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Geophysical Investigations of Marie Byrd Land Lithospheric Evolution (GIMBLE) The PIs propose to use airborne geophysics to provide detailed geophysical mapping over the Marie Byrd Land dome of West Antarctica. They will use a Basler equipped with advanced ice penetrating radar, a magnetometer, an airborne gravimeter and laser altimeter. They will test models of Marie Byrd Land lithospheric evolution in three ways: 1) constrain bedrock topography and crustal structure of central Marie Byrd Land for the first time; 2) map subglacial geomorphology of Marie Byrd Land to constrain landscape evolution; and 3) map the distribution of subglacial volcanic centers and identify active sources. Marie Byrd Land is one of the few parts of West Antarctica whose bedrock lies above sea level; as such, it has a key role to play in the formation and decay of the West Antarctic Ice Sheet (WAIS), and thus on eustatic sea level change during the Neogene. Several lines of evidence suggest that the topography of Marie Byrd Land has changed over the course of the Cenozoic, with significant implications for the origin and evolution of the ice sheet. Two seasons were flown. ICP5 operated from Byrd Camp using Basler C-GJKB and the HiCARS2 radar in January 2013, and ICP6 operated from WAIS Divide Camp using Basler C-FMKB and the MARFA radar in late 2014, both supported by the US Antarctic Program and Kenn Borek Air. ICP6 experienced issues with data overflow on the MARFA system, with resulted in missing radar records and timing ambiguities. GIMBLE data can be found at https://www.usap-dc.org/view/project/p0000435. Dataset organization Transects are provided a P/S/T nomenclature, organized by the Project they are flying in, the acquisition System (typically named after the aircraft) and the Transect within the Project. Transects were collected in preplanned systems with the following parameters: MBL corridor (MBL/MKB##/X|Y###) rotated from the EPSG:3031 polar stereographic projection at 61.75 degrees and separated by 7.5 km in the Y direction and 5 km in the X direction, with an origin of X -579.6 km and Y -803.3 km Untargeted transit lines used the name of the expedition (ICP5|ICP6) as the project, and used the flight and the increment within the flight to name the Transect (eg (ICP6/MKB2l/F19T01a). Processing These data represent focused VHF radargrams. The data are from the HiCARS2/MARFA radar system, a 60 MHz ice penetrating radar system that has operated in several different guises over the years. HiCARS2/MARFA operates with a 1 microsecond chirp with a design bandwidth of 15 MHz, allowing for ~8 range resolution. The record rate after onboard stacking is 200 Hz. High and low gain channels are collected from antennas on each side of the aircraft, for MARFA the antennas are recorded separately. In ground processing, the data was processed using focusing SAR over a range delay of 100 nsec following Peters et al, 2007 (doi:10.1109/TGRS.2007.897416). Where data loss in ICP6 prevented the generating of focused data, simpler unfocused 'pik1' data was substituted, with 10 coherent stakes and 5 incoherent stacks. Data format These data collection represents georeferenced, time registered instrument measurements (L1B data) converted to SI units. The data format are netCDF3 files, following the formats used for NASA/AAD/UTIG's ICECAP/OIB project at NASA's NSIDC DAAC (10.5067/0I7PFBVQOGO5). Metadata fields can be accessed using the open source ncdump tool, or c, python or matlab modules. A Keyhole Metadata Language (KML) file with geolocation for all transects is also provided. See https://www.loc.gov/preservation/digital/formats/fdd/fdd000330.shtml for resources on NetCDF-3, and https://nsidc.org/data/IR2HI1B/versions/1 for a description of the similar OIB dataset. Acknowledgements This field work was supported by NSF grant 1043761 to Young; ICP5 aircraft lease costs were supported by NASA Operation Ice Bridge grant NNX11AD33G. Data processing costs were supported by a gift from the G. Unger Vetlesen Foundation and the Open Polar Radar project (NSF grant 2127606) 

Abstract. The discovery of Antarctica's deepest subglacial troughbeneath the Denman Glacier, combined with high rates of basal melt at thegrounding line, has caused significant concern over its vulnerability toretreat. Recent attention has therefore been focusing on understanding thecontrols driving Denman Glacier's dynamic evolution. Here we consider theShackleton system, comprised of the Shackleton Ice Shelf, Denman Glacier,and the adjacent Scott, Northcliff, Roscoe and Apfel glaciers, about whichalmost nothing is known. We widen the context of previously observed dynamicchanges in the Denman Glacier to the wider region of the Shackleton system,with a multi-decadal time frame and an improved biannual temporal frequencyof observations in the last 7 years (2015–2022). We integrate newsatellite observations of ice structure and airborne radar data with changesin ice front position and ice flow velocities to investigate changes in thesystem. Over the 60-year period of observation we find significant riftpropagation on the Shackleton Ice Shelf and Scott Glacier and notablestructural changes in the floating shear margins between the ice shelf andthe outlet glaciers, as well as features indicative of ice with elevatedsalt concentration and brine infiltration in regions of the system. Over theperiod 2017–2022 we observe a significant increase in ice flow speed (up to50 %) on the floating part of Scott Glacier, coincident with small-scalecalving and rift propagation close to the ice front. We do not observe anyseasonal variation or significant change in ice flow speed across the restof the Shackleton system. Given the potential vulnerability of the system toaccelerating retreat into the overdeepened, potentially sediment-filledbedrock trough, an improved understanding of the glaciological,oceanographic and geological conditions in the Shackleton system arerequired to improve the certainty of numerical model predictions, and weidentify a number of priorities for future research. With access to theseremote coastal regions a major challenge, coordinated internationallycollaborative efforts are required to quantify how much the Shackletonregion is likely to contribute to sea level rise in the coming centuries.