USAP-DC

In this project we investigated glaciers that drain ice from the East Antarctic Ice Sheet through the Transantarctic Mountains into the present-day Ross Ice Shelf. The outlet glaciers that flow through the Transantarctic Mountains have thinned significantly over the past 15,000 years, especially as they retreated from Last Glacial Maximum highstands to their present-day grounding lines. At certain locations and for certain glaciers, rocks or bedrock have been sampled to provide constraints on the timing of when ice retreated from these locations. In the locations where geochronological data are available we can use these data as direct constraints on ice-flow models that simulate ice elevation change over time. The intellectual merit of this work is using ice-flow models to spatially and temporally extrapolate between these limited geochronological data points, which enables new understanding of glacier evolution. The mountainous topography in this region is complex, and there are limited measurements of the topography beneath the ice of the Transantarctic outlet glaciers. Since the topography of the glacier bed is an important control on ice flow and is a necessary boundary condition in models we developed a new gridded bed product at Beardmore Glacier, the one location where sufficient data were available, and we compared this to continent-scale gridded bed products. We found that for this glacier, the BedMachine v1 product was reasonably similar to the Beardmore Glacier bed topography measurements; our limited evaluation suggests that the BedMachine product may be sufficient at other Transantarctic outlets where bed measurements are not available, but that other compilations of bed topography data that do not include information about ice flow directions do not provide reliable results. Using these data and available geochronological constraints we investigated Beardmore Glacier evolution since the Last Glacial Maximum using simplified (flowline) models of ice flow. In addition to flowline modeling at Beardmore Glacier, we developed a flow-model setup using the open-source 'icepack' model that uses the shallow stream equations and resolves flow in both the x and y directions. The key value added over flowline (or parameterized flowband) models is that this can capture converging and diverging ice flow, variable side wall and bottom drag, and other geometric complexities. In these simulations we can evaluate the past accumulation, ice influx, and ice outflux to compare controls on deglaciation to data constraints on the chronology of deglaciation. We also used a flowline model to investigate the Darwin-Hatherton Glacier System. Exposure ages and radiocarbon ages of glacial deposits at four locations alongside Hatherton and Darwin glaciers record several hundred meters of late Pleistocene to early Holocene thickening relative to present. Deglaciation was relatively complex at this site, and we also found that Byrd glacier likely contributed ice to the catchment of the Darwin-Hatherton glacier system during the last glacial maximum, and that subsequent convergent flow from Byrd and Mulock glaciers during deglaciation complicated the response of the Darwin-Hatherton system. These new insights can be used on their own to better understand local deglaciation, and can also be used to evaluate regional or continent-scale model calculations. Separately, we investigated the general response of outlet glaciers to different sources of climate forcing. We found that outlet glaciers have a characteristically different response over time to surface-mass-balance forcing applied over the interior than to oceanic forcing applied at the grounding line. Our models demonstrated that ocean forcing first engages the fast, local response and then the slow adjustment of interior ice, whereas surface-mass-balance forcing is dominated by the slow interior adjustment. These insights contributed to our general understanding of how outlet glaciers may have evolved over time. Our new model investigations provide a framework that can be applied at other Transantarctic outlet glaciers where geochronological data are available. In particular, our 'icepack' setup is an archived and documented resource for the community. These tools are available for future investigations, including additional investigations at Beardmore Glacier and at other Transantarctic Mountain outlet glaciers. Scientific broader impacts include that this contributes to our understanding of the past behavior of East Antarctic ice, which provides an important constraint on the future evolution of Antarctica. Our team has engaged in public outreach and has engaged students in this research. Two graduate students led in aspects of this work, and have since gone on to research positions after their PhD.

Person	Role
Koutnik, Michelle	Investigator and contact
Smith, Ben	Co-Investigator
Conway, Howard	Co-Investigator
Shapero, Daniel	Investigator

Antarctic Glaciology

Award # 1542756

USAP-1542756_1

None in the Database

Repository	Title (link)	Format(s)	Status
GitHub	Beardmore Glacier model in 'icepack'	Not Provided	exists

1. Hillebrand, T. R., Stone, J. O., Koutnik, M., King, C., Conway, H., Hall, B., … Gillespie, M. K. (2020). Holocene thinning and grounding-line retreat of Darwin and Hatherton Glaciers, Antarctica. (doi:10.5194/tc-2020-356)
2. Hillebrand, T. R., Stone, J. O., Koutnik, M., King, C., Conway, H., Hall, B., … Gillespie, M. K. (2021). Holocene thinning of Darwin and Hatherton glaciers, Antarctica, and implications for grounding-line retreat in the Ross Sea. The Cryosphere, 15(7), 3329–3354. (doi:10.5194/tc-15-3329-2021)
3. Christian, J. E., Robel, A. A., Proistosescu, C., Roe, G., Koutnik, M., and Christianson, K. (2020). The contrasting response of outlet glaciers to interior and ocean forcing. 14. The Cryosphere, 14. 2515. (doi:10.5194/tc-14-2515-2020)