Methods of Dating: Types of Relative Dating Methods and Absolute Dating Methods

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Methods of Dating՚s

Dating techniques are procedures used by scientists to determine the age of rocks, fossils, or artifacts.

2 Types:

  • Relative dating methods,
  • Absolute dating methods
Methods of Dating՚s
Methods of Dating՚s

Types of Relative Dating Methods

  • Relative dating methods determine whether one sample is older or younger than another.
  • Very useful technique.
  • Dating is done in relation to same other materials.

Types

  • Stratigraphy
  • Fluorine Dating
  • Pollen Dating or Palynology

Stratigraphy

  • Study of layers of rocks
  • Principle method of relative dating
  • Deeper layers were deposited earlier, and thus are older, than more shallow layers.
  • The smallest of these rock units that can be matched to a specific time interval is called a bed.
  • At the end of 18th century, geologists began to realize that the earth՚s rocks were stratified or laid down in layers, one after another.

Fluorine Dating

  • Used to determine the amount of time an object has been underground.
  • Items such as bone that are in the soil will absorb fluoride from the groundwater over time.
  • Amount of fluorine deposited in bones is proportional to their age. The oldest bones contain the greatest amount of fluorine and vice versa.
  • As not all objects absorb fluorine at the same rate.
  • In 1953 this test was used to easily identify that the ‘Piltdown Man’ was forged, almost 50 years after it was originally ‘unearthed.’

Pollen Dating or Palynology

  • Each year seed-bearing plants release large numbers of pollen grains.
  • Pollens are small grains or particles released by different flowering plants.
  • Pollen is preserved for very long time/periods of time and can be used to reconstruct prehistoric environments.
  • Pollen may also become fossilized in arid conditions if the soil is acidic or cool.

Absolute Dating Methods

  • Absolute dating is the term used to describe any dating technique that tells how old a specimen is in years.
  • Give the dates in absolute terms.
  • Dates are expressed in units of years (AD & BC) .
  • Gives dates in absolute terms with range of plus or minus margin.
  • It is based on solar years.

Types

  • Dendrochronology or Tree Ring Dating
  • Radiometric Dating Method

Types of Radiometric Dating

  • Carbon-14 Dating
  • Potassium Argon Dating
  • Amino Acid Racemization

Dendrochronology or Tree Ring Dating

  • Principle: In certain climates trees add a new growth ring to their trunk every year.
  • Method: counting rings and joining in long sequences from different trees & correlating with fragmentary beams far back as the first century B. C.
  • Advantage: applicable to wooden objects.
  • Limitations:
    • Applicable only to wooden objects.
    • Only useful in regions where trees are of great age.
    • Only useful in regions where rainfall is seasonal & growth of rings regular.

Carbon-14 Dating

  • Invented by J. R. Arnold and W. F. Libby.
  • Best known and most widely used.

Principle

It is based on the fact that all living organisms absorb radioactive carbon.

Chains of Events

  • Solar radiation to radioactive carbon atoms.
  • Both C-14 & C-12 link up with O2 to form CO2.
  • C-14 decomposes into C-12 at a constant rate (5568 years) .
  • As we know the rate of decline, we can measure this ratio in remains of any plant or animal, compare it to ratio of living organism and compute the time it had been died.
  • The age is calculated by counting the beta rays emitted by remaining C-14.

Uses

  • Age up to 50000 years old can be calculated.
  • Most effective for sites dating between 50000 to 2000 years before present.

Limitations

  • Variation of plus minus 180 years in dating.
  • Result depends upon rate of C-14 produced in atmosphere.

Potassium Argon Dating

  • Based upon decay of potassium 40.
  • P-40 has a half-life of 1.3 billion years.
  • Potassium is a common constituent of rocks.
  • Only 11.2 % of P-40 decay into Ar-40 under intense heating condition e. g. : volcanic eruption.
  • As time goes on, the amount of P-40 decreases while amount of Ar-40 increases.
  • In a sample of cooled volcanic rocks, ratio of P-40 to Ar-40 is measured with the help of spectrometer.

Amino Acid Racemization

It occurs into two forms:

  • L-Amino Acid: form of amino acid found in living forms.
  • D-Amino Acid

When an organism dies the L-amino slowly turns into D-Amino. this process is known as Racemization.

  • Each amino acid is associated with a characteristic speed of racemization at a given temp. e. g. : In aspartic acid, racemization is 0.333 at 20-degree centi. in about 15000 to 20000 years.
  • It can be used to date material older than that which can be dated by C-14 method.
  • Much less of the fossil material is needed for a determination of date than radio carbon determinations.

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