Proterozoic (the next two billion years): Life Evolves Flashcards
when was the Proterozoic?
2.5-0.54 Ga
what were the main events of the Proterozoic?
Growth and stabilisation of continents
Great Oxidation Event - oxygenation of atmosphere
The ‘Snowball’ Earth (entire Earth glaciated)
Emergence of complex lifeforms, eukaryotes
what was the great oxidation event?
increase in atmospheric oxygen
Why was there a time lag between the first photosynthesis and the great oxidation event?
BIFs - Banded Iron Formations
what are Banded Iron Formations (BIFs)?
Chemical sedimentary rocks (from dissolved ions)
Alternating layers: silica & iron oxides precipitated from seawater
Don’t form today
How did BIFs form?
Dissolved O2 bonded with dissolved Fe -> iron oxide
and
Dissolved O2 bonded with Si -> silica
when do we see BIF formation slowing (and stopping)?
when atmospheric O2 rises
what gases do volcanoes emit?
H2O CO2 SO2 (oxidised S) H2S (reduced S) CO H2
what was volcanism like before 2.5 Ga?
fewer continents
most volcanism was subaqueous
volcanic degassing under high pressure (100+ bars at seafloor)
H2S> SO2 in gases (more reduced S = larger O2 sink -> anoxia)
iron transported long distances in ocean -> BIFs form, use up oxygen
what was volcanism like after 2.5 Ga?
increased continental mass
more subaerial eruptions
volcanic degassing under low pressure (1 bar)
oxidised gases (SO2>H2S) = less O2 sink -> less BIFs
free O2in atmosphere
why was the level of atmospheric oxygen constant near the beginning of the great oxidation event?
Oxygen used to oxidise subaerial rocks (it took time)
what triggered the early Proterozoic glaciation?
Great Oxidation Event 2.5 Ga?
OR coincident event with similar cause?
what was the great oxidation event’s effect on life?
O2–rich atmosphere
Mass extinction?
Complex life forms?
what were the evolutions of eukaryotes (around 1.8 Ga)?
Enabled by increased O2 in the atmosphere
Single-celled life with nuclei
Can reproduce sexually -> increased genetic variation
what was the climate during the late Proterozoic?
Major global glaciations (even to the equator)
Led to complex life?
what is some of the evidence (and possible causes) for snow ball earth?
800 Ma - supercontinent grouped at the equator
Neoproterozoic glacial deposits on all continents (low latitudes)
Glacial diamictite: sedimentary deposit - formed when clasts (“dropstones”) drop from ice floes into underlying mud -> occurrence requires ice
Glacial deposits on continents that were at the equator?
Glacial deposits in Namibia overlain by limestones
Cooler climate -> ice at lower latitudes -> planetary albedo would rise
Ice beyond a critical latitude -> positive feedback -> Surface temperatures plummet -> a completely frozen planet! (incl. 100 m ice on ocean)
How to get out of a Snowball state?
During glaciation:
Plate tectonics continue -> volcanism continues to supply CO2 to atmosphere & oceans
No liquid water on the continents, no exposed rocks -> no chemical weathering to use up CO2
CO2 builds up to incredibly high levels
Eventually CO2-induced warming offsets ice albedo
Glaciation ends (quickly, catastrophically)
‘Freeze’ then ‘fry’
what is some evidence for global freeze/fry?
limestone (reefs) shortly after ice deposits (at all locations)
how can the evolutionary biology of the late Proterozoic be explained?
Body plans of nearly all living animals now living appeared in v. short interval: 600 Ma to 525 Ma
Evolutionary biology requires more time
Did global “freeze-fry” accelerate evolutionary rates in some species?
- Crash in populations expected with global glaciation
- Re-population in selective environments (refugia) -> favour emergence of new life forms
when did the first couple life forms begin to appear?
Latest Proterozoic (Ediacaran) Diverse, soft-bodied multi-celled eukaryotes -> link between single-celled algae & complex shelled organisms of Cambrian
is there a missing Neoproterozoic fossil record?
no evident jump in the quantity/quality of the stratigraphic record to support this view
Did animals originate earlier as tiny free-floating organisms that left no fossil record? -> Bias in the fossil record?
