286 research outputs found
The precision of radar-derived subglacial bed topography: a case study from Pine Island Glacier, Antarctica
Recent advances in the measurement of bedforms beneath active ice streams have been made using ground-based grid profiling using impulse radar systems operating with centre frequencies in the 3–5 MHz range. Surveys of Rutford Ice Stream and Pine Island Glacier have shown that features such as mega-scale glacial lineations with topographic relief of as little as 3 m can be traced for many kilometres downstream under more than 2 km of fast-moving ice. In the discussion of these data, it is often asked ‘How is it possible to map such fine-scale topography with such a low-frequency radar’. In answering that question, the key point is the distinction between the precision of a radar range measurement to a single, isolated reflective interface and the ability to resolve the presence of two closely-spaced interfaces of similar reflectivity (commonly referred to as the vertical resolution). This paper will discuss and illustrate this distinction and use the case study of data acquired over Pine Island Glacier to examine the limits of precision of the radar range measurement
Constraining the recent mass balance of Pine Island and Thwaites glaciers, West Antarctica, with airborne observations of snow accumulation
In Antarctica, uncertainties in mass input and output translate directly into uncertainty in glacier mass balance and thus in sea level impact. While remotely sensed observations of ice velocity and thickness over the major outlet glaciers have improved our understanding of ice loss to the ocean, snow accumulation over the vast Antarctic interior remains largely unmeasured. Here, we show that an airborne radar system, combined with ice-core glaciochemical analysis, provide the means necessary to measure the accumulation rate at the catchment-scale along the Amundsen Sea coast of West Antarctica. We used along-track radar-derived accumulation to generate a 1985–2009 average accumulation grid that resolves moderate- to large-scale features (>25 km) over the Pine Island–Thwaites glacier drainage system. Comparisons with estimates from atmospheric models and gridded climatologies generally show our results as having less accumulation in the lower-elevation coastal zone but greater accumulation in the interior. Ice discharge, measured over discrete time intervals between 1994 and 2012, combined with our catchment-wide accumulation rates provide an 18-year mass balance history for the sector. While Thwaites Glacier lost the most ice in the mid-1990s, Pine Island Glacier's losses increased substantially by 2006, overtaking Thwaites as the largest regional contributor to sea-level rise. The trend of increasing discharge for both glaciers, however, appears to have leveled off since 2008
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Inversion of IceBridge gravity data for continental shelf bathymetry beneath the Larsen Ice Shelf, Antarctica
A possible cause for accelerated thinning and break-up of floating marine ice shelves is warming of the water in the cavity below the ice shelf. Accurate bathymetry beneath large ice shelves is crucial for developing models of the ocean circulation in the sub-ice cavities. A grid of free-air gravity data over the floating Larsen C ice shelf collected during the IceBridge 2009 Antarctic campaign was utilized to develop the first bathymetry model of the underlying continental shelf. Independent control on the continental shelf geologic structures from marine surveys was used to constrain the inversion. Depths on the continental shelf beneath the ice shelf estimated from the inversion generally range from about 350 to 650 m, but vary from 1000 m. Localized overdeepenings, 20–30 km long and 900–1000 m deep, are located in inlets just seaward of the grounding line. Submarine valleys extending seaward from the overdeepenings coalesce into two broad troughs that extend to the seaward limit of the ice shelf and appear to extend to the edge of the continental shelf. The troughs are generally at a depth of 550–700 m although the southernmost mapped trough deepens to over 1000 m near the edge of the ice shelf just south of 68°S. The combination of the newly determined bathymetry with published ice-draft determinations based on laser altimetry and radar data defines the geometry of the water-filled cavity. These newly imaged troughs provide a conduit for water to traverse the continental shelf and interact with the overlying Larsen C ice shelf and the grounding lines of the outlet glaciers
Revealing the former bed of Thwaites Glacier using sea-floor bathymetry: Implications for warm-water routing and bed controls on ice flow and buttressing
Abstract. The 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.
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Geostatistical methods for improved quantification of ice mass bed topography
Contribution to global mean sea level rise by ice sheets, ice caps and glaciers is accelerating. The total volume of water stored globally in terrestrial ice is estimated by a multitude of methods but principally by the interpolation of icethickness data. For the ice sheets and large Arctic ice caps, ice thickness is predominantly measured by airborne radio-echo sounding surveys which use radio waves to detect the bed of the surveyed ice mass. While such surveys are now extensive, large portions of ice masses are generally unsurveyed due to their size. In order to quantify ice thickness and subsequently ice volume over the entirety of an ice mass, interpolation of the input measurements is used. Throughout this whole process, uncertainties arise. Initially, from the radio-echo sounding (RES) survey and subsequently, in the interpolation. Compounding this is the absence of ground-truthing for measurements and interpolations due to the inaccessibility of ice mass beds. Hence, there is a requirement to find alternative means of quantifying uncertainty in ice thickness measurements and subsequently derived bed topography, and analyses made from these data to reduce the uncertainty in sea level change projections. This thesis develops and applies methods which aim to reduce uncertainty in ice thickness and bed topography datasets. Using high-resolution elevation data, this study exploits the likely similarity between currently ice-covered topography and formerly glaciated topography in the Arctic to generate datasets which provide alternative validation for ice mass bed topography. For the first time topographic error in RES surveying is quantified and corrections are formulated for treating future and historic ice thickness and bed topography data. Additionally, the propagation of these uncertainties through interpolations of bed topography is quantified and reduced, focussing on the Greenland Ice Sheet. Finally, the full suite of methods is applied to ice caps in the Canadian Arctic to generate, for the first time, ice cap wide topography for ice caps in the region that hold approximately a third of the freshwater outside of the continental ice sheets. By quantifying and reducing uncertainty in datasets of bed topography and ice thickness this thesis assesses the perceived stability of the continental ice sheets and large ice Arctic ice caps. From this, the implications of this for near and far term global mean sea-level rise are investigated
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Flow Characteristics, Setting, and Basal Boundary Condition of North Greenland Outlet Glaciers
Temporal variability in the partitioning of mass loss from the Greenland Ice Sheet suggests a complex response to recent climatic change. The contribution to sea level rise from tidewater glaciers is also spatially variable across the ice sheet, and observations reveal their contrasting behaviour at a regional scale. Whilst ice-ocean interaction may explain their dynamics in the south of Greenland, the warm subtropical waters responsible for this extend only to mid-latitudes. In this study, the geometric setting and bed condition is therefore investigated to better understand the flow characteristics and dynamics of high-latitude tidewater glaciers in Greenland. New ice thickness measurements are combined with a novel mass conservation method to reconstruct high resolution (<300 m) bed topographies for 4 tidewater glaciers in northern Greenland. A 3D inversion using control methods is then applied to each glacier with the full-stokes model, Elmer/Ice, to calculate basal drag. The bed topographies reveal that these glaciers are underlain by deeply incised channels, which extend more inland and descend further below present sea level than previously thought. The results of the inversions show spatially variable drag across the beds of these glaciers, but highlight a close relationship with local changes in bed elevation. This may be attributed to form drag by topographic highs, or the infilling of depressions with soft sediment and water. Regardless, the results of this study imply that these glaciers are far more vulnerable to future sea level rise than previously recognised, and may themselves contribute significantly to this in response to a relatively small, future perturbation
Bed properties and three-dimensional topography from radar at Rutford Ice Stream, West Antarctica
Outlet glaciers and ice streams of the Antarctic Ice Sheet provide dis-charge pathways, transporting >90% of the continents ice into the oceans. Elongated landforms beneath fast flowing ice streams form as a result of ice-bed interactions. Understanding their link to ice flow dynamics will better inform subglacial processes and allow these processes to be correctly implemented in predictive numerical flow models, thus improving predictions of future contributions to sea level rise. In this thesis, a section of the bed of Rutford Ice Stream (West Antarctica), containing numerous elongated subglacial landforms, was analysed using a suite of 2D and 3D radar data with repeat surveys. Bed properties vary spatially over a 100 m scale and imply the pat-tern of inferred basal motion in this area is more complex, and basal sliding dominated areas are more extensive, than previously assumed. Local erosion rates are high (1 m/a), indicating a mobile bed, whereas most of the bed shows no temporal change, implying stability of the basal environment. Observations of landforms shortening, and previ-ous observations of landforms extending, highlight that landforms are a non-static part of the bed. Isolated landforms appear to consist of a more rigid sediment at their upstream end with softer sediment down-stream. Some landforms contain a water body (up to 10 km length) along their crest. 3D processed data reveal a so far unseen moat (de-pression) around one landform. Dimensions of the upstream part of the moat are comparable to dimensions of the upstream end of the landform (<50 m height, <300 m width). Observations suggest land-forms are depositional features, while the moat was likely eroded. The radar and other data analysed provide detailed landform and moat ar-chitecture, at a resolution comparable to digital elevation models of deglaciated terrain, and together with interpreted properties give a solid basis for testing existing landform formation theories
Bed topography of Jakobshavn Isbrae, Greenland, and Byrd Glacier, Antarctica
This is the published version. Copyright 2015 International Glaciological SocietyThis paper presents the bed topography of Jakobshavn Isbrae, Greenland, and Byrd Glacier, Antarctica, derived from sounding these glaciers with high-sensitivity radars. To understand the processes causing the speed-up and retreat of outlet glaciers, and to enable the development of next-generation ice-sheet models, we need information on bed topography and basal conditions. To this end, we performed measurements with the progressively improved Multichannel Coherent Radar Depth Sounder/Imager (MCoRDS/I). We processed the data from each antenna-array element using synthetic aperture radar algorithms to improve radar sensitivity and reduce along-track surface clutter. We then applied array and image-processing algorithms to extract the weak bed echoes buried in off-vertical scatter (cross-track surface clutter). At Jakobshavn Isbrae, we observed 2.7 km thick ice ∼30 km upstream of the calving front and ∼850 m thick ice at the calving front. We also observed echoes from multiple interfaces near the bed. We applied the MUSIC algorithm to the data to derive the direction of arrival of the signals. This analysis revealed that clutter is dominated by the ice surface at Jakobshavn Isbrae. At Byrd Glacier, we found ∼3.62 km thick ice, as well as a subglacial trench ∼3.05 km below sea level. We used ice thickness information derived from radar data in conjunction with surface elevation data to generate bed maps for these two critical glaciers. The performance of current radars must be improved further by ∼15 dB to fully sound the deepest part of Byrd Glacier. Unmanned aerial systems equipped with radars that can be flown over lines spaced as close as 5 m apart in the cross-track direction to synthesize a two-dimensional aperture would be ideal for collecting fine-resolution data over glaciers like Jakobshavn near their grounding lines
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