Proterozoic (the next two billion years): Life Evolves

Question 1

when was the Proterozoic?

Answer 1

2.5-0.54 Ga

Question 2

what were the main events of the Proterozoic?

Answer 2

Growth and stabilisation of continents
Great Oxidation Event - oxygenation of atmosphere
The ‘Snowball’ Earth (entire Earth glaciated)
Emergence of complex lifeforms, eukaryotes

Question 3

what was the great oxidation event?

Answer 3

increase in atmospheric oxygen

Question 4

Why was there a time lag between the first photosynthesis and the great oxidation event?

Answer 4

BIFs - Banded Iron Formations

Question 5

what are Banded Iron Formations (BIFs)?

Answer 5

Chemical sedimentary rocks (from dissolved ions)
Alternating layers: silica & iron oxides precipitated from seawater
Don’t form today

Question 6

How did BIFs form?

Answer 6

Dissolved O2 bonded with dissolved Fe -> iron oxide
and
Dissolved O2 bonded with Si -> silica

Question 7

when do we see BIF formation slowing (and stopping)?

Answer 7

when atmospheric O2 rises

Question 8

what gases do volcanoes emit?

Answer 8

H2O
CO2
SO2 (oxidised S)
H2S (reduced S)
CO
H2

Question 9

what was volcanism like before 2.5 Ga?

Answer 9

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

Question 10

what was volcanism like after 2.5 Ga?

Answer 10

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

Question 11

why was the level of atmospheric oxygen constant near the beginning of the great oxidation event?

Answer 11

Oxygen used to oxidise subaerial rocks (it took time)

Question 12

what triggered the early Proterozoic glaciation?

Answer 12

Great Oxidation Event 2.5 Ga?

OR coincident event with similar cause?

Question 13

what was the great oxidation event’s effect on life?

Answer 13

O2–rich atmosphere
Mass extinction?
Complex life forms?

Question 14

what were the evolutions of eukaryotes (around 1.8 Ga)?

Answer 14

Enabled by increased O2 in the atmosphere
Single-celled life with nuclei
Can reproduce sexually -> increased genetic variation

Question 15

what was the climate during the late Proterozoic?

Answer 15

Major global glaciations (even to the equator)

Led to complex life?

Question 16

what is some of the evidence (and possible causes) for snow ball earth?

Answer 16

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)

Question 17

How to get out of a Snowball state?

Answer 17

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’

Question 18

what is some evidence for global freeze/fry?

Answer 18

limestone (reefs) shortly after ice deposits (at all locations)

Question 19

how can the evolutionary biology of the late Proterozoic be explained?

Answer 19

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

Question 20

when did the first couple life forms begin to appear?

Answer 20

Latest Proterozoic (Ediacaran)
Diverse, soft-bodied multi-celled eukaryotes -> link between single-celled algae & complex shelled organisms of Cambrian

Question 21

is there a missing Neoproterozoic fossil record?

Answer 21

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?