Nick Holschuh



West Antarctic Stability and the Interior Basins
The West Antarctic Ice Sheet (WAIS) rests on a bed almost entirely below sea-level. Its geometry makes it prone to retreat for several reasons:

1) Modern Antarctic mass loss is controlled by warm ocean water, which thins the ice shelves at the margins. Unlike East Antarctica, where the ice sheet would retreat up out of the ocean, into the rocky highlands of the interior, the ocean will have continued access to the margins of WAIS.

2) The flux of ice across the grounding line (the grounded/floating transition) scales with the thickness of the ice. The bed deepens inland of the modern margin, so grounded ice upstream of the current groundingline is, by necessity, thicker than ice at the modern grounding line. Any retreat will increase the flux of ice to the ocean and accelerate retreat.

Now that the initiation of retreat has been observed, it is important that we think carefully about the interior basin geometry and the factors that will control the timing and maximum inland retreat of collapse. I have previously used radar to better constrain the geometric boundary conditions for Marie Byrd Land, and investigated the long term stability and nucleation potential for an ice-cap on a deeper bed than previously estimated.
Antarctic Surface Elevation Antarctic Basal Topography Antarctic Surface Velocity

Diagnosing Stable Bed Properties using Internal Ice Structures

Several studies state that the dynamic retreat of WAIS has already begun at its margins along the Amundsen Sea, and will continue until the interior has been fully evacuated. The frictional properties of the bed control this process, as high friction beds (or low power rheologies) restrain ice flow from the interior of the continent out to its margins, where it melts and contributes to sea-level rise. The timing of the system response and modeled rates of sea-level rise are highly dependent on the prescribed bed properties that define the model boundary – higher power rheologies (i.e. plastic beds) spread coastal perturbations far inland and contribute to more near-term sea-level rise, but they limit the coastal thinning that is known to precede ice-sheet collapse. Low power rheologies limit thinning in the interior at present, but promote grounding line retreat, that leads to rapid sea-level rise once initiated. Developing a geophysical method for diagnosing the frictional and rheologic properties of the bed, in order to distinguish between these two scenarios, is a critical need of the discipline.

Spatially variable flow rates leave a record in the internal structure of the ice, detectable using radar. As the ice flows over either resistive or lubricated beds, it compresses or stretches, leaving characteristic synformal and antiformal structures in the internal layers of the ice sheet. While challenging to interpret, these structures are some of the few data that directly reflect the frictional resistance of the system, and have the potential to inform our understanding of the basal boundary conditions. One goal of my research is to be able to use measurable features in these structures to invert for the frictional characteristics at the bed. Working toward this objective involves observational studies (collecting and examining radar data over areas of known or expected variability in the bed properties) as well as modeling exercises (cataloging the suite of possible and expected internal structures). The Amundsen Sea sector of West Antarctica is a perfect area to test this method, as the frictional properties of the system are thought to be geologically controlled, and therefore spatially and temporally stable over the glacial cycle.

Investigating Transient Features, and Evaluating the Assumption of Steady State to Improve Projection of Future Sea Level Rise
Parameterized View of the Ice Sheet System The Actual Ice Sheet System The Composition and Material Properties of the Bed Ice-Ocean Interaction and the role of Circumpolar Deep Water The Basal Hydrologic System
Predictive modeling of the ice sheets starts with a spin-up process that reproduces modern observations given today’s atmospheric and ocean boundary conditions. A fundamental assumption of most model spin-up is that our observations today represent a steady state configuration, despite the overwhelming evidence that the modern ice sheets are not in equilibrium with the current environment. While the accuracy of “steady state” modeling presents a philosophical question within glaciology, there are observed features in radar data that point toward transient processes affecting the modern ice sheet. My long-term plans include using modern geophysical tools to better characterize and understand these transient features. One of many specific targets I have is to re-image the features described by Bell et al. [4] using low frequency, ground based radar together with the deployment of surface GPS grids, in an effort to better resolve the deep structure and determine the impact of these features on the present ice flow regime. I also intend to investigate the timing and duration of the impacts of subglacial lakes, the migration of shear margins, and the grounding or ungrounding of ice rises based on internal structures imaged by ground radar surveys.

Novel Seismic and Radar Processing Algorithms
Through the research I did at Chevron, I've become interested in developing novel signal processing techniques for improved interpretation and description of seismic and radar data. At chevron I worked on time-lapse seismic data, looking for a robust method to realign the monitor and baseline surveys to extract changes in the acoustic impedance of the system. I continue to work on these issues, as well as expand them into the world of airborne radar data processing.

As computational power increases, and the volume of airborne geophysical data collected exceeds an amount that any individual could parse by hand, it is important that we come up with reliable, automated methods for analyzing geophysical data. This means both from a data processing standpoint (improving our methods for resolving complex internal structures in radar data), and from an analysis perspective (performing automatic inversions for basal properties given radar return power, englacial geometry, and radar wave attenuation). My work to date has focused on analyzing the airborne radar data collected through Operation Ice Bridge, using instruments developed by the Center for Remote Sensing of Ice Sheets (CReSIS), however I have also looked at developing new time shift methods for 4D seismic data with Chevron, and am interested in all time-series analysis techniques that could extract new insight from existing data sets.

© Nick Holschuh - August 2016