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CHAPTER 11

Ice Cores

See Fig. 11-1. Ice cores drilled from the highest point of the ice downward will yield the oldest data. Two important climate signals are the greenhouse gases carbon dioxide (CO₂) and methane (CH₄).
See Fig. 11-3. When snow falls onto the ice sheet, it is fluffy so air exists between flakes. As more and more weight of snow is added above, the older snow is compressed into solid ice. Air bubbles are trapped in this ice as a result. This process is called sintering and will provide a permanent record of the atmosphere at the time of sintering.
Fig. 11-4 shows the ice core records of CO₂ and CH₄ along with actual measured amounts of these gases in modern times.
Ice core data shows that CO₂ concentrations were about 280 ppm (parts per million) for thousands of years but began to increase in the middle of the 18th century. Present-day measurements show that the concentration now is around 365 ppm.
The smooth transition from ice core data to actual measurements shows that the ice core data is very reliable.
The methane concentration curve shows a similar trend. Ice core data shows CH₄ concentrations were stable at 700 ppb to 750 ppb for thousands of years but once the Industrial Revolution began, concentrations have increased rapidly. Modern day values are around 1700 ppb.

Orbital-Scale Changes in Methane

See Fig. 11-5 which shows data from the Vostok ice core (Antarctica).
The methane concentration cycle appears to closely match the 23,000 year summer monsoon cycle. Keep in mind that the 23,000 year summer monsoon cycle occurs at low latitudes. Some possible explanations:
1. When summer monsoons are strong, there is much more precipitation in southeast Asia. This results in much more standing water in bogs because the ground is saturated and cannot absorb more water. Decaying vegetation in these bogs uses up all the available oxygen. Once oxygen is used up, chemical processes that produce methane gas dominate. (MORE LIKELY)
2. Methane can also be generated at high latitudes when summer bogs are formed due to brief summer melting. During colder cycles, this methane remains locked up in frozen materials (groundwater and sediment.)
Because there appears to be no 41,000 year cycle in the ice core, and the 41,000 year tilt cycle should dominate this high latitude location, the former hypothesis (#1 above) is much more likely than the latter (#2 above.)

Orbital-Scale Changes in CO₂

Fig. 11-6 shows long-term CO₂ changes from the 400,000 year Vostok ice core compared to the δ¹⁸O records.
There is no 23,000 year cycle but there is an obvious 100,000 year cycle. Recall from Chapter 10 that in the last 600,000 years, there appears to be a 100,000 year glacial cycle.
Abrupt increases in CO₂ occur during times of rapid ice melt (δ¹⁸O decreasing) while slower CO₂ decreases match the slower ice buildup (δ¹⁸O increasing.)
It appears that there is some type of cause and effect relationship between CO₂ concentrations and ice volume. (Discussed in Chapter 12.)
There appears to be a 90 ppm drop in CO₂ concentrations during glacial periods. Some possible explanations:
1. Changes in the ocean temperature and salinity: Cold water can dissolve more CO₂ which would account for a net loss of atmospheric CO₂. However, when there are glaciers, more fresh water is locked up which makes the oceans more salty. CO₂ dissolves less efficiently in saltier water.
2. Table 11-1 shows the net effect. This accounts for 11 of the 90 ppm drop in CO₂ concentrations. 79 ppm is still unaccounted for.
See Fig. 11-8. Large changes in CO₂ on a time scale of thousands of years cannot be geologic (rock reservoirs.) However, carbon can exchange quickly between air, water, and vegetation.
During glaciation, there is much less vegetation because much of the land is covered by ice. Furthermore, due to the cooler climate, there was less precipitation. This drier climate led to more grasslands instead of forests. Therefore, it would have been very difficult for vegetation to remove atmospheric carbon. That leaves the ocean as the last remaining carbon remover.
Surface water exchanges all of its carbon with the air in just a few years. Therefore, the deep ocean must account for the missing 79 ppm of carbon. See Fig. 11-10. The deep ocean must have stored 2.7% more carbon during glacial times. So how did the excess carbon get into the deep ocean?
Carbon moves through Earth's reservoirs in one of two forms:
1. Organic carbon - living or dead organic matter
2. Inorganic carbon - CO₂ gas in the air and dissolved carbon ions (HCO₃^- and CO₃^-2)
Carbon isotopes (¹²C and ¹³C) can be used to track carbon through the climate system. The ¹²C isotope accounts for more than 99% of all the carbon on Earth while ¹³C makes up most of the other 1%.
δ¹³C values that are positive mean that the substance is ¹³C enriched while δ¹³C values that are negative mean that the substance is ¹³C depleted.
Ocean inorganic carbon averages δ¹³C values around 0%.
When inorganic carbon is transferred into organic carbon by photosynthesis, the ¹²C isotope is more easily incorporated into living tissue. That causes the organic material to have more negative δ¹³C values than the inorganic material it came from. Therefore, ocean plankton make the δ¹³C values more negative.
See Fig. 11-10 for examples of δ¹³C values.
Organic carbon in land vegetation averages δ¹³C values of -25% whereas the large amount of inorganic carbon in the oceans averages δ¹³C values of 0%.
See Fig. 11-13. During glacial climates, there is less land vegetation so the ¹²C would have to be stored in the deep ocean. During warmer climates where there was more land vegetation, more ¹²C would leave the oceans to be stored inside the vegetation.

Helpful Links:

Geology 150 - Milankovitch Theory and Ice Sheets
Glaciation