Absolute Dating HistoricCarbon-14Potassium-Argon Fission Track Obsidian Hydration LuminescenceMagnetism

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. The extent of documentation varied considerably in 'historical' cultures and the information that survives is determined by a variety of factors.  E.G. If a context containing burnt debris and broken artefacts 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

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)  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 Range of absolute age

Fission Track  Charged particles from radioactive decay pass through mineral's crystal lattice and leave trails of damage called FISSION TRACKS. These trails are due to the spontaneous fission of uranium. Procedure to study:  Enlarge tracks by etching in acid (so that they may be visible with light microscope)  See readily with electron microscope  Count the etched tracks (or note track density in an area)  Useful in dating:  Micas (up to 50,000 tracks per cm squared)  Tektites  Natural and synthetic (manmade) glass  Reheating "anneals" or heals the tracks.  The number of tracks per unit area is a function of age and uranium concentration.

How Fission Track works: Spontaneous Fission TracksInduced Fission Tracks With the information gained from both spontaneous and induced fission an approximate thermal age can be calculated through this equation as long as the neutron dose is known.

Obsidian hydration dating  Obsidian was a popular alternative to flint for making flaked tools in many parts of the world.  As soon as a fresh surface of obsidian is exposed, during the process of making it into a tool, a microscopically thin hydration rim begins to form as a result of the absorption of water.  archserve.ucsb.edu/Anth3/Courseware/Chro nology/Movies/Hydration_Basics.html archserve.ucsb.edu/Anth3/Courseware/Chro nology/Movies/Hydration_Basics.html archserve.ucsb.edu/Anth3/Courseware/Chro nology/Movies/Hydration_Basics.html

Luminescence Dating  The physical phenomenon of luminescence may be used to date artifacts that were made from (or include) crystalline minerals which have been subjected to strong heating.  The first successful application was to clay fired to make pottery, but it is commonly used now for dating flint tools that have been burnt, for example by being dropped accidentally into a fire.  Also know as Thermoluminescence dating.

Thermoluminescence dating  Thermoluminescence dating is used for rocks, minerals and pottery.  It dates items between the years ,000B.P.  It is based on the fact that almost all natural minerals are thermoluminescent.  Energy absorbed from ionizing radiation frees electrons to move through the crystal lattice and some are trapped at imperfections.  Later heating releases the trapped electrons, producing light.

Thermoluminescence To test the date, three steps are taken: 1. Measure sample’s intensity of luminescence. 2. Relate luminescence intensity to radiation dose irradiate sample with a calibrated radioactive source. 3. Determine the dose per year that the sample has been exposed to. The formula used in this equation is: Age=accumulated dose ______________ dose rate Dose Rate=dose accumulated each year tltech.htm

Archaeomagnetic dating  The position of magnetic North wanders around the North Pole, and even reverses completely to the South Pole for extended periods on a geological time-scale.  From any reference point its position is measurable in terms of two components: movement up or down (inclination or 'dip') and from side to side (declination).  The earth's magnetic field is indeed dynamic and does shift. At present the declination for London changes by approximately 1degree every decade.

How does it work?  Magnetic information is recorded in ferromagnetic elements in baked clay which have kept their position on cooling from the last firing of the clay.  This means that baked clay, used for thousands of years in the construction of hearths, ovens and kilns, contains a weak but permanent magnetization which can be measured to determine the magnetic intensity and declination at the time of its last cooling.  The thermoremnent magnetism (TRM) of baked clay is gained from the magnetic properties of magnetite and hematite, iron- oxides that make up on the average of 6-8% of the earth's crust.  Before clay is baked these properties are orientated in random directions. If the temperature is raised to over several hundred degrees Celsius, the thermal agitation of the crystals allows some of the domains to be aligned by the earth's magnetic field. When the clay cools their directions remain fixed, and there is a weak permanent magnetization in the same direction as the earth's field.

Archaeomagnetic Example Roman fort at Chester-le-Street in County Durham (GB). The north wall of the officer's quarters was burnt during construction and it was decided to remove a few stones from the wall for archaeomagnetic dating. Prior to lifting, bubble levels were glued in place to permit their exact re-orientation in the laboratory (archaeomagnetic dating depends upon the known drift of magnetic north, which is fixed when an object is heated above a certain temperature).