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.

The Labrador Sea is a climatically important region for the entire globe. As one of the main locations of deep water formation, this subpolar sea helps modulate the strength of the Atlantic Meridional Overturning Circulation. The deep water formation rate is influenced by freshwater originating from the Arctic Ocean as well as from the Greenland Ice Sheet. The freshwater entering the sea is then distributed across the Labrador Sea by boundary currents and eddy circulation. In this study we analyze a suite of simulations using a high resolution earth system model, the Regional Arctic System Model (RASM), to explore the freshwater forcing methods needed to properly represent the Labrador Sea dynamics.

The first simulation, the H-case, uses an actively coupled ocean and sea ice models only, with sea surface salinity (SSS) restored to mean climatology. The G-case removed the SSS restoring method and instead incorporated land runoff fluxes from the Coordinated Ocean-Ice Reference Experiments version 2 (COREv2) to evolve SSS. The final simulation, the R-case, is a fully coupled run that relies minimally on reanalysis forcings, but instead allows the coupled model physics to drive the simulation, with air-sea-land fluxes that develop and grow without artificial forcing.

The critical oceanographic conditions that permit deep water formation can be assessed in our model results by analyzing the strength of water column stratification within the Labrador basin. We explore three methods for calculating the mixed layer depth (MLD) methods. The MLD serves as a measure of the vertical extent of conditions amenable to deep convection and as an indicator of the deep water formation rate. We find that all three methods create a MLD signal that overpredicts, as compared to the existing observations, the spatial and temporal extent of water stratification conducive to deep convection in the Labrador Sea. The maximum density gradient (MDG) method produces on average the lowest MLD values, and hence may best represent the intermittent nature of the deep convection process in the region. Out of the three numerical experiments considered in this study, the R-case shows the most realistic depictions of the extent and depth of MLDs when compared to the G and H-cases. However, even in the R-case, the results of this study highlight the need for further improvements of the ability of the model to produce realistic levels of vertical stratification across this critical high latitude ocean basin.

Beyond the MLD analysis, we find that the R-case outperforms the G and H-cases in a number of surface ocean dynamics measures compared to observations. First, it displays a stronger West Greenland Current System (WGCS) from which Irminger Current Anticyclones are generated and buoyancy is distributed across the rest of the basin. The eddy activity in the R-case is not only more frequent, but the eddies are also more effective at supplying freshwater to the interior Labrador Sea compared to the other two simulations. We find that the salinity gradient within the WGCS and the sea surface height (SSH) gradient across the west Greenland shelf are the strongest in the R-case. These characteristics stem from freshwater entering the region through boundary currents and from local sources. The R-case also displays a realistic seasonal variation of surface hydrography, and it more closely follows the evolution of sea ice on a mean annual basis compared to the G and H-cases. This shows that the coupled modeling framework provides great improvement to the surface dynamics of the Labrador Sea.