Update: 2018-05-15 06:25 PM -0400

TIL

Proterozoic Eon

archean.htm

from Wikipedia: - https://en.wikipedia.org/wiki/Proterozoic 180425

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 
 - http://www.tuninst.net , www.romabama.blogspot.com

index.htm | | Top
geol-indx.htm

Contents of this page

UKT 180515: Similar to Phanerozoic Eon, this folder should be dealing with "Eon > Era > Period".
However, remember that Proterozoic Eon belongs to Precambrian Supereon, and
Phanerozoic Eon belongs to the Present Supereon. Because of the difference in Supereons,
you won't be seeing the "Eon > Era > Period" divisions.
¤ The word
- Phanerozoic comes from Greek <phaneró(s)> 'manifest, visible' + zo- + -ic
- Proterozoic from comb. form of Greek <proteros> 'before, former, anterior'
  - http://www.dictionary.com/ 180515

Don't runaway with the notion that the boundary between Supereans are "sharp". Life in the form of blue-green algae appear in the later part of Precambrian Supereon, and there may be other life-forms such as those which can withstand high temperatures and the lack of molecular Oxygen. See Life in this file.

Introduction
1. Proterozoic record
2. Accumulation of Oxygen
3. Subduction processes
4. Tectonic history (supercontinents)
5. Life

The following are still linked to the original article:
6 See also
7 References
8 External links

 

UKT notes
 

 

Contents of this page

Introduction

The Proterozoic [1] [2] is a geological eon representing the time just before the proliferation of complex life on Earth. [UKT ¶]

The name Proterozoic comes from Greek and means "earlier life": the Greek root "protero-" means "former, earlier" and "zoic-" means "animal, living being". [3]  [UKT ¶]

The Proterozoic Eon extended from 0.250 - 0.541 Ga ago, and is the most recent part of the Precambrian Supereon (4.6 - 0.54 Ga ago). [UKT ¶]

UKT 180306: Precambrian Supereon is the only supereon defined. The second supereon would be the one to which Pheneozoic Eon, and would belong. Logically, we should define the next supereon as the Present Supereon .

"The Precambrian Supereon or Pre-Cambrian, sometimes abbreviated , or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian Supereon is so named because it preceded the Cambrian Period, the first Geologic Period of the Phanerozoic Eon." - https://en.wikipedia.org/wiki/Precambrian 180211

Proterozoic Eon can be also described as the time range between the appearance of [diatomic] oxygen O2 in Earth's atmosphere [and dissolved oxygen in the oceans and in streams] and the appearance of first complex life forms* (like trilobites or corals). It is subdivided into three geologic eras (from oldest to youngest): the Paleoproterozoic, Mesoproterozoic, and Neoproterozoic. [4]

* UKT 180426: Earliest known life forms : https://en.wikipedia.org/wiki/Earliest_known_life_forms 180426
"The earliest known life forms on Earth are putative fossilized microorganisms found in hydrothermal vent precipitates. [1] The earliest time that ife forms first appeared on Earth is unknown. They may have lived earlier than 3.77 Ga ago, possibly as early as 4.28 Ga ago, [1] not long after the oceans formed 4.41 Ga ago, and not long after the formation of the Earth 4.54 Ga ago. [1] [2] [3] [4] The earliest direct evidence of life on Earth are fossils of microorganisms permineralized in 3.465-Ga-old Australian Apex chert rocks. [5] [6]

UKT 180425: Remember not all life-forms require O2 , and there are life-forms that do not need oxygen.
See: The Earliest Anaerobic and Aerobic Life, by M. Rozotti, Biological Science Fundamentals and Systematics, vol. 1
- MRozotti-EarliAnaerobAerobLife<Ô> / Bkp<Ô> (link chk 180425)
"The first cells are thought to have been heterotrophic, i.e. to have fed on organic molecules present in the body of liquid water in which the cells originated. Reduction in the availability of dissolved organic molecules would have constituted a selection pressure favoring the emergence of autotrophy — the capacity to synthesize such molecules from carbon dioxide and other simple inorganic molecules. The first autotrophy would have occurred without oxygen."

UKT 180426: The term "organic" means "derived from organism" implying "life-forms". Since life-forms are yet to evolve, I suggest "Carbon-based molecules".

The well-identified events of this eon were the transition to an oxygenated atmosphere during the Paleoproterozoic; several glaciations, which produced the hypothesized Snowball Earth during the Cryogenian Period in the late Neoproterozoic Era; and the Ediacaran Period (635 to 541 Ma) which is characterized by the evolution of abundant soft-bodied multicellular organisms and provides us with the first obvious fossil evidence of life on earth.

Contents of this page

Proterozoic record

The geologic record of the Proterozoic Eon is more complete than that for the preceding Archean Eon. In contrast to the deep-water deposits of the Archean, the Proterozoic features many strata that were laid down in extensive shallow epicontinental seas (inland seas); furthermore, many of those rocks are less metamorphosed than are Archean ones, and many are unaltered. [5]:315 Studies of these rocks have shown that the eon continued the massive continental accretion that had begun late in the Archean Eon. [UKT ¶]

UKT 180426: Epicontinental seas (inland seas): https://en.wikipedia.org/wiki/Inland_sea_(geology) 180426
"An inland sea (also known as an epeiric sea or an epicontinental sea) is a shallow sea that covers central areas of continents during periods of high sea level that result in marine transgressions."

UKT 180426: My further quest: the inland sea which had covered the Himalaya mountains and Irrawaddy basin of modern Myanmarpré, and when did that happened. It is this sea that makes my birth country rich. As in the case of Grand River Canyon, area in question might have been covered not only once but many times during geologic time. 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 180426)

The Proterozoic Eon also featured the first definitive supercontinent cycles and wholly modern mountain building activity (orogeny). [5]:315–18, 329–32

There is evidence that the first known glaciations occurred during the Proterozoic. The first began shortly after the beginning of the Proterozoic Eon, and evidence of at least four during the Neoproterozoic Era at the end of the Proterozoic Eon, possibly climaxing with the hypothesized Snowball Earth of the Sturtian and Marinoan glaciations. [5]:320–1, 325

Contents of this page

Accumulation of Oxygen

One of the most important events of the Proterozoic Eon was the accumulation of oxygen in the Earth's atmosphere*. Though oxygen is believed to have been released by photosynthesis as far back as Archean Eon, it could not build up to any significant degree until mineral sinks of unoxidized sulfur [elemental S] and iron [elemental Fe] had been filled. [UKT ¶]

*UKT 180426: Whenever we think of the atmosphere, always do in terms of Earth's System of Spheres: Atmosphere, Hydrosphere, Lithosphere, and Biosphere. See a downloaded text in the TIL HD-PDF and SD-PDF libraries:
- UniDallas-EarthSysSphere<Ô> / Bkp<Ô> (link chk 180426)

UKT 180426: The term "accumulation of oxygen" needs to be clarified. The oxygen is the oxygen needed by Aerobic life-forms, and must be O2 in the atmosphere and in its dissolved state in the hydrosphere - i.e. oceans and streams. We always remember that "oxygen" can be O3 (Ozone), which is formed from O2 during electric discharge of thunder and lightning between clouds in the atmosphere, or between land or objects in water below. Ozone O3, is lethal to Aerobic life-forms. Other lethal gases such as oxides of nitrogen may also be produced during thunder and lightning.
See : https://en.wikipedia.org/wiki/Paleolightning 180426
"Paleolightning is the study of lightning activity throughout Earth's history. Some studies have speculated that lightning activity played a crucial role in the development of not only Earth's early atmosphere, but also early life."

Until roughly 2.3 Ga ago, oxygen O2 was probably only 1% to 2% of its current level. [5]:323 The Banded iron formations, which provide most of the world's iron ore, are one mark of that mineral sink process. Their accumulation ceased after 1.9 Ga ago, after the iron in the oceans* had all been oxidized. [5]:324

*UKT 180426: Since elemental iron and the bulk of its compounds are insoluble in water the phrase iron in the oceans seems a little strange. See: https://www.princeton.edu/~cebic/ironIIvsIII.html 180426
"Like many elements, iron Fe can exist in more than one oxidation state. The two most common forms for iron are Fe(II), in which the iron ion shares two of its electrons, and Fe(III), in which it shares three electrons.
   The oxidation state of iron is interesting and important, because it dramatically effects the solubility of iron in sea water. Very long ago, in the earliest days of life on earth, there was little or no oxygen (O2). As a result, iron was most often found in the Fe(II) state. Fe(II) is quite soluble in water, so in those days iron was probably readily available. In those days cyanobacteria, photosynthetic microorganisms responsible for fixing much of the carbon in those early days and, concurrently, manufacturing the earth's oxygen-rich atmosphere, probably had no need for special machinery for harvesting the iron it needed grow, reproduce, and to do this important work. But as atmospheric oxygen increased, iron began to prefer the Fe(III) state, which is quite insoluble in sea water. Acquiring iron then became difficult, and microorganisms began to develop specialized machinery for iron acquisition.
   This difference in solubility between Fe(II) and Fe(III) also means that iron acquisition tends to be much more of a problem for aerobic organisms than for anaerobic organisms, since anaerobic environments favor the more soluble Fe(II)."

Red beds, which are colored [reddish] by hematite Fe2O3, indicate an increase in atmospheric oxygen, O2, 2 Ga ago. Such massive iron oxide [hemtite Fe2O3] formations are not found in older rocks. [5]:324 [UKT ¶]

UKT 180427: It is said that the most important human activity in the Iron Age in which we live in, is the production of metallic iron from haematite, and preventing implements of iron from rust - going back to being haematite. We must remember that the Iron Age wars are more brutal that than those of preceding Bronze Age. The Hindu Epic Mahabharata is probably a historical account of the end of Bronze Age and the beginning of Iron Age, and coincides with the beginning of Kali Yug that we now live in. See Wikipedia:
- https://en.wikipedia.org/wiki/Yuga 180427
"Yuga in Hinduism is an epoch or era within a four-age cycle. A complete Yuga begins with the Satya Yuga, via Treta Yuga and Dvapara Yuga into a Kali Yuga. [1] Our present time is a Kali Yuga, which started at 3102 BCE with the end of the Kurukshetra War (or Mahabharata war). [2]

One of my aims of my study of Sanskrit written in Devanagari script (one of the BEPS languages) is to read Sanskrit - the language of IE speakers encroaching into our region of Tib-Bur (Tibeto-Burmese) speakers of present-day India and Népal extending into my birth-country - the present-day Myanmarpré. Our area was and still is populated by peoples speaking languages related to the Asokan (Brahmi) script. My aim is the same as that of our celebrated Buddhist monk, Shin Kicsi:

 See Section 7 Sanskrit dictionaries and grammars : Section 7 > MC-indx.htm (link chk 180427)

The oxygen buildup was probably due to two factors: a filling of the chemical sinks, and an increase in carbon burial, which sequestered organic compounds that would have otherwise been oxidized by the atmosphere. [5]:325

Contents of this page

Subduction processes

The Proterozoic Eon was a very tectonically active period in the Earth’s history. The late Archean Eon to Early Proterozoic Eon corresponds to a period of increasing crustal recycling, suggesting subduction. [UKT¶]

Evidence for this increased subduction activity comes from the abundance of old granites originating mostly after 2.6 Ga. [6] The appearance of eclogites, which metamorphic rocks created by high pressure (>1 GPa), are explained using a model that incorporates subduction. The lack of eclogites that date to the Archean Eon suggests that conditions at that time did not favor the formation of high grade metamorphism and therefore did not achieve the same levels of subduction as was occurring in the Proterozoic Eon. [7] As a result of remelting of basaltic oceanic crust due to subduction, the cores of the first continents grew large enough to withstand the crustal recycling processes. The long-term tectonic stability of those cratons is why we find continental crust ranging up to a few billion years in age. [8] It is believed that 43% of modern continental crust was formed in the Proterozoic, 39% formed in the Archean, and only 18% in the Phanerozoic. [6] Studies by Condie 2000 [citation needed] and Rino et al. 2004 [citation needed] suggest that crust production happened episodically. By isotopically calculating the ages of Proterozoic granitoids it was determined that there were several episodes of rapid increase in continental crust production. The reason for these pulses is unknown, but they seemed to have decreased in magnitude after every period. [6]

Contents of this page

Tectonic history (supercontinents)

Evidence of collision and rifting between continents raises the question as to what exactly were the movements of the Archean cratons composing Proterozoic continents. Paleomagnetic and geochronological dating mechanisms have allowed the deciphering of Precambrian Supereon tectonics. It is known that tectonic processes of the Proterozoic Eon resemble greatly the evidence of tectonic activity, such as orogenic belts or ophiolite complexes, we see today. Hence, most geologists would conclude that the Earth was active at that time. It is also commonly accepted that during the Precambrian Supereon, the Earth went through several supercontinent breakup and rebuilding cycles ( Wilson cycle). [6] [UKT ¶]

In the late Proterozoic Eon (most recent), the dominant supercontinent was Rodinia (~1000–750 Ma). It consisted of a series of continents attached to a central craton that forms the core of the North American Continent called Laurentia. An example of an orogeny (mountain building processes) associated with the construction of Rodinia is the Grenville orogeny located in Eastern North America. Rodinia formed after the breakup of the supercontinent Columbia and prior to the assemblage of the supercontinent Gondwana (~500 Ma). [9] The defining orogenic event associated with the formation of Gondwana was the collision of Africa, South America, Antarctica and Australia forming the Pan-African orogeny. [10]

Columbia was dominant in the early-mid Proterozoic and not much is known about continental assemblages before then. There are a few plausible models that explain tectonics of the early Earth pre-Columbia, but the current most plausible theory is that prior to Columbia, there were only a few independent craton formations scattered around the Earth (not necessarily a supercontinent formation like Rodinia or Columbia). [6]

Contents of this page

Life

The first advanced single-celled, eukaryotes and multi-cellular life, Francevillian Group Fossils, roughly coincides with the start of the accumulation of free oxygen. [11] This may have been due to an increase in the oxidized nitrates that eukaryotes use, as opposed to cyanobacteria. [5]:325 It was also during the Proterozoic that the first symbiotic relationships between mitochondria (found in nearly all eukaryotes) and chloroplasts (found in plants and some protists only) and their hosts evolved. [5]:321–2

The blossoming of eukaryotes such as acritarchs did not preclude the expansion of cyanobacteria; in fact, stromatolites reached their greatest abundance and diversity during the Proterozoic, peaking roughly 1200 million years ago. [5] :321–3

Classically, the boundary between the Proterozoic eon and the Phanerozoic eon was set at the base of the Cambrian Period [of Paleozoic Era] [UKT ¶]

[It is at this boundary] when the first fossils of animals including trilobites and archeocyathids appeared. In the second half of the 20th century, a number of fossil forms have been found in Proterozoic rocks, but the upper boundary of the Proterozoic has remained fixed at the base of the Cambrian Period, which is currently placed at 541 Ma.

 

Contents of this page

UKT notes

 

 

Contents of this page

End of TIL file