Transect 8 – The Minch

Transect Leader:

Tom Bradwell (University of Stirling)

Personnel:

D Fabel, D Small, R Chiverrell, M Burke, A Medialdea

Fieldwork activities:

  • 3-10 May 2013 – 28 rock samples collected from Trotternish-Raasay-Stoer-Cape Wrath.

  • 14-23 Sept 2013 – 55 rock samples collected from Islivick-Uig-Butt of Lewis-Ness-Bragar-Tolsta-Ranish.

  • May 2014 – Fieldwork season

See Picture Gallery

 

Background

The Minch palaeo ice stream A: Google Earth image of NW Scotland showing the main topographic and bathymetric features. Study site (transect) area defined by transparent grey polygon. B: Footprint and flow path of the Minch palaeo-ice stream. Flow lines and ice stream catchment deduced from geomorphological evidence (Bradwell et al., 2007). Maximum ice sheet limit taken from Bradwell et al. (2008). Shelf bathymetry at 50 m intervals. C: Merged bathymetric-topographic surface model showing the broad cross-shelf extension of The Minch trough and some associated large-scale glacial features. TMF = (Sula Sgeir) trough mouth fan; GZW = North Minch grounding zone wedge complex; LSB = large streamlined bedforms. Solid white lines show approximate positions of ice sheet margin during retreat from the LGM. Limits are currently undated but are based on all the available evidence. Yellow circles – TCN exposure ages currently being processed; green circles – published TCN exposure ages (Bradwell et al., 2008; Ballantyne, 2009) [Seabed surface – Olex database; land surface – NEXTMap Britain hillshaded DSM]. D: Simplified Quaternary geology of the continental shelf around NW Scotland (data from BGS, 2011). Note the widespread occurrence of mud grade sediment along the flow path of the Minch palaeo-ice stream. Potential targets for terrestrial cosmogenic nuclide sampling (TCN), and potential marine core sampling sites are also shown. Solid line shows main transect for offshore cruise, from Raasay Deep to Sula Sgeir Fan.

The Minch palaeo ice stream
A: Google Earth image of NW Scotland showing the main topographic and bathymetric features. Study site (transect) area defined by transparent grey polygon.
B: Footprint and flow path of the Minch palaeo-ice stream. Flow lines and ice stream catchment deduced from geomorphological evidence (Bradwell et al., 2007). Maximum ice sheet limit taken from Bradwell et al. (2008). Shelf bathymetry at 50 m intervals.
C: Merged bathymetric-topographic surface model showing the broad cross-shelf extension of The Minch trough and some associated large-scale glacial features. TMF = (Sula Sgeir) trough mouth fan; GZW = North Minch grounding zone wedge complex; LSB = large streamlined bedforms. Solid white lines show approximate positions of ice sheet margin during retreat from the LGM. Limits are currently undated but are based on all the available evidence. Yellow circles – TCN exposure ages currently being processed; green circles – published TCN exposure ages (Bradwell et al., 2008; Ballantyne, 2009) [Seabed surface – Olex database; land surface – NEXTMap Britain hillshaded DSM].
D: Simplified Quaternary geology of the continental shelf around NW Scotland (data from BGS, 2011).
Note the widespread occurrence of mud grade sediment along the flow path of the Minch palaeo-ice stream.
Potential targets for terrestrial cosmogenic nuclide sampling (TCN), and potential marine core sampling sites are
also shown. Solid line shows main transect for offshore cruise, from Raasay Deep to Sula Sgeir Fan.

Good 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.

Ongoing work

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.

Rationale

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.

transect8bbRecent 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).

Sampling programme

 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