Notes: Fri. 22 Nov

Finishing glaciation

reviewed the terms involved in valley glaciation (cirque, are^te, horn, hanging valley, tarn, tributary glacier, crevasse, lateral / medial / end moraines, U-shaped and V-shaped valleys) from last lecture.

viewed slides

  1. the Grand Tetons
  2. U-shaped valleys with wide, flat bottoms - a sign that they were created by glaciers
  3. Yosemite, with its hanging valley waterfalls, and Half Dome, which was once a whole dome but half of it was eroded away by a glacier
  4. striations and cross cutting
  5. polishing and plucking by a glacier on the up- (stoss) and down- (lee) side of a rock
  6. roche moutonee

Climate change and the Pleistocene

Within recent history, global temperatures have been rising. But they have not always done so. Remember from the units on Sun-Earth relationships, there was a period called the little ice age which corresponded with low sunspot activity. Is it the sun that causes all climate change? There may be a correlation, but that does not mean that is the cause.

We can examine the last period of strong change in global temperatures: the Pleistocene, which is the time period from about 2 million years ago till 10,000 years ago. It was a period of ice ages during which hominids were evolving. The Pleistocene is a division of the Cenozoic time period (from about 65 million years ago - after the dinosaurs - till today). During the Cenozoic, there was an overall trend towards cooler and drier climates, culminating in the Pleistocene ice ages.

Effects of glaciation on biomes:
During the Pleistocene, glaciers did not grow continuously, but rather in a cycle of about 100,000 years. Along with the change in temperatures and the advance / retreat of glaciers, you could expect that global biomes shifted, too. For instance, the following is a cross-section of typical biomes found today going from mid-latitudes to polar regions:

equator<---deciduous forest - mixed forest - boreal forest - birch - shrubs --->North, where glaciers are today.

But if a glacier was advancing from the north, some of these biomes may have been crammed further south or destroyed:

equator<---shrubs [-------big glacier here----------------]--->North

Other reasons for studying glaciation: understand climate change and development of humans
To get the record of climate change from continents is difficult because erosion has eliminated some of the rock record. Some of the clues that can be used to figure out climate change and human evolution/spreading are fossils and pollen.

We are also interested in glaciation because it influences sea level and therefore the spread of hominids. During periods of glaciation, sea level is low because a lot of water is tied up in glaciers. And we think that during periods of low sea level, more land is exposed so hominids could travel to different regions.

The record of climate change from oceans is much more continuous because erosion does not occur underwater. Scientists analyze drill cores from the ocean floor. As you go deeper in a core, the rock is older; as you get closer to the surface, the sediment is younger. So analyzing the chemistry of the cores gives clues as to what global temperatures were through time. For example, oxygen isotopes are used to find out paleotemperatures (that is, temperatures in the past). The process is as follows:

As mentioned before, the average cycle of ice ages in the Pleistocene is 100,000 years long. Since the Pleistocene was about 2 million years long, that implies about 20 interglacial periods occurred in the Pleistocene. The last glacial maximum was 18,000 years ago. If we use the oxygen isotope curve to look at temperatures through the Pleistocene, we would see O-18 increasing slowly and then suddenly dropping off, increasing slowly and suddenly dropping off, and so on. This implies then that temperatures on Earth also cooled slowly and then suddenly got warm, over and over, with a periodicity of 100,000 years. Plotting temperature over time, we see a sawtooth-type pattern reflecting the sudden warming and slow cooling.

We also have data on the change in sea surface temperature (SST) through time from faunal data. These temperatures are an important constraint on global circulation models (GCMs)

The important points are

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