Radar evidence of ponded subglacial water in Greenland

Abstract

The thermal state at the bed of a large ice sheet is a critical boundary condition governing its future evolution. Radar surveys provide an opportunity for direct but remote observation of the ice-sheet bed, and therefore offer a means of constraining numerical ice-sheet models at the ice–bed interface. Here we have processed results of radar surveys of the Greenland Ice Sheet undertaken by the Program for Arctic Regional Climate Assessment (PARCA) between 1999 and 2003, to explore this opportunity. We consider the robustness of the measurements in the context of uncertain dielectric losses in the ice sheet, concluding that the observed radar signal characteristics reflect the character of the bed itself rather than that of uncertain englacial absorption. However, the identification of thaw is restricted to areas where subglacial water has sufficient depth to influence the radar reflection. We derive a map of inferred areas of subglacial thaw, and compare our results with other studies predicting regions with temperate bed. We show that in many areas the radar inferences of ponded water lie within areas predicted to be thawed by modelling and radiostratigraphy. There is clear disagreement in certain areas, suggesting the presence of high geothermal flux anomalies.

Subglacial roughness of the Greenland Ice Sheet: relationship with contemporary ice velocity and geology

“…High waveform abruptness (A) values, here normalised across radar sounders as Λ, have when combined with radar bed-echo reflectivity, been used to discriminate basal thermal state where larger, contiguous regions have been associated with bodies of, electrically-deep, water Gogineni, 2008, 2012;Oswald et al, 2018). However, recent comparison alongside ice-core temperature data and a synthesis for the likely basal thermal state (MacGregor et al, 2016) in north-west Greenland,…”

“…Here, using Ω, we map the distribution of roughness anisotropy across the GrIS and assess the relationship between |v| and Ω Bed-echo waveform properties are related to electromagnetic scattering from the glacier bed and, hence, also provide information about subglacial roughness (Oswald and Gogineni, 2008;Oswald et al, 2018;Jordan et al, 2017). Radar bed-echoes range from sharp pulse-like returns (associated with specular reflections from a smooth glacier bed), to echoes that have a trailing edge that extends greatly over the original pulse length (associated with diffuse scattering from a rough glacier bed).…”

Biogeochemical Connectivity Between Freshwater Ecosystems beneath the West Antarctic Ice Sheet and the Sub‐Ice Marine Environment

“…The beds of the Greenland Ice Sheet and AIS store large quantities of liquid water (e.g., Palmer et al, 2013;Siegert et al, 2016) in saturated sediments and sediment cavities. In Greenland, basal melt combined with supraglacial water that is directly transported to the bed (Das et al, 2008;Willis et al, 2015) cause subglacial "ponding" in areas where the ice is not frozen to the bed (Oswald et al, 2018). There are at least 400 subglacial lakes (Siegert et al, 2016) and extensive groundwater at the base of the AIS (Priscu et al, 2008) that are isolated from the surface by the~1 to 4 km of ice.…”

Brief Communication: Outburst floods triggered by periodic drainage of subglacial lakes, Isunguata Sermia, West Greenland

“…Storage of water in firn (Forster et al, 2013), damaged englacial ice (Kendrick et al, 2018) and both supraglacial (Selmes et al, 2011) and subglacial lakes (Palmer et al, 2013;Oswald et al, 2018;Bowling et al, 2019) can delay the drainage of meltwater through the ice sheet to the ocean, while the rapid drainage of stored water can overwhelm the drainage system and perturb ice flow (e.g. Das et al, 2008).…”

“…Siegfried & Fricker, 2018) due to steeper hydraulic gradients, dominance of surface inputs and more efficient subglacial water routing, 1000s of subglacial lakes have been predicted and over 50 identified beneath the Greenland Ice Sheet (Livingstone et al, 2013;Bowling et al, 2019). This includes stable lakes above 40 the Equilibrium Line Altitude (ELA) but away from the interior, hydrologically active lakes near the ELA recharged by surface meltwater, and small seasonally active lakes below the ELA which form during winter and drain during the melt season (Palmer et al, 2013(Palmer et al, , 2015Howat et al, 2013;Willis et al, 2015;Chu et al, 2016;Oswald et al, 2018;Bowling et al, 2019). Landsat 8 OLI satellite images acquired before and after the 2015 ice-surface subsidence (anomaly 2) reveal a major change in the 1.8 km wide proglacial braided river system ( Fig.…”