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improved significantly and the new dates are at least an order of magnitude more
precise than the earlier dates. In time scale calculations the weight of individual age
determinations is approximately proportional to the inverse of their measurement
variance (
2
). Consequently, a modern age date, which is ten times more precise
than a pre-1990 age determination as obtained by the Rubidium-Strontium or
Potassium-Argon method, is weighted 100
˃
stronger in the statistical calculations.
This implies that a single modern date roughly receives at least as much weight as
100 earlier dates. New high-precision dates continue to become available regularly.
Since 1990, Felix Gradstein has led several international teams of scientists
engaged in constructing new time scales including GTS2004 (Gradstein
et al.
2004
), which became used by geologists worldwide, and GTS2012 (Gradstein
et al.
2012
) that has replaced GTS2004. Construction of GTS2012 involved over
65 geoscientists and other experts. Stage boundary age estimates in GTS2004 and
GTS2012 are accompanied by approximate 95 % confidence intervals shown as
for periods and Cenozoic epochs in Table
9.14
. The largest difference between
GTS2012 and GTS2004 in this table is 3.2 Ma and occurs at the base of the
Devonian.
Constructing a numerical time scale consists of converting a relative geological
time scale such as the age thickness scale used by Holmes into a linear time scale
along which samples and events are measured in millions of years. Geologists
realized early on that for clarity and international communication the rock record
was to be subdivided into a chronostratigraphic scale of standardized global
stratigraphic units such as “Cambrian” and “Miocene”. This became possible
because many events in geological history affected the entire surface of the Earth
or very large regions. Numerous examples of methods to define the original
chronostratigraphic boundaries could be cited. One example is Lyell's early sub-
division of the Cenozoic that was partly based on a quantitative model. The early
editions of Lyell's (
1833
) topic contain a 60-page appendix with presence-absence
information on 7,810 species of recent and fossil shells. By counting for each Series
(Epoch) the number of fossil shells of species living today and re-computing the
resulting frequencies into percentage values, Lyell established the first subdivision
of the Tertiary Period into Pliocene, Miocene and Eocene. Later, Paleocene,
Oligocene and Pleistocene were added, thus providing the break-down of the
Cenozoic in the geologic time table using names that reflect the magnitudes of
these percentage values. Another example is the Maastrichtian-Paleocene boundary
marked by an iridium anomaly caused by bolide impact now estimated to have
taken place 66.0 million years ago (
cf
. Table
9.14
).
The latest international Geologic Time Scale is shown in Fig.
9.35
(from
Gradstein et al.
2012
). An international initiative that has been helpful to establish
GTS2004 and GTS2012 is the definition of GSSPs or “golden spikes”. The first
GSSP (“Global Boundary Stratotype Section and Point”) fixed the lower limit of the
Lochkovian Stage (Silurian-Devonian boundary) at the precise level in an outcrop
with the name Klonk in the Czech Republic (Martinsson
1977
). Each GSSP must
meet certain requirements and secondary desirable characteristics (Remane
et al.
1996
, Table 2.1). Before a GSSP is formally defined, its correlation potential
in practice is thoroughly tested. GTS 2004 made use of 28, 8 and 7 GSSP's for the
2
˃
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