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Radar sounding is a powerful geophysical approach for characterizing the subsurface conditions of terrestrial and planetary ice masses at local to global scales. As a result, a wide array of orbital, airborne, ground-based, and in situ instruments, platforms and data analysis approaches for radioglaciology have been developed, applied or proposed. Terrestrially, airborne radar sounding has been used in glaciology to observe ice thickness, basal topography and englacial layers for five decades. More recently, radar sounding data have also been exploited to estimate the extent and configuration of subglacial water, the geometry of subglacial bedforms and the subglacial and englacial thermal states of ice sheets. Planetary radar sounders have observed, or are planned to observe, the subsurfaces and near-surfaces of Mars, Earth's Moon, comets and the icy moons of Jupiter. In this review paper, and the thematic issue of the Annals of Glaciology on 'Five decades of radioglaciology' to which it belongs, we present recent advances in the fields of radar systems, missions, signal processing, data analysis, modeling and scientific interpretation. Our review presents progress in these fields since the last radio-glaciological Annals of Glaciology issue of 2014, the context of their history and future prospects. [ABSTRACT FROM AUTHOR]

Airborne radar sounding can measure conditions within and beneath polar ice sheets. In Antarctica, most digital radar-sounding data have been collected in the last 2 decades, limiting our ability to understand processes that govern longer-term ice-sheet behavior. Here, we demonstrate how analog radar data collected over 40 y ago in Antarctica can be combined with modern records to quantify multidecadal changes. Specifically, we digitize over 400,000 line kilometers of exploratory Antarctic radar data originally recorded on 35-mm optical film between 1971 and 1979. We leverage the increased geometric and radiometric resolution of our digitization process to show how these data can be used to identify and investigate hydrologic, geologic, and topographic features beneath and within the ice sheet. To highlight their scientific potential, we compare the digitized data with contemporary radar measurements to reveal that the remnant eastern ice shelf of Thwaites Glacier in West Antarctica had thinned between 10 and 33% between 1978 and 2009. We also release the collection of scanned radargrams in their entirety in a persistent public archive along with updated geolocation data for a subset of the data that reduces themean positioning error from 5 to 2.5 km. Together, these data represent a unique and renewed extensive, multidecadal historical baseline, critical for observing and modeling ice-sheet change on societally relevant timescales. [ABSTRACT FROM AUTHOR]

Flood-carved landforms across the deglaciated terrain of Victoria Land, East Antarctica, provide convincing geomorphological evidence for the existence of subglacial drainage networks beneath the Antarctic ice sheet, and motivate research into the inaccessible environment beneath the contemporary ice sheet. Through this research, our understanding of Antarctic subglacial hydrology is steadily building, and this paper presents an overview of the current state of knowledge. The conceptualization of subglacial hydrological behaviour was developed at temperate and Arctic glaciers, and is thus less mature in the Antarctic. Geophysical and remote sensing observations have demonstrated that many subglacial lakes form part of a highly dynamic network of subglacial drainage beneath the Antarctic ice sheet. Recent research into subglacial water flows, other than those directly concerned with lakes, has discovered potentially significant impacts on ice stream dynamics, ice sheet mass balance, and supplies of water to the ocean potentially affecting circulation and nutrient productivity. Despite considerable advances in understanding there remain a number of grand challenges that must be overcome in order to improve our knowledge of these subglacial hydrological processes. [ABSTRACT FROM AUTHOR]

Swift, D. A., Sanderson, D. C. W., Nienow, P. W., Bingham, R. G. & Cochrane, I. C. 2010: Anomalous luminescence of subglacial sediment at Haut Glacier d'Arolla, Switzerland - a consequence of resetting at the glacier bed? Boreas, Vol. 40, pp. 446-458. 10.1111/j.1502-3885.2010.00196.x. ISSN 0300-9483. Luminescence has the potential to elucidate glacial geomorphic processes because primary glacial sediment sources and transport pathways are associated with contrasting degrees of exposure to light. Most notably, sediment entrained from extraglacial sources should be at least partially reset, whereas sediment produced by glacial erosion of subglacial bedrock should retain substantial luminescence commensurate with a geological irradiation history. We set out to test the validity of this assumption at Haut Glacier d'Arolla, Switzerland using sediment sampled extraglacially and from the glacier bed. Contrary to our expectations, the subglacial samples exhibited natural signals that were substantially lower than those of other sample groups, and further (albeit limited) analyses have indicated no obvious differences in sample-group luminescence characteristics or behaviour that could account for this observation. For glaciological reasons, we can eliminate the possibilities that the subglacial sediment has been extraglacially reset or exposed in situ to heat or light. We therefore advocate investigation of possible resetting processes related to subglacial crushing and grinding, and speculate that such processes, if more generally present, may enable the dating of subglacially deposited tills using luminescence-based techniques. [ABSTRACT FROM AUTHOR]

This review presents a perspective on recent trends in glacial geomorphological research, which has seen an increasing engagement with investigating glaciation over larger and longer timescales facilitated by advances in remote sensing and numerical modelling. Remote sensing has enabled the visualization of deglaciated landscapes and glacial landform assemblages across continental scales, from which hypotheses of millennial-scale glacial landscape evolution and associations of landforms with palaeo-ice streams have been developed. To test these ideas rigorously, the related goal of imaging comparable subglacial landscapes and landforms beneath contemporary ice masses is being addressed through the application of radar and seismic technologies. Focusing on the West Antarctic Ice Sheet, we review progress to date in achieving this goal, and the use of radar and seismic imaging to assess: (1) subglacial bed morphology and roughness; (2) subglacial bed reflectivity; and (3) subglacial sediment properties. Numerical modelling, now the primary modus operandi of 'glaciologists' investigating the dynamics of modern ice sheets, offers significant potential for testing 'glacial geomorphological' hypotheses of continental glacial landscape evolution and smaller-scale landform development, and some recent examples of such an approach are presented. We close by identifying some future challenges in glacial geomorphology, which include: (1) embracing numerical modelling as a framework for testing hypotheses of glacial landform and landscape development; (2) identifying analogues beneath modern ice sheets for landscapes and landforms observed across deglaciated terrains; (3) repeat-surveying dynamic subglacial landforms to assess scales of formation and evolution; and (4) applying glacial geomorphological expertise more fully to extraterrestrial cryospheres. [ABSTRACT FROM AUTHOR]

Abstract: Glaciated landscapes consist of complex assemblages of landforms resulting from ice flow dynamic regimes and ice-sheet history, superimposed over, and in turn modifying, preglacial topography, lithology and geological structure. Insights into the formation of glaciated landscapes can, in principle, be obtained by analysing modern ice-sheet beds, but terrain analyses beneath modern ice sheets are restricted by the inaccessibility of the bed. It is, however, possible to quantify roughness, the vertical variation of the subglacial interface with horizontal distance, along two-dimensional images of the bed obtained from radio-echo sounding (RES). Here we collate several case studies from Antarctica, where roughness calculations have been used as a glaciological tool to infer basal processes and ice-sheet history over large (>500km2) areas. We present two examples from West Antarctica, which demonstrate the utility of bed roughness in determining the presence and extent of subglacial sediments, glacial dynamics and former ice-sheet size. We also present two examples from East Antarctica, which illustrate how roughness provides knowledge of ice-sheet dynamics in the interior and pre-Quaternary ice-sheet histories. In modern ice-sheet settings, characterising bed roughness along RES tracks has the twin advantages of being relatively simple to calculate while producing informative subsurface data, and is especially powerful at furthering understanding when coupled with knowledge of ice flow from field, satellite and modelling investigations. The technique also offers significant potential for the comparison of modern and former ice-sheet terrains, contributing to an improved understanding of the formation and evolution of glaciated landscapes. [Copyright &y& Elsevier]

In southern South America, well-dated glacial geomorphological records constrain the last glacial cycle across much of the former Patagonian Ice Sheet, but its northeastern sector remains comparatively understudied and unconstrained. This knowledge gap inhibits our understanding of the timing of maximum glacier extent, the duration of the glacial maximum, the onset of deglaciation, and whether asynchronies exist in the behaviour of the former ice sheet with latitude, or with location (east or west) relative to the ice divide. Robust glacial reconstructions from this region are thus required to comprehend the mechanisms driving Quaternary glaciations at the southern mid-latitudes. We here present 10 Be surface exposure ages from five moraine sets along with Bayesian age modelling to reconstruct a detailed chronology of Last Glacial Maximum expansions of the Río Corcovado glacier, a major former ice conduit of northern Patagonia. We find that the outlet glacier reached maximum expansion of the last glacial cycle during the global Last Glacial Maximum at ∼26.5–26 ka, and that at least four subsequent advances/stillstands occurred over a 2–3 ka period, at ∼22.5–22 ka, ∼22–21.5 ka, ∼21–20.5 ka and 20–19.5 ka. The onset of local ice sheet deglaciation likely occurred between 20 and 19 ka. Contrary to several other Patagonian outlet glaciers, including from similar latitudes on the western side of the Andes, we find no evidence for MIS 3/4 advances. Exposure dating of palaeo-shoreline cobbles reconstructing the timing of proglacial lake formation and drainage shifts in the studied region indicate three glaciolacustrine phases characterised by Atlantic-directed drainage. Phase one occurred from 26.4 ± 1.4 ka, phase two between ∼21 and ∼19 ka and phase three between ∼19 ka and ∼16.3 ka. Exposure dating of ice-moulded bedrock in the interior of the cordillera indicates local disintegration of the Patagonian Ice Sheet and the Atlantic-Pacific drainage reversal had occurred by ∼16.3 ka. We find that local Last Glacial Maximum glacier expansions were coeval with Antarctic and southern mid-latitude atmospheric and oceanic cooling signals, but out of phase with local summer insolation intensity. Our results indicate that local Patagonian Ice Sheet deglaciation occurred 1–2 ka earlier than northwestern, central eastern and southeastern Patagonian outlet glaciers, which could indicate high regional Patagonian Ice Sheet sensitivity to warming and drying during the Varas interstade (∼22.5–19.5 ka). • The Río Corcovado glacier experienced five advances during the Last Glacial Maximum. • These advances occurred over a 6–7 ka period, during Marine Isotope Stage 2. • We find no evidence for more extensive local Marine Isotope Stage 3/4 advances. • Local Patagonian Ice Sheet deglaciation started at ∼19–20 ka. • Local Atlantic/Pacific reversal of proglacial lake waters occurred at ∼16.3 ± 0.3 ka. [ABSTRACT FROM AUTHOR]