Geoscience Reference
In-Depth Information
the no more existing reaction between the plant (or the animal) and the atmosphere
is a gradual decrease of
14
C concentration in the dead body. One-half of the
14
C
atoms decay after 5,730 years - the time period denoted as its half-life. Using its
half-life value in an exponential equation, we obtain the time of death of the body
being analyzed in our study. The accuracy of the analytical method allows the age
of the body to be estimated for times not more than roughly 60,000 years BP (before
present) with the year 1950 taken as the present time. Most frequently, the method
is used for estimating the age of unconsolidated sediments, shells, some limestones,
and organic materials. According to the half-life of
14
C, the radiocarbon method is
applicable back to the middle of the last glacial period, denoted as Würm in the
Central and Alpine Europe, or Wisconsin in the USA, or Weichselian in the north
Europe, or Merida in Venezuela.
The age of older Quaternary sediments and buried soils has been determined by
optical methods which translate the time of stored luminescence into age. Although
several methods have been developed, the most frequently used procedure is that
called optically stimulated luminescence (OSL). Exposure to sunlight resets the
luminescent signal and so the time period between recent time and the time when
the soil was buried can be calculated. The methods are applicable for ages between
500 and 100,000 years.
Determination of the age of “very young” paleosols developed in Holocene
requires more accurate methods. For example, the spores of mycorrhizae are used
for identifying buried remnants of A horizon below a recent soil or of its top horizon
when a relatively small age up to 5,000 years BP is the object of the research.
In reports on sediments and soils in the time span between recent and about
170,000 years BP, a detailed time scale is described by the term MIS (Marine
Isotope Stage), and in less frequent time named Oxygen Isotope Stage by some
authors. It is based upon the ratio of oxygen isotopes,
18
O, or ratio of “heavy” oxy-
gen to “light” oxygen (
18
O/
16
O) in studies of ocean bottom sediments sampled by
core drilling. Up to now, over 100 stages have been identifi ed reaching slightly over
6 million years before present, i.e., the stages cover the youngest part of Miocene,
Pliocene, Pleistocene, and Holocene. The cycles in the isotope ratio correspond to
glacials and interglacials. Even-numbered stages have high levels of oxygen-18 and
represent cold glacial phases, while the stages with odd numbers have a relatively
low content of oxygen-18 and they represent warm
interglacial
intervals. MIS 1 is
for Holocene, ending with Younger Dryas. The last glacial (Wisconsin or Würm or
Weichsel glacial) contains MIS 2-5. Some stages are subdivided into substages, like
MIS 5, where 5a, 5c, and 5e denote relatively warm substages and 5b and 5d are
cold substages. Substage 5d is the end of last interglacial (115 kyr BP) and starts the
last glacial, while MIS 5e denotes part of the last interglacial. The numeric values
continue into a deeper past, and, e.g., the end of the Pliocene is denoted by MIS 103
(2.588 myr BP).
The estimation of a larger time BP is also performed on the principle of stable
isotopes being related to the unstable isotope of the same element. Relatively fre-
quent is the use of potassium isotope method. The name of the element in English
was derived from “pot ash” that refers to the method of obtaining potash during and
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