GEOL 102 Historical Geology
Spring Semester 2017
The Proterozoic Eon
Proterozoic Eon: 2.5 - 0.542 Ga
|Neoproterozoic ||1000 - 542 Ma
|Mesoproterozoic ||1.6 - 1.0 Ga
|Paleoproterozoic ||2.5 - 1.6 Ga
Global and Regional Geology of the Proterozoic
Proterozoic is distinct from Archean in:
- Clear evidence of "modern-style" (continent-scale) plate tectonics
- Decrease in abundance of komatiites & greenstone belts
- HUGE deposits of Banded Iron Formations during 1rst half of Paleoproterozoic
- Complex unicellular, and first multicellular life
- Indications of higher levels of oxygen
The 2.5 Ga boundary is somewhat arbitrary: shift from "Archean" to "Proterozoic"-style
crust begins at 2.95 Ga in southern Africa, but not until 2.6 in North Ameirca, and
after 2.45 in some other regions.
Lithologies of the Proterozoic:
- Stromatolites VERY abundant throughout
- BIFs VERY abundant during first half of Paleoproterozoic (92% of all BIFs deposited
from 2.5-2.0 Ga); reappear in brief intervals associated with Neoproterozoic
- Rare greenstone belts
- Paleoproterozoic glacial deposits, especially the Huronian Glaciations (at least 3 pulses between 2.45 and 2.2 Ga in central Canada):
- Some of the oldest known tillites deposited on striations is Gowganda Formation
(c. 2.3 Ga, Great Lakes region, Canada)
- Possible older one at 2.7 Ga in Bruce Formation of Ontario, and diamictite in 2.9 Ga Pongola Supergroup in southern Africa
- Also, some carbon isotope evidence for large scale glaciation in Paleoproterozoic
- Last of the detrital uranite & pyrite at 2.3 Ga
- First red beds (arkoses, reddish shales, etc.) at 2.0-1.8 Ga
The Great Oxidation Event: Prokaryotic photosynthesizers, included the newly-evolved cyanobacteria (and eventually eukaryotic algae) release more and more oxygen into atmosphere. Between 2.7 and 2.4
Ga, most of this oxygen got absorbed by the copious levels of dissolved iron in the ocean water. This produced "rust", which accumulated on
the sea floors as Banded Iron Formations (or BIFs).
When the dissolved iron was all used up, BIF production stopped and the oxygen began to add to the atmosphere (and as dissolved oxygen in the
water). Atmospheric levels of oxygen rises to about 10% of modern levels. The Great Oxidation Event produced the modern oxygen-nitrogen
- Reducing components (methane, ammonia, etc.) were oxidized and scrubbed out of the skies
- On the surface of the land, iron-rich minerals rusted (which is why they turn red) and some material which was once stable became instable
- A curious side effect: the oceans would have been green with iron, and the skies orange with methane, prior to the Great Oxidation Event. After
the event, both skies and oceans turned blue
- Loss of greenhouse gasses result in lowering Earth's temperature, likely responsible for Huronian and related glaciations. Greatest of these,
at about 2.2 Ga, is the Makganyene event: oldest of the Snowball Earth events.
The Great Oxidation Event would have devastated anaerobic organisms, which from that point onward would survive only in "extreme" environments.
Paleoproterozoic saw the suturing together of many small Archean cratons to form much
larger continents: the formation of the large cratons of today:
- Kenoran Orogeny:
- Archean-Proterozoic boundary
- Evidenced only by granitic intrusives into granitoid-greenstone complex in Superior
Province (an Archean craton), Canada
- Oldest known evidence of major orogeny
- Wopmay Orogeny:
- In Slave Province (an Archean craton), Canada
- Oldest well-studied modern style orogeny with passive margin -> flysch -> molasse
-> passive margin sequence
- Caused by collision of Slave Province with some other craton, 2.0 Ga
Recent work has suggested the following Paleoproterozoic continents and supercontinents had formed by accretion and suturing of Archean provinces:
- 2.5 Ga: Arctica (perhaps associated with the Kenoran orogeny?): various northern and central Canadian, Greenland, and Siberian cratons
- 2.1 Ga: Baltica, northwestern Europe
- 2.0 Ga: Atlantica: various cratons in modern South
America and Africa, later incorporated into western Gondwana
- 1.8 Ga: Laurentia: the core of North America and
- 1.6 Ga: Nena/Nuna/Columbia, fusion of Laurentia, Siberia, and Baltica (forshadowing later Laurussia and Laurasia).
- Many other cratons established during this Era
The Paleoproterozoic Era has recently been subdivided into the Siderian ("iron", after banded iron: 2.50 - 2.30 Ga), Rhyacian ("streams of lava": 2.30 - 2.05 Ga), Orosirian ("mountain building": 2.05 - 1.80 Ga), and Statherian ("stabilization": 1.80 - 1.60 Ga) Periods. The boundaries are arbitrarily defined, but their names are derived from prominent geologic processes occurring at that time.
Recently the Mesoproterozoic Era has been divided into the following Periods (with arbitrarily defined boundaries): Calymmian ("covering", after the development of platforms: 1.60 - 1.40 Ga), Ectasian ("extension", both of platforms and of the beginnings of the rifting events: 1.40 - 1.20 Ga), and Stennian ("narrowing", after narrow belts of intense metamorphism and deformation, such as the Grenville Orogeny: 1.20 - 1.00 Ga).
Mesoproterozoic saw high amounts of igneous activity. It may be due to large-scale mantle "superplumes" forming underneath the first continental or supercontinental (not yet observed) granitic masses.
- Keweenawan (failed) Rift:
- Mid-continental rift through middle of Laurentia, about 1.2-1.0 Ga
- Had potential to split Laurentia in two, but did not finish the job
- Giant basaltic flows (flood basalts) around 1.3 Ga.
- Dike "swarms" throughout much of Laurentia, and elsewhere, at this time (1.267 Ga); was an earlier similar event at 2.45 Ga).
Belt Supergroup: a huge (16 km thick!) sequence of terrestrial mudstones, shallow marine sandstones & limestones & turbidites in western North America, deposited from 1.45 to 0.85 Ga. A huge filled-in down-warped basin of western Laurentia.
During late Mesoproterozoic, assembly of the first well-known supercontinent
Rodinia (newer reconstruction; requires online access to Lyell collection):
- Grenville Orogeny:
- Down eastern ridge of Laurentian craton, represents continent-continent collision, c. 1.3-1.0 Ga
- Laurentia apparently near center of Rodinia: other cratons assembled around it.
- The Grenville Orogen + the pre-orogeny Laurentian craton represent 75% of North
America + Greenland; everything else added during Phanerozoic.
[Someone has created the Rodinian National Anthem]
(Despite all this, the period from 1.8 to 0.8 Ga (basically the Mesoproterozoic Era and Tonian Period) are nicknamed "The Boring Billion", because much less major change in Earth's geology, atmosphere, and biology than the time before or the time afterwards).
During Neoproterozoic, Rodinia rifts apart around 750 Ma: birth of Pacific Ocean (although generally we call it the Panthalassic Ocean
until the break up of Pangaea in the Mesozoic).
From this point onward, western Laurentia never has contact with continental-size masses (although much of the far western part of North America was accreted during the Phanerozoic).
The Pan-African Orogeny (and related orogenies):
The Neoproterozoic Era has recently been divided into three Periods: Tonian ("stretching", after continued expansion of the platform covers": 1000 - 720 Ma), Cryogenian ("ice origins", after the Snowball Earth glaciations: 720 - 635 Ma), and Ediacaran (After Ediacara Springs, Australia: 635 - 542 Ma). The boundaries of the Tonian are arbitrarily defined, while the uppermost boundary of the Cryogenian is the the end of the Marinoan glaciation, and the uppermost boundary of the Ediacaran is the first appearance of the trace fossil Trichophycus (or Treptichnus) pedum (and thus the oldest biostratigraphically-determined boundary).
Climates and Atmospheres of the Proterozoic
Good evidence of rising levels of oxygen in atmosphere (and hence oceans):
Suggests atmosphere of about 1% of atmosphere was oxygen at end of Archean, rising to 10% (or half of modern level) by end of Proterozoic.
- BIFs may require extremely low levels of oxygen, but clearly cannot form at higher levels: their presence in Archean and early Paleoproterozoic and almost total absence after 2.0 Ga suggests that levels had risen too high by 2.0 Ga
- Uranite and pyrite are unstable at surface temperatures and pressures in presence of atmospheric oxygen; disappear by 2.3 Ga
- Red beds require oxygen to form: do not appear until 2.0-1.8 Ga
- Stromatolites (and thus potential oxygenators of atmosphere) common throughout Proterozoic
During Paleoproterozoic, good evidence of widespread glaciation (tillites,
striations, carbon shifts).
During Neoproterozoic (in particular, from c. 720-580 Ma), evidence for superglaciations:
- Carbon isotope shifts far greater than seen during Phanerozoic glaciations
- Tillites, striations, etc. in paleoequatorial regions
Strong evidence for a Neoproterozoic Snowball Earth
- Break-up of Rodinia leaves many smaller continents near equator
- Climate change due to new weather patterns: lots of carbon dioxide sucked out of atmosphere due to erosion
- Temperature drops due to reduced greenhouse, pack ice builds up on poles and continental glaciers in mountains
- Positive feedback loop: ice increases albedo, drops temperature, ice spreads, albedo increases, etc.
- Pole-to-equator-to-pole glaciation:
- Hydrologic cycle stops (and thus erosion stops)
- Ice thickens to 100s of m to 1 km, so photic zone forbidden to life; oceans become anoxic (reappearance of BIFs)
- Life clings to refuges in ice cracks & along mid-ocean ridges
- Glaciers cannot build up after end of hydrologic cycle (no new snow!), and gradual creep off of continents
- Glacial conditions persist until slow (many Myrs or so?) build up of carbon dioxide due to volcanic activity to form a supergreenhouse (many MANY times modern levels)
- EXTREMELY fast melting, superhot world, massive rapid deposition of carbonates, massive blooms of organisms
- Cycle seems to have happened at least three (possibly four or more times in Neoproterozoic: the Sturtian (recently dated from 717-660 Ma), Marinoan (~640-635 Ma), and Gaskiers (~580 Ma) are the best studied and established, but there may be others.
The reason Neoproterozoic superglaciations stop and haven't been seen again may be due to the rise of complex animals, which were able to liberate the carbon in the sediment through bottom-feeding (previously was simply trapped in sediment). However, another factor may be the ever-increasing brightness of the Sun.
Life in the Proterozoic
Stromatolites still common, becoming even more common c. 2.2 Ga, becoming more complex c. 1.2 Ga.
Origin of eukaryotes by endosymbiosis:
- Some large prokaryote (almost certainly an archaean) has close association (symbiosis) with a smaller bacterium (specifically a proteobacterium):
the latter provides chemical energy as a byproduct of its metabolism
- The larger prokaryote absorbs the smaller bacterium into its cell: the smaller proteobacterium is the ancestor of mitochondria
- Presence of mitochondria allows for more energetic heterotrophic life style
- In one branch of new eukaryotes (the ancestor of plants), a second case of endosymbiosis occurs when cyanobacteria become absorbed into the cell: origin of chloroplasts
- Eukaryotic fossils date back to about 1.68-1.78 Ga (chemical signatures from 2.7 Ga or so, but these may be the prokaryote ancestors of eukaryotes)
Eukaryotes remain unicellular for most of the Proterozoic. Life remains entirely aquatic, but food chains get more complex with diversifying levels of heterotroph consumers and detritivores and phototroph producers.
Fossils from Bitter Springs Formation of Australia (1 Ga) looks as if they show cells dividing.
Oldest known acritarchs (fossils of uncertain origin: some likely cysts of some kind of fossil photosynthesizers, possibly dinoflagellates, others are eggs of animals) at 1.4 Ga; become complex around 600 Ma; the main index fossils for the Proterozoic
Metabionts: multicellular organisms include:
- Rhodophyta (red algae, some make calcareous skeletons): may form a larger group Primoplantae with the viridiplants (green plants) and theunicellular glaucophytes
- Phaeophyta (brown algae or kelp; a multicellular group within Chromista, and thus relatives of diatoms and coccolithophorids)
- Viridiplantae, or green algae and plants: basal forms are unicellular, but paraphyletic Charophyta (one of the green algae) and Plantae or Embryophyta (true plants) are multicellular
- Fungi: together with animals and various unicellular groups form larger clade Opisthokonta
- Metazoa (animals): closest relatives are unicellular choanoflagellates; animal phylogenetic interrelationships are a matter of a LOT of study!
Some of these groups have records back to the Proterozoic:
- Possible rhodophytes from 1.25 Ga, but only clearly present from Cambrian
- Possible phaeophytes from Ediacaran Period but not particularly convincing: better possible Ordovician forms, and oldest definite kelps not until Cenozoic
- Possible charophytes from Neoproterozoic, but not definite until Cambrian (true plants do not show up until Ordovician)
- Fungi not clearly present until Ordovician: no strong argument for multicellular fungi in Proterozoic (unless some of the "vendobionts" arelichens)
Animal record is somewhat better, but still much debate:
- Molecular data suggests many true animal lineages had diverged at around 1 Ga
- Some controversial trace fossils from c. 800 Ma: only animals (with muscles) can move sediment around
- Possible animal-generated trace fossils by 570 Ma (but might be generated by giant single-celled eukaryotes!!)
- Phosphatized clusters of cells from Doushantou Fm., China from 600 Ma were thought to be animal embryos and eggs, but new evidence from later show these are most likely the cysts of protist-grade eukaryotes rather than animals
- Tiny (1 mm or smaller) calcified shells:
- The Ediacaran fauna (from Ediacara Hills, Australia, and elsewhere from c. 570 Ma onward)
The Ediacaran fauna is preserved only as impressions in sediment. Indicate a variety of organisms. Originally were pigeon-holed into modern groups, but new evidence suggests that a number of types of animals are present:
None of the Ediacaran fauna apparently burrowed into the sediment, nor had preservable hard parts.
- Early cnidarians (members of the coral-sea anemone-jellyfish group), large polyps with their butts in the sand
- A unique (and extinct) collection of multicellular forms with a quilt-like body construction: see here and here and here.
- May have fed by osmotrophy (absorbing food over their body surface), since there is no evidence for mouths or anus in these creatures
- Some up to 1 m or more across, but thinner than a slice of bread
- Recent work suggests that there are several branches of these, some outside of true Metazoa, some within Metazoa
- Kimberella: apparently an early mollusk or mollusk-relative; known from impressions of its foot and probable grazing marks
Interestingly, almost all the evidence for multicellular organisms comes AFTER the Gaskiers Glaciation. Some speculate that selective pressures from these hard times led to development of complex life; others that the appearance of creatures able to mobilize the carbon in the sea sediments kept atmospheric carbon dioxide from getting low enough to trigger the super ice ages.
Recent studies suggest that the
size (and complexity) increases seen with the rise of endosymbiosis and of multicellularity both coincide with major increases in the
amount of oxygen available.
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Last modified: 18 January 2017