Handout, page 1

Under items to be explained: We want to know why extensive glaciation came in the Pleistocene, and why did glaciation cycle throughout the Pleistocene with a period of about 100,000 years?

Under causes intrinsic to the Earth: it is not convincing that volcanoes spewed enough junk into the atmosphere that would reflect enough light away to cool the Earth and cause a glaciation. Volcanoes have existed throughout time and they certainly have erupted during our lifetimes, but that hasn't caused an ice age.

It is also not convincing that carbon dioxide has anything to do with glaciation. Bubbles in ice cores have shown that there is low CO2 during ice ages, suggesting some kind of link between increased cold and decreased carbon dioxide. However, this relationship does not occur extensively in all samples.

It is conceivable that the movement of Earth's plates changes climate. The Earth's outer layer, the lithosphere, is made of plates that move. Edges of plates are often marked by earthquakes and volcanoes. Whether these plates are all together or are far apart can affect albedo, atmospheric and oceanic circulation, etc. You may have heard of Pangea, Gondwana, and Laurasia. These were supercontinents of the past whose orientation may have drastically affected climate.
Specifically, moving continents affects temperature distributions and circulation. You need a continent at high latitudes to get a long- lived glacier, because you depend on the continentality effect to keep temperatures cold.

Similarly, mountain building can affect climate. Lifting up land causes a topographic barrier and changes atmospheric circulation patterns. For instance, it can redirect air flow, or change precipitation patterns in front and behind it (rainshadow effect). During the Pleistocene, there was 2 km of uplift in the Alps, 2 km in the Sierra Nevadas, and 3 km in the Himalayas.

Shallow and deep water in ocean basins have different circulation patterns. Some scientists have linked glacial episodes with changes in the path of travel of oceanic water. And in the Pleistocene there has been a lot of change in the geometry of ocean basins due to plate tectonics.

Under causes external to the Earth: 1 and 2 are self-explanatory. #3 explains the Milankovitch mechanism. It is based on the fact that the Earth-sun geometry discussed early in the semester is not always in that particular geometry. 3 things change regularly over time: orbital eccentricity (how elliptical the orbit is), precession of the equinoxes (wobble of the axis, or in other words, the way the axis of a spinning top is turns around), and axial tilt (the tilt of the Earth's axis is 23.5 degrees now but through time it varies between 21.5 and 24.5 degrees).

Each of these three components causes fluctuations in the amount of energy reaching the Earth, and where it reaches the Earth. Summing these three components together produces the sawtooth temperature curve that is in fact preserved in the ocean floor record. The Milankovich cycles show warm peaks about every 90,000-100,000 years, which could explain why glacial cycles seem to end abruptly every 100,000 years.


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