Scientific enigma
puzzle them out with 14C dating
I. Discover the age of the mummified seal from Wright Valley.
Read in a Scientific Newspaper: »Teacher Discovers a dead seal,far away from the sea in
Wright Valley, Antarctica »...
...« Eventually the scientists requested a 14C radioactive dating test done on the seal's
body. To their astonishment the test found that for every 100,000,000 12C atoms present
in the seal's body,4,100 radioactive 14C atoms were present, instead of the 10,000 atoms
expected for a recently deceased seal. »
How long ago did the seal die? _________________________.
You will be able to determine the age of the seal by using the decay of carbon 14 isotope.
Information required: 14C half-life is approximately 5,700 years.
II. Murder Above the Arctic Circle
The body of a man wearing the traditional clothes of the Saami people was found at the
bottom of one of the many peat bogs that remain from the last glacial retreat. Buried in the
back of his skull was a stone ax, made in the archaic style. The stone ax head was sharpened
by chipping and the blade was bound to a forked wooden handle by crisscrossed hide strips.
Bog acid has tanned the man's skin. Although his skin is wrinkled and tightly drawn over his
facial bones, his features are still distinguishable and his clothes and internal organs are
still intact and available for police analysis. The withered condition of the body has
convinced the police that the homicide happened at least 10 yeas ago. From forensic
evidence, we can conclude that he was murdered in a different place, dragged to the bog
and tossed in.
After months of investigation no new evidence or information was turned up. Eventually the
police requested a 14C radioactive dating test done on the victim's body and clothes.To
their astonishment the test found that for every 100,000,000 12C atoms present in the
man's body, and clothes only 3,000 radio-active 14 C atoms were present, instead of the
10,000 atoms expected for a recently deceased person. This has greatly confused the police
who had assumed that the murder was a recent event.
How long ago did the murder take place ?
Information required: C14 half-life is approximately 5,700 years.
Assessment
1. How old is the buried ice in Beacon Valley, Antarctica? Potassium/Argon Dating was
performed. See background for half-life information. It was discovered that for every
100,000 atoms of Potassium 40, there were 1500 atoms ofArgon-40.
2. A 12,0 mg carbon-sample from linen threads of Saint-Louis d’Anjou's « chape » produces
now(2007) a mean value of 236,87±0,60 decays per day.
Useful information :
14
C / 12C in the living organisms : 1,3.10-12, half-live of 14C : 5730 years,
Avogadro number : 6,02.1023.
a. Work out the age of this « chape ».
This age is actually known with great accuracy thanks to the style of the embroideries on it.
It was weaved between 1280 and 1290.
b. Compare and comment your result and this theoretical age.
Background information :Radioactive Half-Life and Dating Techniques
There are several methods available to the geologist for acquiring an absolute date. A
suitable material must be available to date, however.
Radiometric Dating. The minerals that make up rocks may contain radioactive elements
(usually in small quantities!!). These radioactive elements spontaneously decay, meaning that
they release electrons or neutrons and protons and subsequently form a different element.
The average rate of decay is constant. No chemical or physical process alters this decay
rate (except substantial re-heating to reform the mineral). When an igneous rock is
created, it has a set amount of a radioactive material and that material begins to decay.
The dating comes from examining the rock and finding the proportions of the original
element (parent element) and the newly formed element (daughter element). If the decay
rate is known, the scientists can calculate the time when only parent elements and no
daughter elements existed.
Scientists speak in terms of half-lives. A half-life is when one half of the original parent
element has decayed into the daughter element. In the first half life, half of the parent
element has decayed and half remains. At the end of the second half life, a quarter of the
parent remains; at the end of the third half life, 1/8 of the parent remains, and so on.
The instruments that measure the amount of parent material and daughter material are
sensitive. However, there are limits to how small of a quantity they can measure. Usually the
amount of parent material remaining after five half lives is starting to be too small to
measure (1/32 of the original quantity of the parent element). Thus, depending on the rate
of decay, some radioactive elements offer a long time span over which they can be
measured, and others offer only a short time span because they decay so quickly.
Some of the more important radioactive isotopes and their useful time measurements are
listed below. Potassium/Argon and Carbon 14 (discussed below) are the most important for
dating glacial events of the Cenozoic in Antarctica. The other methods have been used to
date the age of the oldest rocks on earth (3.7 billion years). Because of the slow decay rate
of the uranium group, they have not decayed sufficiently to be measurable in the most
recent glacial sediment.
Parent................Daughter.......Half-Life...........Useful Dating Range
Uranium 238.......Lead 206.........4510 million......>5 million years
Uranium 235.......Lead 207..........713 million......>60 million years
Potassium 40......Argon 40.........1300 million......>50,000 years
Carbon 14..........Nitrogen 14.......5,730 year........40,000 - 70,000 years
Short-hand for Carbon 14 is 14C; Carbon-12 is written as 12C.
The 14C Dating Method.
The Carbon 14 method is the most widely used method for dating sedimentary materials
less than 70,000 years old. It is a radiometric decay process, where the 14C (pronounced
carbon-14) isotope decays into more stable nitrogen-14 and releases an electron in the
process.
Carbon-14 is incorporated with the other carbon isotopes (12C and 13C) by plants and
animals. When the organism dies, it stops taking up 14C. The 14C begins to decay into 14N at
a constant rate. The decay rate is rapid. The half-life of 14C is approximately 5730 years.
Carbon-14 can be measured accurately to approximately 40,000 years. Recent developments
of parcticularly sensitive instruments have allowed successful detection of the parent
element in materials as old as 70,000 years.
Radioactive Decay of Carbon-14
Years
14
C atoms remaining
since death
per 1.0 x 108 12C atoms
0
10,000
5700
5000
11,400
2500
17,100
1250
22,800
625
28,500
312
34,200
156
39,900
78
45,600
39
51.300
20
57,000
10
62,700
5
68,400
2