Week 14 Flashcards
Ice age =
Long period geological t with tendency for cold T and ice accumulation
Glaciation =
Shorter duration period within ice age
Phanerozoic eras
Cenozoic
Mesozoic
Palaeozoic
Cenozoic periods
Quaternary
Tertiary
Mesozoic periods
Cretaceous
Jurassic
Triassic
Palaeozoic periods
Permian
Carboniferous
Devonian
Silurian
Ordovician
Cambrian
Precambrian
Proterozoic
Archaean
Cenozoic
65-present
Mesozoic
250-65
Palaeozoic
543-50ma
Cretaceous
145-65ma
Jurassic
199-145ma
Triassic
251-199ma
Permian
299-251ma
Carboniferous
359-299ma
Devonian
416-359ma
Silurian
443-416ma
Ordovician
488-433ma
Cambrian
542-488ma
Proterozoic
2.5-0.54ga
Archaean
4-2.5ma
Archaean overview
32% earth’s history
Erosion = -geochemical evidence
Hydrosphere present (Greenland rounded clasts)
Probably low continental land area (small and independent; end = joined)
Blue green algae = stromatolites precipitation
Likely reducing atmosphere (no free O, CH4/CO2 dominated) - outgassing of earth’s atmosphere = free stable O at end
Life (Cyanobacteria - photosynthesis = O, red beds) 3.6-3.8Ga
Archaean atmosphere
During Hadean H released to space
‘Cold trap’/inversion/CH4 smog
—> water-rich air trapped and low UV
Global “haze”
Faint young sun paradox
Luminosity varies due to molecular mass
- over t H —> He = increases
Early sun 70% modern brightness
SO WHY IS 1ST GLACIATION ~2.7Ga and LOTS OF EVIDENCE FOR LIQUID WATER?
Explanations for faint young sun paradox
- Volcanic outgassing = more GHG
- Impact heat 4.2-3.9Ga
- Dust in atmosphere due to impacts
- Less heat loss to atmosphere
- Less weathering (less continents) = less CO2
- Lower T = less chemical weathering = less CO2
- Fewer plant species = C precipitation not forced
Forms of chemical weathering
Hydrolysis (key for removing CO2)
Dissolution
Requirements for chemical weathering
Silicate minerals (in CC)
Rainwater
CO2 from atmosphere
What are weathering rates controlled by?
Temperature
- +10’C = x2
Vegetation
- 10x compared to bare
Soil microbiology
Rain/warm/humid = more vegetation/photosynthesis/orogenic
—> negative feedback = stability
Proterozoic summary
No land plants/animals
Primitive sea life
Continents = barren rock
END- deglaciation, Cambrian explosion and rodinia splitting (W Gondwana, E Gondwana, Laurentia)
Odd thing about Proterozoic glaciations
3 major low latitude glacial events
What/when were the 3 major low latitude G events?
730-700Ma
Older cryogenian
Sturtian
665-635Ma
Younger cryogenian
Marinoan
635-542Ma
Ediacaran
Explanations for low latitude glaciations
Snowball earth
Very high obliquity
Continental unzipping
Slushball earth
Snowball earth, texts
Millions of years glaciations
Frozen seas
Runaway albedo positive feedback?
Snowball earth freeze phase
Low latitude continents have higher albedo>tropical oceans
—> extreme tropical weathering (reduces CO2)
Once ice is present from 30’ onwards = +ve feedback = snowball earth
Snowball earth thaw phase
Ice covered surfaces don’t react with volcanic CO2
Oceans frozen 5-30Ma
CO2 accumulation = super greenhouse = melt
Equator open ocean absorb more sunlight (lower albedo) = positive feedback
Evidence for snowball earth
GLACIAL
PALAEOMAGNETISM
BIFS
CAP CARBONATES
N.B possible explanations:
- rapid hothouse earth deposition
- high atmospheric CO2 after snowball earth = rapid continental weathering
- upwelling alkalinity-charged ocean deep waters
Issues with snowball earth
Could be low latitude alpine style G
Could be local O-deprived basins
Large errorbars for dates
Recovery?!
No biological record gap
- Refugia?!
Palaeomagnetism sites ~reliable
How the alternatives to snowball earth work:
V HIGH OBLIQUITY
>54’ —> strong equatorial seasonality
Lower mean T at equator than piles
But recovery?!
CONTINENTAL UNZIPPING
Rodinia break up
High plateaus = accumulate ice
Rift zone basins = BIFS
SLUSHBALL EARTH
Life can survive
Water around equator (for sedimentary rock deposition and G ice streams requiring space to increase velocity)
