Four decades of radar-echo sounding: The past, present, and future of radar applications for understanding subglacial environments

<p>Airborne radar sounding observations have been instrumental in understanding subglacial environments and basal processes of ice sheets. Since the advent of analog radar-echo sounding (RES) system in the early 1970s, there have been tremendous innovations in both RES hardware and signal processing techniques. These technological advancements have provided high-resolution ice thickness measurements, improved detection and characterization of subglacial hydrology, as well as improved understanding of basal thermal conditions, bed roughness and geomorphology, and other processes that govern the basal boundary of the polar ice sheets. In this talk, I will provide an overview of the recent developments in radar processing approaches and system designs and highlight some of the new understanding of ice sheet subglacial processes that emerge from these breakthroughs. I will end by discussing areas where future radar applications and discoveries may be possible, including the utilization of machine learning algorithms, space-borne radar missions, and ground-based passive radar platforms to provide long-term monitoring of ice sheet subglacial environments.</p>

Hydrology and biogeochemistry of subglacial environment of Greenland ice sheet

<p>The Greenland ice sheet surface melt has increased substantially in intensity and spatial extent over the recent decades. The rapid migration of melt towards upstream areas of Greenland ice sheet is expected to incur major changes in hydrological behaviour of the ice sheet and outlet glaciers along with changes in export fluxes of carbon, methane, and other nutrient fluxes, which, in turn, will further affect the downstream ecosystem of rivers, fjords and oceans. Subglacial environments are emerging as ecological hotspots, urging detailed understanding of interaction between subglacial hydrology and biogeochemistry. However, due to their inaccessibility, the hydrology and geochemistry of subglacial environment thus far lacks a detailed understanding. As such this area is now the focus of many major projects in Greenland and Antarctica. </p><p> </p><p>Under the NuttI (<strong>Nut</strong>rient fac<strong>t</strong>ories under the <strong>I</strong>ce) project, we aim to develop a hydrological-biogeochemical model framework to investigate seasonal and inter-annual evolution of subglacial hydrology system and quantify carbon and nutrient export from subglacial environments to proglacial rivers. We use the subglacial hydrology model GlaDS (Glacier Drainage System model) to simulate seasonal and interannual evolution of distributed and channelized subglacial water flow while calculating subglacial water storage, residence time, water flux and effective pressure. A subglacial erosion scheme is coupled to the model to calculate physical weathering occurring especially in early melt and peak melt season due to glacier sliding and higher water flow, respectively. All these parameters are used in a geochemical model to quantify subglacial chemical weathering fluxes. The meltwater becomes chemically enriched subglacially and reaches the glacier outlet through subglacial channels. We also intend to further develop the model to investigate processes such as subglacial cycling of silica and production of methane.</p><p> </p><p>We primarily use the coupled model to simulate Leverett glacier, a land-terminating outlet glacier in southwest Greenland which has been well studied with different geophysical measurements and long-term monitoring. The model output is validated with the in-situ measurement of discharge and export fluxes in the proglacial river of the land-terminating glacier. </p>

Repeat Subglacial Lake Drainage and Filling Beneath Thwaites Glacier

<p>Active subglacial lakes have been identified throughout Antarctica, offering a window into subglacial environments and their impact on ice sheet mass balance. We use high-resolution altimetry measurements over the Thwaites Glacier to show that a lake system underwent a second episode of drainage activity in 2017, only four years after another substantial drainage event. Our observations suggest significant modifications of the drainage system between the two events, with 2017 experiencing greater upstream discharge, faster lake-to-lake connectivity, and the transfer of water within a closed system. Measured rates of lake recharge during the inter-drainage period are significantly larger than modelled estimates, suggesting processes which drive subglacial melt production are currently underestimated. Our study highlights new methods of exploring subglacial environments through the application of altimetry, with potential applications for studying subglacial lakes across Antarctica</p>

Iron transport by subglacial meltwater indicated by stable iron isotopes in fjord sediments of King George Island, Antarctica

<p>Polar regions are critical for future climate evolution, and they experience major environmental changes. A particular focus of biogeochemical investigations in these regions lies on iron (Fe). This element drives primary productivity and, thus, the uptake of atmospheric CO<sub>2</sub> in vast areas of the ocean. Due to the Fe-limitation of phytoplankton growth in the Southern Ocean, Antarctica is a key region for studying the change of iron fluxes as glaciers progressively melt away. The respective climate feedbacks can currently hardly be quantified because data availability is low, and iron transport and reaction pathways in Polar coastal and shelf areas are insufficiently understood. We show how novel stable Fe isotope techniques, in combination with other geochemical analyses, can be used to identify iron discharges from subglacial environments and how this will help us assessing short and long term impacts of glacier retreat on coastal ecosystems.</p><p>Stable Fe isotopes (δ<sup>56</sup>Fe) may be used to trace Fe sources and reactions, but respective data availability is low. In addition, there is a need to constrain δ<sup>56</sup>Fe endmembers for different types of sediments, environments, and biogeochemical processes.</p><p>δ<sup>56</sup>Fe data from pore waters and sequentially extracted solid Fe phases at two sites in Potter Cove (King George Island, Antarctica), a bay affected by fast glacier retreat, are presented. Close to the glacier front, sediments contain high amounts of easily reducible Fe oxides and show a dominance of ferruginous conditions compared to sediments close to the ice-free coast, where surficial oxic meltwater discharges and sulfate reduction dominates. We suggest that high amounts of reducible Fe oxides close to the glacier mainly derive from subglacial sources, where Fe liberation from comminuted material beneath the glacier is coupled to biogeochemical weathering. A strong argument for a subglacial source is the predominantly negative δ<sup>56</sup>Fe signature of reducible Fe oxides that remains constant throughout the ferruginous zone. In situ dissimilatory iron reduction (DIR) does not significantly alter the isotopic composition of the oxides. The composition of the easily reducible Fe fraction therefore suggests pre-depositional microbial cycling as it occurs in subglacial environments. Sediments influenced by oxic meltwater discharge show downcore trends towards positive δ<sup>56</sup>Fe signals in pore water and reactive Fe oxides, typical for in situ DIR as <sup>54</sup>Fe becomes less available with increasing depth.</p><p>We found that a quantification of benthic Fe fluxes and subglacial Fe discharges based on stable Fe isotope geochemistry will be complicated because (1) diagenetic processes vary strongly at short lateral distances and (2) the variability of δ<sup>56</sup>Fe in subglacial meltwater has not been sufficiently well investigated yet. However, isotope mass balance models that consider the current uncertainties could, in combination with an application of ancillary proxies, lead to a much better quantification of Fe inputs into polar marine waters than currently available. This would consequently allow a better assessment of the flux and fate of Fe originating from the Antarctic Ice Sheet.</p><p><strong>Henkel et al. (2018)</strong> Diagenetic iron cycling and stable Fe isotope fractionation in Antarctic shelf sediments, King George Island. GCA 237, 320-338.</p>

Possible Transport of Basal Debris to the Surface of a Mid-Latitude Glacier on Mars.

<p><strong>Introduction:</strong> We observe internal flow structures within a viscous flow feature (VFF; 51.24°W, 42.53°S) interpreted as a debris-covered glacier in Nereidum Montes, Mars. The structures are exposed in the wall of a gully that is incised through the VFF, parallel to its flow-direction. They are near to the glacier terminus and appear to connect its deep interior (and possibly its bed) to arcuate flow-transverse foliations on its surface. Such foliations are common on VFF surfaces, but their relation to VFF-internal structures and ice flow is poorly understood. The VFF-internal structures we observe are reminiscent of up-glacier dipping shear structures that transport basal debris to glacier surfaces on Earth.</p><p>Subglacial environments on Mars are of astrobiological interest due to the availability of water ice and shelter from Mars’ surface radiation environment. However, current limitations in drilling technology prevent their direct exploration. If debris on VFF surfaces contains a component of englacial and/or subglacial debris, those materials could be sampled without access to the subsurface. This could reduce the potential cost and complexity of future missions that aim to explore englacial and subglacial environments on Mars.</p><p><strong>Methods: </strong>We use a 1 m/pixel digital elevation model (DEM) derived from 25 cm/pixel High Resolution Imaging Science Experiment (HiRISE) stereo-pair images, and a false-colour (merged IRB) HiRISE image. We measured the dip and strike of the VFF-internal structures using ArcGIS 10.7 and QGIS software. We also input the DEM (and an inferred glacier bed topography derived from it) into ice flow simulations using the Ice Sheet System Model, assuming no basal sliding and present-day mean annual surface temperature (210K).</p><p><strong>Results and Discussion: </strong>The VFF-internal structures dip up-glacier at ~20° from the bed. This is inconsistent with their formation by bed-parallel ice-accumulation layering without modification by ice flow. The VFF-internal structures and surface foliations are spectrally ‘redder’ than adjacent VFF portions, which appear ‘bluer’. This could result from differences in debris concentration and/or surficial dust trapping between the internal structures and the bulk VFF. Modelling experiments suggest that the up-glacier-dipping structures occur at the onset of a compressional regime as ice flow slowed towards the VFF terminus.</p><p>In cold-based glaciers on Earth, up-glacier-dipping folds are common approaching zones of enhanced ice rigidity near the glacier margin. Where multiple folds co-exist, the outermost typically comprises basal ice with a component of subglacial debris entrained in the presence of interfacial films of liquid water at sub-freezing temperatures. In polythermal glaciers, debris-rich up-glacier-dipping thrust faults form where sliding wet-based ice converges with cold-based ice.</p><p><strong>Conclusions: </strong>We propose that the observed up-glacier-dipping VFF-internal structures are englacial shear zones formed by compressional ice flow. They could represent transport pathways for englacial and subglacial material to the VFF surface. The majority of extant mid-latitude VFF on Mars are thought to have been perennially cold-based; thus we favour the hypothesis that the VFF-internal structures are folds formed under a cold-based thermal regime. Under this mechanism, the outermost surface foliation, and its corresponding VFF-internal structure, is the most likely to contain subglacial debris.</p>

Viable cold-tolerant iron-reducing microorganisms in geographically diverse subglacial environments

Subglacial environments are known to harbour metabolically diverse microbial communities. These microbial communities drive chemical weathering of underlying bedrock and influence the geochemistry of glacial meltwater. Despite its importance in weathering reactions, the microbial cycling of iron in subglacial environments, in particular the role of microbial iron reduction, is poorly understood. In this study we address the prevalence of viable iron-reducing microorganisms in subglacial sediments from five geographically isolated glaciers. Iron-reducing enrichment cultures were established with sediment from beneath Engabreen (Norway), Finsterwalderbreen (Svalbard), Leverett and Russell glaciers (Greenland), and Lower Wright Glacier (Antarctica). Rates of iron reduction were higher at 4 °C compared with 15 °C in all but one duplicated second-generation enrichment culture, indicative of cold-tolerant and perhaps cold-adapted iron reducers. Analysis of bacterial 16S rRNA genes indicates Desulfosporosinus were the dominant iron-reducing microorganisms in low-temperature Engabreen, Finsterwalderbreen and Lower Wright Glacier enrichments, and Geobacter dominated in Russell and Leverett enrichments. Results from this study suggest microbial iron reduction is widespread in subglacial environments and may have important implications for global biogeochemical iron cycling and export to marine ecosystems.