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An example is shown in Figure 8.16; radiocarbon dates from wood fragments in glacial sediments have been used to estimate the time of the last glacial advance along the Strait of Georgia. Figure 8.16 Radiocarbon dates on wood fragments in glacial sediments in the Strait of Georgia [SE after Clague, J, 1976, Quadra Sand and its relation to late Wisconsin glaciation of southeast British Columbia, Can. In most cases, we cannot use isotopic techniques to directly date fossils or the sedimentary rocks they are found in, but we can constrain their ages by dating igneous rocks that cut across sedimentary rocks, or volcanic ash layers that lie within sedimentary layers.Isotopic dating of rocks, or the minerals in them, is based on the fact that we know the decay rates of certain unstable isotopes of elements and that these rates have been constant over geological time.This is determined by drawing a horizontal line from 0.95 to the decay curve line, and then a vertical line from there to the time axis.

Feldspar does not have any argon in it when it forms.

It has a half-life of 1.3 billion years, meaning that over a period of 1.3 Ga one-half of the Figure 8.14 The decay of 40K over time.

Each half-life is 1.3 billion years, so after 3.9 billion years (three half-lives) 12.5% of the original 40K will remain.

C) can be applied to many geological materials, including sediments and sedimentary rocks, but the materials in question must be younger than 60 ka.

Fragments of wood incorporated into young sediments are good candidates for carbon dating, and this technique has been used widely in studies involving late Pleistocene glaciers and glacial sediments.

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Some of the other important pairs are listed in Table 8.2, along with the age ranges that they apply to and some comments on their applications.