Geology/Chronology

Geochronology is the science of applying dates in the past to rocks. These rocks receive dates because they contain constituents that can be used as chronometers.

Geologic time

This clock representation shows some of the major units of geological time and definitive events of Earth history. Credit: Woudloper.

On the right is a geologic clock representation. It shows some of the major units of geological time and definitive events of Earth history. The Hadean eon represents the time before fossil record of life on Earth; its upper boundary is now regarded as 4.0 Ga (billion years ago).^[1] Other subdivisions reflect the evolution of life; the Archean and Proterozoic are both eons, the Palaeozoic, Mesozoic and Cenozoic are eras of the Phanerozoic eon. The two million year Quaternary period, the time of recognizable humans, is too small to be visible at this scale.

The following four timelines show the geologic time scale. The first shows the entire time from the formation of the Earth to the present, but this compresses the most recent eon. Therefore the second scale shows the most recent eon with an expanded scale. The second scale compresses the most recent era, so the most recent era is expanded in the third scale. Since the Quaternary is a very short period with short epochs, it is further expanded in the fourth scale. The second, third, and fourth timelines are therefore each subsections of their preceding timeline as indicated by asterisks. The Holocene (the latest epoch) is too small to be shown clearly on the third timeline on the right, another reason for expanding the fourth scale. The Pleistocene (P) epoch. Q stands for the Quaternary period.

Units in geochronology and stratigraphy^[2]
Segments of rock (strata) in chronostratigraphy	Time spans in geochronology	Notes to geochronological units
Supereonothem	Supereon	1 total, four billion years or more (e.g. Precambrian)
Eonothem	Eon	4 total, half a billion years or more
Erathem	Era	10 defined, several hundred million years
System	Period	22 defined, tens to ~one hundred million years
Series	Epoch	34 defined, tens of millions of years
Stage	Age	99 defined, millions of years
Chronozone	Chron	subdivision of an age, not used by the ICS timescale

Notations

Let

ALMA represent the Asian Land Mammal Age,
b2k represent before AD 2000,
BP represent before present, as the chart is for 2008, this may require an added -8 for b2k,
ELMMZ represent the European Land Mammal Mega Zone,
FAD represent first appearance datum,
FO represent first occurrence,
Ga represent Gegaannum, billion years ago, or -10⁹ b2k,
GICC05 represent Greenland Ice Core Chronology 2005,
GRIP represent Greenland Ice Core Project,
GSSP represent Global Stratotype Section and Point,
HO represent highest occurrence,
ICS represent the International Commission on Stratigraphy,
IUGS represent the International Union of Geological Sciences,
LAD represent last appearance datum,
LO represent lowest occurrence,
Ma represent Megaannum, million years ago, or -10⁶ b2k,
NALMA represent the North American Land Mammal Age,
NGRIP represent North Greenland Ice Core Project, and
SALMA represent South American Land Mammal Age.

"The term b2 k [b2k] refers to the ice-core zero age of AD 2000; note that this is 50 years different from the zero yr for radiocarbon, which is AD 1950 [...]."^[3]

Stratigraphy

Orbitally forced cyclicity

Argon–argon dating

Cathodoluminescences

Chemostratigraphy

Cosmogenic radionuclide dating

Dendrochronology

Electron spin resonances

Fission track dating

Geomagnetic Polarity Time Scale

Ice cores

Lichenometry

Magnetostratigraphy

Marker horizons

Optically stimulated luminescences

Paleomagnetic dating

Palynology

Potassium–argon dating

Radiocarbon dating

Tephrochronology

Thermoluminescences

Uranium–lead dating

Uranium-thorium dating

Varves

Phanerozoic

The Phanerozoic eon includes the Paleozoic, Mesozoic, and Cenozoic.

Cenozoic

Mesozoic

Paleozoic

Precambrian

Hypotheses

Each time frame or span of time in geochronology has at least one dating technique.
Late Jurassic is a time frame and Upper Jurassic is a stratigraphic frame so both are different time frames.
The overall size of—or efficiency of carbon export from—the biosphere decreased at the end of the Great Oxidation Event (GOE) (ca. 2,400 to 2,050 Ma).

References

↑ stratigraphy.org. International Commission on Stratigraphy 2008. http://www.stratigraphy.org/column.php?id=Chart/Time%20Scale. Retrieved 9 March 2009.
↑ Cohen, K.M.; Finney, S.; Gibbard, P.L. (2015). "International Chronostratigraphic Chart" (PDF). International Commission on Stratigraphy.
↑ Mike Walker, Sigfus Johnsen, Sune Olander Rasmussen, Trevor Popp, Jørgen-Peder Steffensen, Phil Gibbard, Wim Hoek, John Lowe, John Andrews, Svante Björck, Les C. Cwynar, Konrad Hughen, Peter Kershaw, Bernd Kromer, Thomas Litt, David J. Lowe, Takeshi Nakagawa, Rewi Newnham and Jakob Schwander (2009). "Formal definition and dating of the GSSP (Global Stratotype Section and Point) for the base of the Holocene using the Greenland NGRIP ice core, and selected auxiliary records". Journal of Quaternary Science 24 (1): 3-17. doi:10.1002/jqs.1227. http://www.stratigraphy.org/GSSP/Holocene.pdf. Retrieved 2015-01-18.

External links

Global Boundary Stratotype Section and Point (GSSP) of the International Commission on Stratigraphy
WiscAr Geochronology Laboratory, University of Wisconsin-Madison
UC Berkeley press release: "Precise dating of the destruction of Pompeii proves argon-argon method can reliably date rocks as young as 2,000 years"
New Mexico Geochronology Research Laboratory
Argon Isotope Facility of the Scottish Universities Environmental Research Council
Open University Ar/Ar and Noble Gas Laboratory
Argon Laboratory / Australian National University

[ICS-1] stratigraphy.org. International Commission on Stratigraphy 2008. http://www.stratigraphy.org/column.php?id=Chart/Time%20Scale. Retrieved 9 March 2009.

[2] Cohen, K.M.; Finney, S.; Gibbard, P.L. (2015). "International Chronostratigraphic Chart" (PDF). International Commission on Stratigraphy.

[Walker-3] Mike Walker, Sigfus Johnsen, Sune Olander Rasmussen, Trevor Popp, Jørgen-Peder Steffensen, Phil Gibbard, Wim Hoek, John Lowe, John Andrews, Svante Björck, Les C. Cwynar, Konrad Hughen, Peter Kershaw, Bernd Kromer, Thomas Litt, David J. Lowe, Takeshi Nakagawa, Rewi Newnham and Jakob Schwander (2009). "Formal definition and dating of the GSSP (Global Stratotype Section and Point) for the base of the Holocene using the Greenland NGRIP ice core, and selected auxiliary records". Journal of Quaternary Science 24 (1): 3-17. doi:10.1002/jqs.1227. http://www.stratigraphy.org/GSSP/Holocene.pdf. Retrieved 2015-01-18.

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