The Antarctic Ice Sheets
Past and present changes in ice sheet dynamics provide valuable clues to what the future might bring.
Understanding how ice sheets have behaved in the past is critical for reducing uncertainties over their response to a future changing climate. One approach is to use the remains of former ice sheets preserved on mountain slopes as glaciological 'dip sticks'. These features may be glacial moraines, ramparts of rubble that accumulated at the former edge of the ice, or erratics, rocks with a distinctive mineralogy that had been eroded and transported by ice to their new found location. Taken together, these remnants preserve moments when the mountains were covered by ice and therefore a measure of changes in the surface elevation of the ice sheet. The question is when. In a cold arid environment like Antarctica, traditional dating techniques such as radiocarbon (or 14C) do not work well due to limited production of organic material. An alternative approach is called exposure rock dating, which measures the build up of rare cosmogenic isotopes in the lattice structure of rocks created from their exposure to cosmic rays on the Earth’s surface. Even 20 metres of ice can provide a shield to incoming radiation, allowing the rocks to start accumulating a signal once they are near the surface and providing a powerful tool for reconstructing past ice sheet change.
With support from Antarctic Logistics and Expeditions (ALE) and funded through the Australian Research Council (ARC), we have been able to study the history of the Rutford and Institute ice streams, two major ice streams on either side of the Ellsworth Mountains that drain the interior of the West Antarctic Ice Sheet (often abbreviated to WAIS). Our new terrestrial cosmogenic constraints (using in situ 14C and 10Be) allow us to reconstruct detailed past surface profile changes in the Rutford and Institute ice streams entering the Weddell Sea Embayment (or WSE). The data reveal that although these two adjacent ice streams exhibited similar surface geometries at the end of the Last Glacial Maximum (around 21,000 years ago), they behaved markedly differently during the onset of warming into the current interglacial (the Holocene), showing asynchronous thinning trajectories as they approached present-day configurations some 6000 years ago.
To understand the mechanism for these different trends, we use a high-resolution ice-sheet model, forced with a warming ocean and rising sea-levels, to simulate grounding-line retreat in the WSE. We have discovered that the divergence of the surface trajectories of the two ice streams was driven by differences in the rate of grounding-line retreat across the region, resulting in the extensive Institute Ice Stream switching direction by 90˚ and discharging ice into the Thiel Trough during the early Holocene. This is quite different to today where the Institute Ice Stream discharges into the Rutford Depression. We are testing these ideas by working with palaeoceanographers to directly compare our records to marine cores from the Southern Ocean that preserve evidence of iceberg discharge and climate changes over the last 100,000 years.
Our findings highlight that spatial variability in ice flow can trigger marked changes in the pattern, flux and direction of extensive ice streams on sub-millennial timescales, decoupling them from direct climate forcing and, importantly, changing ice-sheet mass balance over large areas of the ice sheet. Given the sensitivity of ice streams such as the Institute to marine ice-sheet instability and projected regional ocean warming in the Thiel Trough and eastern Weddell Sea, further work to disentangle the different controls which drive these abrupt shifts is critical to better predict future West Antarctic Ice Sheet dynamics.
Other key collaborators: Michael Weber (University of Bonn), Dylan Rood (SUERC, Glasgow), Kristina Hippe (ETH Zurich), Lukas Wacker (ETH Zurich), Rainer Weiler (ETH Zurich), John Woodward (Northumbria University), Winter (Northumbria University), and Keith Fifield (Australian National University).
If you would like to learn more, check out some of our recent research papers:
Fogwill, C.J., Turney, C.S.M., Golledge, N.R., Etheridge, D.M., Rubino, M., Thornton, D.P., Baker, A., Woodward, J., Winter, K., van Ommen, T.D., Moy, A.D., Curran, M.A.J., Davies, S.M., Weber, M.E., Bird, M.I., Munksgaard, N.C., Menviel, L., Rootes, C.M., Ellis, B., Millman, H., Vohra, J., Rivera, A. and Cooper, A. (2017) Antarctic ice sheet discharge driven by atmosphere-ocean feedbacks during the Last Glacial Termination. Nature Scientific Reports, 7, 39979.
Winter, K., Woodward, J., Dunning, S.A., Turney, C.S.M., Fogwill, C.J., Hein, A.S., Golledge, N.R., Bingham, R.G., Marrero, S.M., Sugden, D.E., Ross, N. (2016) Assessing the continuity of the blue ice climate record at Patriot Hills, Horseshoe Valley, West Antarctica. Geophysical Research Letters, 43, 2019-2026.
Jones, R.S., Mackintosh, A.N., Norton, K.P., Golledge, N.R., Fogwill, C.J., Kubik, P.W., Christl, M., Greenwood, S.L. (2015) Rapid Holocene thinning of an East Antarctic outlet glacier driven by marine ice sheet instability. Nature Communications 6, 8910.
Fogwill, C.J., Turney, C.S.M., Golledge, N.R., Rood, D.H., Hippe, K., Wacker, L., Wieler, R., Rainsley, E.B., Selwyn Jones, R. (2014) Drivers of abrupt Holocene shifts in West Antarctic ice stream direction from combined ice sheet modelling and geologic signatures. Antarctic Science, 26, 674-686.
Golledge, N.R., Levy, R.H., McKay, R.M., Fogwill, C.J., White, D.A., Graham, A.G., Smith, J.A., Hillenbrand, C.-D., Licht, K.J., Denton, G.H. (2013) Glaciology and geological signature of the Last Glacial Maximum Antarctic ice sheet. Quaternary Science Reviews 78, 225-247.
Fogwill, C.J., Hein, A.S., Bentley, M.J., Sugden, D.E. (2012) Do blue-ice moraines in the Heritage Range show the West Antarctic ice sheet survived the last interglacial? Palaeogeography, Palaeoclimatology, Palaeoecology 335, 61-70.
Bentley, M.J., Fogwill, C.J., Le Brocq, A.M., Hubbard, A.L., Sugden, D.E., Dunai, T.J., Freeman, S.P.H.T. (2010) Deglacial history of the West Antarctic Ice Sheet in the Weddell Sea embayment: Constraints on past ice volume change. Geology 38, 411-414.
Fogwill, C., Bentley, M., Sugden, D., Kerr, A., Kubik, P., 2004. Cosmogenic nuclides 10Be and 26Al imply limited Antarctic Ice Sheet thickening and low erosion in the Shackleton Range for >1 m.y. Geology 32, 265-268.
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