Precambrian Climate

Question 1

Climate and broad scale geological time?

Answer 1

• Most of the climate over geologic time is inferred
• Very low resolution
• Huge shifts in climate that make Quaternary glaciations seem trivial
• No high-resolution proxies outside Cenozoic, few outside Quaternary
• Very little known about Proterozoic + Archean
Hadean predates but we know almost nothing

Question 2

Ice ages

Answer 2

Ice Age’: long period of geological time with the tendency towards cold temperatures and ice accumulation
• ‘Glaciation’: a shorter duration period within an ‘Ice Age’

Question 3

The Archaean

Answer 3

(4.0 to 2.5 Ga)
• Much of geochemical evidence has been removed by erosion, we know very little of what occurred
• Probably reduced continental land area
• Some rounded clasts found in Greenland suggest presence of hydrosphere
• Blue-green algae likely resulted in precipitation of stromatolites
• Likely to be a reducing atmosphere (no free oxygen) dominated by CO2 and methane
• Beginning defined as the age of oldest rocks
• Ending defined as when free oxygen stable in the atmosphere
• The atmosphere results from outgassing from the Earth’s interior
• Atmosphere may have been CO2 (carbon dioxide) or CH4 (methane) dominated giving a strong greenhouse effect
• Continents were small and independent, the Archaean ended with their gradual consolidation
• Life starts around 3.6 to 3.8 Ga with cyanobacteria dominating
• Cyanobacterial photosynthesis creates oxygen
• Oxygen is consumed in the oxidation of the lithosphere (red beds)
• Eventually free oxygen remains and gradually increases in the atmosphere
When oxygen becomes freely available in atmosphere  allows for accumulation

Question 4

Possible model of atmosphere at the beginning of the Archaean?

Answer 4

• During Hadean hydrogen escaped to space
• ‘Cold trap’, or inversion, during Archaean may have held water-rich air below zone where it could be lost to space
Methane ‘smog’ acting as UV filter (after James Lovelock) ‘Proto-UV’ Filter

Question 5

‘Faint Young Sun Paradox’

Answer 5

• First brought up by Carl Sagan and George Mullen in 1972
• Luminosity (and heat produced) by the Sun depends on mean molecular mass of material in Sun
• Over time H is converted to He in Sun, increasing mean molecular mass
• Early Sun 70% as bright as modern Sun
• Decrease in solar luminosity of 5-10% from present values should result in global glaciation
• Ice-albedo feedback could maintain this state even if luminosity values were to increase to present values
• Earth should have been covered by ice
• But the first glaciers only occurred ~2.7 Ga ago? And there is plenty of evidence for liquid water?

Question 6

Possible solutions to ‘Faint Young Sun Paradox’?

Answer 6

• More Greenhouse gases (GHG) due to volcanic outgassing (CO2 or H2O, maybe NH3)
• Less weathering (fewer continents) so less
• removal of CO2 from atmosphere
• Lower temperatures would reduce chemical weathering rates thus reducing amount of CO2 – negative feedback
• Few plant species to force precipitation of carbon

Early Precambrian atmospheric CO2 concentrations may have been 30 to 300x as great as modern values
• Mean temperature of 57°C at 4.2 Ga
• Some estimates say between 85-110°C (Kasting and Ackerman, 1986)

Question 7

Chemical Weathering

Answer 7

• A major control on climate
• Two types of chemical weathering (hydrolysis and dissolution)
• Hydrolysis is the key mechanism for removing CO2 from atmosphere
• Hydrolysis requires silicate minerals in continental rocks, rainwater, and CO2 from the atmosphere
• Most of the continental crust consists of rocks made of silicate minerals such as feldspar
• Removes CO2 from atmosphere
• CO2 forms H2CO3 in soil/surface water
• Reacts with silicate rocks (weathering)
• Deposited in CaCO3 shells of marine organisms
• Finally incorporated into ocean sediments

This reaction rate is about 10 times faster than for silicates…
But…
CO2 is released back to atmosphere, therefore no net removal of CO2 and no net effect on climate

Question 8

Carbonate system

Answer 8

CO2 gas to CO2 Aqueous
CO2 (aq) + h2O = H2CO3 (carbonic acid)
H2CO3 = H+ + HCO3- (bicarbonate)
HCO3- = H+ + CO3- (carbonate)

Question 9

Silicate weathering controls

Answer 9

Temp
Vegetation
Soil microbiology

Veg + soil micro not present duing Archean

Question 10

Chemical weathering relationships?

Answer 10

Chemical Weathering:
• Chemical weathering rates roughly double for each 10°C increase in temperature.
• Rainfall is necessary for chemical weathering
• Vegetation increases chemical weathering by 10x compared to bare land.
• Also the warmer and the more humid the climate, the more vegetation and the more photosynthesis.

The Gist:
• Chemical (and mechanical) weathering drawdown CO2 from atmosphere
• Increased orogenic activity provides ample opportunity for erosion
• Works over geologic timescales

Question 11

The Proterozoic?

Answer 11

2.5 to 0.54 Ga
• Beginning defined as reflecting the rise of atmospheric O2
• No land plants or animals
• Relatively primitive life in seas
• Continents mostly barren rock
• Less atmospheric oxygen
• Ending very-well defined as the end of global glaciation and the ‘Cambrian Explosion’
• Oldest rocks of glacial origin ~2.5 Ga (late Archaean/early Proterozoic)
• Evidence of 3 major low-latitude glacial events (730-700 Ma (Older Cryogenian (Sturtian)), 665-635 Ma (Younger Crogenian (Marinoan)), and 635-542 Ma (Ediacaran).
Gowganda Formation, Huronian Supergroup, Ontario: 2.5 Ga

Question 12

Snowball Earth Theory

Answer 12

• One theory attempting to explain the presence of apparently low latitude glacial features
• There are others…
• The ‘pure’ Snowball Earth theory predicts completely frozen seas
• Glaciations lasting millions of years
• Triggered by a runaway albedo positive feedback?
• Followed by ‘Hothouse Earth’ after ice melting

Question 13

Freeze

Answer 13

• Continents at low latitudes cause higher albedo than tropical oceans
• Tropical weathering is extreme – chemically eroding silicate rocks and drawing down CO2 (no negative feedback on weathering because continents are in the tropics)
• Once ice covers Earth past 30 degrees latitude, positive feedback kicks in, resulting in ice covered planet

Question 14

Thaw

Answer 14

• Ice covered surface cannot react with atmospheric CO2 released by volcanoes
• Ice covered oceans would remain frozen for between 5-30 million years
• CO2 accumulates and becomes ‘Super Greenhouse’ melting the ice – estimates are that atmospheric CO2 would have to be 350x modern values
• Open ocean around equator would absorb more sunlight (lower albedo), thus initiating positive feedback melting remaining glaciers

Question 15

Evidence types of Snowball Earth

Answer 15

• Should survive only in places where easily preserved
• Glacial drift (moraines, tills, etc)
• Scratched surfaces
• Dropstones
• Buried under sedimentary rock
• Evidence relatively widespread
• Geochemistry
Palaeogeography seems to indicate glaciation near equator
Palaeomagnetic data suggest low-latitude ice sheets

Question 16

Snowball Evidence: Palaeomagnetism

Answer 16

• Most important piece of evidence
• Difficult to determine if rocks have been deformed, tilted, buried, and heated for 700 million years
• Rocks can be corrected – if all measurements agree high likelihood of correctness
• Many measurements seem to indicate latitudes less than 30º from equator
• None further than 60º from equator
• Apparently, no land at all at poles
Most land masses were gathered along equator (Rodinia)

Question 17

Snowball Evidence: BIFs

Answer 17

• Banded Iron Formations: sedimentary deposits rich in iron
• First piece of evidence that led to ‘Snowball’ suggestion
• BIFs first found in Neoproterozoic oceanic sediment by Joe Kirschvink
• BIFs occur above glacial deposits around the world
• None found for a billion years previously in rock record, none have formed since
• Their formation requires the accumulation of large amounts of dissolved Fe in water
• This doesn’t happen because Fe immediately reacts with O and precipitates out of water column: Fe2+ (aqueous) + O2 ßà Fe(3+)O2 (solid)
• Reasoning goes that Fe2+ is added to oceans by volcanism and it accumulates under the cover of sea ice
• Fe oxidised rapidly when ice melts and deposits a BIF

Question 18

Snowball Evidence: Cap Carbonates

Answer 18

• Immediately overlying glacial deposits are sedimentary limestone units termed ‘cap carbonates’
• Typically, 3-30 meters thick
• Occurred on platforms, shelves and slopes worldwide
• Occurred even in regions otherwise lacking carbonate strata
• “knife-sharp” contacts with underlying glacial deposits (diamictites, debris flows)
• Show characteristic features of rapid deposition
• May contain abundant iridium (Bodiselitsch et al., 2005, Science)
• Comes from meteors
• What contribution did space make?
Normal grey and green units such as diamictite
Then instantaneous cap dolostone catastrophic deposition

Question 19

Issues with Snowball Earth?

Answer 19

• Could the glaciation have occurred at low latitudes but be restricted to ‘Alpine’-style glaciers?
• Could the BIFs have been deposited in a local oxygen- deprived basin?
• How does the Earth recover from being completely ice covered?
• Earth’s temperature has remained in a relatively narrow band for most of Earth’s history
• Some estimates put mean temperature during Neoproterozoic glaciations at close to -50ºC
What could have caused such an amazing departure from the norm?
Large uncertainties in dating and palaeolatitude

Biggest issue:
• Almost no gap in biological record
• Should most life not have gone extinct?
• Faunal diversity nearly unchanged through ‘Snowball Earth’ events
• Stromatolite deposition unperturbed
Stromatolites (that like warm, shallow water) are seen to be unaffected

Question 20

New palaeolaltuitude techniques?

Answer 20

• Producing better dates
• Re-Os dates with reasonably low errors +/- ~10 million years
• “…refutes previous…studies which correlated the formation with a series of middle (ca. 715 Ma) Cryogenian glacials.”
• One glaciation took place at
• 662.4 +/- 3.9 Mya
• Lasted 55 million years

Question 21

Alternate Snowball Earth explanations?

Answer 21

• ‘Very high obliquity’:
– (>54°) would cause strong equatorial seasonality
– Would cause lower mean annual T at equator than at poles
– Reasonable at early stage in Earth history (chaotic orbit)
• ‘Continental unzipping’:
– Break-up of Rodinia
– Would cause high plateaus where ice could accumulate
– Restricted basins in rift zones could have chemistry sufficient to produce BIFs

Question 22

‘Slushball Earth’:

Answer 22

– Essentially the same ideas as ‘Snowball Earth’ except with oceans that are not completely frozen
– Would allow life to persist more easily
– Water would surround planet at equator
– Some sedimentary rocks deposited at the time need open ocean
– Some glacially derived rocks need fast flowing ice: this would need space to gain velocity