Freezing Cold Case Files

http://www.colbertnation-q.mtvi.com/the-colbert-report-videos/183049/june-13-2007/freezing-cold-case-files--otzi

Archaeological Dating Methods

Relative

Absolute

Relative Dating

Geologic TimeStratigraphic Correlation

BiostratigraphyExample: Life in the Pleistocene

SeriationDendrochronology

Composite Geologic Section

Geologic Time: Calculating Metaphors

Stratigraphic Correlation

Following criteria help establish stratigraphic correlation within a region and between different regions. Order of Superposition Palaeontology Geophysics Structural Geology Petrography

Order of Superposition 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 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

Ice Age

Example: Pleistocene

Extinct Animals

Sabertooth Tiger

Pleistocene

Mastodon

Example: Pleistocene

Extinct Animals

Giant Beaver

Giant beaver Modern Black Bear

Other Pleistocene Mammals

Wooly Rhino

Irish Elk

Example: Pleistocene

Human Species (1.8m ya-10,000 ya) Homo antecessor

(800,000 ya) Homo heidelbergensis

(780,000 ya) Homo neanderthalensis

(200,000-32,000 ya) Homo sapiens (200-

150,000 ya)

Hunting Large Mammals

Seriation

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. 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

Tombstones design changes through time.

1872

Dendrochronology Method

Find a tree w/ known date of cutting Define sets of distinctive ring patterns for series of

years (slow, fast growth) 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" U.S. SW (pine), NW Europe (oak)

Dendrochronology

Dendrochronology and Cross-Dating

© 2002 by Henri D. Grissino-Mayer.

Absolute DatingHistoric

Carbon-14Potassium-Argon

Historical Dating 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 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 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? 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 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 MinimumCharcoal 30 gms 1.7 gmsShell 100 gms 7 gmsWood 100 gms 7 gmsPeat 100 gms15 gms Organic Sediment 1-2 kilogramsBone 500 gms 200 gmsDung 30 gms 7 gmsWater:BaCO3, SrCO350 gms7 gms

Material Recommended MinimumCharcoal 50 mgs 5 mgsShell 100 mgs 30 mgs Forams 100mgs 15 mgsWood, seeds100 mgs10 mgsOrganic Sediment10 gmsPeat100 mgs 15 mgsWater:BaCO3, SrCO 350 mgs 15 mgs Bone 30 gms 2 gmsPlant Material l50 mgs10 mgs

Potassium-argon (40K/40Ar) 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