British and Irish Ice Sheet Flashcards
What is a brief history of glaciation in Britain? (intro)
Covered by several “Ice Ages” over the last 500,000 years.
Most recent one ended only 10,000 years.
Glaciers and ice sheets scoured the landscape, wearing away rocks to form glacial landscapes in the Scottish Highlands, Lake Distract and N. Wales.
In the coldest periods, the ice would have been hundreds of metres thick, and reached as far south as London.
What were the two extensive glaciations in Britain?
Anglian Glaciation
Anglian Glaciation 450 ka
Largest glaciation to affect us during the Quaternary with ice extending across two-thirds of Britain and Ireland down to Oxfordshire and north London, and laterally to the edge of the continental margin from western Ireland around to Norway.
Both the North Sea and Irish Sea were land and occupied by glacier ice.
What were the two extensive glaciations in Britain?
Late Devensian
The second glaciation with a maximum ice sheet extent (Last Glacial Maximum) achieved at around 27ka ago.
British North Sea ice reached the Wash but there is no direct evidence for ice ever reaching into the East Midlands or beyond.
Both the North Sea and Irish Sea were land and occupied by glacier ice.
What global climate was present during the Anglian Glaciation?
‘Mid-Pleistocene Transition’
Smaller, more frequent, ‘precession’ and ‘obliquity’ orbital cycles –> higher magnitude 100ka ‘eccentricity cycles’.
Far colder during ‘cold stages’ and reduce ‘seasonality’.
Pushed Polar Front further southwards and for much longer periods of time leading to parts of Britain becoming arctic tundra with the ground frequently becoming either permanently or seasonally frozen (permafrost).
What global climate was present during the Late Devensian?
100ka ‘eccentricity’ orbital cycle dominates and drives big oscillations in climate between 5 warm temperate interglacial stages and 6 cold glacial stages.
During cold stages, the effect of this climate forcing is to promote the rapid build-up of ice volume within high and mid-latitude areas.
Hubbard et al., 2009
What does the model do?
Numerical modelling procedure to see the effect of ice streams, model looks at changes of air temperature and grounding lines.
Understanding the locations and dynamics of palaeo-ice streams is important for understanding palaeo-ice sheets.
This is because their large ice flux would have effected ice-sheet configurations; investigations on former ice-streams helps understand glacial processes
Hubbard et al., 2009
What does the model find?
The modelled BIIS is extremely dynamic, drained mainly by a number of transient but recurrent ice streams which dynamically switch and fluctuate in extent and intensity on a centennial time-scale.
A series of binge/purge, cycles are identified which correspond to alternating periods of relatively cold-based ice, and wet-based ice with a lower aspect ratio, characterised by streaming- of similar periodicity to Laurentide ice sheet.
Hubbard et al., 2009
What do the phases of predominant streaming activity coincide with?
Periods of maximum ice extent and are triggered by abrupt transitions from a cold to relatively warm climate, resulting in major iceberg/melt discharge events into the North Sea and Atlantic Ocean.
Hubbard et al., 2009
The broad chronology of the modelled BIIS indicates a maximum extent at…
∼20 ka, with fast-flowing ice across its western and northern sectors that extended to the continental shelf edge.
Boulton, 2010
What does the model do?
Analysis of subglacial drainage systems within the BIIS, Vale of York ice stream analysis.
Developed ice stream flow velocity modelling for different areas of the ice sheet to deduce the extent of Devensian ice stream termini.
Boulton, 2010
What does the model find?
- The ice sheet lobe advanced to its maximum extent driven by a very low gravitational shear stress of about 5–8 kPa.
- As such large ice fluxes could not be sustained over the whole terminal zone of the ice sheet, such a high velocity must have been restricted to features such as the Vale of York lobe, which must therefore have acted as an ice stream. There is currently no evidence, however, whether this stream was ephemeral, the result of a surge event or was sustained for a longer period.
- The effective pressure at the base of the ice stream in its terminal zone was very low. The ice stream was therefore very close to a buoyant state.
- The subglacial drainage system was a self-organised system largely adapted to the seasonally varying pattern of surface water recharge to the bed.
Names
Clark et al., 2012- Remote sensing data
Bowden et al., 2002- New dating methods
Hubbard et al., 2009- Ice streams
Boulton, 2010- Vale of York ice stream analysis.
Clark et al., 2010 (timings)
What was done?
Reconstructed BIIS from mapping paleoecological data in the BRITICE database alongside cosmic (imminence) dating to determine age maps of retreat in the UK between 27 and 15 ka BP.
Remote sensing data presented maps of moraines, meltwater channels, eskers, and drumlins and a methodology of how to interpret and bring them together.
For the continental shelf, numerous large moraines were discovered recording an extensive pattern of retreat stretching from SW Ireland to the Shetland Isles.
What is cosmogenic nuclide dating?
Cosmogenic nuclide dating uses the interactions between cosmic rays and nuclides in glacially transported boulders or glacially eroded bedrock to provide age estimates for rock at the Earth’s surface.
Cosmogenic nuclides are rare nuclides that form in surface rocks because of bombardment by high-energy cosmic rays.
When particular isotopes in rock crystals are bombarded by these energetic cosmic rays neutrons, a spallation reaction results.
Spallation reactions are those where cosmic-ray neutrons collide with particular elements in surface rocks, resulting in a reaction that is sufficiently energetic to fragment the target nucleus.
These cosmic rays do not penetrate deep into the earth’s surface. This is important for glacial geologists, as it means that surfaces that have had repeated glaciations with repeated periods of exposure to cosmic rays can still be dated, as long as they have had sufficient glacial erosion to remove any inherited signal.
Clark et al., 2010 (timings)
What was found?
27 ka- maximum extent
23 ka- ice ice withdrawing along N boundaries and advanced along southern
19 ka- widespread marine based ice loss in both North and Irish Sea
17 ka- ice no longer marine- all terrestrial- Final collapse of all marine sectors occurred by 17 ka BP and with most margins beginning to back-step onshore.
There is a lack of understanding when the ice bridge of the UK-Norway collapsed.
