Search

Antarctica preserves Earth’s largest ice-sheet, which in response to climate warming, may lose ice mass and raise sea level by several metres. The ice-sheet bed exerts critical controls on dynamic mass loss through feedbacks between water and heat fluxes, topographic forcing, till deformation and basal sliding. Here we show that sedimentary basins may amplify critical feedbacks that are known to impact ice-sheet retreat dynamics. We create a high-resolution subglacial geology classification for Antarctica by applying a supervised machine-learning method to geophysical data, revealing the distribution of sedimentary basins. Hydro-mechanical numerical modelling demonstrates that during glacial retreat, where sedimentary basins exist, the groundwater discharge rate scales with the rate of ice unloading. Antarctica’s most dynamic ice streams, including Thwaites and Pine Island glaciers, possess sedimentary basins in their upper catchments. Enhanced groundwater discharge and its associated feedbacks are likely to amplify basal sliding and increase the vulnerability of these catchments to rapid ice retreat and enhanced dynamic mass loss.

Is groundwater abundant in Antarctica and does it modulate ice flow? Answering this question matters because ice streams flow by gliding over a wet substrate of till. Water fed to ice-stream beds thus influences ice-sheet dynamics and, potentially, sea-level rise. It is recognized that both till and the sedimentary basins from which it originates are porous and could host a reservoir of mobile groundwater that interacts with the subglacial interfacial system. According to recent numerical modelling, up to half of all water available for basal lubrication, and time lags between hydrological forcing and ice-sheet response as long as millennia, may have been overlooked in models of ice flow. Here, we review evidence in support of Antarctic groundwater and propose how it can be measured to ascertain the extent to which it modulates ice flow. We present new seismoelectric soundings of subglacial till, and magnetotelluric and transient electromagnetic forward models of subglacial groundwater reservoirs. We demonstrate that multifaceted and integrated geophysical datasets can detect, delineate and quantify the groundwater contents of subglacial sedimentary basins and, potentially, monitor groundwater exchange rates between subglacial till layers. The paper thus describes a new area of glaciological investigation and how it should progress in future.

We use borehole optical televiewing (OPTV) to explore the internal structure of Larsen C Ice Shelf (LCIS). We report a suite of five ~90 m long OPTV logs, recording a light‐emitting diode‐illuminated, geometrically correct image of the borehole wall, from the northern and central sectors of LCIS collected during austral spring 2014 and 2015. We use a thresholding‐based technique to estimate the refrozen ice content of the ice column and exploit a recently calibrated density‐luminosity relationship to reveal its structure. All sites are dense and strongly influenced by surface melt, with frequent refrozen ice layers and mean densities, between the depths of 1.87 and 90 m, ranging from 862 to 894 kg m−3. We define four distinct units that comprise LCIS and relate these to ice provenance, dynamic history, and past melt events. These units are in situ meteoric ice with infiltration ice (U1), meteoric ice which has undergone enhanced densification (U2), thick refrozen ice (U3), and advected continental ice (U4). We show that the OPTV‐derived pattern of firn air content is consistent with previous estimates, but that a significant proportion of firn air is contained within U4, which we interpret to have been deposited inland of the grounding line. The structure of LCIS is strongly influenced by the E‐W gradient in föhn‐driven melting, with sites close to the Antarctic Peninsula being predominantly composed of refrozen ice. Melting is also substantial toward the ice shelf center with >40% of the overall imaged ice column being composed of refrozen ice. Larsen C Ice Shelf is composed of distinct material units recording its dynamic and melt historyExisting firn air model estimates commonly neglect the presence of buried low‐density continental ice>40% of the firn zone of Larsen C is composed of refrozen ice in its central and northern sectors

Strategies available to evaluate the performance of in situpermeable reactive barriers are currently not well developed and often rely on fluid and media sampling directly from the permeable reactive barrier (PRB). Here, we investigate the utility of the self‐potential (SP)method as a technique to monitor in situPRB performance. Our field study was conducted at in situbiological PRB in Portadown, Northern Ireland, UK, which was emplaced to assist in the remediation of groundwater contamination (e.g., hydrocarbons, ammonia) that resulted from the operations and waste disposal practices of a former gasworks. Borehole SPmeasurements were collected during the injection of contaminant groundwater slugs in an attempt to monitor/detect the response of the microbial activity associated with the breakdown of the added contaminants into the PRB. In addition, an uncontaminated groundwater slug was injected into a different portion of the PRB as a ‘control’ and SPmeasurements were collected for comparison to the SPresponse of the contaminant slugs. The results of the SPsignals due to the contaminant injections show that the magnitude of the response was relatively small (<10 mV) yet showed a consistent decrease during both contaminant injections. The net decrease in SPrecorded during the contaminant injections slowly rebounded to near background values through ~44 hours post‐injection. The SPresponse during the uncontaminated injection showed a slight, albeit negligible (within the margin of error), 1 mV increase in the measured SPsignals, in contrast to the contaminant injections. The results of the SPsignals recorded from the uncontaminated groundwater injection also persisted through a period of ~47 hours after injection but show a net increase in SPrelative to pre‐injection values. Based on the difference in SPresponse between the contaminated and uncontaminated injections, we suggest that the responses are likely to be the result of differences in the chemistry of the injection types (contaminated versus uncontaminated) and in situgroundwater. We argue that the SPsignals associated with the contaminated injections are dominated by diffusion (electrochemical) potential, possibly enhanced by a microbial effect. While the results of our investigation show a consistent SPresponse associated with the contaminant injections that is dominated by diffusional effects, further studies are required in order to better understand the effect of microbial activity on SPsignals and the potential utility for the SPmethod to detect/monitor changes that may be indicative of biological PRB performance.