The Nature and Impact of the Last Ice Age
Importance
The most recent major global environmental change
Significant impact on landscape, the biosphere and humans
Sets the context for current environmental change
Objectives
To identify the general features of the Last Ice Age
To evaluate processes, concepts and impacts

The Last Glacial Maximum
A note on dates:
Use radiocarbon dates but precise ages vary
Last glacial maximum – 18,000 yrs BP in radiocarbon dates
21,000 years ago in “real years”
Lower insolation at the beginning of the ice age
By the maximum, isolation was the same as today
Ice sheets and lower greenhouses gases influential

CLIMAP
CLImate MApping and Prediction Project (CLIMAP)
Maps to show surface temperatures
Today, Antarctic and Greenland ice sheets cover 3% of Earth (10% of land surface)
Glacial maximum – 7% of Earth (25% of land)
4 degrees colder than today
North Atlantic – 8 degrees cooler


Quaternary: Period comprising 2 Epochs
Pleistocene = most recent Ice Age
In general - 30% Earth’s surface covered by ice (today = 10%)
There were several minor retreats during this period (INTERSTADIALS)
Major retreat = INTERGLACIAL
Holocene (Recent) since 10,000 years
The Quaternary Period & its Subdivisions

Palaeomagnetism
Today Earth has a “normal” field
but magnetic  fields can reverse
Some rocks and sediments preserve the signal at deposition
Enables global dating system
Short intervals of normal or reverse polarity (150,000 years) = EVENTS
Longer intervals = EPOCHS

Evidence
Polar and continental ice cores have also given evidence of environmental change (especially in last 160 K years)
Numerous PHASES
 most recent in North American WISCONSIN

Ice sheets
Laurentide ice sheet
Contained the most ice, centered on east-central Canada
Cordilleran ice sheet
Smaller, centered on Rockies
Scandinavian ice sheet
Northern Europe
Barents ice sheet
Northern Eurasia
Also expanded Greenland and Antarctic ice sheet and smaller mountain glaciers

Volume of Ice
Still debated due to uncertain lines of evidence
e.g. how large were the ice shelves?
Especially concerning ice thickness
Strongly influences sea level
Ice sheets weigh down underlying bedrock by 30% of the ice thickness
At the last glacial maximum sea levels dropped by ~110 metres

Sea Level Changes
EUSTATIC CHANGES
changes in sea level which are a consequence of variations in the amount of water in the oceans
GLACIOEUSTATIC
changes in sea level specifically a result of ice sheet activity
ISOSTATIC MOVEMENTS
movements in sea level due to adjustments of Earth”s crust

Winds and Dust
Erosive glaciers produce a great deal of loose debris
Transported to margins which have little vegetation
Winds remove fine particles ‘
Creates extensive deposits (LOESS)
Also evident as dust layers in ice cores
Lot of sodium and chlorine (salts from stormy seas)
More from low latitudes (expansion of deserts)

Deserts
Reasons for expansion:
Northward movement of African plate
Uplift of Tibetan plateau creating a dry easterly jet stream (air subsides)
Build up of ice created a steeper temperature gradient
Cooling of the ocean reduced evaporation

Temperature Gradients
Difference between the temperatures at the equator and the poles
Determines the location of the subtropical high and the intertropical convergence zone
For every 15 degree increase in the temperature gradient
A 10 degree shift (South) in the location of the subtropical high
Amplified in the ITCZ – so it moves even further South
Hence desert areas are broader

Atmospheric Composition
Carbon dioxide
Greenhouse gas absorbs longwave terrestrial radiation
Low levels lead to cooling
Cooler oceans absorb more CO2
Last glacial maximum ~30% lower levels of CO2
Methane (CH4)levels are also lower during glacials
Perhaps because high latitude bogs are covered by glaciers
Last glacial maximum ~50% lower levels of CH4

The Biosphere in the Last Ice Age
Use of pollen data to reconstruct vegetation
Preserved in sediments that can be 14C dated
Much of North America was covered by boreal forests
Dominated by pine and spruce
Europe- mostly tundra
Beringia – also mostly tundra


Fall in sea-level
Migrations between land masses
E.G. Between Alaska and Sibera, and Australia and New Guinea
Also in response to warming and cooling, moves to north and to south
Glacial advances also associated with increased aridity, but in some cases some areas became wetter

Environmental Change in the Late Glacial Period
At the end of the last glaciation temperatures began to rise
But there were two cold periods
Older Dryas
Younger Dryas

Environmental Change in the Late Glacial Period
Ice-covered ... re-established ecosystems (soils)
Non-ice-covered ... also ecosystem changes (trees)
Timing varies regionally
Record shows floral, faunal, soil and human changes

The Interglacial Cycle
Fossil evidence for end of last glacial similar to previous occaisions
Important to use small sampling intervals in pollen analysis (1 cm) to resolve short timescale changes
Iverson’s model of ecological processes in interglacials
Cycle of 4 stages

Ecological change in Interglacials
1. Cryocratic - cold, continental
2. Protocratic - warming
3. Mesocratic - temperate
4. Telocratic - cooling
Plants may not be the same for different interglacials
May be evident as changes in altitudes of plant communities

Ecological change in Interglacials
1. Cryocratic - Alkaline soils, Alpine plants
2. Protocratic - Grasslands, open woodlands
3. Mesocratic - Fertile, brown soils, temperate deciduous woodland
4. Telocratic - Acidic, nutrient-depleted soils and peats, conifers

Climate Changes
Ice ages end abruptly (few ‘000 years)
From radiocarbon dating
From palaeo evidence
End of glacial - cold - before the holocene (younger dryas period)
Increase in O 18 in ocean sediments (cold)
Not due to the milankovitch cycles

Effects on Insects
- Useful for reconstruction because of sensitivity to climate
Indicate rate of temperature change e.G. Warming at 13K 2.6 degrees per century
Pollen data disagrees because of time lag
Respond slowley and also influenced by poor soils (e.G. Oak, elm)
Problems:wind blown pollen, insect response to microclimates

Late glacial temperatures
High temperatures13 to 12K, and 10K
Correlates with increased snow in greenland
Changes in the position of the polar front
The boundary where warm air rises over cold polar air
Where mid-latitude depressions form here

The Polar Front
As ice sheets retreat, it moved north-east (NW europe warmed)
It therefore also produced more snow in greenland
During the cooling (younger dryas ~12 to 10K) moved back south
As the temperatures across the front change rapidly, its movement explains why we see such sudden climate changes

The Younger Dryas in Canada
Also experienced late glacial warming and cooling trends
E.G. Onset of cooling from ~ 12K to ~ 10K
Glaciers readvanced in quebec
Considerable cooling in new brunswick (proximity to atlantic)
Other regions also affected

Worldwide changes in the Younger Dryas
Numerous regions also show climate changes
E.G. South america, vegetation belts fall in the mountains by 300-400 m
Reduced rate of sea level rise (reduction in ice sheet melting)
Also lake level declines (aridity)

Faunal Changes in the Later Glacial Period
Much evidence suggests the role of humans
The late glacial is more than just a period of warming (two cooler periods)
Role of climate change is not simple
Faunal Changes in the Later Glacial Period
Loss of plants during younger dryas may have particularly affected herbivores
Most extintions occur between 12 to 8 K
E.G. Deer in ireland (younger dryas) before humans colonised
Extinct Irish Elk