applications of such Page 1 of 41 approaches 0 must confront. (e predict that improvements in bed-data quality will reduce the need for quality control procedures and facilitate increasin ly robust insi hts from empirical data. eywords. overdeepenin s/ automated landform mappin / lacial erosion/ landscape evolution/ Antarctica/ Greenland 1. Introduction .he mechanisms by which laciers and ice sheets form spectacular alpine and fjord landscapes are well known, and such landscapes have been e)ploited widely for purposes of palaeo laciolo y and process understandin ,e. ., Glasser and 1ennett, 2004-. Recently, this understandin has been aided by implementation of simple ice-erosion laws within numerical models, which are able to simulate patterns of lacial incision with compellin success ,e. ., Harbor, 1992/ 7acGre or et al., 2000/ 8essler et al., 2008-. However, the mechanisms that produce overdeepenin s ,:i . 1- remain unclear, and the implementation of candidate processes within ice-erosion models has met with limited success ,e. ., E holm et al., 2012-. An important constraint on the development of process understandin is the absence of quantitative studies that have characterised overdeepenin distribution and morpholo y. .his contrasts sharply with the advances that have taken place with re ards to the understandin of other sub lacial phenomena, such as drumlins ,e. ., Clark et al., 2009-. In particular, improvements in remote sensin mean these studies have provided robust morpholo ical data that has motivated development and testin numerical models that simulate ice sheet and landscape evolution processes ,e. ., :owler, 2009, 2010/ :owler et al., in press-. Cook and Swift ,2012- have ar ued that the laciolo ical si nificance of the reverse-bed radient that occurs in the presence of an overdeepenin ,:i . 1C- is such that a complete understandin of overdeepenin formation and morpholo y is essential to elucidate and understand critical ice-bed processes and to inform predictions of past and present ice-mass behaviour. :irst, empirical ,e. ., Iverson et al., 1995- and theoretical work ,e. ., R=thlisber er and Lan , 1987/ Creyts and Clarke, 2010- demonstrates that overdeepenin s raise sub lacial water pressures by reducin draina e system transmissivity, thus affectin ice-bed couplin and basal slidin . Second, the amplitude and wavelen th of topo raphy at the ice sheet bed is of fundamental si nificance to ice sheet slidin laws ,Schoof, 2005-/ and third, marine outlet systems that terminate on adverse slopes are vulnerable to rapid ?collapse’ ,Schoof, 2007/ Aick et al., 2009-. :urther, the processes of overdeepenin evolution are poorly understood. Cook and Swift ,2012- proposed that patterns of sub lacial erosion and deposition, which are dictated by the ice surface radient ,cf. Hooke, 1991/ Alley et al., 2003-, mean all lacier and ice sheet beds should tend toward a uniformly overdeepened lon -profile, with deepest Page 2 of 41 erosion near the lon -term avera e equilibrium line altitude ,ELA- ,e. ., 1oulton, 199C/ Hallet et al., 199C/ Anderson et al., 200C-. However, other observations indicate that ice-water-sediment-erosion ,I(SE- feedbacks produce very localised, deep lacial erosion ,e. ., Hooke, 1991/ Alley et al., 2003- and may therefore act as an important constraint on overdeepenin siDe and morpholo y ,Cook and Swift, 2012-. .he increasin availability of hi h-resolution sub lacial topo raphy data sets for the e)tant ice sheets ,e. ., 1amber et al., 2013/ :retwell et al., 2013- indicates potential now e)ists to achieve major advances. :or e)ample, the ability to relate morpholo ical characteristics to ice sheet parameters means that e)pectations of overdeepenin influence on ice dynamics are eminently testable. :urther, parameterisation of I(SE processes in numerical ice-erosion models that couple the flow of ice, water, and sediment ,e. ., E holm et al., 2012- 0 and increasin ly sophisticated treatments of the ice flow ,E holm et al., 2011-, erosion ,e. ., Iverson, 2012-, and water flow ,e. ., (erder et al., 2013- 0 will benefit from quantitative data on overdeepenin location and morpholo y that facilitates evaluation of model-based landscape evolution simulations. (e therefore develop a computationally efficient GIS-based method for mappin overdeepenin s, and, in anticipation of the data that may be required to test empirical relationships and model-based outputs, we develop additional methods that enable characterisation of overdeepenin shape, includin lon - and cross-profiles that pass throu h the deepest point. .hese methods require delimitation of the overdeepenin perimeter and identification of in- and outflow points, and also require identification of overdeepenin s within overdeepenin s ,i.e., nested overdeepenin s/ :i . 1D-. :rom these phenomena, further metrics 0 includin elon ation ratio ,cf. Clark et al., 2009-, normal and adverse slope len ths and radients, and planform area 0 can then be derived ,:i . 2-. .o illustrate a possible application of our method, we seek to evaluate a probable relationship between overdeepenin elon ation ratio and ice velocity usin sub lacial topo raphy data sets and ice surface velocity data for the Antarctic and Greenland ice sheets. Such applications are limited by the quality of the metrics that present data sets permit, and we use this e)ample to motivate ,i- the development of quality control criteria appropriate to current data set uncertainties and ,ii- a discussion of the sources of uncertainty
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