Dating
Stratigraphy
• As early as 1668 in De solido intra solidum
naturaliter…the Danish scholar Niels Stensen
(renamed Nicolaus Steno in college) argued that
seashells and shark’s teeth found in rocks far
from the sea were the remains of ancient
animals.
• In the same work he recognized the fact that
geological strata represent past depositional
events. The vertical order of strata represented a
sequence of deposition. This is the “priniciple of
superposition.”
Younger
Older
Faunal Correlation
A fossil with an extensive geographical distribution and
relatively short span of existence on earth can be used
to establish the contemporaneity of strata that are
spatially remote. A fossil used in this way is called an
index fossil.
Faunal Correlation at
Boxgrove, Sussex, England
Paleomagnetism
• Lava contains large quantities of iron.
• As lava cools the iron crystallizes, and
becomes magnetized through exposure to
the earth’s magnetic fields.
• The earth’s magnetic polarity of that time
thereby becomes locked into the rock.
• As we can date igneous rock by 40K/40Ar,
we have become aware that the earth’s
magnetic field periodically reverses itself.
Radiocarbon Dating
• Radiocarbon is an unstable isotope. An
isotope is a version of an atom that varies
in mass.
14
C6
Mass number
(total number of
nucleons)
Atomic
number (just
the number of
protons)
• A radiocarbon atom has the same number of nucleons
as Nitrogen, but the ratio of neutrons to protons is 8/6 =
14. There would be only 6 electrons.
14CO
2
Decay Curve of Radiocarbon
100
% of original 75
radioisotope
50
remaining
12.5%
25
0
Half Lives 1
Years B.P.
5730
2
11,460
3
4
17, 190 22,920
5
28,650
6
34,360
7
40,110
8
45,840
Decay: the reversion of a radioisotope to a stable form.
Half-life: the amount of time it takes for one half of a quantity of a
radioisotope to decay.
Dr. Rainer Berger in the UCLA Radiocarbon Lab.
1986.
• In the early 1960’s, older
radiocarbon dates (before 1200
BC) were shown to be inaccurate.
The inaccuracy was due to the
fact that the production of
radiocarbon in the atmosphere
varied year in, year out.
• Hans Suess (the real Dr. Suess)
found a way to correct the
problems by measuring the
amount of yearly radiocarbon
variation in the tree rings of the
bristlecone pine growing in
California. He then constructed a
calibration curve.
The calibration of dates these days
is done with a computer program.
Raw radiocarbon dates are
indicated by a small bc, and
calbrated dates by BC in capital
letters.
Argon-Argon
• Postassium contains a large amount of
Potassium 39.
• When Potassium is irradiated and fused, it
gives off both Argon 39 and Argon 40
gasses. The amount of Argon 40
represents the length of time that has
transpired since the mineral potassium
had been created.
Carl Swisher of UC Berkeley went to Indonesia to test
this technique on fossils that had been excavated
between the 1930’s and 1960’s.
A small sample extracted
from the pumice matrix
inside a child’s
cranium, excavated in
1936, at Mojokerto,
Java, dated to 1.81
mya.
Dating
techniques
used at
Boxgrove,
Sussex,
England
Oxygen Isotopes
Oxygen occurs in two common isotopic
forms: 16O and 18O. They are commonly
found in water molecules: H2O.
16o
18O
CaC18O3
16O
• During a cold period
water rich in 16O
becomes locked into
ice sheets.
• As a consequence,
sea water becomes
richer in 18O.
• This oxygen isotope
shows up in the
shells of shellfish,
whose shells are
composed of calcium
carbonate, CaCO3.
Optically Stimulated
Luminescence
• Electrons are trapped in imperfections in
the crystal lattice of quartz.
• Exposure to sunlight releases these
electrons.
• Burial of the quartz leads to reabsorption
of electrons from soil.
• Exposing the quartz to infrared light
releases these trapped electrons.