Erathem

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

Eras mapped into Eons
Eras in the Phanerozoic Eon[1][2]
Geologic EraSpan of YearsNotes:
Cenozoic
present – 065.5 (+/- 0.3) Mya
many GSSP points
Mesozoic
065.5 (+/- 0.3) Mya – 251.0 (+/- 0.4) Mya
many GSSP points
Paleozoic
251.0 (+/- 0.4) Mya – 542.0 (+/- 1.0) Mya
mostly GSSP points
Eras in the Proterozoic Eon
542.0 (+/- 1.0) Mya – 2500 Mya[1][2]
Neoproterozoic
542.0 (+/- 1.0) Mya – 1000 Mya
few GSSP points
Mesoproterozoic
1000 Mya – 1600 Mya
all GSSA points
Paleoproterozoic
1600 Mya – 2500 Mya
all GSSA points
Eras in the Archean Eon
2500 Mya – years > 3600 Mya
rocks older than 2.5 Billion years – rocks older than 3.6 Billion years[1][2]
Neoarchean
2500 Mya – 2800 Mya
(only GSSA points)
Mesoarchean
2800 Mya – 3200 Mya
Paleoarchean
3200 Mya – 3600 Mya
Eoarchean
3600 Mya – 4000 Mya
Earth's crust solidifies
ca 3800 Mya[3]
Note: Rocks older than ca. 2500 Mya old are rare due to tectonic activity recycling the Earth's crust.

It can therefore be used as a chronostratigraphic unit of time which delineates a large span of years – less than a 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).[3] The atmosphere was a mix of noxious and poisonous gases (Methane, Ammonia, Sulphur compounds, etc.[3] – a so-called reducing atmosphere[4] 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[3] 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 subdivisions of eonothems and are themselves subdivided into systems, dating experts prefer the finer resolution of smaller spans of time when evaluating strata.

Erathems have the same names as their corresponding eras.

The Phanerozoic eonothem can thus be divided into a
Cenozoic, a Mesozoic and a Paleozoic 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.[1][2]

See also

Multidiscipline comparison
Units in geochronology and stratigraphy[5]
Segments of rock (strata) in chronostratigraphy Time spans in geochronology Notes to
geochronological units
EonothemEon4 total, half a billion years or more
ErathemEra10 defined, several hundred million years
SystemPeriod22 defined, tens to ~one hundred million years
SeriesEpoch34 defined, tens of millions of years
StageAge99 defined, millions of years
ChronozoneChronsubdivision of an age, not used by the ICS timescale

Notes
  1. International Commission on Stratigraphy, by Gabi Ogg. "International Stratigraphic Chart" (PDF). Archived from the original (PDF) on 2008-05-29. Retrieved 2008-06-17.
  2. F.M. Gradstein, J.G. Ogg, A.G. Smith, et al., "A Geologic Time Scale", (2004; Cambridge University Press).
  3. "Rockman's Geologic Time Chart". Retrieved 2008-06-17.
  4. basis for the Miller–Urey experiment
  5. Cohen, K.M.; Finney, S.; Gibbard, P.L. (2015), International Chronostratigraphic Chart (PDF), International Commission on Stratigraphy.

References
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