Dating of Sediments in Rocks (With Diagram)

One of the most frequent questions a Palaeobotanist or Palaeontologist hears concerns the method for dating sediments containing fossil plants and animals. Present knowledge is based on a long series of efforts to date the ages of various rocks.

At the present time, the best absolute dating involves the use of naturally occurring radioactive isotopes contained in various minerals that make up a rock. These radioactive isotopes are sometimes referred to as “geological clocks.”

It was demonstrated that there are radioactive isotopes of certain elements that decay at a constant rate irrespective of heat, pressure and any other factor in the environment. Radioactive isotopes like U²³⁸, U²³⁶, Thorium²³², K⁴⁰, C¹⁴ have been used in making ace determination. U and Th are found most frequently in an igneous rock while K⁴⁰ and C¹⁴ are components of some sedimentary rocks.

Radiocarbon Dating:

Amongst the physical methods, the C¹⁴ dating technique for dating organic remains is still unsurpassed in accuracy Normally its dating range is 50,000 years for its short half-life. The technique of C¹⁴ was developed by W.F.Libby (1955).

The method is based on the fact that C¹⁴ atoms are continuously produced in the atmosphere as a result of neutron (n), proton (p) reaction induced by slow neutrons of the cosmic ray on the atmospheric nitrogen cycle (N¹⁴):

The newly formed carbon is oxidised to ¹⁴CO₂ and rapidly mixes with atmospheric carbon dioxide (¹²CO₂). Part of the atmospheric ¹⁴CO₂ and ¹²CO₂ enter plant tissue as a result of photosynthesis. Animals partake this carbon through the consumption of vegetable matter. The larger part of the ¹⁴CO₂ goes to the ocean where it gets incorporated in the marine carbonates. From the atmosphere which is its birth place, C is distributed globally through the carbon cycle.

All living matter on earth is thus labelled by radiocarbon atoms at a constant level (activity per gm of Carbon). The amount of ¹⁴C present in this system is about 1×10 per atom of ordinary carbon (¹²C). ¹⁴C atom will follow the radioactive decay where a neutron is converted to a proton by the ejection of a negatively charged beta (β) particle called a negatron. As a result the nucleus loses a neutron but gains a proton and will converted to a stable Nitrogen atom.

Radioactive decay is a spontaneous process and it occurs at a definite rate characteristic of the source. This rate always follows an exponential law. Thus the number of atoms disintegrating at any time is proportional to the number of atoms of the isotope present at that time (Fig 13.4).

So the exponential curve will give the equation:

Thus the rate of change in the number of radioactive atoms is porportional to the number of atoms present (N) multiplied by the decay constant (λ). This constant is a characteristic of a given isotope and is defined as the fraction of an isotope decaying in unit time (t^-1).

By integrating the above equation it can be converted to a logarithmic form:

1_t = 1₀e^-λt

By measuring the radioactivity of plant samples freshly formed, 1₀ is obtained, remembering that the rate of synthesis of ¹⁴C is constant. The present day radioactivity (It) is measured with the sample. It is thus possible to find out ‘t’, the age of the sample, knowing that half life of ¹⁴C is 5568 ± 30 years.

An age limit of about 50,000 years applies to this technique because of the short half-life of ¹⁴C. This technique obviously has somewhat limited usefulness in Palaeobotany and Palaeontology because bulk of the fossil plant and animal records are much older.

Human influence on the earth has even altered the usefulness of the ¹⁴C dating method because combustion of fossil fuels and nuclear testing have artificially altered the ¹⁴C content of the total carbon reservoir. Loss or addition of ¹⁴C to specimens and apparent fluctuations of past atmospheric ¹⁴C abundance also impose limitations on this dating method.

Uranium Dating:

When the U²³⁶, U²³⁸ disintegrate they ultimately produce a stable form of lead, helium and heat. Because isotopes of uranium decay by emitting alpha particles (α). An alpha particle is a helium nucleus in that it consists of two protons and two neutrons. Emission of a α-particle results in a decrease in atomic number of two and a decrease in the mass number of four.

Thus U²³⁶ after passing through 14 intermediate stages will yield Pb²⁰⁶ plus 8H and heat. The rate at which 1 gm of U²³⁶ decays into Pb²⁰⁶ is 1/7600,000 gm of lead in 1 year. Knowing this constant rate of decay, it is clear that the ratio of lead to the remaining uranium can be used to determine the age of the rock.

Another way of portraying the same idea is to express the rate of decay in terms of half-life of the isotopes. U²³⁸ has a half-life of 4.5 x 10⁹ years. If the Pb²⁰⁶ is the product of the decay of U²³⁸, then the ratio of U²³⁶ to Pb²⁰⁶ when related to half-life of U²³⁸,will give us an indication of the age of the rock by putting the same formula as applied to ¹⁴C dating.

One difficulty in employing this dating technique is that radioactive isotopes occur more commonly in igneous and metamorphic rocks and most fossils occur in sedimentary exposures. Today direct isotopic dating for sedimentary rocks is possible. One of these is glauconite, a silicate minerals that contains potassium. Since the K in part contain K⁴⁰, the K-A method can be used.

Potassium Argon Dating:

This depends on the decay of the naturally occurring radioactive Potassium (K⁴⁰) isotope to Argon (A⁴⁰) and Calcium (Ca⁴⁰).

Half-life of K⁴⁰ is 1.26 x 10⁹ years. About 88% of the K⁴⁰ decayed to Ca⁴⁰ and remaining 12% to an inert gas A⁴⁰. Hence the ratio of K⁴⁰ and Ca⁴⁰ would be helpful in determining the age of the rock by putting the same formula.

In this way samples as old as Silurian or even Precambrian age can be dealt with.

Biological Correlation:

Though radiometric dates are not available for all sequences of rocks in specific geographic regions, so it becomes necessary to be able to position a given rock unit accurately relative to its absolute age. One means by which a given sequence of sedimentary rocks can be grouped according to age is through the use of index fossils.

Typically an index fossil should be:

i) Distinguishable from other fossils and easily identifiable,

ii) Existed during a relatively short period of geologic time,

iii) Abundant,

iv) Widely distributed geographically, and

v) Lived in different sedimentary environ­ments.

So that it may be preserved in different sedimentary rocks. Obviously, not many fossils fulfill all these requirements, and assemblages of several fossil taxa are typically more useful than a single species.

Foraminifera, diatoms, Dinoflagellates, etc., are very useful in detection of age and subsequent environment in shallow-water marine deposits. Some of the best types of plant index fossils used across different fades are pollen grains and spores. Some of these palynomorphs are therefore especially important in providing correlation between marine and freshwater sediments.