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Archean Eon

archean.htm

from Wikipedia: - https://en.wikipedia.org/wiki/Archean 180306, 180423

Downloaded and edited by U Kyaw Tun (UKT) (M.S., I.P.S.T., USA), and staff of Tun Institute of Learning (TIL) . Not for sale. No copyright. Free for everyone. Prepared for students and staff of TIL Research Station, Yangon, MYANMAR 
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Introduction
The first supercontinent
 Columia (2.5 to 1.5 Ga ago) and Rodinia (1.3–0.9 to 0.75–0.63 GA ago)
1. Etymology and changes in classification
2. Geology
3. Environment
4. Early life

 

The following are still link to original Wikipedia article:
4 Early life
5 See also
6 References
7 External links

 

UKT notes
 

 

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Introduction

- UKT 180306: Archean Eon is the second Eon of the Precambrian Supereon. We can imagine it to be further cooling of the molten globe. Land masses are being formed and therefore from age of the oldest rocks we can know the duration of this eon. However only the oldest life forms would be there.

From Wikipedia:  - https://en.wikipedia.org/wiki/Archean 180306

The Archean Eon (aka Archaean) is a geologic eon, 4.0 to 2.5 Ga ago (4,000 to 2,500 Ma ago), that followed the Hadean Eon and preceded the Proterozoic Eon. During the Archean Eon, the Earth's crust had cooled enough to allow the formation of continents.

The oldest of those supercontinents is called Rodinia and was formed during Precambrian time some one billion years ago. Another Pangea-like supercontinent, Pannotia, was assembled 600 million years ago, at the end of the Precambrian. Present-day plate motions are bringing the continents together once again. Google - Mar 21, 2018

UKT 180423: Precambrian is a supereon - the "Precambrian time" is misleading.

 

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The first supercontinent

Wikipedia: https://en.wikipedia.org/wiki/Columbia_(supercontinent) 180424

Columbia, also known as Nuna and Hudsonland, was one of Earth's ancient supercontinents. It was first proposed by Rogers & Santosh 2002 [1] and is thought to have existed approximately 2.5 - 1.5 Ga ago ( 2,500 to 1,500 Ma ago ) in the Paleoproterozoic Era. Zhao et al. 2002 [2] proposed that the assembly of the supercontinent Columbia was completed by global-scale collisional events during 2.1–1.8 Ga.

Columbia consisted of proto- cratons that made up the cores of the continents of Laurentia, Baltica, Ukrainian Shield, Amazonian Shield, Australia, and possibly Siberia, North China, and Kalaharia as well.

The evidence of Columbia's existence is based upon geological [2] [3] and paleomagnetic data. [4]

Wikipedia: https://en.wikipedia.org/wiki/Rodinia 180424
UKT 180424: What about an older supercontinent Columbia ?

Rodinia (from Russian: родить, 'rodít - 'to beget, to give birth', [2] or родина", 'ródina - 'motherland, birthplace'  [3] [4] is a Neoproterozoic supercontinent that was assembled 1.3–0.9 GA ago and broke up 750–633 MA ago. [5] [UKT ¶]

Valentine & Moores 1970 were probably the first to recognise a supercontinent of Precambrian Supereon, which they named 'Pangaea I'. [5] It was renamed 'Rodinia' by McMenamin & McMenamin 1990 who also were the first to produce a reconstruction and propose a temporal framework for the supercontinent. [6]

Rodinia formed at c. 1.23 Ga by accretion and collision of fragments produced by breakup of an older supercontinent, Columbia, assembled by global-scale 2.0–1.8 Ga collisional events. [7]

Rodinia broke up in the Neoproterozoic with its continental fragments reassembled to form Pannotia 633–573 million years ago. In contrast with Pannotia, little is known yet about the exact configuration and geodynamic history of Rodinia. Paleomagnetic evidence provides some clues to the paleolatitude of individual pieces of the Earth's crust, but not to their longitude, which geologists have pieced together by comparing similar geologic features, often now widely dispersed.

The extreme cooling of the global climate around 717–635 million years ago (the so-called Snowball Earth of the Cryogenian Period) and the rapid evolution of primitive life during the subsequent Ediacaran and Cambrian periods are thought to have been triggered by the breaking up of Rodinia or to a slowing down of tectonic processes. [8]

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Etymology and changes in classification

Archean Eon (or Archaean) (ancient Greek Αρχή (Arkhē)), meaning "beginning, origin". Its earliest use is from 1872, when it meant "of the earliest geological age." [1] In earlier literature the Hadean Eon was included as part of the Archean Eon. [citation needed]

Instead of being based on stratigraphy, the beginning and end of the Archean Eon are defined chronometrically. [UKT ¶]

¤ Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. - https://en.wikipedia.org/wiki/Stratigraphy 180308

¤ UKT note 180308: Definition Chronometrically: Stratigraphical dating might be just useless in the Hadeon Eon, because the Earth was still very hot for rocks to be formed. In other words there were no or scant sedimentary and layered volcanic rocks. However, the unstable isotope of Carbon 12C14 was being formed because of cosmic rays bombarding the Earth.
"When cosmic rays enter the atmosphere, they undergo various transformations, including the production of neutrons 0n1. The resulting neutron 0n1 participates in the following reaction (with the Nitrogen 7N17 of the atmosphere):

0n1 + 7N14 --> 6C14 + 1p1

...." - https://en.wikipedia.org/wiki/Carbon-14 180308
   UKT continues: Once Carbon-14 is formed it starts to decay radioactively producing stable non-radioactive Carbon-12 at a fixed rate. If you find any Carbon-14 in any solid sample, rocks or fossils, by measuring the ratio of Carbon-14 to Carbon-12, you can know the age of the sample or when it was formed.

Archean Eon are defined chronometrically.The eon's lower boundary or starting point of 4 Ga ago (4 billion years ago) is officially recognized by the ICS (International Commission on Stratigraphy). [2]

Chronological dating, or simply dating, is the process of attributing to an object or event a date in the past, allowing such object or event to be located in a previously established chronology. This usually requires what is commonly known as a "dating method". Several dating methods exist, depending on different criteria and techniques, and some very well known examples of disciplines using such techniques are, for example, history, archaeology, geology, paleontology, astronomy and even forensic science, since in the latter it is sometimes necessary to investigate the moment in the past in which the death of a cadaver occurred.
- https://en.wikipedia.org/wiki/Chronological_dating - 180423

 

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Geology

When the Archean Eon began, the Earth's heat flow* was nearly three times as high as it is today, and it was still twice the current level at the transition from the Archean to the Proterozoic 2.5 Ga (2,500 Ma) ago). The extra heat was the result of a mix of remnant heat from:
1. planetary accretion [UKT: astrophysics - gaseous particles pulled gravitationally to form planets],
2. formation of the Earth's core, [UKT: outer core - liquid under less pressure, and inner core - solid because of high pressure],
3. produced by radioactive elements [UKT: produced by fission, unlike that of the Sun which is due to fusion.]

*Earth's heat flow aka Earth's internal heat budget is fundamental to the thermal history of the Earth. The flow of heat from Earth's interior to the surface is estimated at 47 TW (1 terawatt = 1012 watts) [1] and comes from two main sources in roughly equal amounts: the radiogenic heat produced by the radioactive decay of isotopes in the mantle and crust, and the primordial heat left over from the formation of the Earth. [2]

UKT 180424: In the pix on the right, Radiogenic heat production is given in terms of the decay rates of 4 radio-isotopes, K40 , U235, U238, Th232  - https://www.orau.org/ptp/PTP%20Library/library/DOE/Misc/table55.PDF 180424
Downloaded table published in 2001 [Sources: Turner (1986), DOE/TIC-11026.T] in TIL HD-PDF and SD-PDF libraries:
- NrcGov-ElementHalfLives<Ô> / Bkp<Ô>  (link chk 180424)

The following table is taken from the above source. As usual I've used Courier New font for the table
Note: You should expect some discrepancies in half-lives, e.g.
- http://w.astro.berkeley.edu/~dperley/areopagus/isotopetable.html 180424
Though Potassium is generally considered to be stable, 19K40 is not.

Element name / Symbol / Atomic number / Radioisotopea / Half-lifeb
• Potassium -- K ------ 19 ------------ 40K ---------- 1.277 × 109 yr
• Uranium ---- U ------ 92 ------------ 235U --------- 7.038 × 108 yr
------------------------------------- 238U --------- 4.468 × 109 yr
• Thorium ---- Th ----- 90 ------------ 232Th -------- 1.405 × 1010 yr

Uranium-238, for example, will undergo 14 radioactive decays to eventually become lead-206 which is stable and no longer radioactive. Some of these radioactive decays will be alpha decays and some of these decays will be beta decays.
- http://teachnuclear.ca/all-things-nuclear/radiation/radioactive-decay/ 180424

Read also Wikipedia - https://en.wikipedia.org/wiki/Decay_chain 180424
UKT 180424: Though most decay ends in stable Lead isotopes, there is at least one that ends in Bismuth:

UKT 180424: Personal note. I'm still interested in Nuclear Energy, because the first assignment given to me (Asst Lecturer) in 1955, by Prof. U Po Tha, of Rangoon Univ. was to analyze the Monazite sand as a source of Thorium. Before, I could finish the assignment, I had to leave the Chemistry department as a Burma Govt. state scholar to the US to study Pulp and Paper Technology. It means a radical change in my career involving a change of ministries from Education to Industry.

The first step of the assignment was to dissolve the Monazite sand in boiling fuming sulphuric acid. The process took me more than 12 hours. It was a time when fume cupboards in the chemistry department were yet to be installed: those that had existed before WWII were no longer functioning due to the campus of the University being occupied by the Japanese Army hospital during the war, and by the British 14th Army hospital for a time after the war. Undaunted I built a temporary mini-fume cupboard - the fumes being cleared by dissolving in water.

See Wikipedia on Monazite sand: - https://en.wikipedia.org/wiki/Monazite 180424
"Monazite is an important ore for thorium, [5] lanthanum, and cerium.[6] It is often found in placer deposits. India, Madagascar, and South Africa have large deposits of monazite sands. The deposits in India are particularly rich in monazite."
My comment: Nobody seems to know about the Monazite sand in Myanmarpré.

Although a few mineral grains are known to be Hadean, the oldest rock formations exposed on the surface of the Earth are Archean. [UKT ¶]

Archean rocks are found in Greenland, Siberia, the Canadian Shield, Montana and Wyoming (exposed parts of the Wyoming Craton), the Baltic Shield, Scotland, India, Brazil, western Australia, and southern Africa. [UKT ¶]

Granitic rocks predominate throughout the crystalline remnants of the surviving Archean crust. Examples include great melt sheets and voluminous plutonic masses of granite, diorite, layered intrusions, anorthosites and monzonites known as sanukitoids. Archean Eon rocks are often heavily metamorphized deep-water sediments, such as graywackes, mudstones, volcanic sediments, and banded iron formations. Volcanic activity was considerably higher than today, with numerous lava eruptions, including unusual types such as komatiite. [UKT ¶]

UKT 180425: See a video in the TIL HD-VIDEO and SD-VIDEO, Geology section,
Geological History of Earth - Geological History Documentary on Grand Canyon
- GeolHistEarthDoc<Ô> / Bkp<Ô> (link chk 180425)

Carbonate rocks are rare, indicating that the oceans were more acidic due to dissolved carbon dioxide* than during the Proterozoic. [3] [UKT ¶]

*UKT 180424: To a chemist the phrase dissolved carbon dioxide raised a red flag. The solubility of a gas in water depends on:
1. the temperature: Since the archean oceans were hotter than today's oceans, the acidity might be due to another gas, sulphur dioxide . That Burma's petroleum crude oil contains less sulphur (sweet oil) than the Arabian crude (sour oil), speaks something of the ancient superocean under which the Irrawaddy basin had lain.
2. the dissolved "salts" of various kinds: the ancient oceans were known to contain less salt, primarily sodium chloride.
Note: I am writing this note based on my memory - I still need to check more.

Greenstone belts are typical Archean formations, consisting of alternating units of metamorphosed mafic igneous and sedimentary rocks. The metamorphosed igneous rocks were derived from volcanic island arcs, while the metamorphosed sediments represent deep-sea sediments eroded from the neighboring island arcs and deposited in a forearc basin. Greenstone belts, being both types of metamorphosed rock, represent sutures between the protocontinents. [4]:302–03

The Earth's continents started to form in the Archean, although details about their formation are still being debated, due to lack of extensive geological evidence. One hypothesis is that rocks that are now in India, western Australia, and southern Africa formed a continent called Ur as of  3.1 GA (3,100 Ma). [5] [UKT¶]

A differing conflicting hypothesis is that rocks from western Australia and southern Africa were assembled in a continent called Vaalbara as far back as 3.6 Ga (3,600 Ma). [6] [UKT¶]

Although the first continents formed during this eon, rock of this age makes up only 7% of the present world's cratons; even allowing for erosion and destruction of past formations, evidence suggests that only 5–40% of the present area of continents formed during the Archean. [4] :301–02

By the end of the Archaean Eon c. 2.5 Ga (2500 Ma), plate tectonic activity may have been similar to that of the modern Earth. There are well-preserved sedimentary basins, and evidence of volcanic arcs, intracontinental rifts, continent-continent collisions and widespread globe-spanning orogenic events suggesting the assembly and destruction of one and perhaps several supercontinents. Liquid water was prevalent, and deep oceanic basins are known to have existed attested by the presence of banded iron formations, chert beds, chemical sediments and pillow basalts.

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Environment

The Archean atmosphere is thought to have nearly lacked free oxygen. Astronomers think that the Sun had about 70–75 percent of the present luminosity, yet temperatures on Earth appear to have been near modern levels after only 500 Ma of Earth's formation (the faint young Sun paradox). The presence of liquid water is evidenced by certain highly deformed gneisses produced by metamorphism of sedimentary protoliths. The moderate temperatures may reflect the presence of greater amounts of greenhouse gases than later in the Earth's history. [7] [8] Alternatively, Earth's albedo may have been lower at the time, due to less land area and cloud cover. [9]

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Early Life

The processes that gave rise to life on Earth are not completely understood, but there is substantial evidence that life came into existence either near the end of the Hadean Eon or early in the Archean Eon.

The earliest evidence for life on Earth are graphite of biogenic origin found in 3.7-billion-year-old metasedimentary rocks discovered in Western Greenland. [10]

UKT 180425: Life:
Whenever the question of Life arises, I remember: "What is Life? / It's not hereafter/ Present mirth has Present laugher/". The lines are from William Shakespeare's Twelfth Night, Act 2, Scene 3: I usually got mixed up with two words Love and Life. Here is the word Life according to Wikipedia: - https://en.wikipedia.org/wiki/Life 180425
"Life is a characteristic that distinguishes physical entities that do have biological processes, such as signaling and self-sustaining processes, from those that do not, either because such functions have ceased, or because they never had such functions and are classified as inanimate. Various forms of life exist, such as plants, animals, fungi, protists, archaea, and bacteria."

Then we have Theravada Buddhist interpretation of killing in the Five Precepts.
" Pāṇātipātā veramaṇī sikkhāpadaṃ samādiyāmi ", where Life is given as Pana {pa-Na.} «pāṇa» not {zi-wa.}. The Pali word {pa-Na.} «pāṇa» as defined in PMD by U Hoke Sein, p0658c1 is according to my interpretation "life form which is breathing". It is a narrow and very specific definition, and excludes plant life and includes animal life only. I usually muddy the situation by asking "what about the chicken egg?"

The earliest identifiable fossils consist of stromatolites, which are microbial mats formed in shallow water by cyanobacteria. The earliest stromatolites are found in 3.48 billion-year-old sandstone discovered in Western Australia. [11] [12] Stromatolites are found throughout the Archean [13] and become common late in the Archean. [4] Cyanobacteria were instrumental in creating free oxygen in the atmosphere. [14]

Further evidence for early life is found in 3.47-billon-year-old baryte, in the Warrawoona Group of Western Australia. This mineral shows sulfur fractionation of as much as 21.1%, [15] which is evidence of sulfate-reducing bacteria that metabolize sulfur-32 more readily than sulfur-34. [16]

Evidence of life in the Late Hadean is more controversial. In 2015, biogenic carbon has been detected in zircons dated to 4.1 billion years ago, but this evidence is preliminary and needs validation. [17] [18]

Earth was very hostile to life before 4.2–4.3 Ga and the conclusion is that before the Archean Eon, life as we know it would have been challenged by these environmental conditions. While life could have arisen before the Archean Eon, the conditions necessary to sustain life could not have occurred until the Archean Eon. [19]

Life in the Archean Eon was limited to simple single-celled organisms (lacking nuclei), called Prokaryota. In addition to the domain Bacteria, microfossils of the domain Archaea have also been identified. There are no known eukaryotic fossils from the earliest Archean, though they might have evolved during the Archean Eon without leaving any. [4]:306,323. No fossil evidence has been discovered for ultramicroscopic intracellular replicators such as viruses.

 

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UKT notes

 

 

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End of TIL file