The end of the last glaciation and the arrival of the Holocene

Overview

Loch Lomond Stadial, stadial and an interstadial and saying the Holocene has been unusually stable.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   When was the Last Glacial Maximum, and what did the ice look like over Britain and Ireland?

   Model answer

   The Last Glacial Maximum, the point of greatest ice extent in the last glacial, was reached roughly 26,000 to 20,000 years ago. At that time an ice sheet covered virtually the whole of Ireland and Scotland and most of England and Wales as far south as a line running approximately across the Midlands, and it was confluent with the Scandinavian ice sheet across the North Sea. Global sea level stood roughly 120 metres below its present position because so much water was locked up in ice, so the Irish Sea and North Sea basins were largely dry land and Britain was joined to the continent. Beyond the ice margin lay permafrost tundra.

2. 2

   What triggered the end of the last glaciation, and what amplified it?

   Model answer

   The trigger was orbital: from about 20,000 years ago the combination of increasing obliquity and a favourable precessional phase raised summer insolation at high northern latitudes, so that summer melting began to exceed winter accumulation and the great ice sheets started to lose mass. The trigger alone was far too weak to remove ice sheets kilometres thick, so feedbacks did the work. Retreating ice exposed darker land and ocean, lowering albedo and increasing absorption of radiation. The warming ocean and changes in circulation released carbon dioxide, which rose from about 180 to about 280 parts per million, strengthening the greenhouse effect. Warmer air held more water vapour, itself a greenhouse gas. Rising sea level also floated and destabilised the marine margins of the ice sheets.

3. 3

   Describe the shape of the temperature curve through the last deglaciation, as recorded in Greenland ice cores.

   Model answer

   It is not a smooth ramp but a stepped and interrupted rise. From about 20,000 years ago there is a slow, irregular warming out of full glacial conditions. At about 14,700 years ago the curve jumps abruptly upward by several degrees within decades into a warm interstadial. It then declines unevenly for a couple of thousand years before plunging back at about 12,900 years ago into a cold stadial almost as severe as the glacial itself, which lasts about 1,200 years. At about 11,700 years ago it rises again just as abruptly, by of the order of ten degrees in a matter of decades, and then flattens into the comparatively level line of the Holocene. The two abrupt steps and the intervening reversal are the diagnostic features.

4. 4

   What was the Loch Lomond Stadial, and when did it happen?

   Model answer

   The Loch Lomond Stadial was a short, sharp return to near-glacial conditions between roughly 12,900 and 11,700 years ago, immediately before the Holocene began, and it interrupted an otherwise continuing warming. It is known internationally as the Younger Dryas. Mean annual temperatures in Britain and Ireland fell by several degrees, permafrost and tundra vegetation returned, and glacier ice regrew after having almost entirely disappeared during the preceding warm interval. It lasted around 1,200 years and ended even more abruptly than it began. It is the clearest demonstration in the record that deglaciation was not a one-way process and that the climate system can flip within decades.

5. 5

   Why is the Loch Lomond Stadial named after Loch Lomond, and what evidence in the landscape shows it happened?

   Model answer

   Because the limits of the ice that regrew during it are marked by a well-preserved arc of moraines around the southern end of Loch Lomond, which allowed geologists to map the extent of the readvance. During the stadial a small ice cap re-formed over the western Scottish Highlands, centred on the Rannoch Moor area, with valley and corrie glaciers radiating from it, and separate corrie glaciers formed in the Lake District, Snowdonia and the Irish uplands. The evidence is the fresh, sharp-crested moraines and drift limits marking those glaciers, which overlie and are much less weathered than the deposits of the main glaciation, together with widespread periglacial features such as fossil ice-wedge casts and solifluction deposits beyond the ice limits.

6. 6

   Explain the leading account of what caused the Loch Lomond Stadial.

   Model answer

   A disruption of the North Atlantic circulation. As the great Laurentide ice sheet over North America melted, enormous volumes of cold fresh meltwater ponded in ice-dammed lakes and were released into the North Atlantic. Fresh water is less dense than salt water, so this pulse capped the ocean surface and prevented the cold, salty surface water of the far North Atlantic from sinking. That sinking is the engine of the thermohaline overturning circulation which drags warm water northwards in the North Atlantic Drift, so the circulation weakened or shut down and the flow of tropical heat to north-west Europe was cut off. The region cooled sharply while the effect elsewhere was much smaller, which is why the event is most strongly expressed around the North Atlantic.

Other ways to work this

The same content, four other ways.

What is inside Levo

Six ways to work the same material.