Transect 8 – The Minch
D Fabel, D Small, R Chiverrell, M Burke, A Medialdea
BackgroundGood examples of entire palaeo-ice stream systems, including well-defined source areas and sediment sinks, are surprisingly rare. The Minch palaeo-ice stream in NW Scotland is an exception (Figs A, B). As such, it presents the ideal natural laboratory for resolving long-standing questions about ice stream retreat, and understanding the rate and style of ice-sheet collapse. First identified by geological mapping, remote sensing imagery and offshore seismic data, its signature was confirmed by multibeam imagery, RADAR elevation models and shelf-wide singlebeam (Olex) bathymetry. This ice stream system, extending from NW Scotland across the continental shelf to the Sula Sgeir Fan on the continental slope (Fig B), dictated the overall geometry and flow pattern of the NW sector of the last British Ice Sheet. Detailed mapping has so far focused on the terrestrial footprint of the ice stream – draining a mountainous catchment in excess of 15,000 km2. As a result, the former flow vectors and onset zones of the Minch ice stream and its tributaries are relatively well defined, although its timing of operation, mechanism of decay and offshore extent remain uncertain.
Studies are currently examining the validity of palaeo-nunataks as ice-sheet thickness indicators in NW Scotland, using a suite of over 50 cosmogenic-nuclide surface-exposure analyses (NERC PhD studentship; & 2 x NERCCIAF awards). (Fig C). In addition, studies are constraining the late stages of ice-sheet decay in the fjords, using cosmogenic-nuclide and radiocarbon dating techniques on moraines combined with seismic stratigraphy of glacial deposits. Multibeam bathymetry data and shallow seabed cores have been collected (by BGS 2005-2007) for a portion of the eastern near-shore waters of The Minch. Aspects of this work are in prep. for publication.
This transect will afford powerful insights on how ice streams govern the dynamics of ice sheets past and present. The Minch ice stream is an excellent palaeo-analogue of a large marine-terminating ice stream draining a well-defined ice-sheet catchment in a climatically sensitive setting.
Recent numerical modelling experiments of the British Ice Sheet closely replicate the empirically derived flow geometry and streaming velocities reconstructed for the Minch ice stream (>800 ma-1). These time dependent, climatically forced, simulations demonstrate that the Minch ice stream was a transient but quasi-stable feature of the last British Ice Sheet during growth and decay. Model runs show rapid changes in ice-stream velocity commonly occurring on centennial timescales or less. Fluctuations in ice stream size and vigour are known to determine the regional surface elevation of ice sheets through mass drawdown. Given that stronger ice streams require more ice to sustain them, ice-divide migration and ice piracy from adjacent catchments to The Minch are likely to have occurred. These non-linear responses have been seen elsewhere to prompt ‘runaway’ feedback effects, destabilizing large sectors of ice sheets through dynamic thinning – particularly in marine-ice-sheet sectors. This hypothesis could be ideally tested in NW Scotland using a combination of onshore mapping, offshore geophysical techniques (multibeam bathymetry, seismic profiling) coupled with onshore and offshore sampling of dateable material (radiocarbon, TCN and OSL) (see Fig D).
TCN (Be-10 in quartz-bearing rocks):
- 12 key locations around the terrestrial periphery of NW Scotland (including the headlands of Butt of Lewis, Cape Wrath and Rona near Raasay) minimum of 3 samples (glacially deposited boulders) at each site.
C-14 (marine fauna):
- 15 marine cores (vibrocores) through glacial, glaciomarine and postglacial sediment (unlithified).
- 4 x AMS analysis of marine fauna (minimum) per core to determine age of seismo-stratigraphically resolved units of importance; constrain timing of ice sheet retreat (+readvances) along main flow line.
OSL (terrestrial sands)
- Field sampling of key glaciofluvial, deltaic and glacio-lacustrine sediments for OSL analysis.
- 8 sites in total: 3 in Wester Ross; 2 in NW Sutherland; 3 in north Lewis