Abstract
AbstractLiquid water can exist at temperatures well below freezing beneath glaciers and ice sheets, where subglacial water systems, fresh and saline, have been shown to host unique microbial ecosystems. Geophysical techniques sensitive to fluid-content contrasts, e.g. electromagnetics, can characterize subglacial water and its salinity. Here, we assess the ground-based transient electromagnetic (TEM) method for deriving the resistivity and salinity of subglacial water. We adapt an existing open-source Bayesian inversion algorithm, which uses independent depth constraints, to output posterior distributions of resistivity and pore fluid salinity with depth. A variety of synthetic models, including a thin (5 m), conductive (0.16 Ωm), hypersaline (147 psu) subglacial lake, are used to evaluate the TEM method for imaging under 800 m-thick ice. The study demonstrates that TEM methods can resolve conductive, saline bodies accurately using external depth constraints, for example, from radar or seismic data. The depth resolution of TEM can be limited beneath deep (>800 m), thick (>50 m) conductive, water bodies and additional constraints from passive electromagnetic (EM) methods could be used to reduce ambiguities in the TEM results. Subsequently, non-invasive active and passive EM methods could provide profound insights into remote aqueous systems under glaciers and ice sheets.
Highlights
Liquid water exists beneath glaciers and large ice masses in a variety of different environments from small alpine glaciers to large polar ice sheets (Bell, 2008; Siegert and others, 2018)
We have investigated the use of groundbased transient electromagnetic (TEM) methods for quantifying conductive subglacial water under resistive polar ice sheets
We present the recently developed Bayesian inversion MATLAB tool MuLTI-TEMP for deriving the salinity of subglacial water using Archie’s Law and standard conversions of conductivity to salinity
Summary
Liquid water exists beneath glaciers and large ice masses in a variety of different environments from small alpine glaciers to large polar ice sheets (Bell, 2008; Siegert and others, 2018). Subglacial water can originate from surface water draining to the bed through moulins, crevasses and fractures (Zwally and others, 2002); melting of the ice-sheet base from geothermal heating (Blankenship and others, 1993) and basal friction (Bell, 2008); or influx from underlying aquifers (Christoffersen and others, 2014). This water flows over or through bedrock and sediment, which can provide reaction sites for chemical and biological weathering, with the drainage configuration determining the length of time for water–rock interactions and solute generation. These terrestrial subglacial water systems can be considered as potential analogs for microbial habitats on extraterrestrial frozen planetary bodies, where liquid water has been inferred beneath the ice (Carr and others, 1998; Lauro and others, 2020)
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