Claire Todd

Cosmogenic-nuclide exposure ages from the Pensacola Mountains adjacent to the Foundation Ice Stream, Antarctica

We describe glacial-geological observations and cosmogenic-nuclide exposure ages from the Schmidt, Williams, and Thomas Hills in the Pensacola Mountains of Antarctica adjacent to the Foundation Ice Stream (FIS). Our aim is to learn about changes in the thickness and grounding line position of the Antarctic Ice Sheet in the Weddell Sea embayment between the Last Glacial Maximum (LGM) and the present. Glacial-geological observations from all three regions indicate that currently-ice-free areas were covered by ice during one or more past ice sheet expansions, and that this ice was typically frozen to its bed and thus non-erosive, permitting the accumulation of multiple generations of glacial drift. Cosmogenic-nuclide exposure-age data from glacially transported erratics are consistent with this interpretation in that we observe both (i) samples with Holocene exposure ages that display a systematic age-elevation relationship recording LGM-to-present deglaciation, and (ii) samples with older and highly scattered apparent exposure ages that were deposited in previous glacial-interglacial cycles and have experienced multiple periods of surface exposure and ice cover. Holocene exposure ages at the Thomas and Williams Hills, upstream of the present grounding line of the FIS, show that the FIS was at least 500 m thicker prior to 11 ka, and that 500 m of thinning took place between 11 and 4 ka. However, exposure-age data from the Schmidt Hills, downstream of the present grounding line of the FIS, show no evidence for LGM thickening of the FIS and, in fact, provide some evidence that the FIS could have been no more than 200 m thicker than present at the LGM. If all these observations are correct, they imply that the LGM and early Holocene ice surface slope in the vicinity of the present grounding line was steeper than present, which is inconsistent with glaciological model predictions of possible LGM ice sheet configurations. Specifically, scenarios in which the LGM grounding line of the FIS advanced to the outer continental shelf appear inconsistent with exposure-age data from the Schmidt Hills, whereas scenarios in which the FIS grounding line did not advance at the LGM appear inconsistent with exposure-age data from the Williams and Thomas Hills.

Radar-detected englacial stratigraphy in the Pensacola Mountains, Antarctica: implications for recent changes in ice flow and accumulation

Abstract We used measurements of radar-detected stratigraphy, surface ice-flow velocities and accumulation rates to investigate relationships between local valley-glacier and regional ice-sheet dynamics in and around the Schmidt Hills, Pensacola Mountains, Antarctica. Ground-penetrating radar profiles were collected perpendicular to the long axis of the Schmidt Hills and the margin of Foundation Ice Stream (FIS). Within the valley confines, the glacier consists of blue ice, and profiles show internal stratigraphy dipping steeply toward the nunataks and truncated at the present-day ablation surface. Below the valley confines, the blue ice is overlain by firn. Data show that upward-progressing overlap of actively accumulating firn onto valley-glacier ice is slightly less than ice flow out of the valleys over the past ∼1200 years. The apparent slightly negative mass balance (-0.25 cm a-1) suggests that ice-margin elevations in the Schmidt Hills may have lowered over this time period, even without a change in the surface elevation of FIS. Results suggest that (1) mass-balance gradients between local valley glaciers and regional ice sheets should be considered when using local information to estimate regional ice surface elevation changes; and (2) interpretation of shallow ice structures imaged with radar can provide information about local ice elevation changes and stability.

Features of the glacial history of the Transantarctic Mountains inferred from cosmogenic 26 Al, 10 Be and 21 Ne concentrations in bedrock surfaces

Abstract This paper describes measurements of concentrations of cosmogenic 26Al, 10Be and 21Ne in quartz from bedrock surfaces in the Transantarctic Mountains where stratigraphic and geomorphic evidence shows that the surfaces were covered by ice in the past, but were not glacially eroded during periods of ice cover. It then explores to what extent this information can be used to learn about past ice sheet change. First, cosmogenic nuclide concentrations in sandstone bedrock surfaces at two sites in the McMurdo Dry Valleys near 77°S are consistent with an equilibrium between nuclide production and loss by surface erosion and radioactive decay. They are most easily explained by a scenario in which: i) sites more than c. 100 m above the present ice surface were almost never ice-covered and eroded steadily at 0.5–1.5 m Ma-1, and ii) sites near the present ice margin experienced similar erosion rates when ice-free, but have been covered by cold-based, non-erosive glacier ice as much as half of the time during the past several million years. Nuclide concentrations in granite bedrock at a site in the Quartz Hills near 85°S, on the other hand, have not reached production-erosion equilibrium, thus retaining evidence of the time they were first exposed to the cosmic ray flux. Nuclide concentrations at these sites are most easily explained by 4–6 Ma exposure, extremely low erosion rates of 5–10 cm Ma-1 during periods of exposure, and only very short periods of cold-based, non-erosive ice cover.

Teaching Anthropogenic Climate Change through Interdisciplinary Collaboration: Helping Students Think Critically about Science and Ethics in Dialogue.

ABSTRACT Anthropogenic climate change is a complicated issue involving scientific data and analyses as well as political, economic, and ethical issues. In order to capture this complexity, we developed an interdisciplinary student and faculty collaboration by (1) offering introductory lectures on scientific and ethical methods to two classes, (2) assigning the same technical and opinion texts about anthropogenic climate change to both classes, and (3) coordinating multidiscipline discussions with students about their common reading assignments. Student learning was documented using identical pre- and postcollaboration surveys. We hypothesized that students would be better prepared to understand and engage in public debate about anthropogenic climate change if they were first taught to distinguish clearly between scientific and ethical claims. Our results from pre- and postcollaboration surveys support our hypothesis; as students showed an increased understanding of the distinction between science and ethics, they were better able to critically analyze popular articles and to develop their own questions about anthropogenic climate change. The results also suggest that our students were more prepared to think critically about scientific inquiry than about ethical inquiry regarding anthropogenic climate change.