Page 46 - Earth's Climate Past and Future

22 PART I • Framework of Climate Science

FIGURE 2-6 Ice cores, corals, and
tree rings Ice cores, corals, and tree
rings are archives of climate change in
more recent Earth history. (National
Paleoclimate Data Center, NGDC,
Boulder, CO.)

Tree rings
Corals
Ice cores

(the daughter). This decay occurs at a known rate, the shown in Figure 2–8: the parent decays away exponen-
decay constant, which is a measure of the likelihood of a tially, while the daughter shows an exactly opposite (and
parent-to-daughter decay per amount of parent present compensating) exponential increase in abundance. The
per unit of time. This rate of decay in effect forms a clock half-life is a convenient measure of the rate at which
with which we can measure age. this process occurs: one half-life is the time needed for
An event of some kind is required to start this clock half the parent present to decay to the daughter. The
ticking. The igneous rock that is most commonly used first half-life reduces the parent to half its initial abun-
for dating is basalt, which cools quickly from molten dance, the second reduces it to half of that half (one-
outpourings of lava. The event that starts the clock tick- quarter), and so on. Notice the similarity of radioactive
ing is the cooling of this material to the point where decay to the response time concept from Chapter 1.
neither the parent nor the daughter isotope can migrate Because radioactive parents have a wide range of
in or out of the molten mass. At this point, the rock half-lives, each is most useful over a different part of
forms a closed system, one in which the only changes Earth’s history (Table 2–1). Radioactive isotopes remain
occurring are caused by internal radioactive decay. useful for at least five or six half-lives after the clock is
In the simplest example of a closed system, the set, but after this point too little of the parent may be left
decay of a parent to a daughter produces the changes to permit reliable dating. The long, slow decay series
from uranium (U) to lead (Pb) is useful for rocks that are
nearly as old as Earth itself. The decay from potassium
(K) to argon (Ar) is widely used for dating much of
Earth’s history.
Several factors can complicate radiometric dating.
Unlike the simple case shown in Figure 2–8, the initial
abundance of the daughter isotope is rarely zero: usu-
ally some amount was already present in the igneous
rock when the decay clock was set. Other problems arise
when the system does not remain fully closed to the
migration of parent or daughter isotopes.
If both igneous and sedimentary rocks are present in
a specific region, the igneous rocks can be used to con-
strain the ages of the sediment sequences. The age of
each layer of sediment can be obtained from the nearby
igneous rocks based on which is older or younger than
the other. For example, a layer of igneous rock that
spreads across the top of a layer of sediment must slightly
postdate the time the sediment was deposited and so it
FIGURE 2-7 Instrument measurements Instruments that provides a minimum age for that layer of sediment.
have been used to measure climate range from the primitive In actual practice, it is rare to find enough igneous
thermometers of the seventeenth century to the multiple rock in any one location to date sediments this way.
sensors flown aboard the TOPEX/Poseidon satellite. (NASA.) Instead, sediment sequences are dated by a combination