British glacial land systems Flashcards
lithostratigraphy
- tills
- interglacial deposits
- fluvial deposits
biostratigraphy
- pleistocene mammals
- pollen, beetles, chironomids
climatostratigraphy
record of warm & cold episodes
chronostratigraphy
construction of timing of events
23ka cycle in Britain characteristics
- recessional cycles
- high N. hemispheres insolation values
- low amplitude climate fluctuations
- glacial extent, minimal/no terrestrial evidence
- maritime coastal processes/low energy fluvial
- soil forming processes
- chemical/biological weathering
- high relative sea levels
evidence for 23ka controlled climate
- East Anglia (shallow marine deposits)
- Corraline Crag
- Wroxham Crag
- Red Crag
- Crag - sandy, shell rich shallow marine deposit
41ka cycle in Britain
- obliquity cycles (tilt)
- more variable N. hemisphere insolation values
- moderate amplitude climate fluctuations
- covers period prior to 0.9Ma to 2.6Ma
- upland glaciation
- periglacial processes now influence
- generation of coarser grained deposits
- higher energy fluvial activity
- maritime coastal processes
- soil forming processes, less influential
Evidence for 41ka controlled climate
Major rive system active
- Thames and Bytham, draining Wales and Midlands
- evidence in Thames of volcanic gravels from wales
- identified by detailed petrographic studies, glacial striations on clasts
- acid igneous rock
- found in terrace deposits MIS 68/1.78 Ma
- suggestions of higher energy.more active geomorphic systems due to enhanced cold/glacier in mountain regions
100ka cycle in Britain
- eccentricity cycles
- highly variable N hemisphere isolation values
- high amplitude extremes of climate fluctuations
- covers period: present to 0.9Ma
- continental scale, lowland glaciation
- Periglacial processes highly influential (permafrost melting, gelifluction)
- meltwater processse
- rivers highly efficient, highly erosive
- soil forming processes in warmer part of cycle
- coastal processes active in warm part of cycle
- considerable landscape changing processes
evidence for 100ka controlled climate
- major glacial episodes in the UK
- 3 main glacial episodes reflected in the stratigraphic record (East Anglia)
- pre anglian
- Anglian (450,000 years BP)
- Devensian (115,000 - 15,000 years BP)
- cold episode post Hoxnian
- evidence wiped out by Devensian ice
Key till units - pre Anglian, Happisburgh Till
- Anglain, lowestoft Till
3 stage British glacial stratigraphy
1) Soils and shallow marine
2) Major rivers, upland glaciation
3) Glacial-interglacial cycles
complex offshore glacial history
Ice Rafted Debris (IRD)
- particles greater than 150 microns deposited by ice bergs
- source ID by chemistry (Crustal geology)
- British sources, Canadian, Greenland, Icelandic sources
Last glacial: UK
- ice development and expanion post 115ka BP (devesian ice)
- Bowen et et. 2002
- identified for the first time that the last British-Irish ice sheet was not static and had multiple advance and retreat phases
- the LGM was one of theses
Last Glacial Maximum: UK
- subsequent more detailed studies have added detail to this
- problems of extent limited by lack of off shore data
- clark et al. 2010 GSR (BRITICE large-scale mapping project)
- maximum extent achieved after 27ka BP
off shore mapping
- fishin boats
- sonar and bathymetry
- ability to identify offshore ice limits
- moraine ridges and outwash fans
onshore mapping
identification of moraine ridges utilisation satellite data:
- digital terrain models
- ice maxima on land
- ice recessional positions
- Meltwater channels
- directional indicators
BRITICE
- 1480 moraine ridges mapped
- large moraines at continental shelf edge represent grounded ice
- many still stands during retreat
- bathymetric contours indicate regions which would have been regarded as terrestrial
LGM subglacial bedforms
- mapped remotely and in the field (drumlins, meltwater channels, eskers)
- 39,000 landforms mapped
- reveal complex patterns of ice flow (Cross cutting relationships switching ice flow patterns)
- hughes et al. 2010 journal of maps
confluent ice masses
- individual ice lobes and flow patterns are traced clearly
- possible to identify where Scandinavian and British ice were confluent and unzipped during deglaciation
timing of glacial maximum
- based on 931 fates around the ice margin
- variety of types of dates
- organic/biological material (beetle and plant fragments along with pollen and lake mud)
- sand (Luminescence)
- Bedrock/boulders (Cosmogenic nuclide dates)
retreat: 27-18ka BP
As ice shank back it:
- reduced size
- changed shape
- broke into different centres of ice dispersal (multiple ice domes)
glacial conditions
- 41ka – enhanced geomorphic processes, fluvial transport of glaciated clasts
- 100ka – direct glacial transport of material from remote regions plus enhanced cold-warm fluctuations
Evidence of maximum extent
- Limited/disturbed in terrestrial realm
* Marine sequences preserve more detail, recording numerous more extensive glaciations than terrestrial record
section summary
• Complex Quaternary history driven by Milankovitch variation
• Limited to very intense geomorphic activity
- British isles have experienced multiple phases of glaciation throughout the Quaternary
- each subsequent phase has re-moulded previously glaciated terrain making it very difficult to unravel patterns
sub-Milankovitch scale
- higher frequency fluctuations
- less than 23ka
- operating over 100s-1000s year timescales
- too rapid to be explained by orbital cyclicity
- time period: lateglacial to holocene
lateglacial period
- 15-8Ka BP
- characterised by many short-lived high magnitude climate shifts
- variable duration
- variable effects on the environment
sub-Milankovitch events: UK
- younger dryas (12000 years BP)
- 8.2 event (8000 years BP)
- little ice age (1550-1850 AD)
sub-Milankovitch drivers
- thermohaline circulation changes
- volcanic aerosols
- solar variations (Maunder minimum)
re-initiation glaciation
- year round snow cover and consequent build up of ice in Scottish mountains
- requires mean annual temp decrease of 2.5-3.5
- ELA currently 100m above summit of Ben Nevis
Traditional view
- glaciation decline through lateglacial interstadial warm period
- no ice at low levels
- younger dryas ice accumulated and expanded from an essentially zero point
alternative view
- large bodies of ice persisted to sea level
- ice then expanded from this position during the younger dryas
rapid communication
- ice caps existed throughout the lateglacial interstadial in northern Scotland
- evidence for this is based on dates obtained on rock exposures ages from terrestrial and submarine moraines ridges
- these yield ages of 13,000-14,000 years BP
late surviving ice unlikely
- glacier response to incursion of warmer oceanic waters
- dating evidence poor
- ages presented are not presented with full errors (i.e. 2 sigma uncertainties)
Wester Ross readvance
- related to cold event in Greenland
- others suggest earlier event
- chronological uncertainties
little ice age
- not thought to be forced by meltwater
- no evidence in marine core records
- variations in solar output rather than orbital parameters Effect on the UK?
- permanent snow in Ben Nevis
- lowering of snowline by 300-400m
- increased strominess
- no glaciers
not a straightforward picture
- ice marginal positions are becoming better defined
- many controversies exit, most are related to being unable to precisely date ice margins
shaping of our landscape by multiple processes
related to glacial activity
- Direct glacial erosion
- Intensified geomorphic activity in the extraglacialregions (i.e. enhanced fluvial activity)
- Periglacial and permafrost activity
Investigation of the mechanisms that forced ice advance and retreat in the past
- Provides us with better handle on how it may occur in the future
- Allows us to mitigate or adapt
