L12 - Natural Climate Change

How are glacial striations evidence of past glaciations?

• Glacial striations = polished rock surface with fairly linear grooves carved into it

• These are produced as glaciers move over the surface of bedrock.

• These are produced by lumps of rock held at the base of a glacier being dragged across the bedrock.

• Can find the direction of movement in these also.

How are erratic blocks evidence of past glaciations?

• Large boulders of a particular geology which is different to the surrounding geology: glaciers carried these rocks

How is glacial topography evidence of past glaciations?

• Glaciers carve U-shaped valleys

• ‘Hanging valleys’

• Erects (sharp, steep-sided mountain ridges

• Horns (mountain peaks which form at the point glaciers move outwards from)

• Below the horns are circular-shaped valleys/corries

How are morraines evidence of past glaciations?

• Poorly consolidated sediment which is scraped up by the glaciers as it is moving along

• This material is pushed to the edge of the glacier or to the front of the glacier

How are oxygen isotopes evidence for natural climate change?

• ¹⁶O and ¹⁸O are stable isotopes.

• As you move to higher atomic numbers, the more unstable isotopes there are.

• Over time, if there is a water body where evaporation occurs, the water will become enriched in ¹⁸O and the water vapour will be enriched with ¹⁶O.

• The extent of that fractionation (changes in ratios) increases at lower temperatures because the system has less energy so that mass difference becomes more important in governing the age of the molecules.

• More fractionation = colder temperatures.

How does oxygen isotopic fractionation take place during an ice age?

• During an ice age, more water falls at the poles which will be trapped in ice and will accumulate

• Oceans will become enriched in ¹⁸O and depleted in ¹⁶O as it is increasingly locked up in ice sheets.

• This is the onset of glaciation.

• The oxygen in the shells of foraminifera reflect past climates

How does the obliquity of the earth influence seasonality?

• The earth has a tilted rotational axis.

• At times, the northern hemisphere will be pointed towards the sun (more radiation = summer) or away from the sun (less radiation = winter_

• This generates tilt seasonality

How does the earth’s orbit influence seasonality?

• The earth has an elliptical orbit.

• There will be times when the earth is further from the sun or closer

• This creates seasonality.

• In the winter, the northern hemisphere is further away from the sun, so receives less sunlight?

How does contrast between tilt season and distance season in the northern hemisphere influence preservation of ice?

• The tilt season dominates but is opposed in the northern hemisphere by the distance seasonality

• Increase in sunlight in the summate in the northern hemisphere in June and a decreased winter

• Better chance of preserving ice in the northern hemisphere due to reduced contrast

How does contrast between tilt season and distance season in the southern hemisphere influence preservation of ice?

• In the southern hemisphere, the distance and tilt seasons reinforce each other.

• This is important for the longevity of ice

• Generally, ice forms in the winter when there is less energy from the sun and has a better chance of surviving through the summer if the seasonal contrast between summer and winter is lower.

What are the precession of the equinoxes?

• The rotation of the axis describes a cone.

• Over time, although the axis remains at a tilt, the direction the tilt is pointing in changes

• Precession causes reinforcement of seasonal variation due to Earth pointing towards or away from the sun to vary over time

How can thermohaline circulation change?

• As temperatures increase and glaciers and ice sheets melt, more freshwater will be released.

• Modelling shows the circulation is very sensitive to changes in temperature.

• An increase in freshwater, non-saline water, will cause the conveyor belt to stop and reconfigure to a different arrangement.

How does the movement of Homo sapiens align with ice ages?

• Ending of the last glacial period coincides with the migration of humans from the Asian and African continents to Northern America.

• This probably happened because the continental ice sheet began to shrink and opened a land pathway to move from Asia to North America.

• Transition from hunter-gatherer to farming and pastoral societies coincided with the end of the last glacial period