IB Biology HL 2.1.8 Dating the First Cells and LUCA

Practise IB Biology HL 2.1.8 by comparing radioisotopes, half-lives and isotope ratios when dating fossils and ancient rocks.

Exam points

Compare carbon-14 and potassium-40 dating by isotope, half-life and suitable fossil age.
Use isotope proportions and known decay rates to justify an estimated fossil age.

[Maximum number: 2]

Outline the methods used for dating fossils and rocks using ${ }^{40} \mathrm{~K}$ .

${ }^{40} \mathrm{~K}$ decays into ${ }^{40} \mathrm{Ar}$ ;
ratio/proportion of ${ }^{40} \mathrm{~K}$ to ${ }^{40} \mathrm{Ar}$ indicates the age of the rock/fossil; half-life of ${ }^{40} \mathrm{~K}$ is 1250 million/ 1.25 billion years;
${ }^{40} \mathrm{~K}$ can (only) be used to date very old samples/older than 100000 years;

Marking guidance:

Do not accept if make reference to age less than 100000 years.

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A2.1.8—Dating first cells and LUCA question 1