CHAPTER 10
What Controls the Size of Ice Sheets?
- See Fig. 10-4. Ice covered North America, Europe, and Asia 20,000 years ago. New York was completely covered!
- The cooling trend over the last 55 My caused these ice sheets to be able to form but there have been periods of growth and melting over orbital-scale time periods.
- Ice sheets grow if summer melting (ablation) is less than winter growth (accumulation).
- Snow accumulates and turns into ice at high latitudes or at high altitudes where summer temperatures remain cool.
- Ice can accumulate at an annual mean temperature of 10oC (50oF) and the rate of growth is typically 0.5 meters per year. In contrast, ice will melt when annual mean temperatures are above -10oC (14oF) which correlates to summer temperatures above 0oC (32oF). Ablation can reach 3 meters per year which is much greater than the growth rate of 0.5 meters per year.
- Therefore, summer insolation is the PRIMARY CONTROL on the size and extent of ice sheets because it determines the rate of ablation (melting.)
- Ablation is caused by three factors:
- Amount of insolation
- Warm air masses or rain
- Calving of icebergs to oceans or lakes (One might be mooooved to say that this is a cattle-ist to ice melt.)
- See Fig. 10-2. According to the Milankovitch Theory, northern hemisphere ice grows when the Earth's tilt is less and the Earth is in the aphelion position in summer. Ice melts when the tilt is greater and the Earth is in the perihelion position in summer.
- SKIP Section 10-2.
- Ice sheets take time to melt so there will be a lag between maximum summer insolation and minimum ice volume. This phase lag is thousands of years. See Fig. 10-9.
- See Fig. 10-10. When ice sheets grow, the weight of the ice begins to depress the bedrock underneath. If the bedrock is depressed, the elevation of the ice surface will be lower. Lower altitudes are warmer than higher altitudes.
- Bedrock depresses in two stages:
- Elastic response (30% of total response) - the "immediate" sinking reaction to the weight of the above ice
- Viscous response (70% of total response) - a much slower sinking to the adjustment of the underlying asthenosphere, around 15,000 years.
- Because the slower viscous response is more important than the faster elastic response, bedrock depression causes a positive feedback. See Fig. 10-11 (top).
- When ice melts and the weight above is less, there is a bedrock rebound similar to the above responses but in an opposite sense. There will be a quick elastic rebound followed by a much slower viscous rebound. Today, parts of Canada (Hudson Bay area) and Scandinavia (Baltic Sea area) are still rebounding from ice melt that occurred 7,000 years ago!
- When ice sheets begin to melt, the slower viscous response keeps the ice at a lower, warmer elevation so there is a positive feedback also. See Fig. 10-11 (bottom).
- Fig. 10-12 shows the full cycle of ice growth and decay. THIS SHOULD BE WELL-UNDERSTOOD!
Evolution of Ice Sheets
- See Fig. 10-13. The long-term evolution of ice sheets reflects the interaction between two factors: the slow global cooling over the last 3 My (when glaciation first occurred) and the shorter-term changes in summer insolation. Ice sheets grow when summer insolation falls below a critical glaciation threshold.
- Fig. 10-14 shows the four possible scenarios.
- Preglaciation - where no ice can accumulate
- Small glaciation - ice only exists during weak summer insolation but melts completely during summer insolation maximum
- Large glaciation - ice exists for much of the period due to "weaker" summer insolation maximum
- Permanent glaciation - ice persists through all insolation maxima
- The best records of glaciation come from the ocean via:
- Ice-rafted debris - a mixture of sediments delivered to the oceans by melting of calved icebergs
- δ18O records from shells - which relates to ocean temperatures and amount of ice sheets (recall Chapter 7 notes.) More positive values mean cooler climate (more ice.)
- See Fig. 10-15 which shows a δ18O record from a North Atlantic Ocean sediment core. This record shows that glaciation has increased in the past 3 My but has cycled around the 41,000 year tilt cycle until about 0.9 My ago. This would be called a small glaciation phase seen in Fig. 10-14B.
- From 0.6 My ago to present there appears to be a 100,000 year glaciation cycle which is much more like the large glaciation phase seen in Fig. 10-14C. The 100,000 year cycle is investigated in CH. 12.
- Skip sections 10-9 and 10-10.
Helpful Links:
Geology 150 - Milankovitch Theory and Ice Sheets
Glaciation
