The Carbon Cycle - Introduction to Carbon Capture and Sequestration - page 131

Box 3.6.1 Proxies

It was not that long ago that fi nding the answer to the question: “What was the tem-
perature in Amsterdam on 21 February 1997?” could not simply be retrieved by typing
this phrase in the Google search bar. As temperatures are now routinely measured and
stored digitally, it has become very easy to obtain large amounts of data on historical
temperatures. This question becomes, however, signifi cantly more diffi cult if one
would like to know the temperature, say, a million years ago, before temperatures
were measured systematically. In these cases one has to rely on indirect measure-
ments. Most of these measurements rely on the temperature dependence of many
biological processes (in trees, corals, pollen, plankton, etc.) and through these proxies
one is able to reconstruct the temperature. Examples of commonly used proxies are:
• Tree rings are wider when conditions favor growth, narrower when times are diffi cult.

These annual rings as well as the maximum latewood density are used to determine
a historical temperature profi le.

The ratios of 16 O to 18 O isotopes in the calcium carbonate shells of foraminifora in
deep sea sediment depends on the temperature, salinity, and the isotopic composi-
tion of the water. If water composition and salinity can be estimated, the study of
foraminifera shells allows the reconstruction of past ocean temperatures.

•

inspection of this fi gure one can see that the CO 2 levels need to stay
below 700 ppm levels to sustain ice in the polar regions.
If we look carefully at the data we see that the Paleocene and the
Eocene eras are separated by a peak in the ocean temperatures. This
peak is know as the Palaeocene-Eocene Thermal Maximum ( PETM ),
which took place about 52.5-55.5 million years ago. Figure 3.6.6 shows
some more detailed data for this event. Figure 3.6.6 (top) gives the iso-
topic composition of 13 C in marine and continental records. The sudden
drop indicates that the 13 C composition of the atmosphere had dropped
dramatically, as would be expected from a massive release of organic
( 13 C-depleted) carbon into the atmosphere. At present, the origins of the
PETM are not fully understood. Figure 3.6.6 (middle) shows that during
the same period the

18 O level of the ocean foraminifera decreased. As

δ

18 O level is a proxy for the ocean temperature (see Box 3.6.1 ), the
13 C and

this δ

18 O levels are consistent with a signifi cant increase in carbon
levels (CO 2 or CH 4 ) in the atmosphere, and an ensuing greenhouse gas
effect, causing the increase in the temperature of the oceans. It has been
estimated that the total carbon release for this time period is on the order

δ