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 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 Irish Elk Wooly Rhino

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

Relative Dating of Building I at Monticello http://www.monticello.org/archaeology/research/mulberryrow/building-l.html

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 Dating Historic Carbon-14 Potassium-Argon Fission Track Obsidian Hydration Luminescence Magnetism

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 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 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 leave trails of damage called FISSION TRACKS. These trails are due to the spontaneous fission of uranium. Useful in dating: Micas (up to 50,000 tracks per cm squared) Tektites Natural and synthetic (manmade) glass The number of tracks per unit area is a function of age and uranium concentration. Spontaneous Fission Tracks

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. http://id-archserve.ucsb.edu/Anth3/Courseware/Chronology/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. It dates items between the years 300-10,000 B.P.

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 http://www.users.globalnet.co.uk/~qtls/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 1 degree every decade.

How does it work? Magnetic information is recorded in iron 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. 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).