Fossils encased in limestone at Makapansgat
click to enlarge
Hominid skull (back) from Makapansgat
One of the biggest challenges facing archaeologists and paleontologists is determining the age of an artifact or fossil. There are two general disciplines in dating methodology:
Relative dating provides the age of an sample in relation to another sample. For example: sample A is older than sample B, but younger than sample C.
Chronometric dating methods assign an age in years to an sample. For example, sample A is 50,000 years old; sample B is 10,000; sample C is 100,000.
Within each discipline are several specific methods, including:
Stratigraphic
Dating
Stratigraphic dating is a relative method based on the law
of superposition: in
a layered deposit, the older materials occur in lower layers and younger materials
occur in upper ones. In other words, the deeper a fossil is buried,
the greater its age. This theory works because weather patterns and climate
changes cause sediments to settle over fossil deposits--several feet (or "layers")
of dust, dirt, and rock can accumulate over the course of millions of years. Using
the example above, sample A lies in the middle layer of the fossil deposit. Sample
B (younger) would be above it, while sample C (older) would be below it. While
it cannot estimate precise ages, Stratigraphic dating is extremely effective
in establishing a sequence to historical events.
Radiocarbon Dating
Radiocarbon dating is a chronometric method
based on the study of a radioactive isotope of the element carbon (in this
case C14). Carbon
is a useful element because it is common in both mineral and organic sources. Isotopes
are forms of elements with specific numbers of atomic particles in their nuclei
(the C14 isotope has 14 atomic particles in its nucleus). Some isotopes
emit radiation that decays at a known rate called a half-life. A half-life
is the amount of time it takes for half of an isotope to decay. The half-life
of C14 is 5730 years. Therefore, after 5730 years, only half
of the original C14 isotope remains (the rest disintegrated). In
5730 more years, one half of the remaining C14 would decay, leaving
1/4 of the original sample. Ensuing half lives would take half of remaining
amounts of C14.
By studying the known mass of carbon, the half-life and specific
radioactivity, the age of a sample can be estimated. Radiocarbon dating
is effective for samples less than 50,000 years old, but it has one major drawback: the
burning of fossil fuels (coal and oil) has pumped large amounts of non-radioactive
carbon into the atmosphere; this can influence the results of younger samples
(i.e. 1000 years or less). Most laboratories factor this into their estimates.
Potassium/Argon (K/Ar) Dating
There are two types of this chronometric method
of dating. One involves measuring the ratio of an
argon isotope (Ar40) produced from the decay of a potassium isotope (K40). This
works because, as K40 decays, it produces measurable amounts of Ar40. The
other involves using radiation to convert a potassium isotope (K39) into an
argon isotope (A39). Measuring the amount of Argon isotopes released
in each process gives scientists a fairly accurate estimate of age. The
two methods can estimate samples as old as 4000 million years and as young
as 300,000 years. Since potassium is usually found around volcanic
activity, this method is limited in its application.
Argon/Argon dating is a
recent improvement of the traditional K39-to-A39 method. By comparing the A39 isotope with
an additional argon isotope (A40), scientists recently estimated the destruction
of Pompeii to within seven years of the known date. This is unprecedented
accuracy; effective for samples as young as 1,000 years old.
Uranium Disequilibrium (Ionium) Dating
This chronometric method involves measuring
the relation of uranium (U235) to thorium (Th231) in a given sample over time. As
Uranium decays, it changes into Thorium, and the ratio can provide an
estimate for samples up to 350,000 years old.
Luminescence Dating
This chronometric process works from the theory
that many minerals contain trace impurities of isotopes of uranium, thorium
or potassium. The
radiation emitted by these isotopes causes damage to the mineral at the molecular
level. The radiation damage--or luminescence--of a sample can be measured
and interpreted using two techniques: thermoluminescence (or heat stimulated)
and optically (light) stimulated luminescence. This process is used to
date pottery, burnt stone artifacts, sedimentary rocks, and minerals like quartz
and feldspar.
Electron Spin Resonance
Like luminescence dating, electron spin resonance
measures the radiation damage in crystalline minerals. This method can
study a wide range of minerals, from quartz crystals to the iron in blood hemoglobin
to tooth enamel.
Chemical Dating
There are several types of chemicals used to
determine the age of a sample--for example, one measures the chemical reactions
within amino acids. There are many variables involved, such as temperature and
variations in the rates of the chemical reactions. Nevertheless, these
processes have been used to date ostrich egg and marine shells accurately.
No single method is absolutely accurate, and most can only estimate older samples to within a few hundred thousand years. That might not be much in geological time, but it's several human lifetimes, and enough to push scientists for more accurate methods. Most researches will combine stratigraphic dating with one or more of the other methods listed above to arrive at an estimate. Recently, researchers in South Africa undertook the monumental task of applying modern methodology to samples collected decades ago. By comparing the flora, fauna and sediments found in the fossils' stratigraphic layers, they were able to re-date several important finds, including the australopithecus fossils known as "Mrs. Ples" and "Little Foot". This re-examination (which makes the pieces anywhere from 500,000 to 1,000,000 years younger than previously thought) could lead to a fundamental shift in the way we understand human evolution.