Impacts of marine instability across the East Antarctic Ice Sheet on Southern Ocean dynamics

Recent observations and modelling studies have demonstrated the potential for rapid and substantial retreat of large sectors of the East Antarctic Ice Sheet (EAIS). This has major implications for ocean circulation and global sea level. Here we examine the effects of increasing meltwater from the Wilkes Basin, one of the major marine-based sectors of the EAIS, on Southern Ocean dynamics. Climate model simulations reveal that the meltwater flux rapidly stratifies surface waters, leading to a dramatic decrease in the rate of Antarctic Bottom Water (AABW) formation. The surface ocean cools but, critically, the Southern Ocean warms by more than 1 degrees C at depth. This warming is accompanied by a Southern Ocean-wide “domino effect”, whereby the warming signal propagates westward with depth. Our results suggest that melting of one sector of the EAIS could result in accelerated warming across other sectors, including the Weddell Sea sector of the West Antarctic Ice Sheet. Thus, localised melting of the EAIS could potentially destabilise the wider Antarctic Ice Sheet

Emergence of the Shackleton Range from beneath the Antarctic Ice Sheet due to glacial erosion

This paper explores the long-term evolution of a subglacial fjord landscape in the Shackleton Range, Antarctica. We propose that prolonged ice-sheet erosion across a passive continental margin caused troughs to deepen and lower the surrounding ice-sheet surface, leaving adjacent mountains exposed. Geomorphological evidence suggests a change in the direction of regional ice flow accompanied emergence. Simple calculations suggest that isostatic compensation caused by the deepening of bounding ice-stream troughs lowered the ice-sheet surface relative to the mountains by ~800m. Use of multiple cosmogenic isotopes on bedrock and erratics (26Al, 10Be, 21Ne) provides evidence that overriding of the massif and the deepening of the adjacent troughs occurred earlier than the Quaternary. Perhaps this occurred in the mid-Miocene, as elsewhere in East Antarctica in the McMurdo Dry Valleys and the Lambert basin. The implication is that glacial erosion instigates feedback that can change ice-sheet thickness, extent, and direction of flow. Indeed, as the subglacial troughs evolve over millions of years, they increase topographic relief; and this changes the dynamics of the ice sheet. © 2013 Elsevier B.V

Student co-generated analogies and their influence on the development of science understanding

University of Technology, Sydney. Faculty of Arts and Social Sciences.Science educators often use analogies to help students develop understanding, but successful learning where students develop their own analogies has rarely been reported (Harrison, 2006). This research sought to investigate how the co-generation of analogies influenced students' learning of science. It stemmed from the author's scholarly interest in helping students understand the more difficult science concepts through analogical activities. The use of analogies as tools for learning encourages students to build on what they already know and understand. This research was underpinned by a constructivist epistemology. A pilot study was conducted and this led to the development of four research questions: a. How do students develop analogies? b. How does the co-generation of analogies influence student engagement with science? c. Do students develop deep understanding through the co-generation of analogies? b. How does a teacher support students in the co-generation of analogies? The literature that underpins the theoretical framework for this study is drawn from two main areas. The first relates to learning science through the construction of meaning (Freyberg & Osborne, 1985) and the second relates to the nature of analogy (Gentner, 1983) and its use in learning science (Harrison & Treagust, 2006). A teaching experiment methodology (Brown, 1992; Confrey & Lachance, 2000) suited this study of learning through analogy in school science because it provides a sound framework for a teacher exploring and scrutinising a teaching approach with his own students during the course of regular timetabled lessons. A large amount and variety of data were collected during 24 episodes of the teaching experiment. The teaching experiment involved the application of a teaching intervention with senior high school, chemistry and/or physics students (16–18 years of age). The intervention required students to develop analogies with the purpose of showing and enhancing their understanding of science concepts. Throughout each application of intervention students were supported by each other and by the teacher. The analogy based activities included role play, model building and writing. The discussions that occurred throughout these activities were integral to the analogy refining process. Hence, the resulting analogies were co-generated. The following conjecture was qualitatively investigated using participatory enquiry. When students develop their own analogies (supported by their teacher) in the process of learning science, they will be able to demonstrate deep understanding about the concepts being studied. This conjecture was founded in the literature; supported by personal experience and a pilot study; and tested through several teaching episodes. A large amount and variety of data were collected during the teaching experiment. These data have been used in providing “rich” (detailed) (Denzin & Lincoln, 2008, p. 16) and “thick” (based on multiple perspectives) (Lincoln & Guba, 1985, p. 316) descriptions of 13 episodes in which students developed their own analogies while learning science. Similar episodes have been grouped together and presented in five vignettes. Findings from the vignettes have been used to formulate conclusions. Data from the episodes reveal that in general, students who participated in the intervention enjoyed becoming actively engaged in analogical learning. In all applications of the intervention the majority of students were able, with support, to develop and use their own analogies to foster and display appropriate deep understandings about complex science concepts. By developing, using and sharing analogies, students made their conceptions and misconceptions 'visible'. In the supportive classroom environment, the identification of and discussion about students' alternative conceptions and misconceptions assisted students to develop appropriate scientific understandings. In general the understandings developed were persistent over long periods of time. The data suggests that co-generating analogies enhances student engagement and leads to deep understanding of challenging science concepts. It is thus concluded that the co- generation of analogies for science phenomena contributes positively to students' learning in science

Assessing the continuity of the blue ice climate record at Patriot Hills, Horseshoe Valley, West Antarctica

We use high resolution Ground Penetrating Radar (GPR) to assess the continuity of the Blue Ice Area (BIA) horizontal climate record at Patriot Hills, Horseshoe Valley, West Antarctica. The sequence contains three pronounced changes in deuterium isotopic values at ~18 cal ka, ~12 cal ka and ~8 cal ka. GPR surveys along the climate sequence reveal continuous, conformable dipping isochrones, separated by two unconformities in the isochrone layers, which correlate with the two older deuterium shifts. We interpret these incursions as discontinuities in the sequence, rather than direct measures of climate change. Ice-sheet models and Internal Layer Continuity Index plots suggest that the unconformities represent periods of erosion occurring as the former ice surface was scoured by katabatic winds in front of mountains at the head of Horseshoe Valley. This study demonstrates the importance of high resolution GPR surveys for investigating both paleo-flow dynamics and interpreting BIA climate records

Decadal-scale onset and termination of Antarctic ice-mass loss during the last deglaciation.

Emerging ice-sheet modeling suggests once initiated, retreat of the Antarctic Ice Sheet (AIS) can continue for centuries. Unfortunately, the short observational record cannot resolve the tipping points, rate of change, and timescale of responses. Iceberg-rafted debris data from Iceberg Alley identify eight retreat phases after the Last Glacial Maximum that each destabilized the AIS within a decade, contributing to global sea-level rise for centuries to a millennium, which subsequently re-stabilized equally rapidly. This dynamic response of the AIS is supported by (i) a West Antarctic blue ice record of ice-elevation drawdown >600 m during three such retreat events related to globally recognized deglacial meltwater pulses, (ii) step-wise retreat up to 400 km across the Ross Sea shelf, (iii) independent ice sheet modeling, and (iv) tipping point analysis. Our findings are consistent with a growing body of evidence suggesting the recent acceleration of AIS mass loss may mark the beginning of a prolonged period of ice sheet retreat and substantial global sea level rise

Greenland ice mass loss during the Younger Dryas driven by Atlantic Meridional Overturning Circulation feedbacks.

Understanding feedbacks between the Greenland Ice Sheet (GrIS) and the Atlantic Meridional Overturning Circulation (AMOC) is crucial for reducing uncertainties over future sea level and ocean circulation change. Reconstructing past GrIS dynamics can extend the observational record and elucidate mechanisms that operate on multi-decadal timescales. We report a highly-constrained last glacial vertical profile of cosmogenic isotope exposure ages from Sermilik Fjord, a marine-terminating ice stream in the southeast sector of the GrIS. Our reconstruction reveals substantial ice-mass loss throughout the Younger Dryas (12.9-11.7 ka), a period of marked atmospheric and sea-surface cooling. Earth-system modelling reveals that southern GrIS marginal melt was likely driven by strengthening of the Irminger Current at depth due to a weakening of the AMOC during the Younger Dryas. This change in North Atlantic circulation appears to have drawn warm subsurface waters to southeast Greenland despite markedly cooler sea surface temperatures, enhancing thermal erosion at the grounding lines of palaeo ice-streams, supporting interpretation of regional marine-sediment cores. Given current rates of GrIS meltwater input into the North Atlantic and the vulnerability of major ice streams to water temperature changes at the grounding line, this mechanism has important implications for future AMOC changes and northern hemisphere heat transport

Rapid thinning of the late Pleistocene Patagonian Ice Sheet followed migration of the Southern Westerlies

Here we present the first reconstruction of vertical ice-sheet profile changes from any of the Southern Hemisphere's mid-latitude Pleistocene ice sheets. We use cosmogenic radio-nuclide (CRN) exposure analysis to record the decay of the former Patagonian Ice Sheet (PIS) from the Last Glacial Maximum (LGM) and into the late glacial. Our samples, from mountains along an east-west transect to the east of the present North Patagonian Icefield (NPI), serve as ‘dipsticks' that allow us to reconstruct past changes in ice-sheet thickness, and demonstrates that the former PIS remained extensive and close to its LGM extent in this region until ~19.0 ka. After this time rapid ice-sheet thinning, initiated at ~18.1 ka, saw ice at or near its present dimension by 15.5 ka. We argue this rapid thinning was triggered by a combination of the rapid southward migration of the precipitation bearing Southern Hemisphere (SH) westerlies and regional warming