The discovery and use a of radioactive decay

Natural radioactive decay was discovered by Henri Becquerel, a French physicist, in 1896. shortly afterward, Ernest Rutherford, a British physicist, described the structure of an atom. These two discoveries are what prompted the idea of using radioactivity as a tool with which to measure geologic time. Then, in 1907, Professor B. B. Boltwood, a radiochemist at Yale university, published the first list of geologic ages of formations based on the use of radioactivity as a true laboratory dating process. His initial list of ages was not completely accurate, but it was still a significant breakthrough because he was moving in the right direction by measuring time in extremely large units of time: hundreds of thousands to millions of years.

since these early achievements, the geologic timetable has been revised as technology has advanced. scientists have made new discoveries, and techniques have become more sophisticated and accurate, allowing more of the geologic mysteries of the earth to be unlocked. As technology continues to advance, so will scientists' understanding of the complex processes that shape the earth.

original number of radioactive parent atoms is left. Because this clock is so consistent—it cannot be influenced or changed by external forces, such as heat, cold, acceleration, pressure, vacuum, or chemical reactions—it has proven to be a reliable way to date materials in the Earth's crust and on the surface.

An event must occur to start these radiometric clocks ticking. If it is rock that is being dated, igneous rock is the most commonly used because it cools very quickly from its molten lava state. As soon as the lava cools to the point that neither parent nor daughter elements can leave or enter the rock, it forms what is called a closed system—nothing leaves or enters; the system stays as it is. Once a system is closed, it cannot be influenced by outside forces; it can be influenced only by internal forces, in this case, internal radioactive decay of the parent element. As the parent exponentially decays to ^ to V . . .) the daughter exponentially increases in abundance. Radioactive isotopes are useful for the first five or six half-lives. After that, there is not enough parent material left to be reliable.

Most radioactive isotopes have rapid rates of decay—or short half-lives—and lose their radioactive properties within just a few days or years, which prevents them from being useful dating tools. There are some isotopes that decay slowly, however, and this allows them to function as extremely useful tools for determining age. Radiometric dating is a versatile dating tool. Different types of radioactive parents have different half-lives, so depending on the age range of an item being dated, various parent elements are used for different parts of the Earth's history. Of course, the parent element must be present in the rock being dated. If the oldest rocks on Earth are being dated, for instance, the uranium-to-lead series is used because it has a very long, slow decay rate.

Occasionally, there can be problems with the dating technique. It is assumed that when the clock is set there is no daughter present, but that is typically not the case, which adds some error—the sample is usually not pure. Also, occasionally, the system is not completely closed, allowing parent or daughter isotopes to leave the system, which can skew the results.

Today more than 40 different radiometric dating techniques are used that are each based on a different radioactive isotope. Isotopes are forms of a chemical element that have the same atomic number but differ in mass. Isotopes with long half-lives decay very slowly, which makes them useful for dating ancient events. The isotopes that have shorter half-lives are not used to date ancient events because there would not be enough parent isotope left to measure. Isotopes with short half-lives are useful to date events over shorter intervals of time. The half-lives are measured by a radiation detector in a lab or by measuring the ratio of daughter to parent atoms in a sample.

The table on page 47 shows some of the most commonly used radio-metric techniques in climate science.

When dating climate records, obtaining the date based on radio-metric techniques is the first step. Once dates are obtained on igneous rocks, they serve as markers when looking at the geological formations and give boundaries on intervals of time as to when sedimentary layers

Radioactive Decay Used to Date Climate Records

PARENT iSOTOPE

DAUGHTER iSOTOPE

USED ON HALF-LIFE

USE ON AGE RANGES

MATERIALS

Uranium-238

Lead-206

4.5 Byr

>100 Myr

Multiple rocks

Uranium-235

Lead-207

704 Myr

>100 Myr

Multiple rocks

Rubidium-87

Strontium-87

48.8 Byr

100 Myr

Granite

Thorium-230

Radon-226*

75,000 years

<400,000 years

Corals

Potassium-40

Argon-40

1.25 Byr

>100,000 years

Basalts

Carbon-14

Nitrogen-14*

5,780 years

<50,000 years

Anything containing carbon

(Notes: * Daughter is an escaped gas that cannot be measured; Byr = Billion years; Myr = Million years) Source: U.S. Geological Survey

were formed. It is within these sedimentary layers that many clues to the Earth's past climate can be found.

Most sedimentary rocks, such as sandstone, shale, and limestone, are related to the radiometric time scale only by bracketing them to datable igneous rock formations. This way sediments can be dated based on their relative positions to the datable igneous formations. In other words, once a radiometric age is obtained for an igneous formation, then the ages of the sedimentary layers are constrained by their positions above, below, or between igneous layers. There are six radiometric dating techniques commonly used in climatology: (1) uranium-lead, (2) rubidium-strontium, (3) dating using thorium-230, (4) methods using lead, (5) potassium-argon, and (6) carbon-nitrogen.

In cases in which there are not enough igneous rocks present to serve as markers, fossils are commonly used. The best types of fossils are those species that did not exist for long on Earth but are found in many places worldwide, making their possible temporal range naturally confined. Areas that have fossil records that record the extinction of one species and the appearance of new species are also good for establishing sound geologic time markers.

Igneous layer 4

Igneous layer 2

Igneous layer 4

Igneous layer 2

4

Sedimentary layer C

Sedimentary layer B

B

2

Sedimentary layer A

3 A

1

Igneous bedrock layer 1

Using dated igneous rock to date sedimentary rock

Oldest (first deposited)

Igneous layer 3

History of Deposition

Youngest (last deposited)

Using dated igneous rock to date sedimentary rock

Igneous layer 4 formed: Obtain radiometric age Sediment layer C deposited on top of layer Br"^,"'

Igneous layer 3 formed: Obtain radiometric age

Sediment layer B deposited on top of layer 2[7y°>u

Igneous layer 2 formed: Obtain radiometric age

Sediment layer A deposited on top of layer 1

Igneous layer 1 formed: Obtain radiometric age

& Infobase Publishing

Sedimentary rock layers can be dated based on their position to igneous rock formations in the geologic column. Once the igneous rocks are given absolute dates through radioactive dating, the sedimentary rocks are given ages constrained by their positions above, below, or between the igneous layers.

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