Seasonal evolution of basal environment conditions of Russell sector, West Greenland, inverted from satellite observation of surface flow

Due to increasing surface melting on the Greenland ice sheet, better constraints on seasonally evolving basal water pressure and sliding speed are required by models. Here we assess the potential of using inverse methods on a dense time series of surface speeds to recover the seasonal evolution of the basal conditions in a well-documented region in southwest Greenland. Using data compiled from multiple satellite missions, we document seasonally evolving surface velocities with a temporal resolution of 2 weeks between 2015 and 2019. We then apply the inverse control method using the ice flow model Elmer/Ice to infer the basal sliding and friction corresponding to each of the 24 surface velocity data sets. Near the margin where the uncertainty in the velocity and bed topography are small, we obtain clear seasonal variations that can be mostly interpreted in terms of an effective-pressure-based hard-bed friction law. We find for valley bottoms or “troughs” in the bed topography that the changes in modelled basal conditions directly respond to local modelled water pressure variations, while the link is more complex for subglacial “ridges” which are often non-locally forced. At the catchment scale, in-phase variations in the water pressure, surface velocities, and surface runoff variations are found. Our results show that time series inversions of observed surface velocities can be used to understand the evolution of basal conditions over different timescales and could therefore serve as an intermediate validation for subglacial hydrology models to achieve better coupling with ice flow models.

The Role of Climate and Bed Topography on the Evolution of the Tasman Glacier Since the Last Glacial Maximum

<p>Mountain glaciers respond to climatic changes by advancing or retreating, leaving behind a potentially powerful record of climate through moraine deposition. Estimates of past climate have been made based on the moraine record alone, using geometrical arguments; however, these methods necessarily ignore the effects of glacier dynamics and bed modification. Here, a one-dimensional coupled mass balance-flowline model is used to place constraints on the climate of the Late-glacial (13.5–11.6 kyr ago) and Last Glacial Maximum (LGM, 28 – 17.5 kyr ago) based on the well-mapped and -dated moraines at Tasman Glacier/Lake Pukaki, South Island, New Zealand. Due to the highly-dynamic nature of the system, distinct longitudinal bed profiles are considered for each of the glaciations modelled; the reconstructions show that terminal overdeepenings are likely present in all bed profiles, and hundreds of metres of sediment has been deposited in the glacier valley since the LGM. Using the coupled model and calculated bed topography, a 2.2°C temperature depression from the present is necessary to reproduce the Lateglacial ice extent, and 7.0°C is required for the early LGM, assuming presentday precipitation. The modelled Late-glacial ice extent is more sensitive to precipitation variability than that during the LGM, but the Tasman Glacier during both periods is primarily driven by temperature changes. While the Tasman Glacier shrank between the early and late LGM, modelling demonstrates that changes in bed topography due to erosion, transport and deposition of sediment are a major driver in reduction of glacier extent; a temperature increase of only 0.1°C is required to cause the transition between the two periods, which may be attributable to interannual, zero-trend climate variability. Thus, the consideration of the coupled glacier-sediment system is critical in accurately reconstructing past climate. Future work focusing on modelling this coupled system, such that the bed profile can evolve interactively with glacier flow, will be critical in better resolving transient events such as the early to late LGM transition.</p>

The Role of Climate and Bed Topography on the Evolution of the Tasman Glacier Since the Last Glacial Maximum

<p>Mountain glaciers respond to climatic changes by advancing or retreating, leaving behind a potentially powerful record of climate through moraine deposition. Estimates of past climate have been made based on the moraine record alone, using geometrical arguments; however, these methods necessarily ignore the effects of glacier dynamics and bed modification. Here, a one-dimensional coupled mass balance-flowline model is used to place constraints on the climate of the Late-glacial (13.5–11.6 kyr ago) and Last Glacial Maximum (LGM, 28 – 17.5 kyr ago) based on the well-mapped and -dated moraines at Tasman Glacier/Lake Pukaki, South Island, New Zealand. Due to the highly-dynamic nature of the system, distinct longitudinal bed profiles are considered for each of the glaciations modelled; the reconstructions show that terminal overdeepenings are likely present in all bed profiles, and hundreds of metres of sediment has been deposited in the glacier valley since the LGM. Using the coupled model and calculated bed topography, a 2.2°C temperature depression from the present is necessary to reproduce the Lateglacial ice extent, and 7.0°C is required for the early LGM, assuming presentday precipitation. The modelled Late-glacial ice extent is more sensitive to precipitation variability than that during the LGM, but the Tasman Glacier during both periods is primarily driven by temperature changes. While the Tasman Glacier shrank between the early and late LGM, modelling demonstrates that changes in bed topography due to erosion, transport and deposition of sediment are a major driver in reduction of glacier extent; a temperature increase of only 0.1°C is required to cause the transition between the two periods, which may be attributable to interannual, zero-trend climate variability. Thus, the consideration of the coupled glacier-sediment system is critical in accurately reconstructing past climate. Future work focusing on modelling this coupled system, such that the bed profile can evolve interactively with glacier flow, will be critical in better resolving transient events such as the early to late LGM transition.</p>

Numerical analysis of hydrodynamics influenced by deformed bed due to near-bank vegetation patch

This study with a 2D hydro-morphological model analyzes hydrodynamics over flat and deformed beds with a near-bank vegetation patch. By varying the patch density, the generalized results show that the hydrodynamics over deformed beds differs a lot from those over flat beds. It is found that the deformed bed topography leads to an apparent decrease in longitudinal velocity and bed shear stress in the open region and longitudinal surface gradient for the entire vegetated reach. However, the transverse flow motion and transverse surface gradient in the region of the leading edge and trailing edge is enhanced or maintained, suggesting the strengthening of secondary flow motion. Interestingly, the deformed bed topography tends to alleviate the horizontal shear caused by the junction-interface horizontal coherent vortices, indicating that the turbulence-induced flow mixing is highly inhibited as the bed is deformed. The interior flow adjustment through the patch for the deformed bed requires a shorter distance, La, which is related to the vegetative drag length, (Cda)−1, with a logarithmic formula (La = 0.4ln[(Cda)−1] + b, with b = 3.83 and 4.03 for the deformed and flat beds). The tilting bed topographic effect in the open region accelerating the flow may account for the quick flow adjustment.

Inverting ice surface elevation and velocity for bed topography and slipperiness beneath Thwaites Glacier

There is significant uncertainty over how ice sheets and glaciers will respond to rising global temperatures. Limited knowledge of the topography and rheology of ice-bed interface is a key cause of this uncertainty, as models show that small changes in the bed can have a large influence on predicted rates of ice loss. Most of our detailed knowledge of bed topography comes from airborne and ground-penetrating radar observations. However, these direct observations are not spaced closely enough to meet the requirements of ice-sheet models, so interpolation and inversion methods are used to fill in the gaps. Here we present the results of a new inversion of surface-elevation and velocity data over Thwaites Glacier, West Antarctica, for bed topography and slipperiness (i.e. the degree of basal slip for a given level of drag). The inversion is based on a steady-state linear perturbation analysis of the shallow-ice-stream equations. The method works by identifying disturbances to surface flow which are caused by obstacles or sticky patches in the bed, and can therefore be applied wherever the shallow-ice-stream equations hold and where surface data are available, even where the ice thickness is not well known. We assess the performance of the inversion for topography with the available radar data. Although the topographic output from the inversion is less successful where the bed slopes steeply, it compares well with radar data from the central trunk of the glacier. This method could therefore be useful as either an independent test of other interpolation methods such as mass conservation and kriging, or as a complementary technique in regions where those techniques fail. We do not have data to allow us to assess the success of the slipperiness results from our inversions, but we provide maps that may guide future seismic data collection across Thwaites Glacier. The methods presented here show significant promise for using high-resolution satellite datasets, calibrated by the sparser field datasets, to generate high resolution bed topography products across the ice sheets, and therefore contribute to reduced uncertainty in predictions of future sea-level rise.

Mapping high-resolution basal topography of West Antarctica from radar data using non-stationary multiple-point geostatistics (MPS-BedMappingV1)

The subglacial bed topography is critical for modeling the evolution of Thwaites Glacier in the Amundsen Sea Embayment (ASE), where rapid ice loss threatens the stability of the West Antarctic Ice Sheet. However, mapping of subglacial topography is subject to high uncertainty. This is mainly because the bed topography is measured by airborne ice-penetrating radar along flight lines with large gaps up to tens of kilometers. Deterministic interpolation approaches do not reflect such spatial uncertainty. While traditional geostatistical simulation can model such uncertainty, it may be difficult to apply because of the significant non-stationary spatial variation of topography over such large surface area. In this study, we develop a non-stationary multiple-point geostatistical approach to interpolate large areas with irregular geophysical data and apply it to model the spatial uncertainty of entire ASE basal topography. We collect 166 high-resolution topographic training images (TIs) to train the gap-filling of radar data gaps, thereby simulating realistic topography maps. The TIs are extensively sampled from deglaciated regions in the Arctic as well as Antarctica. To address the non-stationarity in topographic modeling, we introduce a Bayesian framework that models the posterior distribution of non-stationary training images to the local modeling domain. Sampling from this distribution then provide candidate training images for local topographic modeling with uncertainty, constrained to radar flight line data. Compared to traditional MPS approaches without considering TI sampling, our approach demonstrates significant improvement in the topographic modeling quality and efficiency of the simulation algorithm. Finally, we simulate multiple realizations of high-resolution ASE topographic maps. We use the multiple realizations to investigate the impact of basal topography uncertainty on subglacial hydrological flow patterns.

Deformation motion tracks sliding changes through summer, western Greenland

Surface speeds in Greenland's ablation zone undergo substantial variability on an annual basis which are presumed to mainly be driven by changes in sliding. Yet, meltwater-forced changes in ice–bed coupling can also produce variable deformation motion, which impacts the magnitude of sliding changes inferred from surface measurements and provides important context to flow dynamics. We examine spatiotemporal changes in deformation, sliding and surface velocities over a 2-year period using GPS and a dense network of inclinometers installed in borehole grid drilled in western Greenland's ablation zone. We find time variations in deformation motion track sliding changes through the summer and entire measurement period. A distinct spatial deformation and sliding pattern is also observed within the borehole grid which remains similar during winter and summer flow. We suggest that positively covarying sliding and deformation across seasonal timescales is characteristic of passive areas that are coupled to regions undergoing transient forcing, and the spatial patterns are consistent with variations in the local bed topography. The covarying deformation and sliding result in a 1.5–17% overestimate of sliding changes during summer compared to that inferred from surface velocity changes alone. This suggests that summer sliding increases are likely overestimated in many locations across Greenland.

Geophysical constraints on the properties of a subglacial lake in northwest Greenland

In this study, we report the results of an active-source seismology and ground-penetrating radar survey performed in northwestern Greenland at a site where the presence of a subglacial lake beneath the accumulation area has previously been proposed. Both seismic and radar results show a flat reflector approximately 830–845 m below the surface, with a seismic reflection coefficient of −0.43 ± 0.17, which is consistent with the acoustic impedance contrast between a layer of water and glacial ice. Additionally, in the seismic data we observe an intermittent lake bottom reflection arriving between 14–20 ms after the lake top reflection, corresponding to a lake depth of approximately 10–15 m. A strong coda following the lake top and lake bottom reflections is consistent with a package of lake bottom sediments although its thickness and material properties are uncertain. Finally, we use these results to conduct a first-order assessment of the lake origins using a one-dimensional thermal model and hydropotential modeling based on published surface and bed topography. Using these analyses, we narrow the lake origin hypotheses to either anomalously high geothermal flux or hypersalinity due to local ancient evaporite. Because the origins are still unclear, this site provides an intriguing opportunity for the first in situ sampling of a subglacial lake in Greenland, which could better constrain mechanisms of subglacial lake formation, evolution, and relative importance to glacial hydrology.