Proterozoic

Neoproterozoic 1 -0.545 bybp

Mesoproterozoic 1.6-1 by

Paleoproterozoic 2.5- 1.6bybp

How is the Proterozoic different from the Archean?

• 1st example of plate tectonics: brittle deformation- Wopmay Orogeny; first continental rift: Keweenawan Rift; first assembly of a supercontinent

• Continental accretion: felsic continents expand in size (ex, N.Am)

• First global glaciation: Gowganda Till; subsequent “Snowball Earth”?

• Stromatolite abundance increases; atmospheric oxygen levels build up

• Radiation of eucaryotes; Ediacaran fauna

Examples of plate tectonics• 1. Wopmay Orogeny ~2bybp in

northern Canada.• Same internal structure that we see in modern mtn

ranges: high grade metamorphic rocks; fold and thrust belts, igneous intrusions, undeformed seds

Sequence of deposition that records the Wopmay Orogeny:First, the sediment comes from the erosion of the continent,then from the uplifted mountain:

More evidence of plate tectonics…

• 2. Continental accretion: first continents were small isolated pods of felsic rock (like Iceland). These accreted, or joined together to form larger continents. The Grenville Orogeny ~1.1-0.9 bybp is an example of continental accretion..it is the youngest PC rock on the NAm continent, and it is furthest toward the edge

If you look at a map of the ages of rocks in North America you notice that the rocks get younger as you move away from the interior….this represents material that has accreted onto the edges from erosion and then orogenesis adds thismaterial onto the continental margin

More plate tectonics….• 3. Assembly of the supercontinent of Rodinia ~ .600- .550

bybp. The breakup of this supercontinent ~ 550 mybp left a thick sequence of sediments recording this rifting, exposed in the geology of western Vermont

Plate tectonics, continued…

• 4. Keweenawan Rift: 1.2-1by bp. Formation of normal-fault bounded basins, basalt lava flows, and non-marine sediments (rivers, lakes). This rifting failed, or stopped (whew!)

Proterozoic climate fluctuation• Glaciation: Gowganda Tills in southern

Canada,

Till sits on 2.6by old rocks and is intruded by 2.1 by old igneous rocks. Synchronous till are found elsewhere in the world

Neoproterozoic glaciation

• “Snowball Earth”…the hypothesis that global temps plummeted ~.7 bybp, plunging the Earth into a “icebox” world. The evidence for the snowball Earth includes glaciation at the equator, very elevated ∂O18 values, very elevated ∂C13 values. The Vendian radiation in life (the Ediacaran fauna) immediately followed, suggesting that climate warming as the snowball Earth ended helped trigger this evolutionary event.

You’ll hear the term “snowball Earth” but we now think that climate cooling isnot that drastic, hence the more recent term, “icebox Earth”

Evolution of the Atmosphere

• Once the chemical elements that combine readily with O2 (S and Fe) were satisfied, it could begin to accumulate in the atmosphere.

• When: ~2by bp are youngest BIFs, oxidized iron common after this date

Although atmospheric levels of O2 have fluctuated since then, this fluctuation is a small percentage of the change that occurred through the Paleoproterozoic as O2 levels built up

Changes in the Biosphere

• Paleoproterozoic radiation of the eucaryotes, which are thought to have evolved in the late Archean.

• Eucaryote diversification was probably related to the oxygenation of the oceans (as O2 built up in atmosphere, it dissolved in sea water) and the formation of NO3 which can be absorbed by eucaryotes, in the Paleoproterozoic

Biosphere, continued

• Spectacular evolutionary burst in the Neoproterozoic: multicellular animals- the Ediacaran fauna. Fossils of soft-bodied organisms of ~570my bp.

• Preceded by first “trace fossils” or evidence of animals burrowing through the sediment…worms?

How did we get from stromatolites to jellyfish?

• Amino acids combine to form proteins; proteins combine to form nucleic acids

• Nucleic acids RNA and DNA. RNA appears to fulfill many roles: it can be a catalyst that promotes chemical reactions in cells, it is a “messenger” that carries genetic information for the formation of proteins, and it easily replicates itself…this versatility suggests RNA was the earliest nucleic acid, employed by procaryotes in reproduction and metabolism. = “the RNA world”. DNA probably evolved much later

We have procaryotes with the ability to replicate themselves, then….

• Development of advanced cell structure: the cell nucleus. Did this happen from the union of 2 procaryotic cells, one residing within the other,i.e., one cell absorbed another but did not digest it.

• One cell specializes in metabolism through the creation of mitochondrion OR

• One of the ingested cells was a cyanobacterium, it would then evolve to specialize in photosynthesis (become a chloroplast)

• Both of the above processes would have produced unicellular eucaryotic plants or animals

Model for the evolution of the eucaryotic cell

See work of Lynn Margulis: Organismic and Evo Biology.The most primitive eucaryotic organisms are parasites, who live in the absence of O2 and lack mitochondria, living directly off their hosts.

Summary of the Precambrian: