جدول زمانهای زمین شناسی
Geological time scale
The history of
the Earth can be displayed in the form of a calendar which is based on the
observation of rocks formed through time. Depending on the location, the
formation of rocks has recorded different (regional) histories describing the
magmatic, tectonic, hydrospheric, or biospheric evolutions, and thus different
calendars have been generated. Geologists (stratigraphers) are attempting to
unify these different calendars. Stratigraphers define rock units bracketed
between two boundaries that can be correlated worldwide. The succession of these
modern units is the global geological time scale.
Stratigraphical units
In general, the
evolution of the Earth is continuous, so fixing the location of the unit
boundaries can be achieved only through convention (decisions are made under the
aegis of the International Commission on Stratigraphy). Stratigraphers use a
variety of tools for characterizing the age of a rock, including physical ages,
fossils, magnetism, and chemical properties, all of which have evolved through
time. There are three intervals in the history of the Earth
(Archean-Proterozoic, Phanerozoic, Plio-Quaternary) with each one showing
distinct material available for characterizing rocks; thus, there are three
successive kinds of geological time scale. In the earliest time scale, during
Archean and Proterozoic eons, fossils are rare or absent in most rocks, and the
major tool is the physical dating process based on naturally unstable isotope
decay used by geochronologists. For this interval of time, the boundaries of the
calendar units are defined by selected numerical ages (Fig. 1). These ages are
conventionally abbreviated Ma (Mega anna, or million years, following
recommendation of the Subcommission on Geochronology).
Fig. 1 Agreed-upon geological time scale of the
pre-Phanerozoic history of the Earth. The beginning of the Earth is estimated to
be 4,470 to 4,570 Ma, with a preferred age at 4,550
Ma.
During the
Phanerozoic Eon certain animals and plants elaborated skeletons or exoskeletons
that, when preserved as fossils, provide an additional means of time correlation
(Fig. 2). Up to about 5 Ma, these fossils become the major tool for determining
the relative age of a rock. Together with other physicochemical characteristics,
fossils allow a definition of stages which cover an average duration of about 5
Ma each. For the last 5 Ma, Plio-Quaternary time, there is evidence that the
evolution of the environment on Earth is diversified and well represented in the
geological record.
Fig. 2 Simplified geological time scale of the
Phanerozoic Eon. From about 120 possible stages, only one-fourth are formally
defined in modern terms. Among others, many are mostly used regionally (in
parentheses) with less precise boundaries and content compared to formal ones.
Stages are not shown for some epochs for which no realistic subdivision can be
recommended today. Ages given in two columns are shown without error bar when
they are obtained from an interpolation procedure. For some boundaries, the
error bar is asymmetrical; for example, the Aptian-Albian boundary has a
preferred age at 108 Ma, but this age is constrained only between 111 and 107
Ma. (After G. S. Odin, Geological Time Scale, C. R. Acad. Sci., 318:59–71,
1994)
Most
stratigraphical tools can be used continuously in this young interval, each tool
providing a specific time scale. Because several tools can often be used to
characterize (that is, to date) a single rock, it is easy to connect all these
time scales. A variety of easily interchangeable time scales are useful.
However, the absolute time dimension for all time scales can be calibrated only
by using geochronology. The progress toward a common terminology and geological
time scale benefits from three factors: better definition of the conventional
units, better geochronological calibration, and a potential for extrapolating
between calibrated ages.
Geochronological
calibration
Progress in
geochronological calibration benefits from both new technology and new
geochronological information. During the 1990s, the precision and
reproducibility of the measurements obtained using mass spectrometry have
increased significantly. This improvement offers the possibility of dating
minute quantities of certain material with remarkable precision. Using the
uranium-lead (U-Pb) dating method, the age of a single crystal of zircon, 200
micrometers in length and 10 μm in diameter, can be measured. More
significantly, several portions of this same crystal can be dated separately,
allowing for verification of the internal consistency of the data obtained from
the single crystal. For the potassium-argon (K-Ar) dating method, developments
in the irradiation techniques which transform the original potassium into argon
allow single biotite mica flakes 500 μm in diameter and 10 μm thick to be dated.
Laser heating can also give several ages obtained from different points of a
single crystal.
Explosive
volcanic eruptions producing ash clouds blown over large areas are common. The
ability to date small quantities of material has led to the search for minor
volcanic events within the stratigraphical record. This advance is important
because the same volcanic material covers both marine and continental areas,
sometimes at the scale of a continent, allowing correlation of distant deposits.
In addition, there is great interest in this method since explosive volcanism
often scatters a variety of minerals, including uranium-bearing and
potassium-bearing ones. Thus, the geochronologist can perform measurements on
independent isotopic systems in order to make a reciprocal check of the validity
of the calculated ages. The calibration of the geological time scale is mostly
realized through the study of crystal-bearing volcanic dust discovered in
sediments. Precise dating can also be achieved through a variety of other
datable materials. For example, a few tens of milligrams of microtektite
particles (scattered in wide areas when an asteroid collides with the Earth) can
be dated. Another example is calcite crystallized in paleosols during cyclic
deposition in shallow basins. Because this calcite is associated with organic
matter which favors uranium enrichment, and is formed essentially at the time of
deposition, calcite crystals become a potentially datable material using
uranium-lead methods.
Two examples of
refinement
One example of
recent refinement concerns the dating of the Eocene-Oligocene boundary. That
boundary has long been known as the Grande Coupure (great break) in the history
of European land-mammal evolution. There have been a few European studies which
documented an age at about 34 Ma. But the age of the boundary was assumed to be
about 37–38 Ma by some North American geologists. Paleontologists did not like
the latter age because, in North America, the 38-Ma-old mammals were dated using
contemporaneous volcanic flows and seemed less evolved than those known to be at
the stratigraphical boundary in
Another
significant example of the beneficial combination of improved stratigraphical
definition and modern geochronological dating is given by the
Precambrian-Cambrian boundary. The base of the Cambrian (and of the Phanerozoic
Eon) had long been placed at the first occurrence of skeletalized fossils
including trilobites (arthropods). Later, a Tommotian pretrilobitic stage was
added below it in view of the presence of older faunal remains, such as
archaeocythids (calcitized spongelike forms), which have been well documented on
the Siberian Platform. Before 1980, the earliest skeletalized faunas were
estimated to be between 570 and 590 Ma. However, independent geochronological
data were gathered from northern
Extrapolation
procedures
The direct
geochronological calibration method will never allow for the continuous
calibration of every point of geological history, since datable material is much
too scarce in rocks. However, continuous dating can be refined through the use
of interpolation procedures between geochronologically calibrated points. This
principle consists in combining those tie points with a continuous geological
phenomenon. Commonly used phenomena are rhythmic sedimentation and the oceanic
record of past magnetic fields. When the rhythmic deposition of sediments can be
related to the orbital (Milankovitch) parameters of the Earth, the time scales
of which are reasonably well understood, the duration of deposition can be
estimated when combined with nearby measured ages.
Another
procedure considers the aperiodic change (reversal) of the direction of the
Earth's magnetic field that is recorded in the oceanic plates being continuously
formed at midocean ridges (separating two tectonic plates). For a given plate,
the distance between two magnetic reversals is proportional to the time
durations between reversals and spreading rate (which can be calculated from two
geochronologically dated points). Thus, geological ages can be calculated for
each point of the record, though with some degree of uncertainty.
The geological
time scale is gradually becoming unified through an internationally agreed upon
scale which is replacing a variety of regional scales. It is ironic that such an
important improvement in calibrating this vast expanse of time is linked to the
discovery of volcanic dust interbedded in sedimentary rocks.
See also: Archeological chronology;
Dating methods; Fossil; Geochronometry; Geologic time scale; Index fossil;
Radiocarbon dating; Rock age determination; Sedimentary rocks; Stratigraphy
G. S. Odin
Bibliography
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H. Blatt, W. B. N. Berry, and S. Brande, Principles of Stratigraphic Analysis, 1991
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J. P. Grotzinger et al., Biostratigraphic and geochronologic constraints on early animal evolution, Science, 270:598–604, 1995
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G. S. Odin, Geological time scale, C. R. Acad. Sci., 318:59–71, 1994
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G. S. Odin et al., Numerical dating of Precambrian-Cambrian boundary, Nature, 301:21–23, 1983
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G. S. Odin and A. Montanari, Radio-isotopic age and stratotype of the Eocene/Oligocene boundary, C. R. Acad. Sci. Paris, 309:1939–1945, 198۹
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