Page 47 - Earth's Climate Past and Future

CHAPTER 2 • Climate Archives, Data, and Models 23

Radioactive Stable Stable daughter
isotope isotope product

24 Parent
Number of radioactive isotopes in sample 12 isotope
18

6
isotope
0 Daughter
1 1 2 12 3
Time (in half-lives)

FIGURE 2-8 Radioactive decay Time is determined by measuring the gradual decay of a
radioactive parent isotope to a daughter isotope. The half-life is the time needed for half the
parent to decay. The relative abundances of parent and daughter isotopes follow the trends
shown at the bottom. (D. Merritts et al., Environmental Geology, © 1997 by W. H. Freeman and
Company.)

of dating and correlation using fossils or other features a few areas using nearby igneous rocks, the ages that
in the sediments. The fossil method relies on the fact are obtained can be transferred to sediments in other
that a unique and unrepeated sequence of organisms has regions that contain the same short-lived fossils but lack
appeared and disappeared through Earth’s entire his- radiometric dating. Other physical or chemical features
tory and has left fossilized remains. The most useful that show distinctively varying patterns (like ash layers)
fossils are those that are shortest-lived but geographi- can be used in a similar way.
cally most widespread. If the brief existence of these Radiocarbon In the younger geologic record, a dif-
species can be reliably dated in sediments in at least ferent method, radiocarbon dating, is widely used to

TABLE 2-1 Radioactive Decay Used to Date Climate Records
Parent isotope Daughter isotope Half-life Useful for ages: Useful for dating:

87
87
Rubidium–87 ( Rb) Strontium–87 ( Sr) 47 Byr 100 Myr Granites
Uranium–238 ( 238 U) Lead–206 ( 206 Pb) 4.5 Byr >100 Myr Many rocks
Uranium–235 ( 235 U) Lead–207 ( 207 Pb) 0.7 Byr >100 Myr Many rocks
40
40
Potassium–40 ( K) Argon–40 ( Ar) 1.3 Byr >100,000 years Basalts
Thorium 230 ( 230 Th) Radon–226*( 226 Ra) 75,000 years <400,000 years Corals
14
14
Carbon–14 ( C) Nitrogen–14* ( N) 5,780 years <50,000 years Anything that contains carbon
*Daughter in this case is a gas that has escaped and cannot be measured.