Conventions and standards
Ages are given in years before “Present” (BP). To avoid a
constantly changing datum, “Present” was fixed as AD
1950 (as in
14 C determinations), the date of the beginning
of modern isotope dating research in laboratories around
the world. For most geologists, this offset of official “Present” from “today” is not important. However, for
archeologists and researchers into events during the Holocene (the past 11,500 years), the offset between the “BP”
convention from radiogenic isotope laboratories and
actual total elapsed calendar years becomes significant.
The offset between the current year and “Present” has
led many Holocene specialists to use a “2000 BP,” which
is relative to the year AD 2000.
For clarity, the linear age in years is abbreviated as
a (for annum), and ages are measured in ka, Ma, or Ga
for thousands, millions, or billions of years before present.
Elapsed time or duration is often abbreviated as yr (for
year) and longer durations in kyr or myr. Therefore, the
Cenozoic began at 66 Ma and spans 66 myr (to the present
day, defined as the year AD 2000).
The uncertainties in computed ages or durations are
expressed as standard deviation with 2-sigma (95 %) confidence. The uncertainty is indicated by “Æ” and will have
implied units of thousands or millions of years as appropriate to the magnitude of the age.
Historical overview of geologic time scales
Stitching together the many data points on the loom of
time requires an elaborate combination of earth science
and mathematical/statistical methods. Hence, the time
and effort involved in constructing a new geologic time
scale and assembling all relevant information is considerable. Because of this and because continuous updating in
small measure with new information is not advantageous
to the stability of any common standard, new geologic
time scales come out sparsely.
Since 1981, six successive Phanerozoic GTSs have
been published, each new one achieving higher resolution,
better error analysis, and more users worldwide (Harland
et al., 1982, 1990; Gradstein and Ogg, 1996; Gradstein
et al., 2004 ¼ GTS2004; Ogg et al., 2008; Gradstein
et al., 2012 ¼ GTS2012).
Geologic time scale (GTS2012)
The current standard geologic time scale is GTS2012
(Gradstein et al., 2012), constructed between 2004 and
2012 with a team of over 65 earth science and other
specialists. Many of its coauthors are officers of ICS.
Philosophy, methodology, chronostratigraphy, and
geochronology are laid out in 32 chapters. Three data
appendices detail stages of color coding, all radiometric
dates, and Cretaceous through Cenozoic calcareous
microfossils biochronology.
Construction of GTS2012 may be summarized in four
steps:
Step 1. Construct an updated global chronostratigraphic
scale for the Earth’s rock record.
Step 2. Identify key linear-age calibration levels for the
chronostratigraphic scale using radiogenic isotope age
dates, and/or apply astronomical tuning to cyclic sediment, or scale and interpolate (near) linear segments
of stable isotope sequences.
Step 3. Interpolate the combined chronostratigraphic and
chronometric scale, for example, with a smoothing
cubic spline, where direct information in specific stratigraphic intervals is wanting.
Step 4. Calculate or estimate error bars on the combined
chronostratigraphic and chronometric information to
obtain a geologic time scale with estimates of uncertainty on boundary ages and on unit durations.
The first step, integrating multiple types of stratigraphic
information in order to construct the chronostratigraphic
scale, is the most time-consuming; it summarizes and synthesizes centuries of detailed geological research while
reconciling it with the most up-to-date information. The
second step, identifying which radiogenic isotope and
cycle-stratigraphic studies would be used as the primary
constraints for assigning linear ages, is the one that is
evolving most rapidly since the last decade. Historically,
time scale building went from an exercise with very few
and relatively inaccurate radiogenic isotope dates, as used
by Holmes (1960), to one with many dates with greatly
varying analytical precision (like GTS89).
The new philosophy for Step 2 that was started in
GTS2004 and expanded in GTS2012 is to select stratigraphically meaningful radiogenic isotope dates with high
analytical precision. More than 260 radiogenic isotope
dates were thus selected for their reliability and stratigraphic importance to calibrate the geologic record in linear time. All 260+ GTS2012 age dates are detailed in
Appendix 2 of GTS2012 by Schmitz (2012).
In addition to selecting radiogenic isotope ages based
upon their stratigraphic control and analytical precision,
GTS2012 also applied the following criteria or
corrections:
1. Stratigraphically constrained radiogenic isotope ages
with the U-Pb method on zircons were accepted from
the isotope dilution mass spectrometry (IDMS)
method, but not from the high-resolution ion microprobe (SHRIMP).
2.
40 Ar39
Ar radiogenic isotope ages were recomputed to
be in accord with the revised ages for laboratory monitor standards: 527.0 Æ 2.6 Ma for MMhb-1 (Montana
hornblende), 28.51 Æ 0.06 Ma for TCR (Taylor Creek
sanidine), and 28.201 Æ 0.046 Ma for FCT (Fish Canyon sanidine). Systematic (“external”) errors and
uncertainties in decay constants are partially incorporated (Kuiper et al., 2008; Schmitz, 2012). As in
GTS2004, no glauconite-based dates are used.
The bases of Paleozoic, Mesozoic, and Cenozoic are
bracketed by analytically precise ages at their GSSP or
284
GEOLOGIC TIME SCALE
Ages are given in years before “Present” (BP). To avoid a
constantly changing datum, “Present” was fixed as AD
1950 (as in
14 C determinations), the date of the beginning
of modern isotope dating research in laboratories around
the world. For most geologists, this offset of official “Present” from “today” is not important. However, for
archeologists and researchers into events during the Holocene (the past 11,500 years), the offset between the “BP”
convention from radiogenic isotope laboratories and
actual total elapsed calendar years becomes significant.
The offset between the current year and “Present” has
led many Holocene specialists to use a “2000 BP,” which
is relative to the year AD 2000.
For clarity, the linear age in years is abbreviated as
a (for annum), and ages are measured in ka, Ma, or Ga
for thousands, millions, or billions of years before present.
Elapsed time or duration is often abbreviated as yr (for
year) and longer durations in kyr or myr. Therefore, the
Cenozoic began at 66 Ma and spans 66 myr (to the present
day, defined as the year AD 2000).
The uncertainties in computed ages or durations are
expressed as standard deviation with 2-sigma (95 %) confidence. The uncertainty is indicated by “Æ” and will have
implied units of thousands or millions of years as appropriate to the magnitude of the age.
Historical overview of geologic time scales
Stitching together the many data points on the loom of
time requires an elaborate combination of earth science
and mathematical/statistical methods. Hence, the time
and effort involved in constructing a new geologic time
scale and assembling all relevant information is considerable. Because of this and because continuous updating in
small measure with new information is not advantageous
to the stability of any common standard, new geologic
time scales come out sparsely.
Since 1981, six successive Phanerozoic GTSs have
been published, each new one achieving higher resolution,
better error analysis, and more users worldwide (Harland
et al., 1982, 1990; Gradstein and Ogg, 1996; Gradstein
et al., 2004 ¼ GTS2004; Ogg et al., 2008; Gradstein
et al., 2012 ¼ GTS2012).
Geologic time scale (GTS2012)
The current standard geologic time scale is GTS2012
(Gradstein et al., 2012), constructed between 2004 and
2012 with a team of over 65 earth science and other
specialists. Many of its coauthors are officers of ICS.
Philosophy, methodology, chronostratigraphy, and
geochronology are laid out in 32 chapters. Three data
appendices detail stages of color coding, all radiometric
dates, and Cretaceous through Cenozoic calcareous
microfossils biochronology.
Construction of GTS2012 may be summarized in four
steps:
Step 1. Construct an updated global chronostratigraphic
scale for the Earth’s rock record.
Step 2. Identify key linear-age calibration levels for the
chronostratigraphic scale using radiogenic isotope age
dates, and/or apply astronomical tuning to cyclic sediment, or scale and interpolate (near) linear segments
of stable isotope sequences.
Step 3. Interpolate the combined chronostratigraphic and
chronometric scale, for example, with a smoothing
cubic spline, where direct information in specific stratigraphic intervals is wanting.
Step 4. Calculate or estimate error bars on the combined
chronostratigraphic and chronometric information to
obtain a geologic time scale with estimates of uncertainty on boundary ages and on unit durations.
The first step, integrating multiple types of stratigraphic
information in order to construct the chronostratigraphic
scale, is the most time-consuming; it summarizes and synthesizes centuries of detailed geological research while
reconciling it with the most up-to-date information. The
second step, identifying which radiogenic isotope and
cycle-stratigraphic studies would be used as the primary
constraints for assigning linear ages, is the one that is
evolving most rapidly since the last decade. Historically,
time scale building went from an exercise with very few
and relatively inaccurate radiogenic isotope dates, as used
by Holmes (1960), to one with many dates with greatly
varying analytical precision (like GTS89).
The new philosophy for Step 2 that was started in
GTS2004 and expanded in GTS2012 is to select stratigraphically meaningful radiogenic isotope dates with high
analytical precision. More than 260 radiogenic isotope
dates were thus selected for their reliability and stratigraphic importance to calibrate the geologic record in linear time. All 260+ GTS2012 age dates are detailed in
Appendix 2 of GTS2012 by Schmitz (2012).
In addition to selecting radiogenic isotope ages based
upon their stratigraphic control and analytical precision,
GTS2012 also applied the following criteria or
corrections:
1. Stratigraphically constrained radiogenic isotope ages
with the U-Pb method on zircons were accepted from
the isotope dilution mass spectrometry (IDMS)
method, but not from the high-resolution ion microprobe (SHRIMP).
2.
40 Ar39
Ar radiogenic isotope ages were recomputed to
be in accord with the revised ages for laboratory monitor standards: 527.0 Æ 2.6 Ma for MMhb-1 (Montana
hornblende), 28.51 Æ 0.06 Ma for TCR (Taylor Creek
sanidine), and 28.201 Æ 0.046 Ma for FCT (Fish Canyon sanidine). Systematic (“external”) errors and
uncertainties in decay constants are partially incorporated (Kuiper et al., 2008; Schmitz, 2012). As in
GTS2004, no glauconite-based dates are used.
The bases of Paleozoic, Mesozoic, and Cenozoic are
bracketed by analytically precise ages at their GSSP or
284
GEOLOGIC TIME SCALE