Download Proterozoic

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 this
material 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 is
not 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 animalsthe Ediacaran fauna. Fossils of softbodied 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: