Browsing by Author "Hogan, Kelly A."
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Item Morphometry of bedrock meltwater channels on Antarctic inner continental shelves: Implications for channel development and subglacial hydrology(Elsevier, 2020) Kirkham, James D.; Hogan, Kelly A.; Larter, Robert D.; Arnold, Neil S.; Nitsche, Frank O.; Kuhn, Gerhard; Gohl, Karsten; Anderson, John B.; Dowdeswell, Julian A.Expanding multibeam bathymetric data coverage over the last two decades has revealed extensive networks of submarine channels incised into bedrock on the Antarctic inner continental shelf. The large dimensions and prevalence of the channels implies the presence of an active subglacial hydrological system beneath the past Antarctic Ice Sheet which we can use to learn more about inaccessible subglacial processes. Here, we map and analyse over 2700 bedrock channels situated across >100,000 km2 of continental shelf in the western Antarctic Peninsula and Amundsen Sea to produce the first inventory of submarine channels on the Antarctic inner continental shelf. Morphometric analysis reveals highly similar distributions of channel widths, depths, cross-sectional areas and geometric properties, with subtle differences between channels in the western Antarctic Peninsula compared to those in the Amundsen Sea. At 75–3400 m wide, 3–280 m deep, 160–290,000 m2 in cross-sectional area, and typically 8 times as wide as they are deep, the channels have similar morphologies to tunnel valleys and meltwater channel systems observed from other formerly glaciated landscapes despite differences in substrate geology and glaciological regime. We propose that the Antarctic bedrock channels formed over multiple glacial cycles through the episodic drainage of at least 59 former subglacial lakes identified on the inner continental shelf.Item Rapid retreat of Thwaites Glacier in the pre-satellite era(Springer Nature, 2022) Graham, Alastair G.C.; Wåhlin, Anna; Hogan, Kelly A.; Nitsche, Frank O.; Heywood, Karen J.; Totten, Rebecca L.; Smith, James A.; Hillenbrand, Claus-Dieter; Simkins, Lauren M.; Anderson, John B.; Wellner, Julia S.; Larter, Robert D.Understanding the recent history of Thwaites Glacier, and the processes controlling its ongoing retreat, is key to projecting Antarctic contributions to future sea-level rise. Of particular concern is how the glacier grounding zone might evolve over coming decades where it is stabilized by sea-floor bathymetric highs. Here we use geophysical data from an autonomous underwater vehicle deployed at the Thwaites Glacier ice front, to document the ocean-floor imprint of past retreat from a sea-bed promontory. We show patterns of back-stepping sedimentary ridges formed daily by a mechanism of tidal lifting and settling at the grounding line at a time when Thwaites Glacier was more advanced than it is today. Over a duration of 5.5 months, Thwaites grounding zone retreated at a rate of >2.1 km per year—twice the rate observed by satellite at the fastest retreating part of the grounding zone between 2011 and 2019. Our results suggest that sustained pulses of rapid retreat have occurred at Thwaites Glacier in the past two centuries. Similar rapid retreat pulses are likely to occur in the near future when the grounding zone migrates back off stabilizing high points on the sea floor.Item Revealing the former bed of Thwaites Glacier using sea-floor bathymetry: implications for warm-water routing and bed controls on ice flow and buttressing(Copernicus Publications, 2020) Hogan, Kelly A.; Larter, Robert D.; Graham, Alastair G.C.; Arthern, Robert; Kirkham, James D.; Minzoni, Rebecca Totten; Jordan, Tom A.; Clark, Rachel; Fitzgerald, Victoria; Wåhlin, Anna K.; Anderson, John B.; Hillenbrand, Claus-Dieter; Nitsche, Frank O.; Simkins, Lauren; Smith, James A.; Gohl, Karsten; Arndt, Jan Erik; Hong, Jongkuk; Wellner, JuliaThe geometry of the sea floor immediately beyond Antarctica's marine-terminating glaciers is a fundamental control on warm-water routing, but it also describes former topographic pinning points that have been important for ice-shelf buttressing. Unfortunately, this information is often lacking due to the inaccessibility of these areas for survey, leading to modelled or interpolated bathymetries being used as boundary conditions in numerical modelling simulations. At Thwaites Glacier (TG) this critical data gap was addressed in 2019 during the first cruise of the International Thwaites Glacier Collaboration (ITGC) project. We present more than 2000 km2 of new multibeam echo-sounder (MBES) data acquired in exceptional sea-ice conditions immediately offshore TG, and we update existing bathymetric compilations. The cross-sectional areas of sea-floor troughs are under-predicted by up to 40 % or are not resolved at all where MBES data are missing, suggesting that calculations of trough capacity, and thus oceanic heat flux, may be significantly underestimated. Spatial variations in the morphology of topographic highs, known to be former pinning points for the floating ice shelf of TG, indicate differences in bed composition that are supported by landform evidence. We discuss links to ice dynamics for an overriding ice mass including a potential positive feedback mechanism where erosion of soft erodible highs may lead to ice-shelf ungrounding even with little or no ice thinning. Analyses of bed roughnesses and basal drag contributions show that the sea-floor bathymetry in front of TG is an analogue for extant bed areas. Ice flow over the sea-floor troughs and ridges would have been affected by similarly high basal drag to that acting at the grounding zone today. We conclude that more can certainly be gleaned from these 3D bathymetric datasets regarding the likely spatial variability of bed roughness and bed composition types underneath TG. This work also addresses the requirements of recent numerical ice-sheet and ocean modelling studies that have recognised the need for accurate and high-resolution bathymetry to determine warm-water routing to the grounding zone and, ultimately, for predicting glacier retreat behaviour.