Absolute dating 2-2

Absolute Dating:

Carbon Dating -

• Radiocarbon dating is a chemical analysis used to determine the age of organic materials based on their content of the radioisotope of carbon 14.
• The method was developed by Willard F. Libby 
• up to about 50,000 years.
• By radiocarbon method one can date different types of organic or inorganic materials as long as they consist of carbon. 
• sunlight collides with nitrogen and creates carbon 14 isotope which animals consume in life. 14 and 12 ratio remains constant in life and decrease after death. Based on half life of carbon 14 we can estimate the age of specimen. 
• Require large amount of sample. DK is dependent on statistical probability not always accurate. Costly method. Precision needed. requires dry conditions preservation. Contamination of specimen possible. Requires organic matter.
• Applied in Sarai nahar Rai mesolithic dating. 8400 bc.

Pottassium- Argon (K-A40) Dating - radioactive method.

•  The earth's crust contains potassium of which isotope K40 decays to A40 at a known rate.
• The ratio of potassium to Argon may be measured to ascertain date of minerals and rocks in a deposit.  half life of the radioactive potassium is 1330 million years.
• dating some hominid fossils as employed in the site of Olduvai Gorge in east Africa where the remains were as old as 1.75 million years.
•  works well in case of the sites which are 500,000 years old.
• disadvantage of the method is that it can be applied to only to those rocks and minerals which are rich in potassium. 

Thermoluminescence(TL) -

•  Oxford by M. Aitken. RObert Boyle 1663
• Initially designed to date archaeological ceramics, it was subsequently extended to other mineral materials, such as burnt flint.
• This is based on the fact that objects such as pottery that have been heated in the past can be dated by the measurement of their Thermoluminescence (TL) glow.
• Thermoluminiscence (TL) is the emitted light in the pottery which can be measured. If the ground up pottery is reheated, it emits light. The phenomenon results from radio-active influence of the metallic elements like uranium and potassium present in the clay and surrounding soils.
• enabled in revising some prior assumptions about the evolution of lithic industries and the nature of hominids present in the Near East at various stages of the Middle Palaeolithic. Mousterian culture. Dating. 
• 40 k to 200k range. 
• Limitations. other factors also affect the strength of luminescent rays. Not very accurate.

Obsidian daring 

• 1960 freedman and Robert Smith.
• fixed rate at which freshly exposed obsidian surface absorbs moisture from environment point in the course of making tools, table exposed fresh surface of Rock to the moisture. By measuring how deeply the moisture has penetrated into the obsidian researches candid the length of time since the tool was made.
• Used as relative and absolute dating method both. Less expensive than radiocarbon dating. Abundant findings. 
• Limitations. Local environment various changes in moisture. Error is hai. 

• The age of wooden objects can be determined by means of Dendrochronology or tree ring analysis. It determines the calendar years of tree-ring formation and the felling dates of trees, which helps to determine the age of wooden objects with a great precision.
• Dendrochronology has therefore become well established in the field of archaeology, art history and cultural heritage.
• The method depends on the fact that trees growing in temperate zones have clearly defined annual rings of growth. As these tree rings represent annual growth, merely by counting rings one can count the age of the tree and hence its association.
• This dating method with latest methodological advances helps us in defining the calendar year in which the tree-rings were formed and in interpreting such dating in terms of the age of a wooden object.

Electron Spin Resonance (ESR) -

• In the last 20 years, the Electron Spin Resonance (ESR) dating method has allowed the establishment of a chronological time frame over most of the history of human evolution.
• Despite many difficulties found for ESR dating of bones and carbonates, tooth enamel dated by Electron Spin Resonance (ESR) has been proven as a reliable method in its application to fossil teeth and quartz.
• Both of the latter materials have allowed dating of Early and Middle Pleistocene sites which are not datable using other methods.
• In particular, recent discoveries of human remains in Western Europe have been proposed to be sites of the earliest arrival of humans there, and have been dated to the Early Pleistocene by Electron Spin Resonance (ESR) using quartz and tooth enamel.
• Electron Spin Resonance (ESR) method can be applied to different types of samples in various environments; its contribution to the elaboration of a chronostratigraphic frame is of a great importance for the understanding of the Homo erectus dispersals out of Africa and especially for the first settlements in Europe.
• Electron Spin Resonance (ESR) dates ranging between 1.7 and 2.3 Ma have been proposed for the South African sites Sterkfontein and Swartzkrans, indicating that the whole Quaternary period can be dated.

Palaeomagnetic Dating - Robert DuBois. 1960.

•  earth's magnetic field in several places varies with time.
• When clay is heated to a certain degree, the magnetic elements of baked clay realign themselves along lines dictated by the intensity and character of the magnetic field of the earth at that time.
• On cooling the magnetic elements are frozen and can be recorded as long as the clay is preserved. This is called remnant magnetism.
• When records of past angles of declination and dip have been kept it is possible to compare the values of historic records and arrive at the date of archaeological specimens of fired clay.
• Fluxgate, Spinner magnetometer, Super-conduction magnetometer are the instruments used for measuring magnetic remenence.
• dating Karewa sediments in Kashmir.
• The reliability of the application of the method depends on certain conditions such as
1. The availability of good records of change of magnetic paths near the prehistoric sites,
2. Occurrence of series of already dated baked clay in the area against which objects of unknown dates can be dated,
3. Availability of archaeological samples which are found in their place of first occurrence. Fire places and kiln thus provide best samples for dating.

• This technique dates material ranging from 20 years to 1,000,000,000 years before the present.
• over geological time spontaneous fission of uranium-238 impurities produces minute sub- microscopic damage trails in the most insulating solids, both long period of time.
• As the damage sites are permanent, a simple count of their number allows an age to be measured. The density of track depends on the uranium content as well as the age of the sample. The sample is immersed in the proper chemical reagent, which allows rapid attack of the narrow regions of damage and slower dissolution of the adjacent unchanged material. Such treatment are used to create etch pits of optical size, each one making a single fission site. The method requires sufficient uranium to produce a track density, which can be counted within reasonable time.

Amino Acid Racemisation Technique

• Amino Acid Racemisation, a method of absolute dating applied to human fossils is developed very recently.
• This is based on the fact that all living organisms have L-amino acid in their protein and after death, and over a long period of time all the L-amino acids except glycine undergo change called racemisation and became non protein D-amino acid.
• The proportion of these D-amino acids increases with time. Thus the age of the skeletal materials that are found in archaeological sites can be estimated by determining the amount of change, racemisation that has occurred.
• Amino Acid Racemisation (AAR) is a useful comparative dating method with great potential, but it introduces problems of calibration and the requirement of an intimate knowledge of the palaeoenvironmental conditions of the bone deposition site.

Capabilities and limitations of major Chronometric Dating Method