The Glaciation of Northern England

The Devensian Glaciation of Northern England
I felt I should bite the bullet, and translate some of my papers for the general public. And so, below is what is otherwise:

Davies, B.J., Roberts, D.H., O Cofaigh, C., Bridgland, D.R., Riding, J.B., Phillips, E.R., and Teasdale, D., 2009. Interlobate ice-sheet dynamics during the Last Glacial Maximum at Whitburn Bay, County Durham, England. Boreas 38, pp. 555-578

The paper is available to download from Swetswise (if you have a license / are a member of university). Please use the citation above. Whitburn is a town north of Newcastle, and there is a car park and tea shop next to the beach, making it an easy site to visit.

Introduction
This paper examined the glacial sediments in coastal exposures at Whitburn Bay, Co. Durham, and used heavy minerals and stone lithologies to determine the provenance (source) of the ice lobes that deposited them. These glacial sediments were deposited during the Last Glacial Maximum, that is, the most recent period of maximum extent of the British Ice Sheet. This occurred 29 - 21,000 years before present.

Map showing the location of Whitburn Bay, north-eastern England

The last British-Irish Ice Sheet was a highly dynamic ice sheet. It reacted rapidly to external forcings by the atmosphere and oceanic currents. Modelling by people at Aberystwyth University has suggested that the dynamic cycles of the ice sheet occurred in step with temperature variations recorded in ice cores from Greenland.

The Tyne Valley Glacier
The map above shows the Tyne Gap. Whitburn Bay has two tills; these are sediments deposited directly at the base of a glacier. During the LGM, a glacier flowed along the Tyne Gap and out into the North Sea. It deposited a well-consolidated till with Pennine stones - Carboniferous limestones and shales - indicating its western provenance. Upon recession of the Tyne Glacier, numerous fluvio-glacial landforms were deposited in the Tyne Gap.

A boulder pavement was deposited at the top of this first till. You can clearly see the boulders, at the same height, in the photograph below. The boulders are orientated in the same direction, faceted and striated, and have flat tops. Water-lain gravels are interbedded beside and between the boulders. These boulders melted out from the base of the Tyne Valley glacier as it quiesced, stagnated and melted. Finer material was washed away by abundant meltwater at the ice-bed interface.

Photographs of the lower till from the Tyne Valley glacier at Whitburn Bay.

The North Sea Lobe
After the recession of the Tyne Valley glacier, the North Sea Lobe surged forwards. This glacier had its headlands in northern Scotland, and contains lots of Scottish igneous and metamorphic pebbles and minerals. The North Sea Lobe trapped many proglacial lakes between it and the higher ground inland, including Glacial Lake Wear and Glacial Lake Humber. Dates on mosses at Skipsea, further south, indicate that the North Sea Lobe existed after 21 ka BP. Dates on Glacial Lake Humber in the Humber estuary indicate that it was still in existence at 16 ka BP.

The North Sea Lobe was a strong and dynamic part of the British-Irish ice sheet. The Tweed Valley ice stream was diverted southwards by the North Sea Lobe. Orientation data preserved in the glacial sediments along the northern English coastline show that it consistently flowed onshore.

Ice flow pathways during the Last Glacial Maximum

Ice Marginal Canals
Incised downwards into the upper North Sea Lobe till at Whitburn Bay are numerous bedded sands and gravels. They range from well-sorted, low energy sands and clays to high-energy sands and gravels. Some of them are tightly folded and deformed - see below.

These sands and clays represent a very late-stage ice marginal subglacial drainage system. As the amount of water at the ice bed exceeded the carrying capacity of the groundwater and porewater pressure of the tills, it was evacuated by drainage channels. In places, the groundwater carrying capacity was exceeded dramatically and explosively, with water bursting out as a hydrofracture.

This helps us understand the deglaciation of northern England. During the late stages of the Last Glacial Maximum, the ice sheet was warm based and well lubricated at its bed. Abundant meltwater was rapidly evacuated by subglacial channels. Ice marginal lakes were present to the west of the ice lobe, dammed by the ice to the east and the higher ground to the west. The ice probably melted and retreated rapidly in response to a warming climate.

Scandinavian Ice in the North Sea Basin
A large question is why the North Sea Lobe was flowing onshore during the LGM. Why did it divert the Tweed Ice Stream southwards? I think that the North Sea Lobe was constrained in the North Sea by the Scandinavian ice sheet. Contact in the North Sea did not the lobe to spread eastwards as would have been expected.

In addition, the sediments immediately to the coast of northern England are potentially more 'slippy', aiding faster ice flow in this direction. A forebulge in the North Sea from the Scandinavian ice sheet could have resulted in higher ground to the east of the North Sea Lobe, also directing it southwards.

In my opinion, these factors are only small. There must have been contact between these ice sheets at the Last Glacial Maximum. Dates from open-marine shell fauna in the northern North Sea indicate open-marine conditions at 22 ka BP. I think that the ice sheets were confluent at 29 ka BP, and this provided a mechanism to turn the ice sheet southwards. Upon recession of the Scandinavian ice sheet, the North Sea Lobe continued to flow southwards with minimal eastward relaxation, perhaps aided by slippier sediments and the forebulge. The reaction time of ice sheets is slow enough for the North Sea Lobe to not have had time for all the flow to resume a more natural, eastwards direction.

Antarctica's ice shelves and sea level rise

Ice Shelves of the Antarctic Peninsula
Antarctica is fringed with floating ice shelves. They are floating extensions of the Antarctic Peninsula ice sheet, and can be very large indeed. The Ross Ice Shelf is the size of France - see the map below. However, in recent years, the dramatic and rapid loss of ice shelves fringing the Antarctic Peninsula have hit the news.

The Antarctic Peninsula is the third ice sheet on Antarctica. The West Antarctic Ice Sheet (WAIS) and East Antarctic Ice Sheet are the other two. The Antarctic Peninsula Ice Sheet (APIS) is of great interest because it is the most temperate of the three ice sheets, and contains up to 0.6 m sea level rise (in addition to the +1.0 m sea level rise predicted by the IPCC over the next 100 years).

The Antarctic Peninsula. Credit: USGS

The disintegration of the Antarctic Ice Shelves
The first ice shelf to dramatically disintegrate was the Larsen B ice shelf. Read about it in a British Antarctic Survey press release here. The Larsen B Ice Shelf began to collapse in 1995, and disintegrated over a matter of weeks. An armada of icebergs was dispatched into the Weddell Sea. More recently it appears that the Wilson Ice Shelf has begun to disintegrate. This has been accelerated by a 3C warming over the last 50 years; the Antarctic Peninsula is one of the most rapidly warming places on Earth.

The Impact of the Loss of the Ice Shelves
As ice shelves float at their specific density, their melting has no direct impact on sea level rise. You can test this yourself. Find yourself two water glasses. Put two 'ice bergs' (ice cubes) in one and fill it with water so that the ice cubes float. Mark the water level on the glass and note what happens when the ice melts - it will stay at the same level. With the second glass, put in four or five ice cubes and fill the glass so that most of the ice cubes are covered but they are not floating. When the ice melts, the water level in the glass will rise.

This is an important point to note. Melting ice shelves, like the floating ice cubes, will not cause the water level to rise. However, ice shelves are an important part of the glacial system, and strongly effect the ice dynamics within the continental ice sheet. With their loss, internal continental, non-floating ice speeds up, and calves more ice bergs into the ocean, rising sea level.

This works in the following manner. Ice shelves are pinned and grounded at certain points; they form in large bays and are constrained at their edges. They hold back the onshore fast-flowing ice streams. Ice streams are corridors of faster-flowing ice that is melted at its base. Ice streams drain the majority of the ice from ice caps, and contribute to the majority of ice dynamics. The ice streams are the same as the ice cubes that were not floating, and by adding more ice to the glass, the water level will rise.

Ice streams of the WAIS

Since the collapse of some of the ice shelves fringing the Antarctic Peninsula, outlet glaciers and ice streams in the vicinity have increased in speeds of up to 5 times, releasing large amounts of meltwater and ice bergs into the ocean. This of course is directly relevant to sea level rise. The role of sea ice is important as sea ice protects ice shelves from waves and storms, which can destabilise the ice shelf and aid its collapse.

The collapse of the ice shelves is one of the most visible aspects of modern climate change. It is very important to act now to curb global warming, to protect these fragile systems. Their destruction could result in real and rapid sea level rise, threatening our coastal towns, cities and wildlife habitats. With a large majority of the world's population living near the sea, we cannot afford to ignore this problem.