Erathem

In stratigraphy, paleontology, geology, and geobiology an erathem is the total stratigraphic record deposited during a certain corresponding span of time, an era in the geologic timescale. __NOTOC__

It can therefore be used as a chronostratigraphic unit of time which delineates a large span of years — less than an geological eon, but greater than its successively smaller and more refined subdivisions (geologic periods, epochs, and geologic ages). By 3,500 Million years ago (Mya) simple life had developed on earth (the oldest known microbial fossils in Australia are dated to this figure.cite web
url=http://www.rocksandminerals.com/geotime/geotime.htm
title=Rockman's Geologic Time Chart
accessdate=2008-06-17] The atmosphere was a mix of noxious and poisonous gases (Metane, Ammonia, Sulphur compounds, etc.— a so called reducing atmosphere [basis for the Miller-Urey Experiment] lacking much free oxygen which was bound up in compounds).

These simple organisms, Cyanobacteria ruled the still cooling earth for approximately a thousand million (over a billion) years and gradually transformed the atmosphere to one containing free oxygen. These changes, along with tectonic activity left chemical trails (red bed formation, etc.) and other physical clues (magnetic orientation, layer formation factors) in the rock record, and it is these changes along with the later richer fossil record which specialists use to demarcate times early in planet earth's history in various disciplines.

Erathems are not often used in practice. While they are they are subdivisions of eonothems and are themselves subdivided into systems, dating experts prefer the finer resolution of smaller spans of time when evaluating a strata.

Erathems have the same names as their corresponding eras. :The Phanerozoic eonothem can thus be divided into a:: Paleozoic, a Mesozoic and a Cenozoic erathem or matching era name.

Similarly, the Proterozoic eonothem is divided youngest to oldest into the ::Neoproterozoic, Mesoproterozoic and Paleoproterozoic erathems, :and the Archean eon and eonothem are divided similarly into the::Neoarchean, Mesoarchean, Paleoarchean and the Eoarchean, for which a lower (oldest) limit is undefined.cite web|url=http://www.stratigraphy.org/chus.pdf|accessdate=2008-06-17|title=International Stratigraphic Chart|author=International Commission on Stratigraphy, by Gabi Ogg] Cite Sm|F.M. Gradstein, J.G. Ogg, A.G. Smith, et al., "A Geologic Time Scale 2004", (2004; Cambridge University Press). ]

ee also

Multidiscipline comparison

Related other topics

*Body form
*European Mammal Neogene
*Geologic time scale
**New Zealand geologic time scale
*North American Land Mammal Age
*Fauna (animals)
*Type locality
*List of GSSPs

External links

* [http://www.stratigraphy.org/over.htm The Global Boundary Stratotype Section and Point (GSSP)] : overview

* [http://www.stratigraphy.org/gssp.htm Chart of The Global Boundary Stratotype Sections and Points (GSSP)] : chart

*plainlinks| http://www.rocksandminerals.com/geotime/geotime.htm Geotime chart displaying geologic time periods compared to the fossil record. - Deals with chronology and classifications for laymen (not GSSPs)

Notes and references

References

*Cite journal
last = Gehling
first = James
last2 = Jensen
first2 = Sören
last3 = Droser
first3 = Mary
last4 = Myrow
first4 = Paul
last5 = Narbonne
first5 = Guy
title = Burrowing below the basal Cambrian GSSP, Fortune Head, Newfoundland
journal = Geological Magazine
volume = 138
issue = 2
pages = 213–218
date = March 2001
year = 2001
url = http://www.journals.cambridge.org/action/displayAbstract?fromPage=online&aid=74669
doi = 10.1017/S001675680100509X
id = 1
* Hedberg, H.D., (editor), "International stratigraphic guide: A guide to stratigraphic classification, terminology, and procedure", New York, John Wiley and Sons, 1976
* [http://www.stratigraphy.org/cheu.pdf International Stratigraphic Chart] from the International Commission on Stratigraphy
* [http://www2.nature.nps.gov/geology/usgsnps/gtime/gtime1.html USA National Park Service]
* [http://astro.wsu.edu/worthey/earth/html/md08.html Washington State University]
* [http://www.ucmp.berkeley.edu/help/timeform.html Web Geological Time Machine]
* [http://www.pballew.net/arithm16.html#aeon Eon or Aeon] , [http://www.pballew.net/etyindex.html Math Words - An alphabetical index]