GEOL1060

The Last Great Ice Age

Reading: Text, pp. 368-381

The last Ice Age reached its coldest point about 21,000 years ago, and by 15,000 years ago was well on its way to decline. This period is the most recent example we have of a climate situation that was globally dramatically different. In this lecture, we will see what aspects of climate changed, how we know (or think we know) this, and what factors likely caused the changes.

1. The last ice age: what was so different (and how do we know about it?)

Of course, we do not have instrumental cliamte records that reach back 20,000 years ago. But we have lots of paleoclimatic evidence (that is, evidence from geological and biological systems) that gives us an increasingly clear picture of what the last ice age was like.

*Ice Sheet Extent

Two primary sources of evidence:

1. Glacial deposits (moraines) that contain material that has been radiocarbon-dated to about 21,000 years ago, and other glacial geologic evidence (erratics = boulders dropped by glaciers, and striations = grooves worn in bedrock by the rocks that are contianed in a glacier as the glacier moves over the bedrock)

2. Abundance of different kinds (isotopes) of oxygen atoms in radiocarbon-dated deep sea cores. This requires some explanation:

Oxygen has naturally occurring isotopes, oxygen-16 and oxygen-18. (Recall that an isotope is a form of an element with a specific number of nuclear particles.) Most oxygen (about 98%) is O-16.

When water evaporates from the oceans, water containing O-16 evaporates preferentially relative to water containing O-18. That's because O-18 is heavier.

This water that evaporates from the oceans is the water that eventually forms precipitation, and precipitation at high latitudes is what forms large ice sheets. Thus when large ice sheets form, they contain even more O-16 than average ocean water.

Therefore, when large ice sheets form, they lock up lots of the O-16 and the remaining ocean becomes relatively more enriched in water that contains O-18.

You can measure the relative amount of O-16 to O-18 in the fossils of microscopic plankton, and tell by these ratios whether large ice sheets were present or not. Measurement of long histories of theste ratios give us a chronology of glaciation.

*Air temperature

Air temperatures can be inferred from lots of different sources of evidence:

1. Oxygen isotopes again... isotopic ratios in ice cores depends on air temperatures (much more strongly than on ice volume, incidentally!).

2. Glacial evidence in high mountains: if terminal moraines that are 21,000 years old are found lower down the mountain, we can infer the temperature changes by knowing the average lapse rate (between 6-10 °C per 1km). In the tropics, we see that mountain glaciers were found about 1km lower, meaning that temperatures were about 6-10°C cooler.

*Atmospheric CO2

Bubbles trapped in ice can be analyzed to give us the composition of the atmosphere. The glacial atmosphere had only about 200 ppm CO2 (relative to the modern level of 355 and the pre-industrial level of about 280).

*Atmospheric dust

These same ice cores reveal that during the last glacial period, the atmosphere contained lots more dust than it does today. This was probably a combination of drier conditions, more erosion as glaciers scoured the landscape, and stronger winds that lift the dust higher and transport it further.

*Vegetation patterns

We know what plant distributions looked like during the glacial by studying the remains of plants found in different sources. In general, ecosystmes in the US reflect cooler conditions across the country (for example, spruce forests grew in the Carolinas, whereas now they are only found inthfurthest northern part sothe US and in Canada).

1. Pollen can be identified with respect to what kind of plant it came from. Lake sediments preserve fossil pollen and we can radiocarbon date these sediments, to give us a history of vegetation change.

2. Packrats (yes!) make nests of vegetation that are preserved in dry desert climates; these remains can be found and dated.

This site at the Illinois State Museum shows evidence of different kinds of ecological changes that occurred during the glacial in the midwest, including the distinctive large mammal faunas that were present, such as mammoths and sloths.

*Sea level

Coral terraces that grew at sea level 21,000 years ago (of a species that only grows at the surface) are now found 125m below modern sea level.

*Ocean Circulation

The geochemistry of marine sediments and indicators of past ocean temperature patterns tell us that thermohaline circulation was far weaker during the last glaciation than it is at present. This weakening is likely due to fresher waters in the far north Atlantic, as the large continental ice sheets were discharging fresh icebergs into the ocean (which melted, causing salinity to drop).

2. What caused the ice ages?

Our geologic (paleoclimatic) records show us that over the past 2 million years, the earth has experienced a series of glacial (ice age) and interglacial (like today) periods. These alternate with some regularity (about every 100,000 years).

We think we understand the root cause of the periodic alternation between glacial and interglacial periods. Because these alternations are very long, we need to look at causes that have that same characteristic time scale. Early efforts to explain glacial/interglacial periods focused on long-term irregularities in the Earth's orbit.

The Earth's orbit around the Sun is irregular because of the gravitational pull of other bodies in the solar system (notably the moon, because it is close, and Jupiter, because of its large mass). There are three types of irregularity that we will be concerned with here, and each has its own period over which the variation occurs.

1. Eccentricity: The orbital path is not perfectly circular all the time, and the degree of non-circularity (eccentricity) varies from a perfect circle to a slight oval. This variation takes place with a period of 100,000 years (i.e. every 100,000 years it is a perfect cirle).

2. Tilt: The Earth tilts on its axis, currently at an angle of 23.5° from vertical (with respect to the plane of the orbit). This tile varies from about 22-25° with a period of 40,000 years. This tilt is important to our daily experience: without a tilted axis, there would be no seasons. The earth rotates once around the Sun each year, and for part of that time the northern hemisphere is pointed towards the Sun (NH summer/SH winter) and for other parts the southern hemisphere is pointed towards the Sun (NH winter/SH summer).

3. Precession: The Earth "wobbles" as it revolves around the Sun. If you think of a spinning top, recall how the top does not simply sit vertically and spin - the top of the top (sorry!) rotates around and gives the spinning top a wobble. The Earth wobbles like this also, but with a very slow period (23,000 years). This acts in combination with the eccentricity of the orbit. Today, the NH axis points towards the Sun (i.e. NH summer occurs) when the earth is at the greatest distance from the sun, and the NH winter occurs when the Earth is closes to the sun. At 11,000 years ago, the opposite situation occurred: the NH pointed at the Sun when the Earth was closest to the Sun, and away when it was furthest away. Thus the seasonal contrast in radiation received was greater 11,000 years ago than it is today.

You can see that if the orbit were circular, then precession would not influence the radiation received by the earth.

We call these factors the Milankovich cycles, after the Czech astronomer Milutin Milankovich who first identified their importance and potential for affecting glaciation. You can read another description of them at this site.

The Milankovich cycles (acting all together) produce a pattern of radiation receipt that is strongly correlated to the recurrence of glacial/interglacial variations. Although the pattern is different for different latitudes (imagine, for example, the influence of tilt: much greater at high lats than low), we focus on the latitude of the ice sheets that come and go with glacial periods (about 60°N). The Milankovich variations appear to strongly influence the potential for ice sheets to form and (most importantly) to persist through the warm season at this latitude.