Freezing Cold Case Files
http://www.colbertnation-q.mtvi.com/the-colbertreport-videos/183049/june-13-2007/freezing-coldcase-files--otzi
Archaeological Dating
Methods
Relative
Absolute
Relative Dating
Geologic Time
Stratigraphic Correlation
Biostratigraphy
Example: Life in the Pleistocene
Seriation
Dendrochronology
Composite Geologic Section
Geologic Time:
Calculating Metaphors
Stratigraphic Correlation
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Following criteria help establish stratigraphic
correlation within a region and between
different regions.
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Order of Superposition
Palaeontology
Geophysics
Structural Geology
Petrography
Order of Superposition
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Under normal conditions, rocks laid down on Earth's
surface - sedimentary or volcanic - generally lie flat
horizontal and younger rocks are laid on top of older
ones.
Therefore in successions with simple structures
order of superposition determines the relative ages
of successive layers.
The top layer in such cases is younger than the
bottom layer. However, in successions that have
experienced severe deformation , a part of the
succession under study may actually be an inverted
sequence.
Therefore, order of superposition cannot be used to
assign relative ages in areas of complex structure.
Palaeontology
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Despite advances in geochronology and geophysics,
palaeontology remains a popular tool in stratigraphy.
Therefore life forms that existed once during some
geological time period evolve, but do not repeat
themselves.
Fossil groups are therefore characteristically
associated with geological time units. Some groups
may have existed during longer time periods than
others. Some fossil groups have externally restricted
time period of occurrence.
Such fossils, if found to occur fairly commonly in rocks,
are known as index fossils as they indicate the age of
rock/soil formations much more accurately than
others.
Example: Pleistocene
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Ice Age
Example: Pleistocene
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Extinct Animals
Sabertooth Tiger
Pleistocene
Mastodon
Example: Pleistocene
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Extinct Animals
Giant Beaver
Giant beaver Modern Black Bear
Other Pleistocene Mammals
Wooly Rhino
Irish Elk
Example: Pleistocene
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Human Species (1.8m
ya-10,000 ya)
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Homo antecessor
(800,000 ya)
Homo heidelbergensis
(780,000 ya)
Homo neanderthalensis
(200,000-32,000 ya)
Homo sapiens (200150,000 ya)
Hunting Large Mammals
Seriation
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a relative dating technique based on the chronological
ordering of a group of artifacts or assemblages, where
the most similar are placed adjacent to each other in
the series.
Two types of seriation can be recognized, frequency
seriation and contextual seriation.
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frequency seriation: a relative dating method which relies
principally on measuring changes in the proportional
abundance, or frequency, observed among finds (e.g. counts
of tool types, or of ceramic fabrics).
contextual: a method of relative dating pioneered by Flinders
Petrie in the 19th century, in which artifacts are arranged
according to the frequencies of their co-occurrence in specific
contexts (usually burials).
Frequency Seriation Exercise
The frequency of bottle type C increases gradually to a peak of 75 percent in the
middle of the time sequence, and then gradually declines in frequency as bottle type
D increases in frequency (http://encarta.msn.com/media_461546925/Seriation.html).
Ceramic Seriation
Richard MacNeish used seriation to trace the ancient development
of pottery styles in the Tehuacán Valley of Mexico. This shows a
sequence of pottery styles from about 2850 BC to about AD 1500
(http://encarta.msn.com/media_461531254/Mexican_Pottery_Styles_Over_Time.html).
Contextual Seriation
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Tombstones design changes through time.
1872
Dendrochronology Method
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Find a tree w/ known date of cutting
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Define sets of distinctive ring patterns for series of
years (slow, fast growth)
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best to record >1 set per tree
Match same patterns in rings of older trees
Count backwards across yearly rings from
known-date to matched, overlapping series in
older trees
Build regional "master chronology"
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U.S. SW (pine), NW Europe (oak)
Dendrochronology
Dendrochronology and CrossDating
© 2002 by Henri D. Grissino-Mayer.
Absolute Dating
Historic
Carbon-14
Potassium-Argon
Historical Dating
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Prehistorians sometimes overestimate the
accuracy and detail of frameworks based on
historical evidence; in practice, early written
sources may provide little more information than a
scatter of radiocarbon dates.
E.G. If a context containing burnt debris and
broken artifacts is excavated on a site from a
historical period, it is tempting to search the local
historical framework for references to warfare or a
disaster in the region, and to date the excavated
context accordingly.
Absolute Techniques
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Radiocarbon dating-Carbon 14 and AMS-decay of carbon in
organic materials.
Potassium-argon (40K/40Ar)-decay of potassium into argon in
volcanic materials.
Fission-track dating-microscopic tracks in glassy material.
Obsidian Hydration-hydration rim forms when stone tools are
made from obsidian.
Luminescence dating-heating of crystalline material.
Magnetism-variation in the earth’s magnetic pole.
Radiocarbon dating
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Carbon 14- one peaceful by-product of accelerated
wartime research into atomic physics and
radioactivity in the 1940s. The rate of decay of 14C,
which has a half-life of 5730 (±40) years, is long
enough to allow samples of carbon as old as 45,000
years.
Samples containing 300 milligrams to 4 grams final
carbon
AMS- Accelerator Mass Spectrometry, measures on
the atomic level so can go to 70,000 years.
How does Carbon-14 dating work?
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Cosmic rays from the sun strike Nitrogen 14 atoms in the atmosphere
and cause them to turn into radioactive Carbon 14, which combines
with oxygen to form radioactive carbon dioxide.
Living things are in equilibrium with the atmosphere, and the
radioactive carbon dioxide is absorbed and used by plants. The
radioactive carbon dioxide gets into the food chain and the carbon
cycle.
All living things contain a constant ratio of Carbon 14 to Carbon 12. (1
in a trillion).
At death, Carbon 14 exchange ceases and any Carbon 14 in the
tissues of the organism begins to decay to Nitrogen 14, and is not
replenished by new C-14.
The change in the Carbon 14 to Carbon 12 ratio is the basis for
dating.
Assumes that the rate of Carbon 14 production (and hence the
amount of cosmic rays striking the Earth) has been constant (through
the past 70,000 years).
The Curve of Knowns
Curve developed from dating know samples with radiocarbon
Accelerator Mass Spectrometry
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Accelerator Mass Spectrometry (AMS) is a mass spectrometric
method for quantifying extremely low concentrations of long-lived
radioisotopes, such as the commonly used biochemical tracer
14C.
AMS can measure attomoles of radiocarbon with a precision of
better than 10%.
This corresponds to the need for less than 0.1DPM-equivalent of
labeled agent per gram of biological sample.
AMS has use when sample is limiting, specific activity is very low,
when the level of isotope that can be used is very limited (humans
studies) and when trying to study events that occur with very low
frequency or at very low concentration.
AMS ANALYSIS for smaller samples (100 micrograms to 300
milligrams of final carbon.
Accelerator Mass Spectrometry
Wide angle view of the Center's FN tandem accelerator and mass spectrometer
http://cams.llnl.gov/cams_intro.html
What C-14 and AMS date
Radiometric Technique
Accelerator Mass Spectrometry
Technique (AMS Technique)
Material Recommended Minimum
Charcoal 30 gms 1.7 gms
Shell 100 gms 7 gms
Wood 100 gms 7 gms
Peat 100 gms15 gms
Organic Sediment 1-2 kilograms
Bone 500 gms 200 gms
Dung 30 gms 7 gms
Water:BaCO3, SrCO350 gms7 gms
Material Recommended Minimum
Charcoal 50 mgs 5 mgs
Shell 100 mgs 30 mgs
Forams 100mgs 15 mgs
Wood, seeds100 mgs10 mgs
Organic Sediment10 gms
Peat100 mgs 15 mgs
Water:BaCO3, SrCO 350 mgs 15
mgs
Bone 30 gms 2 gms
Plant Material l50 mgs10 mgs
Potassium-argon (40K/40Ar)
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K is an element that goes into many minerals, like feldspars and
biotite. Ar, which is a noble gas, does not go into minerals when
they first crystallize from a magma because Ar does not bond with
any other atom.
When a K-bearing mineral crystallizes from a magma it will
contain K, but will not contain Ar. With passage of time, the 40K
decays to 40Ar, but the 40Ar is now trapped in the crystal
structure where the 40K once was.
Thus, by measuring the amount of 40K and 40Ar now present in
the mineral, we can determine how many half lives have passed
since the igneous rock crystallized, and thus know the absolute
age of the rock.
Ideal for dating early hominid fossils in East Africa.
They occur in an area that was volcanically active when the
fossils were deposited between one and five million years ago.
How K-Ar works:
Datable material
Datable material
Range of absolute age