Lecture
14
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Lecture
14
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This lecture will outline the main developments in landscape that have occurred as a result of climatic change in the last 20,000 years. Key climatic ‘events’ (e.g. The ‘loch lomond stadial’) will be highlighted and their significance explained.
Sea level change Glaciation Fluctuation Equilibrium Loch Lomond Stadial Palaeohydrological change Periglacial
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| Deglaciation of the Northern Hemisphere - abstract |
The impacts of Quaternary were discussed last week, but in a broad way. The focus was upon key elements, and it was predominantly a Northern Hemisphere focus. This week, we shall focus on the last 20,000 years in terms of landscape processes and landscape development. The last 20,000 years sees the decline of the 'Ice Age' ice sheets and our entry into the Holocene (10,000 years ago). This is important because many of the key landscapes and landforms that we have are a relict of this time. There are a variety of processes that have shaped the landscape. The main themes covered this week are: |
| Northern hemisphere Ice Sheets and global Climates (PDF file). | |
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Permafrost
and Climate Change |
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At the height of the last glacial, 20,000 years ago, pronounced ice sheets and glaciers over much of the Northern Hemisphere. The last 20,000 years have seen the decline and decay of large amounts of ice in response to climate change. This has changed our environmental conditions - some of the other landscape changes are brought about through the indirect effects of deglaciation (e.g. Hydrology, permafrost decline, sea level change).
At 18,000 bp there were 2 main ice centres in mainland Europe, 1 in the Baltic, another in the Alps. Europe also had other centres in the Pyranees, Massif Central, Corsica, Dinaric Alps. The deglaciation history of the last ice sheets in the Russian federation, Scandinavia & British Isles is also known in approximate outline.
Ice decay began withwidespread thinning and ice retreat by 16,000 bp. The pattern of ice marginal oscillation was strongly influenced by the presence of ice marginal lakes. The Baltic ice lake was a very large ice-dammed lake (IDL) that formed between 12,800 and 10,450 bp - the eventual decantation of this lake into the North Sea may have profoundly influenced patterns of oceanic and atmospheric circulation.
The major ice accumulation and dispersal centres were along an axis extending from the mountains of W. Scotland to the uplands of Wales. At the Late Devensian maximum only the Midlands and Southern England were free of ice. Modelled ice volumes indicate that the ice volume was 800,000 km³ and was 1.5 km thick. Some information indicates that the Scandinavian ice sheet and the British ice did coalesce in the north, but an ice free corridor over 100 km wide separated the ice masses in the North Sea basin.
In the mountainous regions of Europe and N. America there was considerable expansion of valley glaciers. In Alpine Europe glacier advances have occurred intermittently over last 10 ka., most significantly during the Little Ice Age (LIA).
Laurentide & Cordilleran ice sheet melting history is known approximately. The Laurentide ice sheet (LIS) was the largest glacier complex in the northern hemisphere at 35,000,000 km³! The western side of the Laurentide ice sheet was arid and prone to retreat, the eastern side was nourished by snow from the western Atlantic. The LIS was characterised by surging across its southern margin. The drainage of ice-dammed lakes was an important dynamic in its retreat. The largest floods were from glacial Lake Missoula, especially prior to 13,000 bp. The LIS had a widespread accelerated retreat phase from 13,000- 11,000 bp. The surging & dynamism associated with the decay of the LIS has been linked to IDL formation. Another IDL, glacial Lake Agassiz also generated large floods, emptying floodwater into the North Atlantic at around 11,000 - 10,000 bp. This may have triggered some global cooling. The final disintegration of the LIS occurred c. 8000 bp. This final break up produced large floods and an instantaneous rise in global sea level of 0.4m. By 7000 bp only small remnants of the LIS remained - in Labrador, Keewatin & Baffin Island (there is still some ice here now).
From 13-11.5 thousand years bp a major globalepisode of glacier melting was underway. A marked increase in the rate of glacier wastage around the maritime fringes of the northern hemisphere ice sheets may have led to the abrupt chilling of the western North Atlantic surface waters resulting in the short-lived climatic deterioration of the Loch Lomond stadial (Younger Dryas). This period of amphi-Atlantic climatic oscillation between 11,000 - 10,000 bp saw marked readvances in the S. & W. fringes of the Scandinavian ice sheet and there was renewed activity throughout the mountain regions of W. Europe. It occured in response to southward migration of the polar oceanic front in N. Atlantic.
In Britain, a large mountain ice field - 400m thick - developed in the Western Highlands of Scotland and smaller glacial complexes were present in the northern Scottish Highlands and the east. ELAs were of the prderof 300m in W. Scotland, but rose 1000m in Cairngorms - due to the dominant position of fronts - most precipitation came from the south rather than the east (due to the effects of continentality).
Elswhere in the British Isles numerous cirque glaciers formed , e.g. In the Southern Uplands, the Lake District (even Valley glaciers re-formed here), the uplands of north and south Wales & the hills of Ireland. This was the last time that glacier ice formed in the UK. The LLS event seems to only affected eastern parts of USA & Canada, but its effects have been detected globally, including in the southern hemisphere (Chile).
There were few areas with coastal stability during the Quaternary - areas adjacent to the ice sheets in the mid-latitudes display pronounced changes in sea level during the Quaternary. sea level changes are of one of two broad types:
Eustatic - global
changes in sea level connected with the abstraction and return of water from
the oceans;
Isostatic - localised crustal deformation associated with the loading and unloading
of glacial ice
As a consequence we often speak of relative sea level change - the position of sea relative to land. Sea level change is often difficult to ascertain due to complex changes in both oceanic water volume and isostasy , we can only work with shoreline data. At 18,000 bp sufficient water had been removed and locked up in glaciers such that sea level had fallen by 130m. Yet eustatic lowering was accompanied by isostatic depression (glacial loading) in Scandinavia, Britain and Canada.
Following wastage of N. Hemisphere ice sheets, global sea levels rose steadily while thinning and eventual wastage of the continental ice sheets resulted in rapid glacio-isostatic recovery. Shorelines associated with the margins of melting ice sheets were raised above sea level as isostatic uplift outstripped eustatic sea level rise. Raised beaches were rapidly tilted away from the centre of isostatic depression.
Some isostatic changes:
Scandinavia - 700m
of uplift in holocene
E. Scotland - 250m of uplift since deglaciation
Laurentia - 900m since deglaciation
Land emergence has continued during the Holocene - in some areas the land is still rising:
0.9 cm yr - N.
Baltic (shoreline formed in Viking times therefore 8m above that of present
day!)
0.2cm yr in Scotland (total uplift since LLS = 40-50m, still incomplete)
1cm from Laurentide ice sheet!
Eustatic change is difficult to determine against this background of isostasy. This is one of the major limitations of shoreline data. Evidence from tropical oceans suggests that global sea-levels rose by40m from 14-13,000 bp. There was a further 40m rise from 11,000 bp. but by this time, ice volumes had been massively reduced and isostatic recovery outstripped the eustatic component of sea level change in many mid-latitude coastal areas. Shorelines forming in this period stand well above the present day shorelines in certain areas!
In the early Holocene, eustatic sea level rise (1cm per year) began to exceed isostatic rebound - this caused a marine transgression around coastline of Scotland (8.5 - 6.5 ka bp). It is thought that eustatic sea levels are still rising... but it is difficult to separate out.
The present periglacial domain covers 25% of the land surface. At the height of the last cold stage periglaciation affected 40-50% of the land surface. In Europe the permafrost limit was in southern France, and permafrost was formed eastwards along the course of the Danube. There is some evidence for this zone extending south westwards - fossil cold climate phenomena occur in the Balearic Islands & Corsica.
By 13, 000 bp, due to warmer global climatic conditions, there was a thawing of permafrost over large areas, but much remained throughout the Late Glacial period, or reformed in the cold period 11-10 ka bp.
During the Loch Lomond Stadial in Britain there was active, if discontinuous permafrost in Scotland, the Isle of Man & Wales, and seasonally frozen ground in the Midlands and eastern England.
At around 10, 000 bp, permafrost disappeared almost completely from western Europe. Today discontinuous permafrost is only present in Scandinavia & Iceland but periglacial conditions exist in many mountain regions of central and northern Europe and the British Isles. A similar pattern exists in and around northern USA following the decay of the Laurentide Ice Sheet. Large areas have of the northern hemisphere land masses have remained under the influence of a periglacial climate - permafrost characterises 80-85% of Alaska and 50% of Canada. In addition around 100,000 km² of Alpine permafrost still exists.
Rivers are very sensitive to climatic change. The principal factor involved in river regime change is precipitation, but the impact of rainfall variations on fluvial systems is moderated by other landscape components- e.g. soils. There have been large variations in river incision, terrace development, channel change and sediment loading over the past 20,000 years. These can all be directly or indirectly related to climate change.
Within hydrology, there is the problem of equifinality. For example, terrace formation results from changes in river level, but the triggers are complex , attempting to infer the major controlling variable involves multiple working hypotheses, eliminating all but one control; but this may not always be possible with field evidence (still we should bear in mnd that “Absence Of Evidence Does Not Mean Evidence Of Absence...”
During the deglacial phase of glacial cycles, rates of sediment yield increased to 10 times the geological norm as a result of meltwater activity. Enormous quantities of outwash are generated. In newly deglaciated areas channels are (characteristically) initially broad and steeply graded with low sinuosity; they then change to high sinuosity low gradient channels. Changes back and forth are typical of increased meltwater discharge. Meltwater channels formed beneath ice or at ice margins were exposed and have become distinctive elements of these formerly glaciated landscapes.
Lakes - e.g. Proto Great Lakes - several hundred km².
Outbursts from IDLs had a huge influence on geomorphology. For example, 40,000 km² of water inundated parts of North America in one event. On any scale, this is an enormous flood.
During the early Holocene stable river channels existed with smaller discharges than when the land was ice covered. The climate became wetter after 7,000 bp, causing increased stream loading, aggradation and incision. During the last 5,000 years, human activity has also had a significant impact on rivers and lakes.
Changes in climate brought changes in dominant process environments and the development of landscape. In northern hemisphere the key changes outlined above were profound, especially over the 20,000 year time scale.
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