Geophysical Identification of Subsurface Water in the McMurdo Dry Valleys Region, Antarctica

In Antarctica, basic information about the shallow subsurface is difficult to determine due to logistical challenges, environmental concerns, and cover by ice and permafrost. The relative obscurity of the subsurface complicates answering basic geological questions such as:• Is there water at depth? Does it harbor life? What is its origin? (Chapter 1)• What is the chemistry of subsurface waters? Is there a hydrological and chemical connection across the landscape? (Chapter 2)• How does heat flow through the subsurface? Where does melt occur? (Chapter 3)In all of these questions, liquid water is a crucial component. Antarctica is unique among the continents in that nearly all its water is in the solid phase as ice. Even so, liquid water is widely found across the continent in the few places where pressure, heat flux and salinity permit the liquid phase. Liquid water and water ice typically have a strong electrical conductivity contrast due to water’s ability to serve as a solvent with electrolytic properties. In the McMurdo Dry Valleys (MDV), a largely ice-free coastal zone, water typically contains a high solute load and so this contrast is exceptionally high (Mikucki et al, 2015). The dissolved salts enable water to remain liquid at temperatures well below 0℃ (Lyons et al, 2019).We exploited this geophysical contrast to distinguish conductive subsurface brines from resistive ice, permafrost, and bedrock. We used a helicopter-borne Time Domain Electromagentic (TEM) system to measure resistivity in the shallow subsurface, ca. <500m below surface, in an initial 2011 survey (Mikucki et al, 2015; Foley et al 2015) and a follow up 2018 survey with an improved sensor (Foley et al, 2019). The TEM system works by inducing secondary EM currents in subsurface conductive materials; the decay of these currents is measured with a magnetometer. We inverted the recorded secondary magnetic field transients for a conductive layer model of the subsurface (see Auken et al, 2009; and Chapter 1). With this approach we mapped the transition from the frozen water ice near the Earth’s surface to liquid water at depth.We used this transition to map the subsurface brine’s extent, and to estimate basic properties like salinity, temperature, and flux. In Chapter 1, we demonstrate that that groundwater is widely present at depths as shallow at 200m and temperatures around -10℃ or warmer. The subsurface liquid water appears to be connected across wide areas in the MDV and potentially to a larger East Antarctic aquifer. In the MDV, it could possibly be connected to surface waters through sub-lake taliks, and it may also be connected to the ocean. In Chapter 2, improved data quality and a focus on the coastal zone all us to explore Darcian flow of this aquifer to the ocean. This pressure head gradient driven flow is just one of the suite of submarine groundwater discharge (SGD) processes, but it is the only one that results in net flux of water from deep inland to the ocean (Santos et al, 2012). We use Chapter 2 to consider the MDV as a window into East Antarctic-wide SGD of a regional aquifer and estimate that slow SGD of such a concentrated brine may seed the coastal zone with gigagrams of bioavailable Fe and Si annually, comparable to other sources of nutrients to the coast.A key factor determining the thickness of permafrost in the region is geothermal heat flux (GHF). In Chapter 3, we developed a new approach to using TEM data to estimate the flux in the study region, where it is not very well constrained. On Ross Island, we measured permafrost thickness and used this to estimate the local geothermal heat flux. Our calculations found a high, but reasonable heat flux of 90 mW/m2 13 mW/m2. A two-dimensional numerical model of heat conduction predicts high lateral heat flux from the relatively warm ocean into coastal permafrost, with implications for permafrost degradation and brine infiltration. The methods in Chapter 3 provide a blueprint for future GHF mapping with airborne TEM in the region and further afield