Transcript SGES 1302 INTRODUCTION TO EARTH SYSTEM

SGES 1302
INTRODUCTION
TO EARTH SYSTEM
LECTURE 10: Absolute/Radiometric Dating
Absolute dating
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Relative dating places fossils in a temporal
sequence by noting their positions in layers of
rocks, known as strata.
As shown in the diagram, fossils found in lower
strata were typically deposited first and are
deemed to be older.
By studying and comparing strata from all over
the world we can learn which came first and
which came next, but we need further evidence
to ascertain the specific, or numerical, ages of
fossils.
Absolute dating relies on the decay of
radioactive elements that gives the actual
number of years that have passed since an
event occurred.
By dating volcanic ash layers both above and
below a fossil-bearing layer, as shown in the
diagram, you can determine “older than X, but
younger than Y” dates for the fossils.
Geologists have assembled a geological time
scale on the basis of numerical dating of rocks
from around the world.
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Atom
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An atom is the smallest particle still characterizing a
chemical element. The atoms are composed of
subatomic particles:
 electrons, which have a negative charge, a size
which is so small as to be currently
unmeasurable, and which are the least heavy
(i.e., massive) of the three;
 protons, which have a positive charge, and are
about 1836 times more massive than electrons;
and
 neutrons, which have no charge, and are the
same size as protons.
Protons and neutrons make up a dense, massive
atomic nucleus. The electrons form the much larger
electron cloud surrounding the nucleus.
Atoms of the same element have the same number
of protons (called the atomic number). Within a single
element, the number of neutrons may vary. The
number of electrons associated with an atom is most
easily changed, due to the lower energy of binding of
electrons.
Atoms are electrically neutral if they have an equal
number of protons and electrons. Atoms which have
either a deficit or a surplus of electrons are called
ions.
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Isotopes
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Elements may exist in different isotopes, with each isotope of an element differing only
in the number of neutrons in the nucleus (proton or atomic no. remains the same).
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Typically, the number of protons and neutrons of an atomic nucleus is the same. In an
isotope, this balance is frequently broken. For example, 238U, the most common state
of uranium, has three more neutrons than 235U.
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A neutral atom has the same number of electrons as protons. Thus, different isotopes
of a given element all have the same number of protons and electrons and the same
electronic structure; because the chemical behavior of an atom is largely determined
by its electronic structure, isotopes exhibit nearly identical chemical behavior. The
main exception to this is the kinetic isotope effect: due to their larger masses, heavier
isotopes tend to react somewhat more slowly than lighter isotopes of the same
element.
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Nuclear/Radioactive Decay
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Protons in the nucleus are positively charged, meaning they repel each other. The
presence of neutrons is necessary to separate these protons slightly, making the
configuration stable. A lack of necessary neutrons makes a nucleus unstable.
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Some nuclides are unstable. That is, at some random point in time, such a nuclide
will be transformed into a different nuclide by the process known as radioactive
decay. This transformation is accomplished by the emission of particles such as
electrons (known as beta decay) or alpha particles.
Nuclide: Species of atom as characterized by the number
of protons, neutrons, and the energy state of the nucleus.
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Half-life
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While the moment in time at which a
particular nucleus decays is random,
a collection of atoms of a radioactive
nuclide decays exponentially at a
rate described by a parameter known
as the half-life, usually given in units
of years when discussing dating
techniques.
After one half-life has elapsed, one
half of the atoms of the substance in
question will have decayed.
Many radioactive substances decay
from one nuclide into a final, stable
decay product (or "daughter")
through a series of steps known as a
decay chain.
Nuclides useful for radiometric dating
have half-lives ranging from a few
thousand to a few billion years.
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Radiometric Dating
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In most cases, the half-life of a nuclide depends solely on its nuclear
properties; it is not affected by external factors such as temperature, chemical
environment, or presence of a magnetic or electric field.
The half-life of any nuclide is believed to be constant through time. Therefore,
in any material containing a radioactive nuclide, the proportion of the original
nuclide to its decay product(s) changes in a predictable way as the original
nuclide decays.
This predictability allows the relative abundances of related nuclides to be
used as a clock that measures the time from the incorporation of the original
nuclide(s) into a material to the present.
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Carbon-14 Dating
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There are a number of dating techniques that have short ranges and are so used for
historical or archaeological studies. One of the best-known is the carbon-14 (C14)
radiometric technique.
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Carbon-14 is a radioactive isotope of carbon, with a half-life of 5,730 years. Carbon14 is continuously created through collisions of neutrons generated by cosmic rays
with nitrogen in the upper atmosphere. The carbon-14 ends up as a trace component
in atmospheric carbon dioxide (CO2).
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An organism acquires carbon from carbon dioxide during its lifetime. Plants acquire it
through photosynthesis, and animals acquire it from consumption of plants and other
animals. When an organism dies, it ceases to intake new carbon-14 and the existing
isotope decays with a characteristic half-life (5730 years). The proportion of carbon14 left when the remains of the organism are examined provides an indication of the
time lapsed since its death. The carbon-14 dating limit lies around 60,000 years.
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How to synchronize clocks: Relative & absolute time scales
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Main problem: not all rocks can be dated radiometrically. Radiometric dating
can be used for igneous and some metamorphic rocks, but not suitable for
sedimentary rocks.
Igneous rocks: formed from crystallisation of magma. All the minerals in the
rock are formed at about the same time and radiometric dating will give that
age.
Metamorphic rock: formed from pre-existing (parent) sedimentary, igneous
or even other metamorphic rocks by the process of metamorphism.
Radiometric ages from metamorphic rocks are difficult to interprete: they
can be ages of parent rock or the time when the metamorphism took place.
Sedimentary rocks: formed from materials derived from pre-existing rocks
by the process of weathering, transportation and sedimentation, together
with materials of organic origin.
They are generally not suitable for radiometric dating. Dating will give the
ages of parent rocks and not the age of sedimentation. A sedimentary rock
may contain mineral grains from parent rocks of diverse ages.
Exception when there are ash beds, volcanic clasts, organic materials (C14) in the sedimentary strata.
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How to synchronize clocks: Relative & absolute time scales
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In 1913 Arthur Holmes published “The Age of the Earth”. He
plotted radioactive ages opposite the stratigraphic time scale by
studying the relationship between sediments dated by fossils and
the crosscutting igneous rocks which were dated by radioactivity.
Since then thousands of more accurate radiometric dating were
made and the ages of various rock strata were interpreted. Ages
of sedimenatry rocks were estimated by related them to
radiometrically dated igneous rock (field observations required).
In 1977, the Global Commission on Stratigraphy (now the
International Commission on Stratigraphy) started an effort to
define global references for geologic periods and faunal stages.
The geologic time scale is revised every few years. The
commission's most recent work is in the 2004.
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