Electromagnetic investigations of submarine and subglacial hydrogeologic systems

• Main Author: Gustafson, Chloe
• Format: Thesis
• Language: English
• Published: 2020
• Subjects: Geophysics; Hydrology; Groundwater; Antarctic; Southern Ocean; West Antarctica; Ross Ice Shelf; Mercer; Whillans; Antarc*; Antarctica; Ice Sheet; Ice Shelf
• Online Access: https://doi.org/10.7916/d8-d3p7-k583

Groundwater systems hidden beneath oceans and ice sheets impact modern Earth processes and hold information about past Earth environments. However, the nature, distribution, and architecture of these systems, as well as the mechanisms by which they influence their surrounding environments, remain poorly understood. This dissertation applies electromagnetic (EM) geophysical methods to investigate the spatial character and salinity content of previously unmapped submarine and subglacial hydrogeologic systems. Chapter 2 of this dissertation examines the first large-scale EM imaging of submarine aquifer systems extending up to 90 km offshore on the U.S. Atlantic margin. Magnetotelluric (MT) and controlled source electromagnetic data collected offshore New Jersey and Martha’s Vineyard, Massachusetts, show that low-salinity groundwater contained in continental shelf sediments continuously extends from the coast out to 90 km offshore. The EM imaging offshore New Jersey agrees with observed low-salinity porewater anomalies from drill cores and shows the spatial extent and continuity of these anomalies. Offshore Martha’s Vineyard, our results provide the first observational constraints on the distribution of offshore groundwater. Combining the EM results with seismic reflection imaging provides further insight into the architecture of offshore groundwater systems, as seismically mapped confining units and clinoforms correspond to the vertical and lateral boundaries of low-salinity groundwater distributions, respectively. We estimate there is 2800 km³ of low-salinity groundwater within the aquifer system between New Jersey and Martha’s Vineyard. This aquifer system is representative of a global phenomenon, as it has been estimated that Earth’s passive margins contain 500,000 km³ of low-salinity groundwater. This dissertation demonstrates that these groundwater systems can be efficiently mapped using marine electromagnetic imaging. This mapping is important for understanding past oceanic and climatic conditions, the delivery of nutrients from the continents to the ocean, physical and biogeochemical continental shelf processes, and water resources globally. Chapters 3 - 6 discuss the first polar MT study focused on investigating the upper 5 km of the subglacial environment of an ice stream and demonstrates that MT is an effective tool for mapping subglacial hydrogeologic systems. We collected MT data on Whillans Ice Plain (WIP), West Antarctica, a region of fast flowing ice that hosts an interconnected network of active subglacial lakes that cyclically fill and drain on monthly to yearly timescales, and eventually drain into the Southern Ocean beneath Ross Ice Shelf. Currently, at WIP and other ice streams, water volumes and exchanges have only been accounted for at the ice-bed interface and within the top 10 m of basal sediment; however, deeper groundwater systems are likely active components of the subglacial hydrologic system and may modulate ice flow. We collected MT data in order characterize subglacial groundwater distribution and salinity beneath an active lake, Whillans Subglacial Lake (hereafter referred to as SLW for historical reasons), and the Whillans Grounding Zone (WGZ), where SLW is thought to drain into the Southern Ocean. We collected one additional station at Mercer Subglacial Lake (hereafter referred to as SLM for historical reasons) to evaluate potential geologic and hydrologic variations across WIP. Chapter 3 discusses the importance of subglacial hydrology and how it is parameterized in models. I also introduce previous glaciological studies done within and around WIP, as well detailed geophysical and direct subglacial access studies at SLW and WGZ. Chapter 4, covers the acquisition and processing of MT data on WIP and examines noise identification and removal techniques. I review existing methods for acquiring MT data in polar surveys and develop new methods that are specific to rapid, short-term polar MT surveys, but may also be applied to longer term surveys. Wind speeds as low as 5 knots introduce broadband noise into the electric field measurements, through the stream of electrically charged ice particles carried by wind. However, this noise may be removed through frequency-dependent wind speed based data editing. Magnetic field data are less affected by wind noise and therefore high quality magnetic field data may be collected during times when wind speeds do not allow for the collection of high quality electric field data. The results from this chapter can be used to guide and optimize future polar MT surveys. Chapter 5 evaluates the magnetotelluric data and 2-D inversion results from the data collected on WIP. The MT data vary laterally across each survey region, suggesting lateral hydrologic and geologic variations. Overall, the data trends at SLW and WGZ are fairly similar, while the single station collected at SLM, is quite different, suggesting there are significant geologic and hydrologic changes within WIP. The inversion results indicate that there is a conductive region within the top 350 to 2000 m of the subglacial environment, which is interpreted to be water saturated sediments. This feature is underlain by a more resistive region, which we interpret to represent bedrock. At WGZ, the MT derived depth to bedrock agrees well with a geologic model of bedrock from a previous ground-based gravity survey in the same location. Chapter 6 uses the resistivity models from Chapter 5 to estimate groundwater volumes and salinities at SLW and WGZ. At both survey locations, this analysis requires extensive saline groundwater systems that extend > 1 km below the ice surface. Our results suggest an “open system” in which deep groundwater may be exchanged with basal water. Directly beneath SLW, we estimate the volume of groundwater is over 40 times greater than the volume of water held in the lake, suggesting that WIP and other Antarctic sedimentary basins likely contain groundwater that represents a significant component of subglacial hydrologic budgets. This mapping of subglacial groundwater volume and salinity may be used to improve understanding of coupled groundwater and ice dynamics, as well as provide insight into observed geothermal variations. It also has implications for biogeochemical reactions throughout the sediment column that may play a role in sub-ice-sheet nutrient cycling.