1. Which radioactive isotope is most useful for determining the age of mastodont bones found in late Pleistocene sediments?

A) Carbon-14

B) Uranium-238

C) Potassium-40

D) Rubidium-87

Correct Answer: Option A - Carbon-14

From the Earth Science Reference Table – Page 8, we have the following information:

According to the Geologic History of NYS chart the late Pleistocene is about (0.01 million years ago) 10,000 years ago. Due to the fact that radioactive carbon-14 may be used to date recent materials up to about 60,000 years in age; C-14 is the best answer. To find the half-life of mastodont bones, we can refer to page 1 of the same table. The C-14 has a half life of 5700 yrs. C-14 may only be halved so many times before the isotope is too small to be detected.

Incorrect Options

Uranium-238 is the most common isotope of uranium found in nature. When hit by a neutron, it becomes uranium-239, an unstable isotope which decays into neptunium-239, which then itself decays, with a half-life of 2.355 days, into plutonium-239. Uranium 238 has a half life of 4.46 billion years.

Potassium is a soft, silver-white metal. An important constituent of soil, it is widely distributed in nature and is present in all plant and animal tissues. Potassium-40 is a naturally occurring radioactive isotope of potassium with a half life of 1.248 x 10⁹ years.

Rubidium-87 is the radioactive isotope of rubidium with a half life of 4.88 x 10¹⁰ years.

Uranium-238, Potassium-40 and Rubidium-87 have very long half lifes. Hence, they cannot be used to date the age of mastodont bones. Hence, they are incorrect.