Ice Sheet Reconstruction

 

Understanding how ice sheets change is critical for better understanding their future stability under a changing climate. One approach to this is to use mountains that protrude through the ice sheet today to provide dip sticks recording past changes in surface elevation of the ice sheet. To do this we need a chronometer, a record of time elapsed since the mountains were covered by ice. In a cold arid environment like Antarctica traditional techniques such as radio carbon do not work well due to limited production of organic material, so we need to find other chronometers.

 

One such technique is called cosmogenic exposure analysis, which measures the build up of rare cosmogenic isotopes in the lattice structure of rocks exposed at the Earth’s surface. Even 20 metres of ice can shield rocks from incoming radiation, so providing the glaciological context of a piece of rock or glacial erratic is well understood it can be used as a powerful tool to reconstruct past ice sheet change.

 

With support from ALE and collaborators from ETH Zurich, Glasgow and Edinburgh we have been able to study the history of the Rutford and Institute Ice Streams, two major ice streams draining the interior of the WAIS either side of the Ellsworth Mountains. Our new terrestrial cosmogenic constraints (using in situ 14C and 10Be) allow us to reconstruct, in detail, past surface profile changes in the Rutford and Institute ice streams entering the Weddell Sea Embayment. The data reveal that although these two adjacent ice streams exhibited similar surface geometries at the end of the Last Glacial Maximum, the pattern of ice surface lowering contrasted markedly during the Last Termination, showing asynchronous thinning trajectories as they approached present-day configurations during the mid-Holocene.

 

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 discover that the decoupling of surface trajectories of the two ice streams was driven by differences in the rate of grounding-line retreat across the WSE, resulting in the extensive Institute Ice Stream switching direction by 90˚ and discharging ice into the Thiel Trough during the Holocene, rather than the Rutford Depression as it does at present.

 

These 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 WAIS stability.

 

Key collaborators: Dr Nick Golledge (University of Victoria, Wellington), Dr Dylan Rood (SUERC, Glasgow), Dr Kristina Hippe (ETH Zurich), Dr Lukas Wacker (ETH Zurich) and Prof. Rainer Weiler (ETH Zurich).

 

If you would like to learn more, check out our recent research paper:

Fogwill, C.J., Turney, C.S.M., Golledge, N.R., Rood, D.H., Hippe, K., Wacker, L., Wieler, R., Rainsley, E.B., Jones, R.S., 2014. Drivers of abrupt Holocene shifts in West Antarctic ice stream direction determined from combined ice sheet modelling and geologic signatures. Antarctic Science 26, 674-686.

 

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